IGCSE NOTES : Biology - The Alimentary canal

Ingestion is the taking of substances such as food and drink into the body through the mouth. Mechanical digestion is the breakdown of food into smaller pieces without chemical change to the food molecules. Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules. Absorption is the movement of small food molecules and ions through the wall of the intestine into the blood. Assimilation is the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells. Egestion is the passing out of food that has not been digested or absorbed, as faeces, through the anus.

Ingestion is the taking of substances such as food and drink into the body through the mouth. Mechanical digestion is the breakdown of food into smaller pieces without chemical change to the food molecules. Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules. Absorption is the movement of small food molecules and ions through the wall of the intestine into the blood. Assimilation is the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells. Egestion is the passing out of food that has not been digested or absorbed, as faeces, through the anus.

Feeding involves taking food into the mouth, chewing it and swallowing it down into the stomach. This satisfi es our hunger, but for food to be of any use to the whole body it has fi rst to be digested. This means that the solid food is dissolved and the molecules reduced in size. The soluble products then have to be absorbed into the bloodstream and carried by the blood all around the to the living cells in all parts of the body such as
the muscles, brain, heart and kidneys. This section describes how the food is digested and absorbed.

Regions of the alimentary canal and their functions

The alimentary canal is a tube running through the body. Food is digested in the alimentary canal. The soluble products are absorbed and the indigestible residues expelled (egested).

The inside of the alimentary canal is lined with layers of cells forming what is called an epithelium. New cells in the epithelium are being produced all the time to replace the cells worn away by the movement of the food. There are also cells in the lining that produce mucus. Mucus is a slimy liquid that lubricates the lining of the canal and protects it from wear and tear. Mucus may also protect the lining from attack by the digestive enzymes which are released into the alimentary canal. Some of the digestive enzymes are produced by cells in the lining of the alimentary canal, as in the stomach lining. Others are produced by glands that are outside the alimentary canal but pour their enzymes through tubes (called ducts) into thealimentary canal. The salivary glands and the pancreas are examples of such digestive glands.

The alimentary canal has a great many blood vessels in its walls, close to the lining. These bring oxygen needed by the cells and take away the carbon dioxide they produce. They also absorb the digested food from the alimentary canal.

Five main processes associated with digestion occur in the alimentary canal. These are ingestion, digestion,
absorption, assimilation and egestion.

Peristalsis

The alimentary canal has layers of muscle in its walls. The fi bres of one layer of muscles run around the canal (circular muscle) and the others run along its length (longitudinal muscle). When the circular muscles in one region contract, they make the alimentary canal narrow in that region. A contraction in one region of the alimentary canal is followed by another contraction just below it so that a wave of contraction passes along the canal, pushing food in front of it.

Diarrhoea

Diarrhoea is the loss of watery faeces. It is sometimes caused by bacterial or viral infection, for example from food or water. Once infected, the lining of the digestive system is damaged by the pathogens, resulting in the intestines being unable to absorb fluid from the contents of the colon or too much fl uid being secreted into the colon. Undigested food then moves through the large intestine too quickly, resulting in insuffi cient time to absorb water from it. Unless the condition is treated, dehydration can occur.

Treatment is known as oral hydration therapy. This involves drinking plenty of fl uids – sipping small amounts of water at a time to rehydrate the body. Other possible causes of diarrhoea include anxiety, food allergies, lactose intolerance, a side-effect of antibiotics and bowel cancer.

Cholera

This disease is caused by the bacterium Vibrio cholera which causes acute diarrhoea. Treatment involves rehydration and restoration of the salts lost (administered by injecting a carefully controlled solution into the bloodstream) and use of an antibiotic such as tetracycline to kill the bacteria. The bacteria thrive in dirty water (often that contaminated by sewage) and are transmitted when the water is drunk or used to wash food. Longterm methods of control are to dispose of human sewage safely, ensuring that drinking water is free from bacteria and preventing food from being contaminated.

