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• Digestion And Absorption

Digestion and Absorption

As we have learned, digestion is the simple process of breaking down food molecules into smaller components. This process begins from the mouth and is then carried on to the stomach, to the small intestine, large intestine and then to the anus. This is the physical process of digestion. Here, let us learn more in detail about the chemical process of digestion involved while digesting biomolecules.

Table of Contents

What is Digestion?

What is absorption, digestion and absorption of carbohydrates, digestion and absorption of proteins, digestion and absorption of lipids, frequently asked questions.

Digestion is the process of breaking large, insoluble food molecules into smaller molecules for absorption into the bloodstream. This process involves the use of many digestive fluids and enzymes such as saliva, mucus, bile and hydrochloric acid, among others.

There are four primary stages of food digestion in the human body that include:

• After the intake of food through the mouth, it makes its way through the stomach into the small intestine, where it is digested.
• The nutrients from the digested food get absorbed into the bloodstream through small pores in the small intestine.
• The remaining undigested food is sent to the large intestine, where any unprocessed water or nutrients are reabsorbed into the body.
• The remaining waste food product is passed out of the body in the form of stools.

Absorption is the process of the absorbing or assimilating substances into the cells or across the tissues and organs through the process of diffusion or osmosis. Also refer: Difference between Osmosis and Diffusion

Carbohydrates are one of the essential nutrients in the human diet. There are two types of carbohydrates that can be digested by the human digestive system – sugar and starch.

Sugar is broken down in the gastrointestinal tract by the small intestine and three enzymes present in the mouth, namely, Lactase, Sucrase, and Maltase.

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Lipids are organic compounds comprising fatty acids, which are insoluble in water. Fats are the most common examples of lipids. The insoluble property of lipids makes the digestion and absorption of fats a complicated process.

Since they are hydrophobic, fats stick together as a large glob of insoluble mass after reaching the stomach. It is broken down with the help of bile juice, which contains bile salts. These broken molecules are then acted upon by pancreatic lipase, the major fat-absorbing enzymes in the body.

Pancreatic lipase breaks down fats into tiny molecules of free fatty acids and monoglycerides, which are small enough for the small intestine to push through into the bloodstream.

Also refer:  Lipids

For more detailed information about Digestion and Absorption, its process or any other related topics and for digestion and absorption class 11 revision notes explore @ BYJU’S Biology .

What is protein digestion?

Protein is one of the essential compounds in our body. Human saliva contains the enzymes lipase and amylase. They mainly digest fats and carbohydrates. As soon as we start chewing, protein digestion begins. Protein digestion first breaks the complex molecule into peptides containing various amino acids and then into individual amino acids.

Where does protein digestion begin and end?

Protein digestion begins when you first start chewing and concludes in the small intestine. To produce more proteins, the body reuses amino acids.

Digestion of both starch and protein is done by ___________.

Digestion of both starch and protein is done by the pancreatic juice. The pancreatic enzymes include lipase, which breaks down triglycerides into fatty acids and monoglycerides. Trypsin, chymotrypsin, and elastase all break down proteins. Amylase breaks down the excess complex carbohydrates into monosaccharides. Therefore, the digestion of carbohydrates, protein, and fat is unattainable without the pancreas.

How are proteins in your food digested?

Proteins undergo hydrolysis, which converts them into amino acids, during digestion. The amino acids are dissolved in our blood and transported to organs and tissues. The amino acids are either converted into energy or put together into proteins using condensation polymerization.

How are proteins digested in small intestine?

The partially digested meal, or chyme, next passes into the small intestine, where intestinal fluids and pancreatic enzymes exist. The pancreatic juices and intestinal secretions induce several chemical reactions. These include the enzymes chymotrypsin, trypsin, and elastase, which break down the protein into smaller peptides. Carboxypeptidase, aminopeptidase, and dipeptidase break down the peptides into free amino acids, which can then enter the bloodstream. The pepsin enzyme dissolves peptide bonds and helps in the initial stages of protein digestion in the stomach.

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7 The Process of Digestion and Absorption

• The Body’s Organ System
• How Your Digestive System Works
• Probiotics and Prebiotics
• Common Digestive Diseases and Disorders

introduction

When you smell coffee or fresh baked cookies, what happens? Do those aromas stimulate your desire to drink or eat? The sight and smell of foods are enough to prime your digestive tract and stimulate saliva production. This chapter will look at the steps of digestion and absorption and how your body breaks down the food into usable components. Additionally, we will investigate digestive diseases and disorders.

Human Anatomy & Systems

Human bodies are made of a system of cells.  Cells are the basic structure and functional unit of all life; groups of cells form tissues, and tissues form organs.  The human body depends on organs working together, as organ systems, to do specific jobs in the body.

The Eleven Organ Systems in Your Body

 The Eleven Organ Systems in the Human Body and Their Major Functions

We obtain nutrition by eating food.  Unlike gasoline, which is perfectly designed to power a car, food is not ready for the body to use in its current form.  The tacos you ate last night doesn’t go right into your body and power your body; A LOT has to happen to it for the body to be able to use it.  The process of transforming food into usable nutrition for the body is called digestion.  There are 4 steps to digestion:

• Break down the food into tiny pieces.
• Absorb nutrition into the body:  move the small particles out of the digestive system and the rest of the body.
• Get rid of the waste, which is anything your body can’t use.

The digestive tract is a tube through the body, starting at the mouth and ending with the anus.  The digestive system includes the mouth, esophagus, stomach, small intestine, pancreas, liver, gallbladder, and large intestine.

Watch the Video on Digestion and Absorption

As food moves through the body, it is broken down by mechanical and chemical breakdown.

Mechanical breakdown is when you physically break food into pieces.  Mechanical breakdown starts in the mouth, with the teeth tearing, ripping, and grinding food into smaller pieces.  The mechanical breakdown also occurs with the muscular action of the esophagus, stomach, and small intestines.  Have you ever bought a bag of grated cheese, and it’s all clumped up, so you whack the bag on the counter or massage it with your fingers to break up all the cheese?  That’s essentially what the body does; the digestive system’s muscles massage and separate and move food along through your body.  This process is strong and happens even if working against gravity; if you drink a glass of water and then quickly do a headstand, your body will still be able to move that water through the digestive system even if you are upside down.

The chemical breakdown is when the body makes enzymes (chemicals) that break food down into small molecules.  Enzymes are added to the mechanically broken down food from the salivary glands, stomach, pancreas, and small intestine.  Additionally, the liver makes a chemical called bile, stored in the gallbladder, which helps the body digest fat.  These chemicals are necessary to break the food down into molecules.

On the first half of the journey of a piece of food through the body, mechanical and chemical breakdown occurs.  However, in the small intestine, absorption is also happening:  the body absorbs the molecules from the food, taking them through the intestine wall and into the blood where the energy and building blocks can be delivered throughout the body.  But hold on, the process of digestion is not done yet!  No breakdown occurs in the large intestine, but some molecules and reabsorption of water are absorbed.  Lastly, the large intestine packages and pushes the remaining undigested and indigestible food out of the body through the anus; it eliminates the waste.  Digestion is complete!

Can You Identify The Digestive Organs?

What should your indigestible food look like as it exits your body?

Bristol Stool Chart

Use the link below to access the Bristol Stool Chart and then compare the categories to your stools.

https://cdhf.ca/digestive-disorders/constipation/signs-and-symptoms/

Your Gut Microbiome

Another feature of the large intestine is that it is where an enormous quantity of bacteria live.  That may seem kind of gross to discover that your gut is home to more bacteria than there are cells in your body, but most of these bacteria are harmless, and some are even beneficial.  You may have heard about probiotics and prebiotics in the news or seen them listed on food labels.  Probiotics are defined as live bacteria that provide health benefits to the host (us), such as warding off diarrhea and reducing the effects of lactose intolerance.  On the other hand, prebiotics is a type of fiber in some foods, which serves as a food source for probiotics.  An analogy is to think of your gut as a farm.  The probiotics are farm animals; the more farm animals on the farm, the better the farm is doing.  But to keep the farm animals in good condition, you have to feed them.  The hay you feed them is the prebiotics.  We call this ‘farm’ your gut biome; the more and the healthier the probiotics, the healthier your gut biome.  Foods such as yogurt contain probiotics, which can add to your gut biome.  Some foods are prebiotics, like onions, bananas, and oats, which can feed your gut biome.  A strong gut biome is thought to provide a host of health benefits.

Watch the Video on Pro/PreBiotics

Digestive Diseases and Disorders

Many people suffer from digestive irregularity from time to time.

• LACTOSE INTOLERANCE:  Lactose intolerance happens when the body can’t break down the sugar found in milk, which causes gas, bloating, pain, and diarrhea.  People can avoid dairy products or take supplements such as Lactaid to digest the sugar (lactose) found in milk.
• DIARRHEA:  Diarrhea is a term used to describe very watery stools.  It typically occurs with stomach pain and frequent trips to the bathroom. Diarrhea can be the symptom of stress, foodborne illness, or other infection.  If a person has diarrhea, they should pay special attention to hydration as they lose more water than normal through diarrhea.  Acute diarrhea often resolves on its own and needs no other treatment than rest, hydration, and over the counter medication.  On the other hand, it can be a symptom of a more severe disorder and disease.
• CONSTIPATION:  Constipation occurs when a person has difficulty passing stool and generally has three or fewer bowel movements a week.  There can be numerous causes of constipation, including underlying digestive disorders and diseases, hormonal imbalances such as thyroid issues, or the slowing of food transit that happens in pregnancy.  The first line of defense against constipation is increasing water, dietary fiber, and activity levels.

