Digestion is the body breaking down food so that it can enter the bloodstream.


The digestive system is a set of organs co-operating in the digestion and absorption of nutrients from food.

Digestion is the breakdown of food into simpler substances for absorption - starch is broken down to glucoses, protein to amino acids and fats into glycerol and fatty acids.

The complex food substances are changed to simpler form that is more suitable for use by the body. The proteins in the food may not be the proteins that our body needs. Therefore by breaking the food proteins down into amino acids we can then rearrange the amino acids into the proteins we require.

The alimentary canal, liver and pancreas make up the digestive system.

The alimentary canal or gut is a specialised tube extending from mouth to anus.

The major parts of the gut are: mouth, oesophagus, stomach, small intestine, large intestine and anus.


Food is taken into the gut through the mouth and there it is digested physically and chemically.

Physical or mechanical digestion is the breaking of lumps of food into smaller pieces by cutting or crushing the food between the teeth.

Chemical digestion is much faster on smaller pieces of food.

When you take some food into your mouth, your mouth waters – it becomes full of saliva. Saliva contains an enzyme called amylase. This acts on starch, breaking it down into malt sugar (maltose). Saliva also contains an enzyme which kills bacteria.

Teeth: physical digestion.

The teeth are very hard peg-like structures cemented into the jaws.

There are four different shaped teeth arranged in special sequence in the jaws - molars, premolars, canine, incisors, canine, premolars, molars – from one side of each jaw to the other. The sequence is the same in the upper jaw and lower jaw

The shape of each type of tooth indicates its role in physical digestion.

Incisor: cutting and biting – sharp straight edge.

Canine: gripping and tearing – sharp pointed tip.

Premolar: crushing and grinding – rounded surfaces.

Molars: crushing and grinding – rounded surfaces.

Amylase: chemical digestion

Amylase is a digestive enzyme that breaks down the multiglucose starch to a double glucose (sugar) called maltose. Maltose is a reducing sugar. Amylase is in the saliva secreted by the salivary glands.


Once swallowed the food passes down the oesophagus. The oesophagus has muscle tissue in its wall. A ring of muscular contraction moves slowly downwards, passing the food in front of it. This is called peristalsis.


The food is held, soaked and churned in the stomach for hours. Hydrochloric acid (HCl) protects us by killing bacteria that entered with the food. Chemical digestion of protein digestion starts but starch digestion stops. The stomach finally squeezes the food into the small intestine.

Small Intestine

The acid food is neutralised and made slightly alkaline.

Chemical digestion of protein continues – protein to amino acids.

Starch digestion restarts – starch to glucoses.

Fat digestion begins – fat to glycerol and fatty acids.

The amino acids, glucoses, glycerol and fatty acids are absorbed into the cells lining the small intestine and passed into blood for transport throughout the body.

At least 90% of the food is digested and absorbed.

Much water is also absorbed by the small intestine.


Pancreatic juice pours from the pancreas into the small intestine. This juice contains a variety of digestive enzymes to digest starch, protein and fat. The juice also has alkaline salts to change the food to a more suitable pH.


Bile, a digestive juice, pours into the small intestine from the liver. The bile does not contain digestive enzymes. Alkaline salts in the bile change the food to a more favourable pH. Other bile salts physically digest fat by breaking it into tiny droplets – this is called emulsification.

Large Intestine

The indigestible material passes from the small intestine into the large intestine. Here, ‘friendly bacteria’ feed on this material, supply us with nutrients (vitamins B and K) and have a role in keeping us healthy.

The material is prepared for egestion (expulsion from the gut) as more water is absorbed back into the blood.

Enzymes An Introduction

An enzyme is a protein that functions as a biological catalyst. A catalyst is a substance that alters the rate of a chemical reaction without being used up by the reaction.

Enzymes speed up specific biochemical reactions. The chemistry of life is very fast. All the chemical reactions of living are speeded up by enzymes. Biochemistry runs about a hundred million, million times faster than normal because of enzyme.

Enzymes speed up the formation of large molecules from smaller ones e.g, formation of starch from glucose.

Enzymes also speed up the breakdown of large molecules into smaller ones e.g. the digestion of protein to amino acids and fats to glycerol and fatty acids.

The substance the enzyme acts on is called the substrate and the final substance is called the product:

Substrate Enzyme Product

Glucose Maltase Maltose

Starch Amylase Maltose

Protein Protease Amino Acids

Enzymes can work in the reverse direction e.g. amylase can link maltoses together to make starch but amylase can also breakdown starch to maltose.

Different reactions need different enzymes e.g. amylase digests starch but does not digest protein

ManPractivity Investigate the Action of Amylase on Starch; identify the substrate, product & enzyme.

1. One third fill two test tubes with starch solution.

2. Add saliva to one tube – the experiment tube.

3. Add a similar volume of water to the other tube – the control tube.

4. Warm each to 37°C in a heated water bath – use a thermometer to check the temperature remains at 37°C.

5. Every minute, using a dropper, take a small from each tube and test for starch using iodine.

Results: control (no amylase)– always blue-black iodine test therefore starch is always present.

experiment (amylase) – yellow-brown after some time so then starch is absent.

Concluson: amylase caused the disappearance of starch.

6. Then test the contents of each tube for reducing sugar using Benedict’s reagent.

Results: control (no amylase)– stays blue therefore reducing sugar is absent.

experiment (amylase) – goes brick-red therefore reducing sugar is present.

Conclusison: amylase caused the formation of reducing sugar.

7. Overall Conclusion: amylase converted starch to reducing sugar.

Substrate: starch Product: maltose (maltose is a reducing sugar). Enzyme: amylase (amylase is in saliva)