How cholera causes diarrhoea

When the Vibrio cholera bacteria are ingested, they multiply in the small intestine and invade its epithelial cells. As the bacteria become embedded, they release toxins (poisons) which irritate the intestinal lining and lead to the secretion of large amounts of water and salts, including chloride ions. The salts decrease the osmotic potential of the gut contents, drawing more water from surrounding tissues and blood by osmosis. This makes the undigested food much more watery, leading to acute diarrhoea, and the loss of body fl uids and salt leads to dehydration and kidney failure.

Mechanical digestion

The process of mechanical digestion mainly occurs in the mouth by means of the teeth, through a process called mastication. Humans are omnivores (organisms that eat animal and plant material). Broadly, we have the same types of teeth as carnivores, but human teeth are not used for catching, holding, killing or tearing
up prey, and we cannot cope with bones. Thus, although we have incisors, canines, premolars and molars, they do not show such big variations in size and shape as, for example, a wolf’s.

Our top incisors pass in front of our bottom incisors and cut pieces off the food, such as when biting into an apple or taking a bite out of a piece of toast.

Our canines are more pointed than the incisors but are not much larger. They function like extra incisors. Our premolars and molars are similar in shape and function. Their knobbly surfaces, called cusps, meet when the jaws are closed, and crush the food into small pieces. Small particles of food are easier to digest than large chunks.

Tooth structure

The part of a tooth that is visible above the gum line is called the crown. The gum is tissue that overlays the jaws. The rest, embedded in the jaw bone, is called the root (Figure 7.17). The surface of the crown is covered by a very hard layer of enamel. This layer is replaced by cement in the root, which enables the tooth to grip to its bony socket in the jaw. Below the enamel is a layer of dentine. Dentine is softer than enamel. Inside the dentine is a pulp cavity, containing nerves and blood vessels. These enter the tooth through a small hole at the base of the root.

Dental decay (dental caries)

Decay begins when small holes (cavities) appear in the enamel. The cavities are caused by bacteria on the tooth surface. The bacteria feed on the sugars deposited on the teeth, respiring them and producing acid, which dissolves the calcium salts in the tooth enamel. The enamel is dissolved away in patches, exposing the dentine to the acids. Dentine is softer than enamel and dissolves more quickly so cavities are formed. The cavities reduce the distance between the outside of the tooth and the nerve endings. The acids produced by the bacteria irritate the nerve endings and cause toothache. If the cavity is not cleaned and fi lled by a dentist, the bacteria will get into the pulp cavity and cause a painful abscess at the root. Often, the only way to treat this is to have the tooth pulled out. Although some people’s teeth are more resistant to decay than others, it seems that it is the presence of refi ned sugar (sucrose) in the diet that contributes to decay.

Western diets contain a good deal of refi ned sugar and children suck sweets between one meal and the next. The high level of dental decay in Western society is thought to be caused mainly by keeping sugar in the mouth for long periods of time.

The best way to prevent tooth decay, therefore, is to avoid eating sugar at frequent intervals either in the form of sweets or in sweet drinks such as orange squash or soft (fizzy) drinks. It is advisable also to visit the dentist every 6 months or so for a ‘check-up’ so that any caries or gum disease can be treated at an early stage.

Gum disease (periodontal disease)

There is usually a layer of saliva and mucus over the teeth. This layer contains bacteria that live on the food residues in the mouth, building up a coating on the teeth called plaque. If the plaque is not removed, mineral salts of calcium and magnesium are deposited on it, forming a hard layer of ‘tartar’ or calculus. If the bacterial plaque that forms on teeth is not removed regularly, it spreads down the tooth into the narrow gap between the gum and enamel. Here it causes inflammation, called gingivitis, which leads to redness and bleeding of the gums and to bad breath. It also causes the gums to recede and expose the cement. If gingivitis is not treated, it progresses to periodontitis; the fibres holding the tooth in the jaw are destroyed, so the tooth becomes loose and falls out or has to be pulled out. There is evidence that cleaning the teeth does
help to prevent gum disease. It is best to clean the teeth about twice a day using a toothbrush. No one method of cleaning has proved to be any better than any other, but the cleaning should attempt to remove all the plaque from the narrow crevice between the gums and the teeth. Rinsing the mouth regularly with mouthwashes helps reduce the number of bacteria residing in the mouth. Drawing a waxed thread (‘dental floss’) between the teeth, or using interdental brushes, helps to remove plaque in these regions

