digestion

The human stomach can hold about 1.5 l/2.6 pt of liquid. The digestive juices are acidic enough to dissolve metal. To avoid damage, the cells of the stomach lining are replaced quickly – 500,000 cells are replaced every minute, and the whole stomach lining every three days.

During the process of digestion food is broken down into increasingly smaller particles. The dietary fibre and other indigestible particles remain unchanged.

The digestion of protein. Protein is broken down into amino acids by the enzymes pepsin (secreted by the stomach) and trypsin and peptidase (in the small intestine).

The digestion of fat. Fat is broken down into fatty acids and glycerol by bile (secreted by the gall bladder) and the enzyme lipase (secreted by the small intestine).

The digestion of carbohydrate. The complex polysaccharide starch is broken down into glucose by the enzymes amylase and maltase (secreted by the small intestine).

Process by which food eaten by an animal is broken down mechanically, and chemically by enzymes, mostly in the stomach and intestines, to make the nutrients available for absorption and cell metabolism. In digestion large molecules of food are broken into smaller, soluble molecules, which are absorbed through the wall of the gut into the bloodstream and carried to individual cells. The first stage of this may involve just the mixing of the food with water and the crushing and chopping of pieces of food by teeth or the mixing of food as it is squeezed along the gut. The second stage is the breakdown of large molecules by enzymes. The uptake of digested foods is mainly by absorption.

The digestion of fats starts with the action of bile. This emulsifies lumps of fat into thousands of tiny droplets, allowing the enzyme lipase to break the fats down into glycerol and fatty acids. Lipase is released from the pancreas and digests fat in the small intestine. Fats are not soluble in water, but glycerol and fatty acids are, and they are easily absorbed into the body.

Not all carbohydrates can be digested, but one important carbohydrate that is digestible is starch. The digestion of starch begins in the mouth. Saliva contains the enzyme amylase, which breaks starch into the complex sugar maltose. Later on, in the small intestine, amylase is released from the pancreas and finishes off the digestion.

Protein is partly digested in the stomach, where the enzyme pepsin breaks large protein molecules into smaller ones. In the small intestine other enzymes break the small proteins into amino acids. Enzymes in different parts of the gut work best at different levels of acidity, so the body makes the conditions in the stomach, for example, very acid, but in the small intestine, slightly alkaline.

In some single-celled organisms, such as an amoeba, a food particle is engulfed by the cell and digested in a vacuole within the cell.