Many diseases and disorders related to the digestive system.  Unbalanced diets can cause/effect disease; for example, high-fat diets can exacerbate GERD, IBS, and IBD symptoms.

• PEPTIC ULCERS: Peptic ulcers are sores in the stomach lining brought on by microbiome disturbances, stress, and infection.  GERD medications are often provided to decrease stomach acid and allow healing.
• GERD:  Gastroesophageal Reflux Disease.  This disease is a type of persistent acid reflux that tends to occur in people with an unbalanced high-fat diet.  Additionally, some food and drink such as spicy foods and chocolate make the symptoms worse.  Treatment for GERD is making lifestyle changes, including dietary modifications to reduce weight and pressure on the stomach.
• IBS:  Irritable Bowel Syndrome.  This syndrome is characterized by muscle spasms in the colon, resulting in abdominal pain, bloating, constipation, and/or diarrhea.  An unbalanced diet and stress primarily cause IBS.  Treatment includes diet and lifestyle modifications.
• IBD:  Irritable Bowel Disease.  This disease is inflammation and damage to the GI tract.  If the damage is mostly in the small intestines, it is called Crohn’s Disease; if it is only in the large intestine, it is called Ulcerative Colitis (UC).  The cause is unknown.  The proper course of treatment is unclear but may include drugs and surgery.
• Celiac Disease:  This disease is an autoimmune disorder that damages the villi in the small intestines, resulting in IBS and IBD symptoms.  Damage is caused by the body overreacting to gluten, a common component in grains and found in most bread.  Once diagnosed properly, a person who avoids gluten will heal their gut and stop the negative symptoms within six months.

What is Gluten?

Learning Objectives

• Identify the parts of the Digestive System:  mouth, esophagus, stomach, small intestine, pancreas, liver, gallbladder, large intestine. (MCCCD Competency 10)
• Define what probiotics and prebiotics and their contribution to health. (MCCCD Competency 6)
• Define common digestive diseases and disorders, including GERD/Gastritis, Crohn’s Disease, Ulcerative Colitis, Constipation, Diarrhea, Celiac disease, and lactose intolerance. (MCCCD Competency 10)

Nutrition Essentials Copyright © 2020 by Stephanie Green and Kelli Shallal is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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Protein Digestion and Absorption

When you eat food, the body’s digestive system breaks down dietary protein into individual amino acids, which are absorbed and used by cells to build other proteins and a few other macromolecules, such as DNA. Let’s follow the path that proteins take down the gastrointestinal tract and into the circulatory system.

Eggs are a good dietary source of protein and will be used as our example as we discuss the processes of digestion and absorption of protein. One egg, whether raw, hard-boiled, scrambled, or fried, supplies about six grams of protein.

In the image below, follow the numbers to see what happens to the protein in our egg at each site of digestion.

Fig. 6.17. Protein digestion in the human GI tract.

1 – Protein digestion in the mouth

Unless you are eating it raw, the first step in digesting an egg (or any other solid food) is chewing. The teeth begin the mechanical breakdown of large egg pieces into smaller pieces that can be swallowed. The salivary glands secrete saliva to aid swallowing and the passage of the partially mashed egg through the esophagus.

2 – Protein digestion in the stomach

The mashed egg pieces enter the stomach from the esophagus. As illustrated in the image below, both mechanical and chemical digestion take place in the stomach. The stomach releases gastric juices containing hydrochloric acid and the enzyme, pepsin , which initiate the chemical digestion of protein. Muscular contractions, called peristalsis, also aid in digestion. The powerful stomach contractions churn the partially digested protein into a more uniform mixture, which is called chyme.

Fig. 6.18. Protein digestion in the stomach

Because of the hydrochloric acid in the stomach, it has a very low pH of 1.5-3.5. The acidity of the stomach causes food proteins to denature, unfolding their three-dimensional structure to reveal just the polypeptide chain. This is the first step of chemical digestion of proteins. Recall that the three-dimensional structure of a protein is essential to its function, so denaturation in the stomach also destroys protein function . (This is why a protein such as insulin can’t be taken as an oral medication. Its function is destroyed in the digestive tract, first by denaturation and then further by enzymatic digestion. Instead, it has to be injected so that it is absorbed intact into the bloodstream.)

Fig. 6.19. In the stomach, proteins are denatured because of the acidity of hydrochloric acid.

Once proteins are denatured in the stomach, the peptide bonds linking amino acids together are more accessible for enzymatic digestion. That process is started by pepsin , an enzyme that is secreted by the cells that line the stomach and is activated by hydrochloric acid. Pepsin begins breaking peptide bonds, creating shorter polypeptides.

Fig. 6.20. Enzymatic digestion of proteins begins in the stomach with the action of the enzyme pepsin.

Proteins are large globular molecules, and their chemical breakdown requires time and mixing. Protein digestion in the stomach takes a longer time than carbohydrate digestion, but a shorter time than fat digestion. Eating a high-protein meal increases the amount of time required to sufficiently break down the meal in the stomach. Food remains in the stomach longer, making you feel full longer.

3 – Protein digestion and absorption in the small intestine

The chyme leaves the stomach and enters the small intestine, where the majority of protein digestion occurs. The pancreas secretes digestive juices into the small intestine, and these contain more enzymes to further break down polypeptides.

The two major pancreatic enzymes that digest proteins in the small intestine are chymotrypsin and trypsin . Trypsin activates other protein-digesting enzymes called proteases , and together, these enzymes break proteins down to tripeptides, dipeptides, and individual amino acids. The cells that line the small intestine release additional enzymes that also contribute to the enzymatic digestion of polypeptides.

Tripeptides, dipeptides, and single amino acids enter the enterocytes of the small intestine using active transport systems, which require ATP. Once inside, the tripeptides and dipeptides are all broken down to single amino acids, which are absorbed into the bloodstream. There are several different types of transport systems to accommodate different types of amino acids. Amino acids with structural similarities end up competing to use these transporters. That’s not a problem if your protein is coming from food, because it naturally contains a mix of amino acids. However, if you take high doses of amino acid supplements, those could theoretically interfere with absorption of other amino acids.

Fig. 6.21. Summary of protein digestion. Note that the lines representing polypeptide chains in the stomach consist of strings of amino acids connected by peptide bonds, even though the individual amino acids aren’t shown in this simplified representation.

Proteins that aren’t fully digested in the small intestine pass into the large intestine and are eventually excreted in the feces. Recall from the last page that plant-based proteins are a bit less digestible than animal proteins, because some proteins are bound in plant cell walls.

What happens to absorbed amino acids?

Once the amino acids are in the blood, they are transported to the liver. As with other macronutrients, the liver is the checkpoint for amino acid distribution and any further breakdown of amino acids, which is very minimal. Dietary amino acids then become part of the body’s amino acid pool.

Assuming the body has enough glucose and other sources of energy, those amino acids will be used in one of the following ways:

• Protein synthesis in cells around the body
• Making nonessential amino acids needed for protein synthesis
• Making other nitrogen-containing compounds
• Rearranged and stored as fat (there is no storage form of protein)

If there is not enough glucose or energy available, amino acids can also be used in one of these ways:

• Rearranged into glucose for fuel for the brain and red blood cells
• Metabolized as fuel, for an immediate source of ATP

In order to use amino acids to make ATP, glucose, or fat, the nitrogen first has to be removed in a process called deamination , which occurs in the liver and kidneys. The nitrogen is initially released as ammonia, and because ammonia is toxic, the liver transforms it into urea. Urea is then transported to the kidneys and excreted in the urine. Urea is a molecule that contains two nitrogens and is highly soluble in water. This makes it ideal for transporting excess nitrogen out of the body.

Because amino acids are building blocks that the body reserves in order to synthesize other proteins, more than 90 percent of the protein ingested does not get broken down further than the amino acid monomers.

Attributions:

• Lindshield, B. L. Kansas State University Human Nutrition (FNDH 400) Flexbook. goo.gl/vOAnR , CC BY-NC-SA 4.0
• “Protein Digestion and Absorption”, section 6.3 from the book An Introduction to Nutrition (v. 1.0), CC BY-NC-SA 3.0

Image Credits:

• Fig 6.17. “Protein digestion in the human GI tract” by Alice Callahan is licensed under CC BY 4.0 ; edited from “Digestive system diagram edit” by Mariana Ruiz, edited by Joaquim Alves Gaspar, Jmarchn is in the Public Domain
• Fig 6.18. “Protein digestion in the stomach” from “Protein Digestion and Absorption,” section 6.3 from An Introduction to Nutrition (v. 1.0), CC BY-NC-SA 3.0
• Fig 6.19. “Denaturation of proteins” by Alice Callahan is licensed under CC BY 4.0 ; edited from “ Process of denaturation ” by Scurran is licensed under CC BY-SA 4.0
• Fig 6.20. “Enzymatic digestion of proteins” by Alice Callahan is licensed under CC BY 4.0 ; edited from “ Process of denaturation ” by Scurran is licensed under CC BY-SA 4.0
• Fig 6.21. “Summary of protein digestion” by Alice Callahan is licensed under CC BY 4.0 ; edited from “ Process of denaturation ” by Scurran is licensed under CC BY-SA 4.0

An acid that is a component of gastric juices; creates an acidic environment in the stomach, killing bacteria and aiding in protein digestion.

