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Essay on the Digestive System (For Students) | Human Physiology

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In this essay we will discuss about the digestive system in humans. After reading this essay you will learn about:- 1. Organs of Digestive System 2. Accessory Glands for Digestion of Foods.

Essay # 1. Organs of Digestive System:

Digestion means simplification of complex foods. It is the process of breaking various foodstuff into simple products. The complex foods like carbohydrates, proteins and fats are converted into glucose, amino acids and fatly acids respectively by the action of digestive enzymes. These simple substances enter into the blood circulation after absorption and then they are utilized by the body.

Digestive system consists of two main organs:

(1) Alimentary Canal

(2) Digestive Glands

1. Alimentary Canal:

This is also known as digestive tract or gastrointestinal tract. It is a long tube of varying diameter which begins at the mouth and ends at the anus. The length of this tube is about 8-9 meters. It opens at both the ends. The alimentary canal starts at the mouth into which cavity, the glands of the mouth pour the juice. As it passes backwards, it spreads into a funnel shaped cavity called-pharynx.

The tube then narrows into a soft muscular tube about ten inches in long, called the food pipe or gullet. This passes down the neck into the chest. It then opens into the stomach by piercing the diaphragm. The stomach is a large bag lying a little to the left under the diaphragm. It has two openings, one where the food pipe ends and the other where the intestines begin. The alimentary canal narrows again and passes into the small intestine which is about twenty two feet in length.

The first ten inches of the small intestine is called as Duodenum which forms a ‘C’ shaped loop. The rest of the small intestine is like a coiling tube, whose ends opens into a wide but comparatively short tube known as large intestine. It is about six feet long. The last part of the Large Intestine is known as Anus.

2. Digestive Glands:

Various digestive glands help in the digestion of foods.

(1) Salivary glands in the mouth,

(2) Gastric glands in the stomach

(3) Pancreas,

(5) Intestinal glands in small intestine.

All these digestive glands secrete digestive juices containing different enzymes which digest carbohydrate, protein and fatly foods.

Digestive juices:

Five digestive juices are secreted from digestive glands of the body. The enzymes present in these juices help in the digestion of different types of foods.

These juices are:

1. Salivary juice from salivary glands in mouth.

2. Gastric juice from Gastric glands in the stomach.

3. Pancreatic juice from Pancreas.

4. Intestinal juice from Small Intestine.

5. Bile juice from Liver.

Why so many digestive juices are necessary for digestion of food?

There are three reasons for the presence of so many digestive juices:

1. One digestive juice cannot digest three types of foods i.e. proteins, fats, and carbohydrates up to their completion.

2. One digestive juice cannot digest one particular type of food up to its completion, because food cannot remain in one place for a longer period of time.

3. The medium of action of enzymes present in different digestive juices are different. Some act on acidic medium and some on alkaline medium.

Digestion in Different Parts of Alimentary Canal:

The alimentary canal consists of the following organs in which foods are digested:

2. Oesophagus

4. Duodenum

5. Small Intestine

6. Large intestine

The mouth cavity is the front spread out end of the food pipe. The sides of the cavity are formed by the cheeks, the roof by the palate, and the floor by the tongue. When closed, it is bound in-front by the upper and the lower sets of teeth meeting in the middle. The opening at the back of the mouth is known as throat on each side of which there is a mass of tissue called tonsils. In the outside of the mouth cavity there is a slit like opening which is bounded by two soft movable lips.

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High school biology

Course: high school biology   >   unit 8.

• Meet the gastrointestinal tract!
• Kidney function and anatomy

The digestive and excretory systems review

• The digestive and excretory systems

The digestive system

Accessory organs.

• Salivary glands : moisten food and begin chemical digestion of starches.
• Liver : creates bile for fat digestion, detoxifies blood, processes absorbed vitamins
• Gallbladder : stores bile produced by the liver
• Pancreas : secretes pancreatic juices to help digestion of proteins and carbohydrates

The excretory system

• the skin , which removes excess water and salt via sweat,
• the lungs , which exhale carbon dioxide, and
• the liver , which breaks down toxic substances in the blood and convert nitrogenous waste into urea

Urinary tract

• Filtration : Blood enters a nephron, which filters out impurities.
• Reabsorption : The impurities move through tubules, while the rest of the blood is reabsorbed through capillary walls into the blood.
• Excretion : Urine is transported from the kidneys through the ureters and into the urinary bladder . It remains stored in the bladder until it is released through the urethra.

Common mistakes and misconceptions

• Incorrect : Digestion begins in the stomach.
• Correct : While some digestion occurs in the stomach, the process actually begins in the mouth, where chewing and salivary amylase act on the food.
• Incorrect : The digestive system produces urine.
• Correct : Some people think that the digestive system has two outlets—one for feces and one for urine. However, urine is a product of the excretory system, not the digestive system.
• Incorrect : The small intestine is shorter than the large intestine.
• Correct : The small intestine is actually longer than the large intestine. In fact, at approximately 20 feet in length, the small intestine is nearly four times as long as the large intestine (5 feet long)! However, the intestines are named for their diameters , not their lengths. The large intestine has a diameter of about 3 inches compared to the small intestine, with a diameter of about 1 inch.

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16.2 Digestive System

Learning objectives.

• Explain the processes of digestion and absorption
• Explain the specialized functions of the organs involved in processing food in the body
• Describe the ways in which organs work together to digest food and absorb nutrients
• Describe the essential nutrients required for cellular function that cannot be synthesized by the animal body
• Describe how excess carbohydrates and energy are stored in the body

All living organisms need nutrients to survive. While plants can obtain nutrients from their roots and the energy molecules required for cellular function through the process of photosynthesis, animals obtain their nutrients by the consumption of other organisms. At the cellular level, the biological molecules necessary for animal function are amino acids, lipid molecules, nucleotides, and simple sugars. However, the food consumed consists of protein, fat, and complex carbohydrates. Animals must convert these macromolecules into the simple molecules required for maintaining cellular function. The conversion of the food consumed to the nutrients required is a multistep process involving digestion and absorption. During digestion, food particles are broken down to smaller components, which are later absorbed by the body. This happens by both physical means, such as chewing, and by chemical means.