Chemical digestion

Digestion is mainly a chemical process and consists of breaking down large molecules to small molecules.The large molecules are usually not soluble in water, while the smaller ones are. The small molecules can be absorbed through the epithelium of the alimentary canal, through the walls of the blood vessels and into the blood. Some food can be absorbed without digestion. The glucose in fruit juice, for example, could pass through the walls of the alimentary canal and enter the blood vessels without further change. Most food, however, is solid and cannot get into blood vessels. Digestion is the process by which solid food is dissolved to make a solution. The chemicals that dissolve the food are enzymes, described in Chapter 5. A protein might take 50 years to dissolve if just placed in water but is completely digested by enzymes in a few hours. All the solid starch in foods such as bread and potatoes is digested to glucose, which is soluble in water. The solid proteins in meat, eggs and beans are digested to soluble substances called amino acids. Fats are digested to two soluble products called glycerol and fatty acids.

The chemical breakdown usually takes place in stages. For example, the starch molecule is made up of hundreds of carbon, hydrogen and oxygen atoms. The first stage of digestion breaks it down to a 12-carbon sugar molecule called maltose. The last stage of digestion breaks the maltose molecule into two 6-carbon sugar molecules called glucose. Protein molecules are digested first to smaller molecules called peptides and finally into completely soluble molecules called amino acids.

The mouth

The act of taking food into the mouth is called ingestion. In the mouth, the food is chewed and mixed with saliva. The chewing breaks the food into pieces that can be swallowed and it also increases the surface area for the enzymes to work on later. Saliva is a digestive juice produced by three pairs of glands whose ducts lead into the mouth. It helps to lubricate the food and make the small pieces stick together. Saliva contains one enzyme, salivary amylase (sometimes called ptyalin), which acts on cooked starch and begins to break it down into maltose. Strictly speaking, the ‘mouth’ is the aperture between the lips. The space inside, containing the tongue and teeth, is called the buccal cavity. Beyond the buccal cavity is the ‘throat’ or pharynx.

Swallowing

For food to enter the gullet (oesophagus), it has to pass over the windpipe. To ensure that food does not enter the windpipe and cause choking during swallowing, the epiglottis (a fl ap of cartilage) guides the food into the gullet. The beginning of the swallowing action is voluntary, but once the food reaches the back of the mouth, swallowing becomes an automatic or refl ex action. The food is forced into and down the gullet by peristalsis. This takes about 6 seconds with relatively solid food; the food is then admitted to the stomach. Liquid travels more rapidly down the gullet.

The stomach

The stomach has elastic walls, which stretch as the food collects in it. The pyloric sphincter is a circular band of muscle at the lower end of the stomach that stops solid pieces of food from passing through. The main function of the stomach is to store the food from a meal, turn it into a liquid and release it in small quantities at a time to the rest of the alimentary canal. An example of physical digestion is the peristaltic action of muscles in the wall of the stomach. These muscles alternately contract and relax, churning and squeezing the food in the stomach and mixing it with gastric juice, turning the mixture into a creamy liquid called chyme. This action gives the food a greater surface area so that it can be digested more efficiently.

It helps in the process of breaking down large protein molecules into small, soluble amino acids. The stomach lining also produces hydrochloric acid, which makes a weak solution in the gastric juice. This acid provides the best degree of acidity for stomach protease to work in and kills many of the bacteria taken in with the food. The regular, peristaltic movements of the stomach, about once every 20 seconds, mix up the food and gastric juice into a creamy liquid. How long food remains in the stomach depends on its nature. Water may pass through in a few minutes; a meal of carbohydrate such as porridge may be held in the stomach for less than an hour, but a mixed meal containing protein and fat may be in the stomach for 1 or 2 hours. The pyloric sphincter lets the liquid products of digestion pass, a little at a time, into the fi rst part of the small intestine called the duodenum.

