Nutrition in Man- A Summary

Nutrition in mammals involves:
1. Ingestion of food into body
2. Digestion of food into smaller molecules that can be absorbed into cells
3. Absorption and uptake of nutrients into cells
4. Assimilation: Use of nutrients for energy or making protoplasm
5. Egestion: Removal of undigested and unabsorbed material from the body

This involves the action of the digestive system, which consists of the gut (tube stretching from mouth to anus) and the other glands such as the liver and pancreas.

Gut structure:
1. Outer layer: Serosa (serous coat), secretes oily fluid to reduce friction due to rubbing of the exterior of the organs against each other.

2. Muscle layer: 2 layers of muscle (stomach has 3 layers) arranged in rings (circular muscle), and lengthwise (longitudinal muscle). These contract and relax to bring about peristalsis to push food along the gut.

3. Submucosa (submucous coat): Layer with blood vessels. Blood vessels are required to bring oxygen to gut cells, and to bring digested food to other parts of body. [Sub just means "under", hence the submucosa is just the layer below the mucosa.]

4. Mucosa (Mucous coat): As the name suggests, the layer contains the glands, which produce mucus (for lubrication of food), as well as digestive enzymes and other substances like acid. This is the layer nearest to the lumen of the gut.

Digestion
Can be classified as:
1. Physical: Mechanical breakdown of food into smaller particles, without the use of enzymes. Eg. Mastication in buccal cavity, churning in stomach, emulsification of fats in small intestine.

2. Chemical: Breakdown of food, involving the breaking of chemical bonds, which hence requires the action of enzymes. Eg. Digestion of amylase into maltose by amylase.

Important note: When describing chemical digestion processes, you must include in your answer:
1. Name of enzyme
2. Source of enzyme (where is enzyme made?)
3. Substrate that enzyme acts on
4. Product formed from reaction

Additional Notes
1. All proteases (enzymes that act on proteins/peptones) usually end with "-in" (pepsin, rennin, trypsin, erepsin).

2. Proteases that act on proteins must be produced in the inactive form inside cells, or the protease will cause the destruction of the cell as it will digest the proteins in the cell. Activation of the proteases depends on the type of protease: Those in the stomach are activated by acid, those in the small intestine (trypsin) is activated by another enzyme, enterokinase (from intestinal glands). This ensures protease is only activated in the lumen of the gut.

3. Bile salts allow for PHYSICAL digestion (emulsification) of fats into smaller fat globules, which increases the surface area available for lipase to act on, hence increasing rate of lipase action. Bile salts also allow for the absorption of fatty acids into epithelial cells of villus. They allow the entry of fatty acids and glycerol, after which they then return to the lumen of the small intestine, where the process is repeated again.

4. Bile pigments DO NOT play a digestive function. They are just excretory products, formed from the breakdown of haemoglobin during the destruction of old red blood cells. Of course, since they are pigments, they give faeces their brown colour (but that's not a function!), and are removed out along with the faeces.

5. Digestion of fats: Fats are physically broken into fat globules (emulsification) by action of bile (produced by liver, stored in gall bladder). This increases surface area for lipase (from pancreas and intestinal glands) to digest the lipids into fatty acids and glycerol (chemical digestion).