One of the challenges in human nutrition is maintaining a balance between food intake, storage, and energy expenditure. Taking in more food energy than is used in activity leads to storage of the excess in the form of fat deposits. The rise in obesity and the resulting diseases like type 2 diabetes makes understanding the role of diet and nutrition in maintaining good health all the more important.

The Human Digestive System

The process of digestion begins in the mouth with the intake of food ( Figure 16.4 ). The teeth play an important role in masticating (chewing) or physically breaking food into smaller particles. The enzymes present in saliva also begin to chemically break down food. The food is then swallowed and enters the esophagus —a long tube that connects the mouth to the stomach. Using peristalsis , or wave-like smooth-muscle contractions, the muscles of the esophagus push the food toward the stomach. The stomach contents are extremely acidic, with a pH between 1.5 and 2.5. This acidity kills microorganisms, breaks down food tissues, and activates digestive enzymes. Further breakdown of food takes place in the small intestine where bile produced by the liver, and enzymes produced by the small intestine and the pancreas, continue the process of digestion. The smaller molecules are absorbed into the blood stream through the epithelial cells lining the walls of the small intestine. The waste material travels on to the large intestine where water is absorbed and the drier waste material is compacted into feces; it is stored until it is excreted through the anus.

Oral Cavity

Both physical and chemical digestion begin in the mouth or oral cavity , which is the point of entry of food into the digestive system. The food is broken into smaller particles by mastication, the chewing action of the teeth. All mammals have teeth and can chew their food to begin the process of physically breaking it down into smaller particles.

The chemical process of digestion begins during chewing as food mixes with saliva, produced by the salivary glands ( Figure 16.5 ). Saliva contains mucus that moistens food and buffers the pH of the food. Saliva also contains lysozyme, which has antibacterial action. It also contains an enzyme called salivary amylase that begins the process of converting starches in the food into a disaccharide called maltose. Another enzyme called lipase is produced by cells in the tongue to break down fats. The chewing and wetting action provided by the teeth and saliva prepare the food into a mass called the bolus for swallowing. The tongue helps in swallowing—moving the bolus from the mouth into the pharynx. The pharynx opens to two passageways: the esophagus and the trachea. The esophagus leads to the stomach and the trachea leads to the lungs. The epiglottis is a flap of tissue that covers the tracheal opening during swallowing to prevent food from entering the lungs.

The esophagus is a tubular organ that connects the mouth to the stomach. The chewed and softened food passes through the esophagus after being swallowed. The smooth muscles of the esophagus undergo peristalsis that pushes the food toward the stomach. The peristaltic wave is unidirectional—it moves food from the mouth the stomach, and reverse movement is not possible, except in the case of the vomit reflex. The peristaltic movement of the esophagus is an involuntary reflex; it takes place in response to the act of swallowing.

Ring-like muscles called sphincters form valves in the digestive system. The gastro-esophageal sphincter (or cardiac sphincter) is located at the stomach end of the esophagus. In response to swallowing and the pressure exerted by the bolus of food, this sphincter opens, and the bolus enters the stomach. When there is no swallowing action, this sphincter is shut and prevents the contents of the stomach from traveling up the esophagus. Acid reflux or “heartburn” occurs when the acidic digestive juices escape into the esophagus.

A large part of protein digestion occurs in the stomach ( Figure 16.7 ). The stomach is a saclike organ that secretes gastric digestive juices.

Protein digestion is carried out by an enzyme called pepsin in the stomach chamber. The highly acidic environment kills many microorganisms in the food and, combined with the action of the enzyme pepsin, results in the catabolism of protein in the food. Chemical digestion is facilitated by the churning action of the stomach caused by contraction and relaxation of smooth muscles. The partially digested food and gastric juice mixture is called chyme . Gastric emptying occurs within two to six hours after a meal. Only a small amount of chyme is released into the small intestine at a time. The movement of chyme from the stomach into the small intestine is regulated by hormones, stomach distension and muscular reflexes that influence the pyloric sphincter.

The stomach lining is unaffected by pepsin and the acidity because pepsin is released in an inactive form and the stomach has a thick mucus lining that protects the underlying tissue.

Small Intestine

Chyme moves from the stomach to the small intestine. The small intestine is the organ where the digestion of protein, fats, and carbohydrates is completed. The small intestine is a long tube-like organ with a highly folded surface containing finger-like projections called the villi. The top surface of each villus has many microscopic projections called microvilli. The epithelial cells of these structures absorb nutrients from the digested food and release them to the bloodstream on the other side. The villi and microvilli, with their many folds, increase the surface area of the small intestine and increase absorption efficiency of the nutrients.

The human small intestine is over 6 m (19.6 ft) long and is divided into three parts: the duodenum, the jejunum and the ileum. The duodenum is separated from the stomach by the pyloric sphincter. The chyme is mixed with pancreatic juices, an alkaline solution rich in bicarbonate that neutralizes the acidity of chyme from the stomach. Pancreatic juices contain several digestive enzymes that break down starches, disaccharides, proteins, and fats. Bile is produced in the liver and stored and concentrated in the gallbladder; it enters the duodenum through the bile duct. Bile contains bile salts, which make lipids accessible to the water-soluble enzymes. The monosaccharides, amino acids, bile salts, vitamins, and other nutrients are absorbed by the cells of the intestinal lining.

The undigested food is sent to the colon from the ileum via peristaltic movements. The ileum ends and the large intestine begins at the ileocecal valve. The vermiform, “worm-like,” appendix is located at the ileocecal valve. The appendix of humans has a minor role in immunity.

