Organisation: Digestion

The human digestive system

The digestive system is an organ system in which several organs work together to digest and absorb food. Food passes from the mouth, down the oesophagus, into the stomach, then into the small intestine, the large intestine and out through the rectum and anus. The liver, gall bladder and pancreas are accessory organs that release substances into the gut.

Digestion breaks large insoluble molecules into small soluble ones that can be absorbed into the bloodstream. Absorption happens mainly in the small intestine; water is absorbed in the large intestine.

Enzymes

Enzymes are biological catalysts — they speed up reactions without being used up. They are large protein molecules, and each has an active site whose shape is complementary to its substrate. Because a particular enzyme only fits a particular substrate, enzymes are specific. This is often called the lock-and-key model.

Digestive enzymes

• Carbohydrase — breaks down carbohydrates to simple sugars. The main one, amylase, breaks starch down into sugars and is produced in the salivary glands, the pancreas and the small intestine.
• Protease — breaks down proteins into amino acids. Produced in the stomach (as pepsin), the pancreas and the small intestine.
• Lipase — breaks down lipids into fatty acids and glycerol. Produced in the pancreas and the small intestine.

The products of digestion are used to build new carbohydrates, lipids and proteins, and some glucose is used in respiration.

Bile is made in the liver and stored in the gall bladder. It is alkaline, so it neutralises the hydrochloric acid from the stomach, and it emulsifies fat into small droplets, increasing the surface area for lipase to work on. Both effects speed up fat digestion. Bile is not an enzyme — a common and costly error.

Required practical: food tests

• Starch — add iodine solution. A blue-black colour shows starch is present.
• Sugars — add Benedict's solution and heat in a water bath. Blue means none; green, yellow, then brick red indicate increasing amounts of sugar.
• Protein — add Biuret solution. A purple or mauve colour shows protein.
• Lipids — shake with ethanol, then add water. A cloudy white emulsion shows lipid.

Factors affecting enzyme action

Enzyme activity rises with temperature up to an optimum, then falls sharply. Beyond the optimum, the active site changes shape and the enzyme is denatured — the substrate no longer fits and the reaction stops. The same happens at an unsuitable pH.

Be precise in the wording. Enzymes are denatured, not "killed" — they are molecules, not living things. Different enzymes have different optimum pH values: stomach protease works at about pH 2, while enzymes in the small intestine work in alkaline conditions.