The small intestine

A digestive juice from the pancreas (pancreatic juice) and bile from the liver are poured into the duodenum to act on food there. The pancreas is a digestive gland lying below the stomach. It makes a number of enzymes, which act on all classes of food. Protease breaks down proteins into amino acids. Pancreatic amylase attacks starch and converts it to maltose. Lipase digests fats (lipids) to fatty acids and glycerol.

The small intestine

A digestive juice from the pancreas (pancreatic juice) and bile from the liver are poured into the duodenum to act on food there. The pancreas is a digestive gland lying below the stomach. It makes a number of enzymes, which act on all classes of food. Protease breaks down proteins into amino acids. Pancreatic amylase attacks starch and converts it to maltose. Lipase digests fats (lipids) to fatty acids and glycerol.

Bile

Bile is a green, watery fluid made in the liver, stored in the gall-bladder and delivered to the duodenum by the bile duct. It contains no enzymes, but its green colour is caused by bile pigments, which are formed from the breakdown of haemoglobin in the liver. Bile also contains bile salts, which act on fats rather like a detergent. The bile salts emulsify the fats. That is, they break them up into small droplets with a large surface area, which are more efficiently digested by lipase. Bile is slightly alkaline as it contains sodium hydrogencarbonate and, along with pancreatic juice, has the function of neutralising the acidic mixture of food and gastric juices as it enters the duodenum. This is important because enzymes secreted into the duodenum need alkaline conditions to work at their optimum rate. Digestion of protein There are actually several proteases (or proteinases) which break down proteins. One protease is pepsin and is secreted in the stomach. Pepsin acts on proteins and breaks them down into soluble compounds called peptides. These are shorter chains of amino acids than proteins. Another protease is called trypsin. Trypsin is secreted by the pancreas in an inactive form, which is changed to an active enzyme in the duodenum. It has a similar role to
pepsin, breaking down proteins to peptides. The small intestine itself does not appear to produce digestive enzymes. The structure labelled ‘crypt’ in is not a digestive gland, though some of its cells do produce mucus and other secretions. The main function of the crypts is to produce new epithelial cells (see ‘Absorption’) to replace those lost from the tips of the villi.

The epithelial cells of the villi contain enzymes in their cell membranes that complete the breakdown of sugars and peptides, before they pass through the cells on their way to the bloodstream. For example, peptidase breaks down polypeptides and peptides into amino acids.

Digestion of starch

Starch is digested in two places in the alimentary canal: by salivary amylase in the mouth and by pancreatic amylase in the duodenum. Amylase works best in a neutral or slightly alkaline pH and converts large, insoluble starch molecules into smaller, soluble maltose molecules. Maltose is a disaccharide sugar and is still too big to be absorbed through the wall of the intestine. Maltose is broken down to glucose by the enzyme maltase, which is present in the membranes of the epithelial cells of the villi. Functions of hydrochloric acid in gastric juice The hydrochloric acid, secreted by cells in the wall of the stomach, creates a very acid pH of 2. This pH is important because it denatures enzymes in harmful organisms in food, such as bacteria (which may otherwise cause food poisoning) and it provides the optimum pH for the protein-digesting enzyme pepsin to work.

Prevention of self-digestion

The gland cells of the stomach and pancreas make protein-digesting enzymes (proteases) and yet the proteins of the cells that make these enzymes are not digested. One reason for this is that the proteases are secreted in an inactive form. Pepsin is produced as pepsinogen and does not become the active enzyme until it encounters the hydrochloric acid in the stomach. The lining of the stomach is protected from the action of pepsin probably by the layer of mucus. Similarly, trypsin, one of the proteases from the pancreas, is secreted as the inactive trypsinogen and is activated by enterokinase, an enzyme secreted by the lining of the duodenum.