Large Intestine

The large intestine reabsorbs the water from indigestible food material and processes the waste material ( Figure 16.6 ). The human large intestine is much smaller in length compared to the small intestine but larger in diameter. It has three parts: the cecum, the colon, and the rectum. The cecum joins the ileum to the colon and is the receiving pouch for the waste matter. The colon is home to many bacteria or “intestinal flora” that aid in the digestive processes. The colon has four regions, the ascending colon, the transverse colon, the descending colon and the sigmoid colon. The main functions of the colon are to extract the water and mineral salts from undigested food, and to store waste material.

The rectum ( Figure 16.6 ) stores feces until defecation. The feces are propelled using peristaltic movements during elimination. The anus is an opening at the far-end of the digestive tract and is the exit point for the waste material. Two sphincters regulate the exit of feces, the inner sphincter is involuntary and the outer sphincter is voluntary.

Accessory Organs

The organs discussed above are the organs of the digestive tract through which food passes. Accessory organs add secretions and enzymes that break down food into nutrients. Accessory organs include the salivary glands, the liver, the pancreas, and the gall bladder. The secretions of the liver, pancreas, and gallbladder are regulated by hormones in response to food consumption.

The liver is the largest internal organ in humans and it plays an important role in digestion of fats and detoxifying blood. The liver produces bile, a digestive juice that is required for the breakdown of fats in the duodenum. The liver also processes the absorbed vitamins and fatty acids and synthesizes many plasma proteins. The gallbladder is a small organ that aids the liver by storing bile and concentrating bile salts.

The pancreas secretes bicarbonate that neutralizes the acidic chyme and a variety of enzymes for the digestion of protein and carbohydrates.

Visual Connection

Which of the following statements about the digestive system is false?

• Chyme is a mixture of food and digestive juices that is produced in the stomach.
• Food enters the large intestine before the small intestine.
• In the small intestine, chyme mixes with bile, which emulsifies fats.
• The stomach is separated from the small intestine by the pyloric sphincter.

The human diet should be well balanced to provide nutrients required for bodily function and the minerals and vitamins required for maintaining structure and regulation necessary for good health and reproductive capability ( Figure 16.8 ).

Link to Learning

Explore this interactive United States Department of Agriculture website to learn more about each food group and the recommended daily amounts.

The organic molecules required for building cellular material and tissues must come from food. During digestion, digestible carbohydrates are ultimately broken down into glucose and used to provide energy within the cells of the body. Complex carbohydrates, including polysaccharides, can be broken down into glucose through biochemical modification; however, humans do not produce the enzyme necessary to digest cellulose (fiber). The intestinal flora in the human gut are able to extract some nutrition from these plant fibers. These plant fibers are known as dietary fiber and are an important component of the diet. The excess sugars in the body are converted into glycogen and stored for later use in the liver and muscle tissue. Glycogen stores are used to fuel prolonged exertions, such as long-distance running, and to provide energy during food shortage. Fats are stored under the skin of mammals for insulation and energy reserves.

Proteins in food are broken down during digestion and the resulting amino acids are absorbed. All of the proteins in the body must be formed from these amino-acid constituents; no proteins are obtained directly from food.

Fats add flavor to food and promote a sense of satiety or fullness. Fatty foods are also significant sources of energy, and fatty acids are required for the construction of lipid membranes. Fats are also required in the diet to aid the absorption of fat-soluble vitamins and the production of fat-soluble hormones.

While the animal body can synthesize many of the molecules required for function from precursors, there are some nutrients that must be obtained from food. These nutrients are termed essential nutrients , meaning they must be eaten, because the body cannot produce them.

The fatty acids omega-3 alpha-linolenic acid and omega-6 linoleic acid are essential fatty acids needed to make some membrane phospholipids. Vitamins are another class of essential organic molecules that are required in small quantities. Many of these assist enzymes in their function and, for this reason, are called coenzymes. Absence or low levels of vitamins can have a dramatic effect on health. Minerals are another set of inorganic essential nutrients that must be obtained from food. Minerals perform many functions, from muscle and nerve function, to acting as enzyme cofactors. Certain amino acids also must be procured from food and cannot be synthesized by the body. These amino acids are the “essential” amino acids. The human body can synthesize only 11 of the 20 required amino acids; the rest must be obtained from food.

Everyday Connection

With obesity at high rates in the United States, there is a public health focus on reducing obesity and associated health risks, which include diabetes, colon and breast cancer, and cardiovascular disease. How does the food consumed contribute to obesity?

Fatty foods are calorie-dense, meaning that they have more calories per unit mass than carbohydrates or proteins. One gram of carbohydrates has four calories, one gram of protein has four calories, and one gram of fat has nine calories. Animals tend to seek lipid-rich food for their higher energy content. Greater amounts of food energy taken in than the body’s requirements will result in storage of the excess in fat deposits.

Excess carbohydrate is used by the liver to synthesize glycogen. When glycogen stores are full, additional glucose is converted into fatty acids. These fatty acids are stored in adipose tissue cells—the fat cells in the mammalian body whose primary role is to store fat for later use.

The rate of obesity among children is rapidly rising in the United States. To combat childhood obesity and ensure that children get a healthy start in life, in 2010 First Lady Michelle Obama launched the Let’s Move! campaign. The goal of this campaign is to educate parents and caregivers on providing healthy nutrition and encouraging active lifestyles in future generations. This program aims to involve the entire community, including parents, teachers, and healthcare providers to ensure that children have access to healthy foods—more fruits, vegetables, and whole grains—and consume fewer calories from processed foods. Another goal is to ensure that children get physical activity. With the increase in television viewing and stationary pursuits such as video games, sedentary lifestyles have become the norm. Visit https://letsmove.obamawhitehouse.archives.gov/ to learn more.

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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.

