Digestion and absorption (3.3.3)

Food is broken down through mechanical and chemical digestion.

Mechanical digestion occurs in the mouth and stomach. This physically breaks food down into smaller pieces, increasing surface area for chemical digestion.

Chemical digestion involves hydrolysing large insoluble biological molecules using enzymes. The products of this are soluble and can be absorbed into the body.

The digestive system is made up of the:

• Mouth: performs mechanical digestion by chewing, and mixes food with saliva which contains the enzyme amylase.
• Oesophagus: connects the mouth to the stomach.
• Stomach: stores and digests food. The layer of muscle in the stomach lining contracts and breaks down food. The lining of the stomach produces peptidase (protease) enzymes, to hydrolyse proteins.
• Pancreas: releases enzymes into the first section of the ileum.
• Ileum (small intestine): a long muscular tube which moves food by peristalsis. The walls of the ileum are folded into villi to increase the surface area. Food is further digested by enzymes in the ileum.
• Large intestine: absorbs water and salts to produce faeces.
• Rectum: stores faeces until it is ready to be excreted.

Amylase is released by the salivary glands in the mouth. The optimum of salivary amylase is 7; it is denatured when food enters the stomach.

The pancreas releases amylase into the ileum.

Amylase hydrolyses the glycosidic bonds in polysaccharides, producing maltose (a disaccharide).

The breakdown of maltose is catalysed by maltase. Maltase enzymes are membrane-bound and attached to the surface of the cells lining the ileum.

Maltase hydrolyses the glycosidic bond in maltose, producing two alpha glucose monosaccharides.

Maltase is specific to the substrate maltose and does not break down other disaccharides.

The breakdown of sucrose is catalysed by sucrase. Sucrase hydrolyses the glycosidic bond in sucrose, producing one alpha glucose and one fructose monosaccharide.

The breakdown of lactose is hydrolysed by lactase. Lactase hydrolyses the glycosidic bond in lactose, producing one alpha glucose and one galactose monosaccharide.

Lipids are emulsified by bile salts which are produced by the liver and stored in the gall bladder. Bile salts are released into the first part of the ileum, and also neutralise stomach acid.

Once lipids have been emulsified, tiny droplets called micelles are produced. The micelles have a large surface area, which increases the area for lipase enzymes to act on.

Lipase enzymes hydrolyse the ester bonds in triglycerides, producing monoglycerides and two fatty acids.

Peptidases (proteases) catalyse the breakdown proteins by hydrolysing peptide bonds.

There are different types of peptidases with different roles in chemical digestion:

• Endopeptidases – catalyse the hydrolysis of peptide bonds in the middle of proteins. This produces shorter peptide chains.
• Exopeptidases – catalyse the hydrolysis of peptide bonds of the terminal amino acids. This releases dipeptides and amino acids.
• Dipeptidases – membrane–bound enzymes attached to the cells lining the ileum. These enzymes catalyse the hydrolysis of dipeptides, producing amino acids.

The lining of the ileum is folded into finger-like structures called villi.

The villi are adapted for diffusion:

• Large surface area for diffusion.
• Thin walls, to reduce diffusion distance.
• Good blood supply to maintain diffusion gradient.
• Contain muscle and are able to move the villi, helping to maintain the diffusion gradient.
• Epithelial cells that line the villi have microvilli on their cell surfaces, which further increase their surface area.

Amino acids and monosaccharides are absorbed from the lumen of the ileum into epithelial cells, then moved into the blood stream by co-transport.

The monoglycerides and fatty acids in micelles are non-polar so easily diffuse from the intestinal lumen across the cell membrane. These are then moved into the endoplasmic reticulum and the triglycerides are reformed.

The triglycerides are combined with cholesterol and lipoproteins to produce chylomicrons. These chylomicrons then move out of the cell by exocytosis and into small branches of the lymphatic system called lacteals.

The chylomicrons then pass into the lymphatic system where they are released slowly into the bloodstream.