An enzyme found in gastric juices; aids in the chemical breakdown of proteins.

When the three-dimensional structure of a protein is unfolded due to a change in the environment (e.g., acid, heat); results in loss of protein function.

An enzyme made by the pancreas; facilitates the chemical breakdown of proteins in the small intestine.

An enzyme that facilitates the chemical breakdown of protein in the small intestine; activates other protein-digesting enzymes.

Enzymes that aid in the chemical breakdown of proteins in the small intestine.

A process that removes nitrogen from amino acids before they are used to synthesize ATP, glucose, or fat.

Nutrition: Science and Everyday Application, v. 1.0 Copyright © 2020 by Alice Callahan, PhD; Heather Leonard, MEd, RDN; and Tamberly Powell, MS, RDN is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License , except where otherwise noted.

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3: Nutrition and the Human Body

3.3: digestion and absorption, learning objectives.

• Sketch and label the major organs of the digestive system and state their functions.

Digestion begins even before you put food into your mouth. When you feel hungry, your body sends a message to your brain that it is time to eat. Sights and smells influence your body’s preparedness for food. Smelling food sends a message to your brain. Your brain then tells the mouth to get ready, and you start to salivate in preparation for a delicious meal.

Figure 3.3.1: The Digestion Process. Digestion converts the food we eat into smaller particles, which will be processed into energy or used as building blocks

Once you have eaten, your  digestive sy stem  (Figure 3.3.1) breaks down the food into smaller components. Another word for the breakdown of complex molecules into smaller, simpler molecules is “catabolism” or a “catabolic reaction”. To do this, catabolism functions on two levels, mechanical and chemical. Once the smaller particles have been broken down, they will be absorbed into the blood and delivered to cells throughout the body for energy or for building blocks needed for cells to function. The digestive system is one of the eleven organ systems of the human body and it is composed of several hollow tube-shaped organs including the mouth, pharynx, esophagus, stomach, small intestine, large intestine (or colon), rectum, and anus. It is lined with mucosal tissue that secretes digestive juices (which aid in the breakdown of food) and mucus (which facilitates the propulsion of food through the tract). Smooth muscle tissue surrounds the digestive tract and its contraction produces waves, known as  peristalsis , that propel food down the tract. Nutrients as well as some non-nutrients are absorbed. Substances such as fiber get left behind and are appropriately excreted.

From the Mouth to the Stomach

There are four steps in the digestion process (Figure 3.3.2). The first step is  ingestion , which is the collection of food into the digestive tract. It may seem a simple process, but ingestion involves smelling food, thinking about food, and the involuntary release of saliva in the mouth to prepare for food entry. In the mouth, where the second step of digestion occurs, the mechanical and chemical breakdown of food begins. The  chemical breakdown  of food involves enzymes, which break apart the components in food. Theses enzymes are secreted by the salivary glands, stomach, pancreas, and small intestine.  Mechanical breakdown  starts with mastication (chewing) in the mouth. Teeth crush and grind large food particles, while saliva initiates the chemical breakdown of food and enables its movement downward. The slippery mass of partially broken-down food is called bolus, which moves down the digestive tract as you swallow. Swallowing may seem voluntary at first because it requires conscious effort to push the food with the tongue back toward the throat, but after this, swallowing proceeds involuntarily, meaning it cannot be stopped once it begins.

Figure 3.3.2: Components of the Human Digestive System. All digestive organs play integral roles in the life-sustaining process of digestion. Image used with permission (CC BY 3.0; OpenStax).

As you swallow, the bolus is pushed from the mouth through the pharynx and into a muscular tube called the esophagus. As it travels through the pharynx, a small flap called the epiglottis closes, to prevent choking by keeping food from going into the trachea. Peristaltic contractions in the esophagus propel the food down to the stomach. At the junction between the esophagus and stomach there is a sphincter muscle that remains closed until the food bolus approaches. The pressure of the food bolus stimulates the lower esophageal sphincter to relax and open and food then moves from the esophagus into the stomach. The mechanical breakdown of food is accentuated by the muscular contractions of the stomach and small intestine that mash, mix, slosh, and propel food down the alimentary canal. Solid food takes between four and eight seconds to travel down the esophagus, and liquids take about one second.

From the Stomach to the Small Intestine

When food enters the stomach, a highly muscular organ, powerful peristaltic contractions help mash, pulverize, and churn food into chyme.  Chyme is a semiliquid mass of partially digested food that also contains gastric juices secreted by cells in the stomach. Cells in the stomach also secrete hydrochloric acid and the enzyme pepsin, that chemically breaks down food into smaller molecules. The stomach has three basic tasks:

• To store food
• To mechanically and chemically break down food
• To empty partially broken-down food into the small intestine

The length of time food spends in the stomach varies by the macronutrient composition of the meal. A high-fat or high-protein meal takes longer to break down than one rich in carbohydrates. It usually takes a few hours after a meal to empty the stomach contents completely.

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3.3.1: DIGESTION VIDEO

This video shows the mechanical and chemical breakdown of food into chyme.

Video  3.3.2: the food machine – great endoscopic images, xray and ultrasound images of food being digested (pretty graphic). Great visuals! Also talks about energy from food

The small intestine is divided into three structural parts: the duodenum (the top), the jejunum (the middle), and the ileum (the last part). Once the chyme enters the duodenum (the first segment of the small intestine), three accessory (or helper) organs: liver, pancreas and gallbladder are stimulated to release juices that aid in digestion. The pancreas secretes up to 1.5 liters of pancreatic juice through a duct into the duodenum per day. This fluid consists mostly of water, but it also contains bicarbonate ions that neutralize the acidity of the stomach-derived chyme and enzymes that further breakdown proteins, carbohydrates, and lipids. The gallbladder secretes a much smaller amount of bile to help digest fats, also through a duct that leads to the duodenum. Bile is made in the liver and stored in the gall bladder. Bile’s components act like detergents by surrounding fats similar to the way dish soap removes grease from a frying pan. This allows for the movement of fats in the watery environment of the small intestine. Two different types of muscular contractions, called peristalsis and segmentation, move and mix the food in various stages of digestion through the small intestine. Similar to what occurs in the esophagus and stomach, peristalsis is circular waves of smooth muscle contraction that propel food forward. Segmentation sloshes food back and forth in both directions promoting further mixing of the chyme. Almost all the components of food are completely broken down to their simplest unit within the first 25 centimeters of the small intestine. Instead of proteins, carbohydrates, and lipids, the chyme now consists of amino acids, monosaccharides, and emulsified fatty acids.

The next step of digestion (nutrient absorption) takes place in the remaining length of the small intestine, or ileum (> 5 meters).

Figure 3.3.3: The way the small intestine is structured gives it a huge surface area to maximize nutrient absorption. The surface area is increased by folds, villi, and microvilli. Digested nutrients are absorbed into either capillaries or lymphatic vessels contained within each microvilli. © Shutterstock

The small intestine is perfectly structured for maximizing nutrient absorption. Its surface area is greater than 200 square meters, which is about the size of a tennis court. The surface area of the small intestine increases by multiple levels of folding. The internal tissue of the small intestine is covered in villi, which are tiny finger-like projections that are covered with even smaller projections, called microvilli (Figure  3.3.1). The digested nutrients pass through the absorptive cells of the intestine via diffusion or special transport proteins. Nutrients that are water soluble (dissolve in water) like amino acids and monosaccharides (sugars) are transported from the intestinal cells into capillaries (blood), but the fat soluble nutrients like fatty acids, fat-soluble vitamins, and other lipids are transported first through lymphatic vessels (lymph), which soon meet up with blood vessels.

From the Small Intestine to the Large Intestine

The process of digestion is fairly efficient. Any food that is still incompletely broken down (usually less than ten percent of food consumed) and the food’s indigestible fiber content moves from the small intestine to the large intestine (colon) through a connecting valve. The main task of the large intestine is to reabsorb water. Remember, water is present not only in solid foods, but also the stomach releases a few hundred millilters of gastric juice and the pancreas adds approximately another 500 milliliters during the digestion of the meal. For the body to conserve water, it is important that the water be reabsorbed. In the large intestine, no further chemical or mechanical breakdown of food takes place, unless it is accomplished by the bacteria that inhabit this portion of the digestive tract. The number of bacteria residing in the large intestine is estimated to be greater than 10 (14) , which is more than the total number of cells in the human body (10 (13) ). This may seem rather unpleasant, but the great majority of bacteria in the large intestine are harmless and some are even beneficial.

TOOLS FOR CHANGE:  KEFIR

There has been significant talk about pre- and probiotic foods in the mainstream media. The World Health Organization defines probiotics as live bacteria that confer beneficial health effects on their host. They are sometimes called “friendly bacteria.” The most common bacteria labeled as probiotic is lactic acid bacteria (lactobacilli). They are added as live cultures to certain fermented foods such as yogurt. Prebiotics are indigestible foods, primarily soluble fibers, that stimulate the growth of certain strains of bacteria in the large intestine and provide health benefits to the host. A review article in the June 2008 issue of the  Journal of Nutrition  concludes that there is scientific consensus that probiotics ward off viral-induced diarrhea and reduce the symptoms of lactose intolerance. Farnworth, E. R. “The Evidence to Support Health Claims for Probiotics.”  J Nutr  138, no. 6 (2008): 1250S–4S.  http://jn.nutrition.org/content/138/6/1250S.long .  Expert nutritionists agree that more health benefits of pre- and probiotics will likely reach scientific consensus. As the fields of pre- and probiotic manufacturing and their clinical study progress, more information on proper dosing and what exact strains of bacteria are potentially “friendly” will become available.