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1.1: Introduction to Digestion and Nutrition

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• Why Do We Eat?
• Food Is Fuel
• Mouth to Stomach in One Swallow
• A Journey through the Intestine
• Food for life
• Staying Healthy
• Food Nutrient Chart

H. Craig Heller, Principal Investigator

Mary L. Kiely, Project Director

Text Authors

H. Craig Heller, James V. Lawry

Activity Authors

James V. Lawry, Stan Ogren, Marjorie Gray, Geraldine Horsma

Project Editor

Dennis McKee

Photo Credits

• 1 (top center): Bernd Wittich/Visuals Unlimited
• 2 (bottom center): Robert E. Daemmrich/Tony Stone Images
• 14 (top center): Phil Degginger/Tony Stone Images
• 24 (top center): AP/Wide World Photos
• 34 (top center): Cabisco/Visuals Unlimited
• 47 (top center): “The Field Museum” #A76851, Chicago, Illinois
• 48 (top center): W.A. Banaszewski/Visuals Unlimited
• 48 (middle left): Corbis-Bettmann
• 48 (bottom left): James Mayo/Chicago Tribune
• 56 (top center): Terry Donnelly/West Stock

Data Sources

• Food Nutrient chart: Morrill, Judi, Sheri Bakun, and Suzanne Murphy. Are You Eating Right? San Jose: Department of Nutrition and Food Science, San Jose State University, 1992. Whitney, Eleanor Noss, and Sharon Rady Rolfes. Understanding Nutrition, 7th ed. St. Paul: West Information Publishing Group, 1996.
• FAO information: Food and Agriculture Organization (FAO), Food Security and Nutrition. Prepared for the 1996 World Food Summit.

Originally Published by Everyday Learning Corporation

Everyday learning development staff.

Steve Mico, Leslie Morrison, Susan Zeitner

Production/Design

Fran Brown, Annette Davis, Jess Schaal, Norma Underwood

Diagram Of The Human Digestive System

The diagram given below represents different parts of the human digestive system that convert food into essential nutrients absorbed by the body.

Parts of the Human Digestive System

The digestive system of the human body comprises a group of organs that work together in converting food into energy and other basic nutrients to power the body. The food we take in is digested and utilized by our body, and the unused parts of the food are defecated.

The digestive system of the human body is the sum of the gastrointestinal tract (GIT; also called alimentary canal) and accessory organs (tongue, liver, pancreas, etc.). These two parts together help in the digestion process.

The alimentary canal is the long tube through which the food that we eat is passed. It begins at the mouth (buccal or oral cavity), passes through the pharynx, oesophagus or food pipe, stomach, small intestines, large intestines, rectum and finally ends at the anus. The food particles gradually get digested as they travel through various compartments of the alimentary canal.

Accessory organs are organs which participate in the digestion process but are not actually a part of GIT. They stimulate the digestion by releasing certain enzymes that help in breaking down the food.

Let us have a detailed look at the digestive system of the human body, along with its parts and functions:

Food starts its journey from the mouth or the oral cavity. There are many other organs that contribute to the digestion process, including teeth, salivary glands, and tongue. Teeth are designed for grinding food particles into small pieces and are moistened with saliva before the tongue pushes the food into the pharynx.

A fibromuscular y-shaped tube attached to the terminal end of the mouth. It is mainly involved in the passage of chewed/crushed food from the mouth through the oesophagus. It also has a major part in the respiratory system, as air travels through the pharynx from the nasal cavity on its way to the lungs.

This is a muscular tube that connects the pharynx, which is a part of an upper section of the gastrointestinal tract. It supplies swallowed food along with its length.

Also Read:  Food Pipe

It serves as a muscular bag which is situated towards the left side of the abdominal cavity, beneath the diaphragm. This vital organ acts as a storage for the food and provides enough time to digest meals. The stomach also produces digestive enzymes and hydrochloric acid that maintains the process of digestion.

Mucous : It is an aqueous secretion produced by the mucous membranes. It functions by protecting the stomach lining and gastric pits from the acid, which is produced by the glands to destroy the bacteria that entered along with the food particles.

Digestive enzymes : They are the group of enzymes which functions by breaking down polymeric macromolecules like biopolymers into their smaller and simpler substances.

Hydrochloric acid : It is the digestive fluid formed by the stomach during the process of digestion. It functions by destroying harmful microorganisms present in the food particles.

The small intestine is a thin, long tube of about 10 feet long and a part of the lower gastrointestinal tract. It is present just behind the stomach and acquires a maximum area of the abdominal cavity. The complete small intestine is coiled and the inner surface consists of folds and ridges.

This is a thick, long tube measuring around 5 feet in length. It is present just beneath the stomach and wraps over the superior and lateral edges of the small intestine. It absorbs water and consists of bacteria (symbiotic) that support the breakdown of wastes to fetch small nutrients.

Also Read:  Large Intestine

Waste products are passed into the end of the large intestine called the rectum and eliminated out of the body as a solid matter called stool. It is stored in the rectum as semi-solid faeces which later exits from the body through the anal canal through the process of defecation.

It is a large gland present just behind the stomach. It is short with its anterior connected to the duodenum and posterior pointing towards the left part of the abdominal cavity. The pancreas releases digestive enzymes to complete the process of chemical digestion.

Read More:  Pancreas

The liver is a roughly triangular, reddish-brown accessory organ of the digestive system located to the right of the stomach. It produces bile , which helps in the digestion of fat in the small intestine. The bile is stored and recycled in the gallbladder. It is a small, pear-shaped organ which is located just next to the liver.

Also Read:  Cellulose in Digestion

The process of digestion begins from the mouth and ends in the small intestine – the large intestines’ main function is to absorb the remaining water from the undigested food and enable bacterial fermentation of materials that can no longer be digested.

The alimentary canal or the gastrointestinal tract is a series of hollow organs and tubes that begins from the mouth cavity and continues into the pharynx, through the stomach, small intestines, large intestines, and finally ending at the anus. Food particles gradually get digested as they travel through various compartments of the gastrointestinal tract.

The digestion process takes place in the following steps.

