Human digestive systemIB MYP Biology: Revision notes

Most of the food we eat is made of large, insoluble molecules such as starch, protein and fat. These cannot pass through the gut wall into the blood, so they must be broken down into small, soluble molecules. This is digestion.

There are two kinds:

• Mechanical digestion physically breaks food into smaller pieces without changing the chemicals, e.g. chewing, churning in the stomach and bile breaking fat into droplets. It increases surface area.
• Chemical digestion uses enzymes to break large molecules into small ones: starch to maltose and then glucose, protein to amino acids, fat to fatty acids and glycerol.

In the mouth the teeth chew food (mechanical) and the salivary glands release saliva containing amylase, which starts to digest starch to maltose.

The oesophagus is the tube from the mouth to the stomach. Its muscles carry out peristalsis: waves of muscle contraction that squeeze food along. Peristalsis also moves food through the rest of the gut.

The stomach churns food with its muscular walls (mechanical) and releases protease, which digests protein. The stomach also releases hydrochloric acid, which kills many bacteria and gives the right pH for stomach protease.

These organs are not part of the food tube, but they help digestion.

• The liver makes bile.
• The gall bladder stores bile and releases it into the small intestine.
• The pancreas makes amylase, protease and lipase and releases them into the small intestine.

Bile has two jobs. It is alkaline, so it neutralises the acid from the stomach and gives the right pH for the enzymes in the small intestine. It also emulsifies fat, breaking large drops into tiny droplets. This gives a much bigger surface area so lipase can digest the fat faster.

The small intestine finishes digestion and absorbs the digested food. Its lining is covered in millions of finger-like villi, which are adapted for absorption:

• a very large surface area
• a wall only one cell thick, so a short diffusion distance
• a rich blood supply (capillaries) that carries glucose and amino acids away and keeps a steep concentration gradient
• a lacteal that absorbs fatty acids and glycerol

Glucose, amino acids, fatty acids and glycerol are absorbed here into the blood and lymph.

Not all food is digested, for example fibre. This undigested food, along with a lot of water, passes into the large intestine. Here water is absorbed into the blood, leaving a more solid waste called faeces. Faeces are stored in the rectum and leave the body through the anus. This is egestion.

If the large intestine cannot absorb enough water, as in diarrhoea, the person loses water and can become dehydrated.