3.3.3 Digestion and absorption

EDIT: Hi guys, I was just going through the blog and I CANNOT work out why the formatting on this post is so strange (the text keeps coming up as different fonts/colours) -  I wrote it all at the same time so I don't see why this is happening. I've tried to resolve but with no luck, sorry:(


Glands in the human digestive system produce enzymes to hydrolyse large insoluble molecules into smaller soluble ones so they can be absorbed. We don’t distinctly need to know the main parts of our digestive system, but nonetheless it may help for understanding so here goes:

• Salivary gland: situated near mouth to hydrolyse starch into maltose with silvery amylase contained in their secretions
• Oesophagus: carries food from mouth to stomach
• Stomach: muscular sac with inner layer producing enzymes. Stores and digests food (particularly proteins)
• Pancreas: large gland situated below the stomach whose secretions contain proteases lipase and amylase
• Ileum (small intestine): produces enzymes in its walls to further digest food. Inner walls folded into villi with microvilli to increase surface area for absorption
• Duodenum (large intestine): absorbs water
• Rectum: faeces stored here before being periodically removed via the anus (digestion)

Digestion occurs in two stages: physical breakdown and then chemical digestion.

Physical breakdown is basically the physical breakdown of food (duh). Large food is broken down into smaller pieces by teeth/stomach providing a larger surface area for chemical digestion.

Chemical digestion is the hydrolysis of large insoluble molecules into smaller soluble molecules carried out by enzymes. More than one enzyme is required to hydrolyse a large molecule as enzymes are specific. The different types of digestion are as follows:

• Carbohydrate digestion: Salivary amylase is produced in the mouth. This hydrolyses alternate starch glycosidic bonds forming lots of maltose disaccharides. Mineral salts in the saliva maintain a neutral pH. The food is swallowed and enters the stomach. pH2 in the stomach denatures the salivary amylase. Food is passed into the ileum and mixed with pancreatic amylase (amylase produced in the pancreas) - this hydrolyses any remaining starch into maltose. Alkaline salts produced by the ileum and pancreas maintain a neutral pH. The epithelial lining of the ileum produces maltase (a membrane bound disaccharidase) which hydrolyses the maltose into alpha glucose.
• Also: sucrase (a membrane bound disaccharidase) hydrolyses sucrose producing glucose and fructose. Lactase (a membrane bound disaccharidase) hydrolyses lactase producing glucose and galactose
• Lipid digestion: Lipids are hydrolysed my lipases. These are produced in the pancreas and hydrolyse the ester bond in triglycerides to produce two fatty acids and a monoglyceride. Firstly, lipids are split up by bile salts (produced by the liver). Monoglycerides and fatty acids remain in association with these bile salts forming micelles - a process known as emulsification which increases the surface area. They do not stick to each other (forming large micelles) as the bile salts arrange themselves with their lipophilic ends in fat droplets and their lipophobic ends sticking out. When the micelles come into contact with the villi (on the ileum lining) they break down releasing the constituent monoglycerides and fatty acids (both of thee are non-polar so can easily diffuse across the cell surface membrane into the epithelial cells that line the ileum). Once inside, monoglycerides and fatty acids are transported to the endoplasmic reticulum where they recombine to form triglycerides. Here and in the Golgi apparatus/body they associate with cholesterol and lipoproteins forming chylomicrons which move out of the epithelial cells by exocytosis and enter lacteals (lymphatic capillaries). From here they pass into the blood system. Triglycerides in chylomicrons are hydrolysed by an enzyme in the endothelial cell of the capillaries.
• Protein digestion: peptidases (also known as proteases) hydrolyse proteins as follows...
• Endopeptidases hydrolyse peptide bonds in the central region of a protein molecule - this forms a load of peptide molecules
• Exopeptidases hydrolyse peptide bonds on the terminal amino acids releasing single amino acids and dipeptides
• Dipeptidases (membrane bound to the ileum) hydrolyse peptide bonds in dipeptides

The digestion of proteins and carbohydrates produces amino acids and monosaccharides respectively. These are absorbed into the bloodstream in the ileum by co-transport.

NOTE: It might get a bit confusing that ENDopeptidases don't hydrolyse the END peptide bonds...sorry don't have any help for this just try not to get confused lol

So, you've heard a lot about the ileum, but what actually is it? It is a long tube whose inner wall is folded forming finger like projections (villi). They have thin walls that are lined with epithelial cells and on the other side is a network of capillaries meaning it ha a rich blood supply. Villi accelerate the rate of absorption because...

• Increase the surface area for diffusion
• Contain muscle therefore are able to move the exchange medium ensuring a concentration gradient is established/maintained
• Well supplied with blood vessels maintain a diffusion gradient
• Thin walled - decreasing the diffusion distance
• Possess microvilli further increasing the surface area for absorption