The very first step involves mastication (chewing). The salivary glands, along with the tongue, helps to moisten and lubricate food, before being pushed down into the food pipe.

It involves the process of lubricating and manipulating food and pushing it down the food through the food pipe (using peristalsis ), and into the stomach.

The stomach, small intestine, liver, and pancreas secrete enzymes and acids to aid the process of digestion. It functions by breaking down food particles into simple components and easily absorbable components.

Also Read:  What is Liver

The process of converting complex food particles into simpler substances in the presence of enzymes and acids secreted by different digestive organs.

Read more: What are digestive juices?

This process begins in the small intestine where most of the nutrients and minerals are absorbed. The excess water in the indigestible matter is absorbed by the large intestines.

The process of removing indigestible substances and waste by-products from the body through the process of defecation.

In a nutshell, the digestion process consists of the six following steps:

Ingestion  ⇒ Mixing and Movement  ⇒  Secretion  ⇒  Digestion  ⇒ Absorption  ⇒ Excretion

Also Read:  Gastrointestinal Tract

Disorders of the Human Digestive System

Vomiting : It is the ejection of stomach contents through the mouth.

Diarrhoea : It is the abnormal watery bowel movement. Prolonged diarrhoea eventually leads to dehydration.

Constipation : A condition in which the faeces are clutched within the rectum due to an irregular bowel movement.

Indigestion : A pain or discomfort in the stomach which is caused when food is not digested properly, resulting in the feeling of fullness.  Indigestion is mainly caused due to inadequate enzyme secretion, food poisoning, anxiety, overeating and eating spicy foods.

Also Read:  Difference between trachea and oesophagus

Functions of the Human Digestive System

Digestion and absorption are the two main functions of the digestive system.

Digestion is necessary for breaking down food particles into nutrients that are used by the body as an energy source, cell repair and growth.

Food and drink need to be converted into smaller molecules of nutrients before it is absorbed by the blood and carried to the cells throughout the body. The body breaks the nutrients present in the drinks and food into carbohydrates, vitamins, fats and proteins.

Human Digestive System Notes

• The human digestive system breaks down food to release energy essential for the body to carry out its activities.
• The process of digestion takes place in 6 major steps.
• The food is ingested by the alimentary canal and is propelled through the body for further processing.
• The autonomous nervous system controls the peristalsis, contraction and relaxation of muscles within the alimentary canal wall.
• The food is passed to the small intestine where it is digested, and the nutrients are absorbed.
• Water, electrolytes and vitamins are absorbed by the large intestine and the waste is defecated.

Also Read:  Mouth and Buccal Cavity

To learn more about the human digestive system parts and functions, as well as related topics such as the digestion process and disorders of the digestive system, keep visiting  BYJU’S Biology or download the BYJU’S app, for further reference.

Frequently Asked Questions

Deduce the function of the human digestive system..

The digestive system consists of the alimentary canal and the accessory organs. Their main function is to break down the ingested food into its components and produce vital nutrients and energy required to sustain life.

What are accessory organs?

Accessory organs are organs which are not part of the digestive system; however, they aid in the digestion process by performing many secondary functions. The main accessory organs of the digestive system are the tongue, liver, pancreas and gall bladder.

Outline the process of digestion, step by step.

The process of digestion involves the following steps, namely:

List out the parts of the digestive system.

• Mouth & Buccal Cavity

Explain the role of the tongue as an accessory organ.

The tongue is not a part of the digestive system, but it provides support functions such as moving and manipulating the food within the buccal cavity. Furthermore, moistening food also helps to swallow and pass through the oesophagus without much resistance.

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Put your understanding of this concept to test by answering a few MCQs. Click ‘Start Quiz’ to begin!

Select the correct answer and click on the “Finish” button Check your score and answers at the end of the quiz

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The Digestive System in the Human Body Essay

Introduction, the stomach, large intestines, small intestines, works cited.

Human biology is a field in biology that deals with nutrition, anthropology, and medicine. The body of a human being contains a digestive system that deals with the digestion and absorption of food nutrients into the body. The stomach, small intestines, and large intestines are part of the digestive system. All of them are located at different positions in the body and have different functions and structures.

The stomach is an organ of digestion located between the esophagus and the duodenum on the left upper part of the abdominal cavity. It is a muscular, elastic pear-shaped bag lying just below the diaphragm. It is made up of five layers. The innermost layer is referred to as mucosa. The four sections of the stomach are the fundus, cardia, body, and pylorus. It is involved in the second phase of digestion after chewing. It secretes enzymes that help in the digestion of proteins. It also secretes some acids that help in the digestion of food. The major secretions are hydrochloric acid and protease enzymes. The work of hydrochloric acid is to kill any bacteria that may be in the food and provide an acidic pH that allows proteases to work effectively (Chiras 95).

The large intestines of the human body are located between the ileocecal junction and the rectum. They are about one and a half meters long and two and a half centimeters wide. The major parts of the large intestines are the caecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal (Tortora and Grabowski 46). The large intestines receive food contents and fluids from the small intestines. The large intestines help in the re-absorption of water into the body system. The water absorbed is used to maintain the fluid balance of the body. Some vitamins that are created by the bacteria in the large intestines are also absorbed. They also help in the transport of waste products in form of stool out of the body.

The small intestines are located between the stomach and the large intestines. Specifically, they start at the pyloric sphincter and end at the ileocaecal valve. They are narrow and long to allow digestion and absorption of food. The standard length of the small intestines in an adult human being is five meters but they may be shorter or longer. The three parts of the small intestines are the duodenum, jejunum, and ileum (Tortora and Grabowski 49). The majority of digestion and absorption of food takes place in the small intestines. Chemical digestion takes place in the small intestines where many enzymes that facilitate digestion are secreted. In the small intestines, proteins and peptides are digested into amino acids. Lipase enzymes are used to degrade fats into glycerol and fatty acids. Some carbohydrates are also digested in the stomach to form simple sugars. The process of absorption takes place where food nutrients are diffused into the bloodstream. The small intestines have many villi that have networks of capillaries to help indigestion. Monosaccharides, amino acids, water, iron, and many others are absorbed here.