Kefir, a dairy product fermented with probiotic bacteria, can make a pleasant tasting milkshake. Image used with permission (CC BY-SA 3.0; Quijote )

You may be interested in trying some of these foods in your diet. A simple food to try is kefir. Several websites provide good recipes, including  http://www.kefir.net/recipes.htm .

From the Large Intestine to the Anus

After a few hours in the stomach, plus three to six hours in the small intestine, and about sixteen hours in the large intestine, the digestion process enters step four, which is the elimination of indigestible food as feces. Feces contain indigestible food and gut bacteria (almost 50 percent of content). It is stored in the rectum until it is expelled through the anus via defecation.

VIDEO 3.3.3: THE STAGES OF DIGESTION

This video reviews the sequence of events during food digestion.

Key Takeaways

• The breakdown of complex macromolecules in foods to simple absorbable components is accomplished by the digestive system. These components are processed by cells throughout the body into energy or are used as building blocks.
• The digestive system is composed of the mouth, pharynx, esophagus, stomach, small intestine, large intestine (or colon), rectum, and anus. There are four steps in the digestion process: ingestion, the mechanical and chemical breakdown of food, nutrient absorption, and elimination of indigestible food.
• The mechanical breakdown of food occurs via muscular contractions called peristalsis and segmentation. Enzymes secreted by the salivary glands, stomach, pancreas, and small intestine accomplish the chemical breakdown of food. Additionally, bile emulsifies fats.

Discussion Starter

Decide whether you want to consume pre- and probiotic foods to benefit your health. Visit the websites below to help in your decision-making process. Defend your decision scientifically.

http://www.health.harvard.edu/fhg/updates/update0905c.shtml

http://nccam.nih.gov/research/results/spotlight/110508.htm

• Digestion and Absorption. Authored by : Medical LibreTexts Contributors. Provided by : LibreTexts. Located at : https://med.libretexts.org/Courses/Sacramento_City_College/SCC%3A_Nutri_300_(Coppola)/Chapters/03%3A_Nutrition_and_the_Human_Body/3.3%3A_Digestion_and_Absorption . License : CC BY-NC-SA: Attribution-NonCommercial-ShareAlike

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34.3: Digestive System Processes

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Skills to Develop

• Describe the process of digestion
• Detail the steps involved in digestion and absorption
• Define elimination
• Explain the role of both the small and large intestines in absorption

Obtaining nutrition and energy from food is a multi-step process. For true animals, the first step is ingestion, the act of taking in food. This is followed by digestion, absorption, and elimination. In the following sections, each of these steps will be discussed in detail.

The large molecules found in intact food cannot pass through the cell membranes. Food needs to be broken into smaller particles so that animals can harness the nutrients and organic molecules. The first step in this process is ingestion . Ingestion is the process of taking in food through the mouth. In vertebrates, the teeth, saliva, and tongue play important roles in mastication (preparing the food into bolus). While the food is being mechanically broken down, the enzymes in saliva begin to chemically process the food as well. The combined action of these processes modifies the food from large particles to a soft mass that can be swallowed and can travel the length of the esophagus.

Digestion and Absorption

Digestion is the mechanical and chemical break down of food into small organic fragments. It is important to break down macromolecules into smaller fragments that are of suitable size for absorption across the digestive epithelium. Large, complex molecules of proteins, polysaccharides, and lipids must be reduced to simpler particles such as simple sugar before they can be absorbed by the digestive epithelial cells. Different organs play specific roles in the digestive process. The animal diet needs carbohydrates, protein, and fat, as well as vitamins and inorganic components for nutritional balance. How each of these components is digested is discussed in the following sections.

Carbohydrates

The digestion of carbohydrates begins in the mouth. The salivary enzyme amylase begins the breakdown of food starches into maltose, a disaccharide. As the bolus of food travels through the esophagus to the stomach, no significant digestion of carbohydrates takes place. The esophagus produces no digestive enzymes but does produce mucous for lubrication. The acidic environment in the stomach stops the action of the amylase enzyme.

The next step of carbohydrate digestion takes place in the duodenum. Recall that the chyme from the stomach enters the duodenum and mixes with the digestive secretion from the pancreas, liver, and gallbladder. Pancreatic juices also contain amylase, which continues the breakdown of starch and glycogen into maltose, a disaccharide. The disaccharides are broken down into monosaccharides by enzymes called maltases , sucrases , and lactases , which are also present in the brush border of the small intestinal wall. Maltase breaks down maltose into glucose. Other disaccharides, such as sucrose and lactose are broken down by sucrase and lactase, respectively. Sucrase breaks down sucrose (or “table sugar”) into glucose and fructose, and lactase breaks down lactose (or “milk sugar”) into glucose and galactose. The monosaccharides (glucose) thus produced are absorbed and then can be used in metabolic pathways to harness energy. The monosaccharides are transported across the intestinal epithelium into the bloodstream to be transported to the different cells in the body. The steps in carbohydrate digestion are summarized in Figure \(\PageIndex{1}\).

A large part of protein digestion takes place in the stomach. The enzyme pepsin plays an important role in the digestion of proteins by breaking down the intact protein to peptides, which are short chains of four to nine amino acids. In the duodenum, other enzymes— trypsin , elastase , and chymotrypsin —act on the peptides reducing them to smaller peptides. Trypsin elastase, carboxypeptidase, and chymotrypsin are produced by the pancreas and released into the duodenum where they act on the chyme. Further breakdown of peptides to single amino acids is aided by enzymes called peptidases (those that break down peptides). Specifically, carboxypeptidase , dipeptidase , and aminopeptidase play important roles in reducing the peptides to free amino acids. The amino acids are absorbed into the bloodstream through the small intestines. The steps in protein digestion are summarized in Figure \(\PageIndex{2}\).

Lipid digestion begins in the stomach with the aid of lingual lipase and gastric lipase. However, the bulk of lipid digestion occurs in the small intestine due to pancreatic lipase. When chyme enters the duodenum, the hormonal responses trigger the release of bile, which is produced in the liver and stored in the gallbladder. Bile aids in the digestion of lipids, primarily triglycerides by emulsification. Emulsification is a process in which large lipid globules are broken down into several small lipid globules. These small globules are more widely distributed in the chyme rather than forming large aggregates. Lipids are hydrophobic substances: in the presence of water, they will aggregate to form globules to minimize exposure to water. Bile contains bile salts, which are amphipathic, meaning they contain hydrophobic and hydrophilic parts. Thus, the bile salts hydrophilic side can interface with water on one side and the hydrophobic side interfaces with lipids on the other. By doing so, bile salts emulsify large lipid globules into small lipid globules.

Why is emulsification important for digestion of lipids? Pancreatic juices contain enzymes called lipases (enzymes that break down lipids). If the lipid in the chyme aggregates into large globules, very little surface area of the lipids is available for the lipases to act on, leaving lipid digestion incomplete. By forming an emulsion, bile salts increase the available surface area of the lipids many fold. The pancreatic lipases can then act on the lipids more efficiently and digest them, as detailed in Figure \(\PageIndex{3}\). Lipases break down the lipids into fatty acids and glycerides. These molecules can pass through the plasma membrane of the cell and enter the epithelial cells of the intestinal lining. The bile salts surround long-chain fatty acids and monoglycerides forming tiny spheres called micelles. The micelles move into the brush border of the small intestine absorptive cells where the long-chain fatty acids and monoglycerides diffuse out of the micelles into the absorptive cells leaving the micelles behind in the chyme. The long-chain fatty acids and monoglycerides recombine in the absorptive cells to form triglycerides, which aggregate into globules and become coated with proteins. These large spheres are called chylomicrons . Chylomicrons contain triglycerides, cholesterol, and other lipids and have proteins on their surface. The surface is also composed of the hydrophilic phosphate "heads" of phospholipids. Together, they enable the chylomicron to move in an aqueous environment without exposing the lipids to water. Chylomicrons leave the absorptive cells via exocytosis. Chylomicrons enter the lymphatic vessels, and then enter the blood in the subclavian vein.

Vitamins can be either water-soluble or lipid-soluble. Fat soluble vitamins are absorbed in the same manner as lipids. It is important to consume some amount of dietary lipid to aid the absorption of lipid-soluble vitamins. Water-soluble vitamins can be directly absorbed into the bloodstream from the intestine.

Art Connection

Which of the following statements about digestive processes is true?

• Amylase, maltase, and lactase in the mouth digest carbohydrates.
• Trypsin and lipase in the stomach digest protein.
• Bile emulsifies lipids in the small intestine.
• No food is absorbed until the small intestine.

Elimination

The final step in digestion is the elimination of undigested food content and waste products. The undigested food material enters the colon, where most of the water is reabsorbed. Recall that the colon is also home to the microflora called “intestinal flora” that aid in the digestion process. The semi-solid waste is moved through the colon by peristaltic movements of the muscle and is stored in the rectum. As the rectum expands in response to storage of fecal matter, it triggers the neural signals required to set up the urge to eliminate. The solid waste is eliminated through the anus using peristaltic movements of the rectum.

Common Problems with Elimination

Diarrhea and constipation are some of the most common health concerns that affect digestion. Constipation is a condition where the feces are hardened because of excess water removal in the colon. In contrast, if enough water is not removed from the feces, it results in diarrhea. Many bacteria, including the ones that cause cholera, affect the proteins involved in water reabsorption in the colon and result in excessive diarrhea.