Absorption

The small intestine consists of the duodenum and the ileum. Nearly all the absorption of digested food takes place in the ileum, along with most of the water. Small molecules of the digested food such as glucose and amino acids pass into the bloodstream, while fatty acids and glycerol pass into the lacteals  connected to the lymphatic system.

The large intestine (colon and rectum)

The material passing into the large intestine consists of water with undigested matter, largely cellulose and vegetable fibres (roughage), mucus and dead cells from the lining of the alimentary canal. The large intestine secretes no enzymes but the bacteria in the colon digest part of the fibre to form fatty acids, which the colon can absorb. Bile salts are absorbed and returned to the liver by the blood circulation. The colon also absorbs much of the water from the undigested residues. About 7 litres of digestive juices are poured into the alimentary canal each day. If the water from these was not absorbed by the ileum and colon, the body would soon become dehydrated. The semi-solid waste, the faeces or ‘stool’, is passed into the rectum by peristalsis and is expelled at intervals through the anus. The residues may spend from 12 to 24 hours in the intestine. The act of expelling the faeces is called egestion or defecation

The ileum is efficient in the absorption of digested food for the following reasons:

  • It is fairly long and presents a large absorbing surface to the digested food.
  • Its internal surface is greatly increased by circular folds (Figure 7.22) bearing thousands of tiny projections called villi (singular = villus) (Figures 7.23 and 7.24). These villi are about 0.5 mm long and may be finger-like or flattened in shape.
  • The lining epithelium is very thin and the fluids can pass rapidly through it. The outer membrane of each epithelial cell has microvilli, which increase by 20 times the exposed surface of the cell.
  • There is a dense network of blood capillaries (tiny blood vessels,  in each villus.

The small molecules of digested food, for example glucose and amino acids, pass into the epithelial cells and then through the wall of the capillaries in the villus and into the bloodstream. They are then carried away in the capillaries, which join up to form veins. These veins unite to form one large vein, the hepatic portal vein. This vein carries all the blood from the intestines to the liver, which may store or alter any of the digestion products. When these products are released from the liver, they enter the general blood circulation.
Some of the fatty acids and glycerol from the digestion of fats enter the blood capillaries of the villi. However, a large proportion of the fatty acids and glycerol may be combined to form fats again in the intestinal epithelium. These fats then pass into the lacteals.

The fluid in the lacteals flows into the lymphatic system, which forms a network all over the body and eventually empties its contents into the bloodstream. Water-soluble vitamins may diffuse into the epithelium but fat-soluble vitamins are carried in the microscopic fat droplets that enter the cells. The ions of mineral salts are probably absorbed by active transport. Calcium ions need vitamin D for their effective absorption.

Amino acids, sugars and salts are, almost certainly, taken up by active transport. Glucose, for example, crosses the epithelium faster than fructose (another monosaccharide sugar) although their rates of diffusion would be about the same. The epithelial cells of the villi are constantly being shed into the intestine. Rapid cell division in the epithelium of the crypts replaces these lost cells. In effect there is a steady procession of
epithelial cells moving up from the crypts to the villi.

Use of digested food

The products of digestion are carried around the body in the blood. From the blood, cells absorb and use glucose, fats and amino acids. This uptake and use of food is called assimilation.

Glucose

During respiration in the cells, glucose is oxidised to carbon dioxide and water (see ‘Aerobic respiration’ in Chapter 12). This reaction provides energy to drive the many chemical processes in the cells, which result in, for example, the building-up of proteins, contraction of muscles or electrical changes in nerves.

Fats

These are built into cell membranes and other cell structures. Fats also form an important source of energy for cell metabolism. Fatty acids produced from stored fats or taken in with the food, are oxidised in the cells to carbon dioxide and water. This releases energy for processes such as muscle contraction. Fats can provide twice as much energy as sugars.

Amino acids

These are absorbed by the cells and built up, with the aid of enzymes, into proteins. Some of the proteins will become plasma proteins in the blood. Others may form structures such as cell membranes or they may become enzymes that control the chemical activity within the cell. Amino acids not needed for making cell proteins are converted by the liver into glycogen, which can then be used for energy.