The digestive system in the human body mainly deals with digestion and absorption of food nutrients and fluids. The stomach is located between the esophagus and the duodenum and helps in the digestion of partially chewed food. The large intestines are located between the ileocecal junction and the rectum. They help in the absorption of water, some nutrients, and the excretion of wastes out of the body. The small intestines are located between the pyloric sphincter and the ileocaecal valve. They help in the digestion of food and absorption of food nutrients into the bloodstream.

Chiras, Daniel. Human Biology. Sixth Edition. London: Jones and Bartlett Punishers, 2009. Print.

Tortora, Gerald and Grabowski, Sandra. Introduction to the human body: the essentials of anatomy and physiology. Volume 1. New York: John Wiley & Sons, 2004. Print.

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Enzymes & Digestion - The Digestive System (GCSE Biology)

The digestive system, the human digestive system.

The digestive system is involved in the digestion and absorption of food.

Digestion is the breakdown of food from large, insoluble molecules, to small, soluble molecules. Absorption involves these small, soluble molecules being taken into the bloodstream through the wall of the intestine. Food moves through the gut by peristalsis .

Ingestion is the intake of substance into the body via the mouth e.g. through eating and drinking.

Assimilation is the movement of digested food molecules into the cells where they are used.

Egestion is the passing of undigested food as faeces through the anus.

Loss of watery faeces is known as diarrhoea and oral rehydration therapy can be used to treat it in order to replace the lost fluid. A cause of this can be cholera in which a bacterium produces a toxin that causes movement of chloride ions into the small intestine and thus, water follows due to osmosis. This leads to diarrhoea, dehydration and loss of salts from the blood.

Breaking Down Food

The human digestive system is responsible for the break down of food and absorption of nutrients. Large, insoluble molecules cannot be absorbed into the bloodstream. The digestive enzymes hydrolyse them into small soluble molecules that can be absorbed. In simple terms, the food is broken down into small pieces, which can cross cell membranes in the small intestine (where food is absorbed).

Mechanical Digestion

• There is physical breakdown of food via mechanical digestion (e.g. teeth, stomach churning) and no chemical changes.
• Incisors – are sharp for biting and cutting
• Canines – are sharp for tearing and gripping
• Premolars – are flat for crushing and tearing
• Molars – are flat for grinding and crushing
• Pulp – soft tissue with blood vessels and nerves
• Dentine – are soft and forms a significant amount of the tooth
• Enamel – outer layer of the tooth that is hard
• Cement – similar to bone around tooth to hold it in place
• Bone – the tooth is rooted here and held in place
• Gum – soft tissue surrounding the tissue and bone to protect them

• Teeth can decay. Bacteria together with sugars from food can coat your teeth and when the bacteria respire and produce acid which dissolves the enamel and dentine.
• Proper care must be taken of your teeth. This includes a good diet with a sugar restriction and regular brushing to clear the coat of bacteria.

Chemical Digestion

• Mechanical digestion alone is not enough. Mechanical digestion is not enough to allow for food to be absorbed. Just chewing your food into small pieces is not enough to allow cells of the digestive system to absorb much needed nutrients.
• Enzyme-driven breakdown is via chemical digestion (e.g. lipase enzymes) to breakdown large insoluble molecules to smaller soluble molecules that can be absorbed.

• The small intestine is the area for absorption of digested food. Water is absorbed both in the small intestine and colon but most occurs in the small intestine.
• The small intestine has villi and microvilli. These are finger-like projections that increase the surface area over which molecules are absorbed.
• Microvilli are associated with a network of capillaries. This allows for efficient absorption.
• Microvilli are also associated with lacteals. These aid in the absorption of fats after digestion. Lacteals carry a fluid called lymph and fats are transported through this to blood vessels.

Digestive Organs

Overview of the digestive system.

The digestive system involves many organs . Digestion can be a complicated topic, so we will begin by giving you a summary of the role of each digestive organ part of the alimentary canal.

Products of Digestion

The products of digestion are absorbed into the bloodstream in the small intestine. When they enter the bloodstream, they can be used for several things, including:

• Building carbohydrates – the simple sugars absorbed (maltose, fructose, glucose) can be used to build new carbohydrates. For example, glucose can be stored as glycogen in the liver.
• Building proteins – the amino acids absorbed can be used for building protein. For example, bodybuilders often eat lots of protein to increase the number of amino acids for muscle synthesis.
• Building lipids – the fatty acid and glycerol molecules absorbed can be used to store fat. For example, if you eat lots of high-lipid foods, you are likely to build up more and more fat stores.
• Respiration – some of the glucose from carbohydrate digestion is used to release energy via respiration. This is why food is so important for daily function and especially for exercise.

Enzymes are biological catalysts that speed up chemical reactions in the body. They are made of proteins and are found in every living cell.

Enzymes play a crucial role in digestion by breaking down large molecules of food into smaller, more manageable molecules that the body can absorb and use for energy. Different enzymes are responsible for breaking down different types of food, such as carbohydrates, proteins, and fats.

The human digestive system is the organ system responsible for breaking down food into small molecules that can be absorbed and used by the body for energy, growth, and repair. The main organs of the digestive system include the mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus. In GCSE biology, students learn about the different organs of the digestive system and their functions, as well as the process of digestion, which involves mechanical and chemical digestion. Mechanical digestion occurs when food is physically broken down into smaller pieces, while chemical digestion involves the use of enzymes to break down food molecules into simpler forms.