Emesis, or vomiting, is elimination of food by forceful expulsion through the mouth. It is often in response to an irritant that affects the digestive tract, including but not limited to viruses, bacteria, emotions, sights, and food poisoning. This forceful expulsion of the food is due to the strong contractions produced by the stomach muscles. The process of emesis is regulated by the medulla.

Digestion begins with ingestion, where the food is taken in the mouth. Digestion and absorption take place in a series of steps with special enzymes playing important roles in digesting carbohydrates, proteins, and lipids. Elimination describes removal of undigested food contents and waste products from the body. While most absorption occurs in the small intestines, the large intestine is responsible for the final removal of water that remains after the absorptive process of the small intestines. The cells that line the large intestine absorb some vitamins as well as any leftover salts and water. The large intestine (colon) is also where feces is formed.

Art Connections

Figure \(\PageIndex{4}\): Which of the following statements about digestive processes is true?

• Amylase, maltase and lactase in the mouth digest carbohydrates.

Co-Transporters

After the larger molecules have been hydrolysed into monomers, they must be absorbed by the cells. Amino acids and monosaccharides are absorbed through epithelial cells. This involves co-transporters.

Active transport of sodium ions

• Sodium-dependent co-transporter proteins are located in the epithelial cell membranes.
• The co-transporter proteins actively transport sodium ions into the blood.
• This causes the concentration of sodium ions in the epithelial cells to decrease.

Diffusion of sodium ions

• The decreased concentration of sodium ions in the epithelial cells causes sodium ions in the ileum to diffuse down their concentration gradient.
• The sodium ions diffuse through a co-transporter protein in the cell surface membrane.

Binding of amino acids & sugars

• When sodium ions bind to a co-transporter protein, amino acids or monosaccharides also bind to the protein.
• Binding of amino acids or monosaccharides causes the transporter protein to undergo a conformational change.
• The amino acids or monosaccharides, along with the sodium ions are transported into the epithelial cell cytoplasm.

• Once inside the epithelial cells, the amino acids or monosaccharides can then be used inside the epithelial cells or absorbed into the bloodstream.

Lipids are absorbed from the ileum through epithelial cells. This involves the formation of micelles.

• After larger, lipid molecules have been broken down into monoglycerides and fatty acids, bile salts are secreted by the liver.
• The bile salts associate with the monoglycerides and fatty acids to form micelles.
• Micelles are circular formations that are made up of phospholipid tails.

Incorporation

• The micelles move through the ileum and come into contact with the epithelial cells.
• The micelles incorporate themselves into the epithelial cell surface membrane and are absorbed.

