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 in the blood vessels. Digestion is the process by which solid food is dissolved to make a solution.
The chemicals that dissolve the food are enzymes. 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 protein 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.
starch ⟶ maltose ⟶ glucose
protein ⟶ peptide ⟶ amino acid
These stages take place in different parts of the alimentary canal. The progress of food through the canal and the stages of digestion will now be described.
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 tongue and teeth, is called the buccal cavity. Beyond the buccal cavity is the ‘throat’ or pharynx.
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 flap 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 reflex 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 travel more rapidly down the gullet.
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.
Glands in the lining of the stomach produce gastric juice containing the protease enzyme. 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 first 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.
Pancreatic juice contains sodium hydrogencarbonate, which partly neutralises the acidic liquid from the stomach. This is necessary because the enzymes of the pancreas do not work well in acid conditions.
All the digestible material is thus changed to soluble compounds, which can pass through the lining of the intestine and into the bloodstream. The final products of digestion are:
Food Final products
starch ⟶ glucose (a simple sugar)
proteins ⟶ amino acids
fats (lipids) ⟶ fatty acids and glycerol
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 form 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 Figure 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 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 the hydrochloric acid in the 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.
Principal substances produced by digestion
|Region of alimentary canal
|Digestive juice produced
|Enzymes in the juice/cells
|Class of food acted upon
|glands in stomach lining
|proteases, such as trypsin
|proteins and peptides
|peptides and amino acids
fatty acids and glycerol
Experiments on digestion
- The action of salivary amylase on starch
- Rinse the mouth with water to remove traces of food.
- Collect saliva in two test-tubes, labelled A and B to a depth of about 15 mm.
- Heat the saliva in tube B over a small flame, or in a water bath of boiling water, until it boils for about 30 seconds and then cool the tube under the tap.
- Add about 2 cm3 of a 2% starch solution to each tube; shake each tube and leave them for 5 minutes.
- Share the contents of tube A between two clean test-tubes.
- To one of these add some iodine solution. To the other add some Benedict’s solution and heat in water bath.
- Test the contents of tube B in exactly the same way.
The contents of the tube A fail to give a blue colour with iodine, showing that the starch has gone. The other half of the contents, however gives a red or orange precipitate with Benedict’s solution, showing that sugar is present.
The contents of tube B still give a blue colour with iodine but do not form a red precipitate on heating with Benedict’s solution.
The results with tube A suggest that something in saliva has converted starch into sugar. The fact that the boiled saliva in tube B fails to do this suggests that it was an enzyme in saliva that brought about the change, because enzymes are proteins and are destroyed by boiling. If the boiled saliva had changed starch to sugar, it would have ruled out the possibility of an enzymes being responsible.
This interpretation assumes that it is something in saliva that changes starch into sugar. However, the results could equally well support the claim that starch can turn unboiled saliva into sugar. Our knowledge of (1) the chemical composition of starch and saliva and (2) the effect of heat on enzymes, makes the first interpretation more plausible.
2. Modelling the action of amylase on starch
- Collect a 15 cm length of Visking tubing which has been softened in water.
- Tie one end tightly. Use a syringe to introduce 2% starch solution into the Visking tubing, to about two thirds full.
- Add 2 cm3 of 5% amylase solution (or saliva if it is permissible).
- Pinch the top of the Visking tubing to keep it closed, before carefully mixing its contents by squeezing the tubing.
- Rinse the outside of the Visking tubing thoroughly with tap water, then place it in a boiling tube, trapping the top of the tubing with an elastic band.
- Add enough distilled water to cover the Visking tubing.
- Take a small sample of the distilled water and the contents of the Visking tubing for starch and reducing sugar, using iodine solution and Benedict’s solution.
- Place the boiling tube in a beaker of water or a water bath at 37∘C.
- After 20 minutes, use clean teat pipettes to remove a sample of the water surrounding the Visking tubing and from inside the Visking tubing.
- Test some of each sample for starch, using iodine solution, and for reducing sugar, using Benedict’s solution. Also test some of the original starch solution for reducing sugar, to make sure it is not contaminated with glucose.
At the start of the investigation the distilled water tests negative for starch (stays brown) and reducing sugar (stays turquoise). The contents of the Visking tubing are positive for starch (blue-black), but negative for reducing sugar (stays turquoise).
After 20 minutes, the contents of the Visking tubing are yellow/brown with iodine solution, but turn orange or brick red with Benedict’s solution, but turns orange or brick red with Benedict’s solution. The water sample stays yellow/brown with iodine solution, but turns orange or brick red with Benedict’s solution.
The amylase digests the starch in the Visking tubing, producing reducing sugar. The complete digestion of starch results in a negative colour change with iodine solution. The presence of reducing sugar (maltose or glucose) causes the Benedict’s solution to turn orange or brick red. The reducing sugar molecules can diffuse through the Visking tubing into the surrounding water, so the water gives a positive result with Benedict’s solution. Starch is a large molecule, so it cannot diffuse through the tubing: the water gives an negative result with iodine solution.
This model can be used to represent digestion in the gut. The starch solution and amylase are the contents of the mouth or duodenum. The Visking tubing represents the duodenum wall and the distilled water represents the bloodstream, into which the products of digestion are absorbed.
3. The action of pepsin on egg-white protein
A cloudy suspension of egg-white is prepared by stirring the white of one egg into 500 cm3 tap water, heating it to boiling point and filtering it through glass wool to remove the larger particles.
- Label four test-tubes A, B, C and D and place 2 cm3 egg-white suspension in each of them. Then add pepsin solution and/or dilute hydrochloric acid (HCL) to the tubes as follows:
A – egg-white suspension + 1 cm3 pepsin solution (1%)
B – egg-white suspension + 3 drops dilute HCL
C – egg-white suspension + 1 cm3 pepsin + 3 drops HCL
D – egg-white suspension + 1 cm3 boiled pepsin + 3 drops HCL
- Place all four tubes in a beaker of warm water at 35∘C for 10-15 minutes.
The contents of tube C go clear. The rest remain cloudy.
The change from a cloudy suspension to a clear solution shows that the solid particles of egg protein have been digested to soluble products. The failure of the other three tubes to give clear solution shows that:
- pepsin will only work in acid solutions
- it is the pepsin and not the hydrochloric acid that does the digestion
- pepsin is an enzyme, because its activity is destroyed by boiling
4. The action of lipase
- Place 5 cm3 milk and 7 cm3 dilute (0.05 mol dm-3) sodium carbonate solution into each of three test-tubes labelled 1 to 3.
- Add six drops of phenolphthalein to each to turn the contents pink.
- Add 1 cm3 of 3% bile salts solution to tubes 2 and 3.
- Add 1 cm3 of 5% lipase solution to tubes 1 and 3, and an equal volume of boiled lipase to tube 2.
In 10 minutes or less, the colour of the liquids in tubes 1 and 3 will change to white, with tube 3 changing first. The liquid in tube 2 will remain pink.
Lipase is an enzyme that digests fats to fatty acids and glycerol. When lipase acts on milk fats, the fatty acids that have been produced react with the alkaline sodium carbonate and make the solution more acid. In acid conditions the pH indicator, phenolphthalein, changes from pink to colourless. The presence of bile salts in tube 3 seems to speed up the reaction, although bile salts with the denatured enzyme in tube 2 cannot bring about the change on their own.