The Digestive System works by breaking down food and absorbing its nutrients through a series of processes that begin in the mouth and end in the rectum. Food is mechanically broken down in the mouth through chewing and mixed with saliva, which contains the enzyme amylase to begin breaking down carbohydrates. The food then travels down the esophagus into the stomach where it is mixed with digestive juices and further broken down. From the stomach, the partially digested food moves into the small intestine where it is mixed with digestive enzymes from the pancreas and bile from the liver. This mixture further breaks down the food, allowing the small intestine to absorb the nutrients. The remaining waste products move into the large intestine, where water and electrolytes are absorbed before being eliminated from the body.

There are several organs involved in the digestive system, including: Mouth: where food is ingested and mechanically broken down by teeth and tongue. Pharynx: the back of the throat which connects the mouth to the esophagus. Esophagus: a muscular tube that connects the pharynx to the stomach. Stomach: a muscular sac that mixes food with digestive juices and breaks it down into a liquid mixture. Small intestine: a long, narrow tube where most of the nutrients from the broken-down food are absorbed into the bloodstream. Large intestine: a wider tube where water and electrolytes are absorbed and the remaining waste material is stored until it is eliminated. Rectum: the lower part of the large intestine where feces are stored. Anus: the opening at the end of the digestive tract where feces are eliminated from the body. There are also several accessory organs that help with digestion, including the liver, pancreas, and gallbladder. These organs produce and release substances that aid in the digestion and absorption of nutrients.

The Digestive System is important because it allows the body to break down food and absorb its nutrients. Without the Digestive System, the body would not be able to get the energy and nutrients it needs to function properly.

There are two main types of digestion in the digestive system: mechanical digestion and chemical digestion. Mechanical Digestion: It is the physical breakdown of food into smaller pieces, making it easier to swallow, move through the digestive tract, and ultimately, absorb the nutrients. This type of digestion begins in the mouth with the action of the teeth and tongue, which break down the food into smaller pieces. Chemical Digestion: It is the process of breaking down food into simpler chemical compounds through the use of enzymes and other digestive secretions. This type of digestion occurs mainly in the stomach and small intestine. The digestive enzymes break down proteins, carbohydrates, and fats into their basic building blocks, such as amino acids, simple sugars, and fatty acids. Both types of digestion work together to ensure that food is properly broken down and nutrients are extracted and absorbed into the body.

The products of digestion depend on the type of food that is being digested. Carbohydrates, proteins, and fats are broken down into their basic building blocks, which can then be absorbed into the bloodstream and used by the body for various functions. Here are some examples of the products of digestion: Carbohydrates: Carbohydrates are broken down into simple sugars, such as glucose, fructose, and galactose. Proteins: Proteins are broken down into amino acids. Fats: Fats are broken down into fatty acids and glycerol. Other products of digestion may include vitamins, minerals, and water. These nutrients are also absorbed into the bloodstream and transported to various organs and tissues throughout the body. The digestive system plays a critical role in breaking down food and extracting the nutrients that the body needs to function properly.

The products of digestion can be used for various functions in the body. Here are some examples: Carbohydrates: Glucose is the main source of energy for the body’s cells. It is used to fuel cellular processes and activities. Proteins: Amino acids are used to build and repair tissues in the body, such as muscle tissue, skin, and hair. They are also used to make enzymes, hormones, and other important molecules. Fats: Fatty acids and glycerol are used to make cell membranes, hormones, and other molecules. They are also stored in adipose tissue and can be used for energy when glucose levels are low. Vitamins and minerals: These nutrients are used for various functions in the body, such as maintaining healthy bones, supporting immune function, and promoting wound healing. Overall, the products of digestion provide the body with the nutrients it needs to function properly and maintain good health.

Yes, enzymes can be affected by changes in temperature or pH. Enzymes work best at a specific temperature and pH, and any changes outside of that range can cause the enzyme to denature, or stop working.

The Human Digestive System can be affected by diseases or conditions, such as acid reflux, ulcerative colitis, or Crohn’s disease. These conditions can cause inflammation, pain, and other symptoms that can impact the ability of the Digestive System to function properly.