1 Biological Molecules

1.1 Monomers & Polymers

1.1.1 Monomers & Polymers

1.1.2 Condensation & Hydrolysis Reactions

1.2 Carbohydrates

1.2.1 Structure of Carbohydrates

1.2.2 Types of Polysaccharides

1.2.3 End of Topic Test - Monomers, Polymers and Carbs

1.2.4 Exam-Style Question - Carbohydrates

1.2.5 A-A* (AO3/4) - Carbohydrates

1.3.1 Triglycerides & Phospholipids

1.3.2 Types of Fatty Acids

1.3.3 Testing for Lipids

1.3.4 Exam-Style Question - Fats

1.3.5 A-A* (AO3/4) - Lipids

1.4 Proteins

1.4.1 The Peptide Chain

1.4.2 Investigating Proteins

1.4.3 Primary & Secondary Protein Structure

1.4.4 Tertiary & Quaternary Protein Structure

1.4.5 Enzymes

1.4.6 Factors Affecting Enzyme Activity

1.4.7 Enzyme-Controlled Reactions

1.4.8 End of Topic Test - Lipids & Proteins

1.4.9 A-A* (AO3/4) - Enzymes

1.4.10 A-A* (AO3/4) - Proteins

1.5 Nucleic Acids

1.5.1 DNA & RNA

1.5.2 Polynucleotides

1.5.3 DNA Replication

1.5.4 Exam-Style Question - Nucleic Acids

1.5.5 A-A* (AO3/4) - Nucleic Acids

1.6.1 Structure of ATP

1.6.2 End of Topic Test - Nucleic Acids & ATP

1.7.1 Structure & Function of Water

1.7.2 A-A* (AO3/4) - Water

1.8 Inorganic Ions

1.8.1 Inorganic Ions

1.8.2 End of Topic Test - Water & Inorganic Ions

2.1 Cell Structure

2.1.1 Introduction to Cells

2.1.2 Eukaryotic Cells & Organelles

2.1.3 Eukaryotic Cells & Organelles 2

2.1.4 Prokaryotes

2.1.5 A-A* (AO3/4) - Organelles

2.1.6 Methods of Studying Cells

2.1.7 Microscopes

2.1.8 End of Topic Test - Cell Structure

2.1.9 Exam-Style Question - Cells

2.1.10 A-A* (AO3/4) - Cells

2.2 Mitosis & Cancer

2.2.1 Mitosis

2.2.2 Investigating Mitosis

2.2.3 Cancer

2.2.4 A-A* (AO3/4) - The Cell Cycle

2.3 Transport Across Cell Membrane

2.3.1 Cell Membrane Structure

2.3.2 A-A* (AO3/4) - Membrane Structure

2.3.3 Diffusion

2.3.4 Osmosis

2.3.5 Active Transport

2.3.6 End of Topic Test - Mitosis, Cancer & Transport

2.3.7 Exam-Style Question - Membranes

2.3.8 A-A* (AO3/4) - Membranes & Transport

2.3.9 A-A*- Mitosis, Cancer & Transport

2.4 Cell Recognition & the Immune System

2.4.1 Immune System

2.4.2 The Immune Response

2.4.3 Antibodies

2.4.4 Primary & Secondary Response

2.4.5 Vaccines

2.4.7 Ethical Issues

2.4.8 End of Topic Test - Immune System

2.4.9 Exam-Style Question - Immune System

2.4.10 A-A* (AO3/4) - Immune System

3 Substance Exchange

3.1 Surface Area to Volume Ratio

3.1.1 Size & Surface Area

3.1.2 A-A* (AO3/4) - Cell Size

3.2 Gas Exchange

3.2.1 Single-Celled Organisms

3.2.2 Multicellular Organisms

3.2.3 Control of Water Loss

3.2.4 Human Gas Exchange

3.2.5 Ventilation

3.2.6 Dissection

3.2.7 Measuring Gas Exchange

3.2.8 Lung Disease

3.2.9 Lung Disease Data

3.2.10 End of Topic Test - Gas Exchange

3.2.11 A-A* (AO3/4) - Gas Exchange

3.3 Digestion & Absorption

3.3.1 Overview of Digestion

3.3.2 Digestion in Mammals

3.3.3 Absorption

3.3.4 End of Topic Test - Substance Exchange & Digestion

3.3.5 A-A* (AO3/4) - Substance Ex & Digestion

3.4 Mass Transport

3.4.1 Haemoglobin

3.4.2 Oxygen Transport

3.4.3 The Circulatory System

3.4.4 The Heart

3.4.5 Blood Vessels

3.4.6 Cardiovascular Disease

3.4.7 Heart Dissection

3.4.8 Xylem

3.4.9 Phloem

3.4.10 Investigating Plant Transport

3.4.11 End of Topic Test - Mass Transport

3.4.12 A-A* (AO3/4) - Mass Transport

4 Genetic Information & Variation

4.1 DNA, Genes & Chromosomes

4.1.2 Genes

4.1.3 A-A* (AO3/4) - DNA

4.2 DNA & Protein Synthesis

4.2.1 Protein Synthesis

4.2.2 Transcription & Translation

4.2.3 End of Topic Test - DNA, Genes & Protein Synthesis

4.2.4 Exam-Style Question - Protein Synthesis

4.2.5 A-A* (AO3/4) - Coronavirus Translation

4.2.6 A-A* (AO3/4) - Transcription

4.2.7 A-A* (AO3/4) - Translation

4.3 Mutations & Meiosis

4.3.1 Mutations

4.3.2 Meiosis

4.3.3 A-A* (AO3/4) - Meiosis

4.3.4 Meiosis vs Mitosis

4.3.5 End of Topic Test - Mutations, Meiosis

4.3.6 A-A* (AO3/4) - DNA,Genes, CellDiv & ProtSynth

4.4 Genetic Diversity & Adaptation

4.4.1 Genetic Diversity

4.4.2 Natural Selection

4.4.3 A-A* (AO3/4) - Natural Selection

4.4.4 Adaptations

4.4.5 Investigating Natural Selection

4.4.6 End of Topic Test - Genetic Diversity & Adaptation

4.4.7 A-A* (AO3/4) - Genetic Diversity & Adaptation

4.5 Species & Taxonomy

4.5.1 Classification

4.5.2 DNA Technology

4.5.3 A-A* (AO3/4) - Species & Taxonomy

4.6 Biodiversity Within a Community

4.6.1 Biodiversity

4.6.2 Agriculture

4.6.3 End of Topic Test - Species,Taxonomy& Biodiversity

4.6.4 A-A* (AO3/4) - Species,Taxon&Biodiversity

4.7 Investigating Diversity

4.7.1 Genetic Diversity

4.7.2 Quantitative Investigation

5 Energy Transfers (A2 only)

5.1 Photosynthesis

5.1.1 Overview of Photosynthesis

5.1.2 Light-Dependent Reaction

5.1.3 Light-Independent Reaction

5.1.4 A-A* (AO3/4) - Photosynthesis Reactions

5.1.5 Limiting Factors

5.1.6 Photosynthesis Experiments

5.1.7 End of Topic Test - Photosynthesis

5.1.8 A-A* (AO3/4) - Photosynthesis

5.2 Respiration

5.2.1 Overview of Respiration

5.2.2 Anaerobic Respiration

5.2.3 A-A* (AO3/4) - Anaerobic Respiration

5.2.4 Aerobic Respiration

5.2.5 Respiration Experiments

5.2.6 End of Topic Test - Respiration

5.2.7 A-A* (AO3/4) - Respiration

5.3 Energy & Ecosystems

5.3.1 Biomass

5.3.2 Production & Productivity

5.3.3 Agricultural Practices

5.4 Nutrient Cycles

5.4.1 Nitrogen Cycle

5.4.2 Phosphorous Cycle

5.4.3 Fertilisers & Eutrophication

5.4.4 End of Topic Test - Nutrient Cycles

5.4.5 A-A* (AO3/4) - Energy,Ecosystems&NutrientCycles

6 Responding to Change (A2 only)

6.1 Nervous Communication

6.1.1 Survival

6.1.2 Plant Responses

6.1.3 Animal Responses

6.1.4 Reflexes

6.1.5 End of Topic Test - Reflexes, Responses & Survival

6.1.6 Receptors

6.1.7 The Human Retina

6.1.8 Control of Heart Rate

6.1.9 End of Topic Test - Receptors, Retina & Heart Rate

6.2 Nervous Coordination

6.2.1 Neurones

6.2.2 Action Potentials

6.2.3 Speed of Transmission

6.2.4 End of Topic Test - Neurones & Action Potentials

6.2.5 Synapses

6.2.6 Types of Synapse

6.2.7 Medical Application

6.2.8 End of Topic Test - Synapses

6.2.9 A-A* (AO3/4) - Nervous Comm&Coord

6.3 Muscle Contraction

6.3.1 Skeletal Muscle

6.3.2 Sliding Filament Theory

6.3.3 Contraction

6.3.4 Slow & Fast Twitch Fibres

6.3.5 End of Topic Test - Muscles

6.3.6 A-A* (AO3/4) - Muscle Contraction

6.4 Homeostasis

6.4.1 Overview of Homeostasis

6.4.2 Blood Glucose Concentration

6.4.3 Controlling Blood Glucose Concentration

6.4.4 End of Topic Test - Blood Glucose

6.4.5 Primary & Secondary Messengers

6.4.6 Diabetes Mellitus

6.4.7 Measuring Glucose Concentration

6.4.8 Osmoregulation

6.4.9 Controlling Blood Water Potential

6.4.11 End of Topic Test - Diabetes & Osmoregulation

6.4.12 A-A* (AO3/4) - Homeostasis

7 Genetics & Ecosystems (A2 only)

7.1 Genetics

7.1.1 Key Terms in Genetics

7.1.2 Inheritance

7.1.3 Linkage

7.1.4 Multiple Alleles & Epistasis

7.1.5 Chi-Squared Test

7.1.6 End of Topic Test - Genetics

7.1.7 A-A* (AO3/4) - Genetics

7.2 Populations

7.2.1 Populations

7.2.2 Hardy-Weinberg Principle

7.3 Evolution

7.3.1 Variation

7.3.2 Natural Selection & Evolution

7.3.3 End of Topic Test - Populations & Evolution

7.3.4 Types of Selection

7.3.5 Types of Selection Summary

7.3.6 Overview of Speciation

7.3.7 Causes of Speciation

7.3.8 Diversity

7.3.9 End of Topic Test - Selection & Speciation

7.3.10 A-A* (AO3/4) - Populations & Evolution

7.4 Populations in Ecosystems

7.4.1 Overview of Ecosystems

7.4.2 Niche

7.4.3 Population Size

7.4.4 Investigating Population Size

7.4.5 End of Topic Test - Ecosystems & Population Size

7.4.6 Succession

7.4.7 Conservation

7.4.8 End of Topic Test - Succession & Conservation

7.4.9 A-A* (AO3/4) - Ecosystems

8 The Control of Gene Expression (A2 only)

8.1 Mutation

8.1.1 Mutations

8.1.2 Effects of Mutations

8.1.3 Causes of Mutations

8.2 Gene Expression

8.2.1 Stem Cells

8.2.2 Stem Cells in Disease

8.2.3 End of Topic Test - Mutation & Gene Epression

8.2.4 A-A* (AO3/4) - Mutation & Stem Cells

8.2.5 Regulating Transcription

8.2.6 Epigenetics

8.2.7 Epigenetics & Disease

8.2.8 Regulating Translation

8.2.9 Experimental Data

8.2.10 End of Topic Test - Transcription & Translation

8.2.11 Tumours

8.2.12 Correlations & Causes

8.2.13 Prevention & Treatment

8.2.14 End of Topic Test - Cancer

8.2.15 A-A* (AO3/4) - Gene Expression & Cancer

8.3 Genome Projects

8.3.1 Using Genome Projects

8.4 Gene Technology

8.4.1 Recombinant DNA

8.4.2 Producing Fragments

8.4.3 Amplification

8.4.4 End of Topic Test - Genome Project & Amplification

8.4.5 Using Recombinant DNA

8.4.6 Medical Diagnosis

8.4.7 Genetic Fingerprinting

8.4.8 End of Topic Test - Gene Technologies

8.4.9 A-A* (AO3/4) - Gene Technology

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Digestion in Mammals

End of Topic Test - Substance Exchange & Digestion

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Adsorption vs Absorption – Differences and Examples

Adsorption and absorption are two sorption processes through which one substance attaches to another. The main difference between them is that adsorption is the adhesion of particles onto a substance, while absorption involves mass transfer into another material. But, adsorption and absorption involve other differences as well.

Here is a comparison of adsorption and absorption, a closer look at their definitions, and examples of each process.

Comparing Adsorption vs Absorption

Usually, when people think about adsorption and absorption, they consider the mass transfer of liquid particles onto (adsorption) or into (absorption) solids. But, these processes can involve plasma , gases , liquids, or dissolved solids where the ions, atoms, or molecules are adsorbed or absorbed by liquids or solids. While both sorption processes share this similarity, they differ in several ways:

Adsorption Definition and Examples

Adsorption occurs when ions, atoms, or molecules adhere to a surface. The substance adsorbed onto the surface is called the adsorbate . The substance with the surface is called the adsorbent . Adsorption is an exothermic process because energy is released when the adsorbate sticks to the adsorbent. The rate of the process depends largely on surface area and temperature. Low temperature promotes adsorption because particles with less thermal energy have less kinetic energy and are more likely to stick to surfaces from covalent bond formation, hydrogen bonding, or other intermolecular forces .

Examples of adsorption include:

• Water adsorbing onto silica gel
• Contaminants adsorbing onto activated charcoal
• Particles adsorbing onto zeolites
• Silver adhering to glass and forming a mirror surface
• Non-stick coatings on pans
• Adsorption chillers used with refrigerants
• Viruses adsorb onto cells and surfaces

Uses of adsorption include water purification, cooling water for air conditioners, heterogeneous catalysts, surface treatments, and ion exchange columns.

Absorption Definition and Examples

Absorption occurs when ions, atoms, or molecules pass into a bulky material. These particles (the absorbate ) diffuse or dissolve into the absorbent substance. A familiar example is a paper towel picking up water. Eventually, water evenly permeates the paper. Absorption occurs passively (diffusion) or actively (facilitated diffusion or active transport) and is an endothermic process . Absorption rate depends on several factors, include concentration, exposed surface area, and pressure.

Examples of absorption include:

• A paper towel absorbing water
• Hair absorbing water
• Oxygen from air dissolving into water
• Sodium hydroxide absorbing carbon dioxide from air
• Cells absorb water and nutrients from their surroundings

Uses of absorption include spill clean-up, hydration, and digestion.

Note that there is another definition of absorption in science, referring to the interaction where matter absorbs energy from light.

• Crini, Grégorio; Badot, Pierre-Marie (2010). Sorption processes and pollution : conventional and non-conventional sorbents for pollutant removal from wastewaters . Besançon: Presses universitaires de Franche-Comté. ISBN 978-2848673042.
• Cussler, E. L. (1997). Diffusion: Mass Transfer in Fluid Systems (2nd ed.). New York: Cambridge University Press. ISBN 978-0-521-45078-2.
• IUPAC (1997). Compendium of Chemical Terminology (the “Gold Book”). Blackwell Scientific Publications. doi: 10.1351/goldbook
• McMurry, John (2003). Fundamentals of Organic Chemistry (5th ed.). Agnus McDonald. ISBN 0-534-39573-2.

