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AQA GCSE Biology · 8461
AQA 8461 · 4.2.2.1 Digestive System Check the specification (PDF) (opens in a new tab)
Describe the digestive system as an organ system for digestion and absorption.
Relate enzyme activity to metabolism and environmental factors like temperature and pH.
Use the 'lock and key' theory to explain enzyme action.
Recall the sites of production and action for amylase, proteases, and lipases.
Describe the breakdown of carbohydrates, proteins, and lipids into simple sugars, amino acids, and fatty acids/glycerol.
Explain the role of bile in neutralising stomach acid and emulsifying fats.
Perform qualitative tests for carbohydrates, lipids, and proteins.
Investigate the effect of pH on the rate of reaction of amylase.
The digestive system is an organ system: several organs work together to digest food and absorb its useful molecules. Many food molecules, including starch, proteins and fats, are too large to be absorbed as they are. Digestion breaks them into smaller molecules. These can then be absorbed through the wall of the small intestine and carried to cells around the body.
Food enters through the mouth, travels down the oesophagus and passes through the stomach and intestines. Organs alongside this route contribute digestive juices or bile. The diagram shows how their jobs connect.
The digestive system combines organs that move food, digest it, supply bile or enzymes, and absorb useful molecules.
In the mouth, chewing breaks food into smaller pieces, while saliva from the salivary glands contains amylase, which starts digesting starch. Food passes down the oesophagus to the stomach. The stomach produces hydrochloric acid, which kills bacteria and provides acidic conditions for a stomach protease to work.
The small intestine receives digestive enzymes from the pancreas and contains enzymes of its own. Digested, soluble molecules are absorbed here into the bloodstream. The large intestine absorbs water from undigested material; the remaining faeces pass through the rectum and leave through the anus.
Digestive enzymes speed up the chemical breakdown of particular food molecules. Carbohydrases break down carbohydrates into simple sugars. Amylase is a carbohydrase: it breaks down starch into maltose, which can be digested further to glucose. Amylase is produced in the salivary glands, pancreas and small intestine. It acts in the mouth and small intestine, rather than in the acidic stomach.
Proteases break down proteins into amino acids. They are produced in the stomach, pancreas and small intestine. A stomach protease acts in the stomach; proteases from the pancreas and small intestine act in the small intestine. Lipases break down lipids, or fats, into fatty acids and glycerol. They are produced in the pancreas and small intestine and act in the small intestine.
These changes can be written as simple word equations:
The absorbed products are carried to cells and can be used to build new carbohydrates, lipids and proteins. Some glucose is used in respiration to release energy.
The liver makes bile, the gall bladder stores it, and bile is released into the small intestine. Bile is alkaline, so it neutralises hydrochloric acid arriving from the stomach. This provides conditions better suited to the enzymes working in the small intestine.
Bile also emulsifies fat: it separates large fat droplets into many smaller ones. The smaller droplets have a greater total surface area for lipase to act on. Suitable alkaline conditions and the increased surface area both increase the rate at which lipase breaks down fat. Bile helps this process, but lipase is the enzyme that chemically digests the fat.
Bile increases the surface area available to lipase by turning a large fat droplet into smaller droplets.
Several organs work together to digest food and absorb the resulting small, soluble molecules. The small intestine is the main site of absorption into the bloodstream; the large intestine absorbs water.
For each digestive enzyme, distinguish where it is produced from where it acts.
Amylase breaks starch into maltose; further digestion produces simple sugars such as glucose.
Bile is not an enzyme. It neutralises acid and emulsifies fats, helping lipase work faster.
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Digestion
The breakdown of food into smaller molecules that can be absorbed.
Absorption
The movement of digested, soluble molecules from the small intestine into the body.
Enzyme
A protein that speeds up a particular chemical reaction without being used up.
Carbohydrase
A digestive enzyme that breaks down carbohydrates into simpler sugars.
Amylase
A carbohydrase that breaks down starch into maltose.
Protease
A digestive enzyme that breaks down proteins into amino acids.
Lipase
A digestive enzyme that breaks down lipids into fatty acids and glycerol.
Bile
An alkaline liquid made in the liver and stored in the gall bladder; it neutralises stomach acid and emulsifies fats in the small intestine.
Emulsification
The breaking of large fat droplets into smaller droplets, increasing their total surface area.
Put your knowledge into practice — try past paper questions for Biology
Digestion
The breakdown of food into smaller molecules that can be absorbed.
Absorption
The movement of digested, soluble molecules from the small intestine into the body.
Enzyme
A protein that speeds up a particular chemical reaction without being used up.
Carbohydrase
A digestive enzyme that breaks down carbohydrates into simpler sugars.
Amylase
A carbohydrase that breaks down starch into maltose.
Protease
A digestive enzyme that breaks down proteins into amino acids.
Lipase
A digestive enzyme that breaks down lipids into fatty acids and glycerol.
Bile
An alkaline liquid made in the liver and stored in the gall bladder; it neutralises stomach acid and emulsifies fats in the small intestine.
Emulsification
The breaking of large fat droplets into smaller droplets, increasing their total surface area.
Digestion products can build new molecules; some glucose is used in respiration.
Made in the liver, stored in the gall bladder and released into the small intestine. Its alkalinity neutralises stomach acid. Emulsification creates smaller fat droplets with greater total surface area, increasing the rate of fat digestion by lipase.