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respiration – animals (gcse biology), cie igcse 13 excretion in humans, osmoregulation & the kidney – kidney transplantation (gcse biology), osmoregulation & the kidney – kidney failure and dialysis (gcse biology), osmoregulation & the kidney – the kidneys and excretion (gcse biology), osmoregulation & the kidney – osmoregulation (gcse biology), cie igcse 14 coordination and response, plant hormones – commercial use of plant hormones (gcse biology), plant hormones – experiments on plant responses (gcse biology), plant hormones – tropisms: phototropism & geotropism (gcse biology), control of blood glucose concentration – diabetes mellitus: type i & ii (gcse biology), control of blood glucose concentration – increasing and decreasing blood glucose levels (gcse biology), control of blood glucose concentration – blood glucose homeostasis (gcse biology), homeostasis – increasing and decreasing body temperature (gcse biology), homeostasis – an introduction (gcse biology), homeostasis – thermoregulation (gcse biology), human endocrine system – hormones: adrenaline and thyroxine (gcse biology), cie igcse 15 drugs, antibiotics – drug resistance, antivirals and antiseptics (gcse biology), antibiotics – drugs: antibiotics and painkillers (gcse biology), lifestyle & disease – effects of smoking and alcohol on health (gcse biology), cie igcse 16 reproduction, asexual and sexual reproduction – sexual reproduction: pros and cons (gcse biology), asexual and sexual reproduction – asexual reproduction: pros and cons (gcse biology), asexual and sexual reproduction – (gcse biology), treating infertility – ivf: development and treatment issues (gcse biology), treating infertility – drugs, ivf and ai for infertility (gcse biology), contraception – hormonal contraception: the pill, patches & implants (gcse biology), contraception – contraception and non-hormonal contraception (gcse biology), hormones in human reproduction – the menstrual cycle: graphs (gcse biology), hormones in human reproduction – the menstrual cycle: hormonal interactions (gcse biology), hormones in human reproduction – the menstrual cycle: hormones (gcse biology), cie igcse 17 inheritance, meiosis – mitosis and meiosis (gcse biology), inheritance – sex determination (gcse biology), inheritance – genetic diagrams (gcse biology), inheritance – genes and inheritance (gcse biology), dna – protein synthesis: translation (gcse biology), dna – an introduction (gcse biology), cell division – stem cell types (gcse biology), cell division – the cell cycle and mitosis (gcse biology), cell division – nucleus and chromosomes (gcse biology), cie igcse 18 variation and selection, ecosystems – extremophiles (gcse biology), ecosystems – adaptations (gcse biology), development and understanding of evolution – evidence for evolution: resistant bacteria (gcse biology), variation – selective breeding (gcse biology), variation – evolution and natural selection (gcse biology), variation – variation and its causes (gcse biology), inheritance – inherited disorders (gcse biology), dna – mutations (gcse biology), cie igcse 19 organisms and their environment, cycles – decomposition & the nitrogen cycle (gcse biology), cycles – the water cycle (gcse biology), cycles – cycles & the carbon cycle (gcse biology), organisation & trophic levels – transfer of biomass (gcse biology), organisation & trophic levels – pyramids of biomass (gcse biology), organisation & trophic levels – trophic levels & food chains (gcse biology), ecosystems – biotic factors (gcse biology), cie igcse 2 organisation of the organism, transport in plants – how plants are adapted for photosynthesis (gcse biology), enzymes & digestion – cell organisation (gcse biology), microscopes & cultures – cell size and area estimations (gcse biology), microscopes & cultures – magnification and unit conversions (gcse biology), introduction to cells – specialised cells: more cells (gcse biology), introduction to cells – specialised cells: sperm cells (gcse biology), introduction to cells – animal and plant cells (gcse biology), cie igcse 20 biotechnology and genetic engineering, variation – genetic engineering (gcse biology), cie igcse 21 human influences on ecosystems, cie igcse 3 movement in and out of cells, simple molecular covalent structures (gcse chemistry), exchange surfaces – surface areas to volume ratios (gcse biology), transport in cells – diffusion – (gcse biology), transport in cells – active transport (gcse biology), transport in cells – measuring the effects of osmosis (gcse biology), transport in cells – osmosis (gcse biology), transport in cells – factors that affect the rate of diffusion (gcse biology), cie igcse 4 biological molecules, dna – its structure (gcse biology), enzymes & digestion – protein and lipids: breakdown (gcse biology), enzymes & digestion – carbohydrates: breakdown and synthesis (gcse biology), cie igcse 5 enzymes, enzymes & digestion – enzyme action: factors that affect it (gcse biology), enzymes & digestion – enzymes: an introduction (gcse biology), cie igcse 6 plant nutrition, plant disease & defence – plant diseases and deficiencies (gcse biology), photosynthesis: greenhouses – (gcse biology), photosynthesis: limiting factors affecting the rate of photosynthesis – (gcse biology), photosynthesis: an introduction – (gcse biology), transport in plants – structure of a plant (gcse biology), cie igcse 7 human nutrition, types of diseases – bacterial diseases: cholera and tuberculosis – (gcse biology), lifestyle & disease – diet and exercise (gcse biology), enzymes & digestion – the digestive system (gcse biology), cie igcse 8 transport in plants, transpiration – plant water loss (gcse biology), transpiration – transpiration rates (gcse biology), transpiration – transpiration in plants (gcse biology), transport in plants – transport systems in plants (gcse biology), cie igcse 9 transport in animals, cardiovascular disease: prophylactic treatment (gcse biology), cardiovascular disease: artificial hearts and transplants (gcse biology), cardiovascular disease: stents and lifestyle (gcse biology), blood and blood vessels: veins and capillaries (gcse biology), blood and blood vessels – white blood cells and platelets (gcse biology), blood and blood vessels – plasma and red blood cells (gcse biology), blood and blood vessels – arteries (gcse biology), the circulatory system – heart: structure and function (gcse biology), circulatory system – the double circulatory system (gcse biology), circulatory system – the single circulatory system (gcse biology), edexcel igcse 1 - the nature and variety of living organisms, types of diseases – viral diseases: hiv (gcse biology), types of diseases – sexually transmitted infections (gcse biology), types of diseases – viral diseases:tmv, measles and ebola (gcse biology), introduction to cells – bacterial cells (gcse biology), edexcel igcse 2 - structure and functions in living organisms, hormones in human reproduction – puberty and hormones (gcse biology), osmoregulation & the kidney – anti-diuretic hormone (gcse biology), edexcel igcse 3 - reproduction and inheritance, development and understanding of evolution – theory of evolution: darwin and lamarck (gcse biology), edexcel igcse 4 - ecology and the environment, organisation & trophic levels – quadrat and transect sampling (gcse biology), edexcel igcse 5 - use of biological resources, variation – cloning (gcse biology).

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Overview of Digestion

Digestion involves the break down of large, biological molecules into smaller molecules. The smaller molecules can then be taken up by the cells and used for a number of functions.

• When an organism eats, it ingests large, biological molecules (e.g. carbohydrates).
• The molecules could be used in the cells for important reactions (e.g respiration) but they are too large to be transported into the cells.
• The large molecules must first be broken down.

• Hydrolysis is the reaction that breaks down large, biological molecules.
• The reaction requires water and splits larger molecules into their smaller components.
• These smaller molecules (e.g. glucose) can easily diffuse into cells or be transported using protein channels.

Products of hydrolysis

• Hydrolysis of protein produces amino acids.
• Disaccharides are two monosaccharides joined together.
• Hydrolysation of lipids produces fatty acids and monoglycerides.

E.g. Hydrolysis of protein

• A human eats a ham and cheese sandwich.
• The sandwich is made up of carbohydrate (in the bread), protein (in the ham) and fat (in the cheese).
• In the stomach, the protein is broken down into amino acids in a hydrolysation reaction.
• The amino acids are taken up by the cells and used to make new proteins for other uses.

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

Digestion in Mammals