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Chapter 1 Introduction to light absorption: visible and ultraviolet spectra

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Light is a form of electromagnetic radiation, usually a mixture of waves having different wavelengths. The wavelength of light, expressed by the symbol λ, is defined as the distance between two crests (or troughs) of a wave, measured in the direction of its progression. The unit used is the nanometre (nm, 10-9 m). Light that the human eye can sense is called visible light. Each colour that we perceive corresponds to a certain wavelength band in the 400-700 nm region. Spectrophotometry in its biochemical applications is generally concerned with the ultraviolet (UV, 185-400 nm), visible (400-700 nm) and infrared (700-15 000 nm) regions of the electromagnetic radiation spectrum, the former two being most common in laboratory practice. The wavelength of light is inversely related to its energy (E), according to the equation: . . . E = ch/ λ . . . where c denotes the speed of light, and h is Planck’s constant. UV radiation, therefore, has greater energy than the visible, and visible radiation has greater energy than the infrared. Light of certain wavelengths can be selectively absorbed by a substance according to its molecular structure. Absorption of light energy occurs when the incident photon carries energy equal to the difference in energy between two allowed states of the valency electrons, the photon promoting the transition of an electron from the lower to the higher energy state. Thus biochemical spectrophotometry may be referred to as electronic absorption spectroscopy. The excited electrons afterwards lose energy by the process of heat radiation, and return to the initial ground state. An absorption spectrum is obtained by successively changing the wavelength of monochromatic light falling on the substance, and recording the change of light absorption. Spectra are presented by plotting the wavelengths (generally nm or μm) on the abscissa and the degree of absorption (transmittance or absorbance) on the ordinate. For more information on the theory of light absorption, see Brown (1) and Chapters 2, 3 and 4. The most widespread use of UV and visible spectroscopy in biochemistry is in the quantitative determination of absorbing species (chromophores), known as spectrophotometry.

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Understanding Global Change

Discover why the climate and environment changes, your place in the Earth system, and paths to a resilient future.

absorption of sunlight

The process through which solar radiation is retained by Earth’s surfaces and converted to longwave infrared radiation, or heat.

Learn more about absorption of sunlight

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3.3: The Digestion and Absorption Process

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Skills to Develop

• Sketch and label the major organs of the digestive system and state their functions.

Digestion begins even before you put food into your mouth. When you feel hungry, your body sends a message to your brain that it is time to eat. Sights and smells influence your body’s preparedness for food. Smelling food sends a message to your brain. Your brain then tells the mouth to get ready, and you start to salivate in preparation for a delicious meal.

Once you have eaten, your digestive system (Figure 3.3.1) breaks down the food into smaller components. Another word for the breakdown of complex molecules into smaller, simpler molecules is "catabolism". To do this, catabolism functions on two levels, mechanical and chemical. Once the smaller particles have been broken down, they will be absorbed into the blood and delivered to cells throughout the body for energy or for building blocks needed for cells to function. The digestive system is one of the eleven organ systems of the human body and it is composed of several hollow tube-shaped organs including the mouth, pharynx, esophagus, stomach, small intestine, large intestine (or colon), rectum, and anus. It is lined with mucosal tissue that secretes digestive juices (which aid in the breakdown of food) and mucus (which facilitates the propulsion of food through the tract). Smooth muscle tissue surrounds the digestive tract and its contraction produces waves, known as peristalsis , that propel food down the tract. Nutrients as well as some non-nutrients are absorbed. Substances such as fiber get left behind and are appropriately excreted.

From the Mouth to the Stomach

There are four steps in the digestion process (Figure 2.3.2). The first step is ingestion , which is the collection of food into the digestive tract. It may seem a simple process, but ingestion involves smelling food, thinking about food, and the involuntary release of saliva, in the mouth to prepare for food entry. In the mouth, where the second step of digestion occurs, the mechanical and chemical breakdown of food begins. The chemical breakdown of food involves enzymes, which break apart the components in food. In the mouth, the enzyme amylase is secreted to begin breaking down complex carbohydrates. Mechanical breakdown starts with mastication (chewing) in the mouth. Teeth crush and grind large food particles, while saliva initiates the chemical breakdown of food and enables its movement downward. The slippery mass of partially broken-down food is called a bolus, which moves down the digestive tract as you swallow. Swallowing may seem voluntary at first because it requires conscious effort to push the food with the tongue back toward the throat, but after this, swallowing proceeds involuntarily, meaning it cannot be stopped once it begins.

As you swallow, the bolus is pushed from the mouth through the pharynx and into a muscular tube called the esophagus. As it travels through the pharynx, a small flap called the epiglottis closes, to prevent choking by keeping food from going into the trachea. Peristaltic contractions in the esophagus propel the food down to the stomach. At the junction between the esophagus and stomach, there is a sphincter muscle that remains closed until the food bolus approaches. The pressure of the food bolus stimulates the lower esophageal sphincter to relax and open and food then moves from the esophagus into the stomach. The mechanical breakdown of food is accentuated by the muscular contractions of the stomach and small intestine that mash, mix, slosh, and propel food down the alimentary canal. Solid food takes between four and eight seconds to travel down the esophagus, and liquids take about one second.

From the Stomach to the Small Intestine

When food enters the stomach, a highly muscular organ, powerful peristaltic contractions help mash, pulverize, and churn food into chyme. Chyme is a semiliquid mass of partially digested food that also contains gastric juices secreted by cells in the stomach. Cells in the stomach also secrete hydrochloric acid and the enzyme pepsin, which chemically breaks down protein into smaller molecules. A thick mucus coat lines the stomach to protect it from digesting itself. The stomach has three basic tasks:

• To store food
• To mechanically and chemically break down food
• To empty partially broken-down food into the small intestine

The length of time food spends in the stomach varies by the macronutrient composition of the meal. A high-fat or high-protein meal takes longer to break down than one rich in carbohydrates. It usually takes a few hours after a meal to empty the stomach contents completely. The sphincter that allows chyme to pass into the small intestine is known as the pyloric sphincter.

Video 3.3.1: Digestion Video

This video shows the mechanical and chemical breakdown of food into chyme.

The small intestine is divided into three structural parts: the duodenum, the jejunum, and the ileum. Once the chyme enters the duodenum (the first segment of the small intestine), three accessory (or helper) organs, the liver, pancreas, and gallbladder, are stimulated to release juices that aid in digestion. The pancreas secretes up to 1.5 liters of pancreatic juice through a duct into the duodenum per day. This fluid consists mostly of water, but it also contains bicarbonate ions that neutralize the acidity of the stomach-derived chyme and enzymes that further breakdown proteins, carbohydrates, and lipids. The gallbladder secretes a much smaller amount of bile to help digest fats, also through a duct that leads to the duodenum. Bile is made in the liver and stored in the gallbladder. Bile’s components act like detergents by surrounding fats similar to the way dish soap removes grease from a frying pan. This allows for the movement of fats in the watery environment of the small intestine. Two different types of muscular contractions, called peristalsis and segmentation, move and mix the food in various stages of digestion through the small intestine. Similar to what occurs in the esophagus and stomach, peristalsis is circular waves of smooth muscle contraction that propel food forward. Segmentation sloshes food back and forth in both directions promoting further mixing of the chyme. Almost all the components of food are completely broken down to their simplest unit within the first 25 centimeters of the small intestine. Instead of proteins, carbohydrates, and lipids, the chyme now consists of amino acids, monosaccharides, and emulsified fatty acids.

The next step of digestion (nutrient absorption) takes place in the remaining length of the small intestine, or ileum (> 5 meters).

The small intestine is perfectly structured for maximizing nutrient absorption. Its surface area is greater than 200 square meters, which is about the size of a tennis court. The surface area of the small intestine increases by multiple levels of folding. The internal tissue of the small intestine is covered in villi, which are tiny finger-like projections that are covered with even smaller projections, called microvilli (Figure 2.3.3). The digested nutrients pass through the absorptive cells of the intestine via diffusion or special transport proteins. Amino acids, minerals, alcohol, water-soluble vitamins, and monosaccharides (sugars like glucose) are transported from the intestinal cells into capillaries, but the much larger emulsified fatty acids, fat-soluble vitamins, and other lipids are transported first through lymphatic vessels, which soon meet up with blood vessels.

From the Small Intestine to the Large Intestine

The process of digestion is fairly efficient. Any food that is still incompletely broken down (usually less than ten percent of food consumed) and the food’s indigestible fiber content moves from the small intestine to the large intestine (colon) through a connecting valve, ileoceceal sphincter. The main task of the large intestine is to reabsorb water. Remember, water is present not only in solid foods but also the stomach releases a few hundred milliliters of gastric juice, and the pancreas adds approximately another 500 milliliters during the digestion of the meal. For the body to conserve water, it is important that the water is reabsorbed. In the large intestine, no further chemical or mechanical breakdown of food takes place, unless it is accomplished by the bacteria that inhabit this portion of the digestive tract. The number of bacteria residing in the large intestine is estimated to be greater than 10 (14) , which is more than the total number of cells in the human body (10 (13) ). This may seem rather unpleasant, but the great majority of bacteria in the large intestine are harmless and some are even beneficial. The bacteria synthesize the essential nutrient, vitamin K, short-chain fatty acids, which are essential for our health, from the undigested fiber. Also, minerals, such as sodium and potassium, are absorbed.

There has been significant talk about pre- and probiotic foods in the mainstream media. The World Health Organization defines probiotics as live bacteria that confer beneficial health effects on their host. They are sometimes called “friendly bacteria.” The most common bacteria labeled as probiotics are lactic acid bacteria (lactobacilli). They are added as live cultures to certain fermented foods such as yogurt. Prebiotics are indigestible foods, primarily soluble fibers, that stimulate the growth of certain strains of bacteria in the large intestine and provide health benefits to the host. Examples of prebiotics would be inulin, soluble fiber, and resistant starch. A review article in the June 2008 issue of the Journal of Nutrition concludes that there is a scientific consensus that probiotics ward off viral-induced diarrhea and reduce the symptoms of lactose intolerance. Farnworth, E. R. “The Evidence to Support Health Claims for Probiotics.” J Nutr 138, no. 6 (2008): 1250S–4S. http://jn.nutrition.org/content/138/6/1250S.long . Expert nutritionists agree that more health benefits of pre- and probiotics will likely reach a scientific consensus. As the fields of pre- and probiotic manufacturing and their clinical study progress, more information on proper dosing and what exact strains of bacteria are potentially “friendly” will become available.

