Digestion, absorption and bile

In the mouth, teeth break food into smaller pieces and saliva mixes with it. The salivary glands produce amylase, so starch digestion begins here. Swallowed food travels down the oesophagus to the stomach, pushed along by contractions of the tube's muscular walls.

The stomach churns food and produces proteases that begin protein digestion. It also releases hydrochloric acid, providing acidic conditions suitable for its proteases.

Food then enters the small intestine, where enzymes complete digestion and the small, soluble products are absorbed. Its lining has projections called villi, which provide a large surface area for absorption. The blood carries absorbed nutrients to cells around the body.

The pancreas produces digestive enzymes and releases them into the small intestine. The liver makes bile, which is stored in the gall bladder before being released into the small intestine. Food does not need to pass through these organs for them to contribute to digestion.

The large intestine absorbs remaining water. Undigested material forms faeces, which are stored in the rectum and leave through the anus.

The enzymes and their products

Different digestive enzymes catalyse the breakdown of different food molecules. Carbohydrases break down carbohydrates into simple sugars. Amylase is one particular carbohydrase: it breaks down starch into sugars. Other carbohydrases complete carbohydrate digestion to produce simple sugars such as glucose.

Proteases break down proteins into amino acids. Lipases break down lipids, also called fats, into glycerol and fatty acids.

These changes can be represented by simple word equations, with the enzyme named above the arrow:

$$\text{starch} \xrightarrow{\text{amylase}} \text{sugars}$$ $$\text{proteins} \xrightarrow{\text{proteases}} \text{amino acids}$$ $$\text{lipids} \xrightarrow{\text{lipases}} \text{glycerol} + \text{fatty acids}$$

The enzyme catalyses the reaction; it is not one of the products. The main production sites and sites of action are:

For example, starch in bread begins to be digested by salivary amylase in the mouth. Digestion continues in the small intestine using enzymes supplied by the pancreas and the intestine itself. This shows how several organs contribute to processing the same food.

How bile helps fat digestion

Bile contains no digestive enzymes, but it helps lipase work more quickly in the small intestine in two ways.

First, bile is alkaline, so it neutralises hydrochloric acid arriving from the stomach. This provides alkaline conditions suitable for enzymes such as lipase in the small intestine. The acidic conditions useful in the stomach would not be suitable for these enzymes.

Second, bile emulsifies fats: it divides large fat droplets into many smaller droplets. The amount of fat is unchanged, but its total surface area increases. Lipase acts at the surface of the droplets, so a larger surface area allows faster fat breakdown.

Emulsification and enzyme digestion are different processes. Bile changes the size of the droplets; lipase breaks down the lipid molecules within them into glycerol and fatty acids.

What happens to the absorbed products?

Digestion supplies small molecules that cells can use as building materials. Simple sugars can be used to build new carbohydrates, amino acids to build new proteins, and glycerol and fatty acids to build new lipids. These products support growth, repair and the normal functioning of cells. For example, amino acids can be assembled into enzymes, while lipids are needed in cell membranes.

Some glucose is used in cellular respiration, releasing energy for cell activities rather than being used to build larger molecules. Digestion therefore connects to metabolism: the body's reactions include both breaking molecules down and building new molecules from their products.

Carbohydrases break down carbohydrates into simple sugars.

Bile

Made in the liver → stored in the gall bladder → released into the small intestine.

• Alkaline → neutralises stomach hydrochloric acid → suitable conditions for lipase.
• Emulsifies fat → smaller droplets → larger total surface area → faster breakdown by lipase.
• Bile is not an enzyme.

Uses of digestion products

• Build new carbohydrates, proteins and lipids.
• Some glucose is used in respiration to release energy.
• These breakdown and building reactions link digestion to metabolism.