You may be interested in trying some of these foods in your diet. A simple food to try is kefir. Several websites provide good recipes, including www.kefir.net/recipes.htm.

From the Large Intestine to the Anus

After a few hours in the stomach, plus three to six hours in the small intestine, and about sixteen hours in the large intestine, the digestion process enters step four, which is the elimination of indigestible food as feces. Feces contain indigestible food and gut bacteria (almost 50 percent of content). It is stored in the rectum until it is expelled through the anus via defecation.

Video 3.3.2: The Stages of Digestion

This video reviews the sequence of events during food digestion.

Processes of Digestion

Digestion involves two processes - physical and chemical. During the physical process, the food is mixed and moved throughout the gastrointestinal tract. This process is also referred to as motility and the partially digested food is propelled by the wave-like action called peristalsis. Ring-like muscular valves called sphincters prevent the back flow of partially digested food and digestive juices. There are sphincters between the esophagus and stomach (esophageal sphincter), between the stomach and small intestine (pyloric sphincter), and small intestine and colon (ileocecal sphincter).

The chemical process of digestion involves the release of water, acid, bicarbonate and enzymes to be mixed with the food to further break it down into smaller subunits. Chemical breakdown starts in the mouth where enzymes break down complex carbohydrates. In the stomach, water and acid are released to begin the breakdown of protein. A mucus lining protects the stomach from the corrosive acid. The mixture, also known as chyme, enters the small intestine where bicarbonate is introduced to neutralize the acid, and enzymes are added to break chemical bonds. Most small intestine digestive enzymes are produced in the pancreas and small intestine.

Regulation of Digestion

Our nervous system and hormones control digestion. The nervous system consists of the central nervous system, and the peripheral nervous system. Our brain and spinal cord make up the central nervous system while the peripheral system lies outside the skull and vertebral column. There are two components to the peripheral system: the somatic system which supplies the skin and muscle, and the autonomic system which supplies the smooth muscle, cardiac muscle, and glands. The autonomic system has two divisions: the parasympathetic (PSNS or PNS) and sympathetic system (SNS). The PSNS supplies signals to maintain normal function and conserve body processes. The SNS provides signals to accelerate the process. Our gastrointestinal tract receives signals from the central and autonomic systems as well as sends signals to these systems.

https://www.youtube.com/watch?v=hWks2wS56Qs

Hormones are also involved in regulating digestion. Your digestive tract secretes hormones to control the release of digestive enzymes and juices. Here is a table of some hormones.

Our appetite and hunger are controlled by a complex process that involves many signals. Here is a brief overview of that process.

https://www.youtube.com/watch?v=bQT17Mifh94

Key Takeaways

• The breakdown of complex macromolecules in foods to simple absorbable components is accomplished by the digestive system. These components are processed by cells throughout the body into energy or are used as building blocks.
• The digestive system is composed of the mouth, pharynx, esophagus, stomach, small intestine, large intestine (or colon), rectum, and anus. There are four steps in the digestion process: ingestion, the mechanical and chemical breakdown of food, nutrient absorption, and elimination of indigestible food.
• The mechanical breakdown of food occurs via muscular contractions called peristalsis and segmentation. Enzymes secreted by the salivary glands, stomach, pancreas, and small intestine accomplishes the chemical breakdown of food. Additionally, bile emulsifies fats.

Discussion Starter

Decide whether you want to consume pre- and probiotic foods to benefit your health. Visit the websites below to help in your decision-making process. Defend your decision scientifically.

http://www.health.harvard.edu/fhg/updates/update0905c.shtml

nccam.nih.gov/research/results/spotlight/110508.htm

Absorption of Water In Plants

Absorption of water in plants is a vital process that is important for plant growth and other metabolic activities. Water absorption in lower plants takes place by the process of osmosis through the whole plant body . In higher plants, the mechanism of water absorption is through the root hairs .

Plants mainly absorb water from the soil by the capillary action . There are five types of water that are found in the soil, namely runway water, gravitational water, hygroscopic water, chemically combined water and capillary water. Among the runaway water, gravitational water, hygroscopic water, chemically combined water, only the capillary water is useful for the plant.

There are some epiphytic plants, which grow on the substratum like rock and soil, while other plants absorb water by their aerial roots . The total water content in the soil is called holard . The water content consumed by the plant is called chesord. Water unconsumed by the plant is called echard .

Content: Absorption of Water In Plants

• Types of water absorption in plants
• Role of root hairs in water absorption
• Mechanism of water absorption in plants
• Factors affecting water absorption in plants

Absorption of water in plants is a biological process, in which the plants uptake capillary water from the soil to the root xylem through the root hairs during various plant processes like respiration, transpiration and osmosis. The water supply is an important factor, which directly or indirectly influences all the plant activities such as photosynthesis, internal water balance etc.

Loss of water in plants may result in loss or turgor, leaf-wilting, closure of stomata, reduction in photosynthetic activity and protoplasm disorganization. In plants, the absorbed water typically exists in two phases, namely apoplastic and symplastic water. Apoplastic water resides with the cell wall and xylem components, whereas symplastic water remains in the cell protoplast.

• Firstly, the two types (active and passive water absorption) were introduced by the scientist named Renner in 1912-1915.
• After the types of water absorption, two theories were introduced to know more about the concept of active absorption of water.
• The osmotic theory was given by the two scientists Atxins and Priestley .
• The non-osmotic theory was given by Bennet , Clark and Thimann in 1951.

Types of Water Absorption in Plants

Plants typically absorb water by the following two methods:

• Active absorption of water
• Passive absorption of water

Active Absorption of Water

This type of water absorption requires the expenditure of metabolic energy by the root cells to perform the metabolic activity like respiration. Active absorption in plant occurs in two ways, namely osmotic and non-osmotic absorption of water.

• Osmotic active absorption of water : In this type, the water absorption occurs through osmosis where the water moves into the root xylem across the concentration gradient of the root cell. The osmotic movement is due to the high concentration of solute in the cell sap and low concentration of the surrounding soil.
• Non-osmotic active absorption of water : Here, the water absorption occurs where the water enters the cell from the soil against the concentration gradient of the cell. This requires the expenditure of metabolic energy through the respiration process. Hence, as the rate of respiration increases, the rate of water absorption will also increase. Auxin is a growth regulatory hormone, which increases the rate of respiration in plants that, in turn, also increase the rate of water absorption.

Passive Absorption of Water

This type of water absorption does not require the use of metabolic energy. The absorption occurs by metabolic activity like transpiration. Passive absorption is the type where the water absorption is through the transpiration pull. This creates tension or force that helps in the movement of water upwards into the xylem sap. Higher is the transpiration rate, and higher is the absorption of water.

Role of Root Hairs in Water Absorption

Root cells, nucleus, and vacuole or cell sap are present inside the cytoplasmic membrane. Soil aggregates contain small droplets of water carried away by the root hairs into the root xylem through different mechanisms, out of which osmosis is most common.

Mechanism of Water Absorption in Plants

The movement of water from the soil to the root xylem occurs through the following stages:

• In the first step, the root hairs of the plant will absorb the water from the surrounding soil through the process of osmosis. The soil has high water concentration than the cell sap. Therefore, the water will move from a high concentration to the low concentration following osmosis through the cytoplasmic membrane of the root hairs.
• After entering into the root hair, the water will cross the epidermis or piliferous layer of the root system.
• Then, the water will move from the epidermis to the root cortex .
• From the root cortex, the water will travel through the endodermis that consists of suberic and passive cells. The further movement of water is facilitated by the passive cells.
• Then, water moves from the pericycle to the root xylem, i.e. perixylem and metaxylem. Water will be stored in the xylem root system, which can be utilized by the plant body to perform various metabolic activities and for its growth.

Factors Affecting

There are two kinds of factors that directly or indirectly influence the activity of water absorption.

Extrinsic factors: It includes external factors or environmental factors like:

• Soil water : Soil carries five different types of water, out of which the capillary water is useful for the biological activity of the plant.
• The concentration of soil solution : The concentration of soil must be less. If there is a high concentration of soil, then it will be called physiologically dry soil. Highly concentrated or dry soil makes the water absorption difficult.
• Soil air : There should be space between the soil particles for the proper air supply. If the quantity of oxygen is less, then the quantity of carbon dioxide will be more, which leads to the anaerobic respiration.
• Soil temperature : The optimum temperature is 20- 35 degrees Celsius.

Intrinsic factors : It involves the metabolic activities like respiration , transcription and the number of root hairs which directly influences the rate of water absorption.

Therefore, water absorption in plants occurs through the root hairs that carry the water present in the soil and forms a zone called the root hair zone. The root hairs absorb water through their wall, which is water-loving “Hydrophilic” in nature. Therefore, the high permeability of root hairs to the water will help in uptake water either through osmosis or transpiration.

Related Topics:

• Mendelian Inheritance
• Hydrophytes
• Adaptations in Hydrophytes
• Diastase Enzyme
• Types of Hydroponic Systems

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