Human health and physiology - Digestion

Taking in food
1. Humans take food into their digestive system through the mouth and esophagus.
2. However, this food is not truly inside the body until it has passed through a layer of cells into the body's tissues.
3. This happens in the small intestine and is called absorption.
4. Small finger like projection from the wall of the small intestine called villi are specially adapted to absorb food molecules.
5. After food has been absorbed it is assimilated - it becomes part of the tissues of the body.
The need for digestion.
1. The food that humans eat contains substances made by other organisms, many of which are not suitable for human tissues.
  1. They must therefore be broken down and reassembled in a form that is suitable.
2. A second reason for digestion is that many of the molecules in foods are too large to be absorbed by the villi in the small intestine.
  1. These large molecules have to be broken down into small molecules that can then be absorbed by diffusion, facilitated diffusion or active transport.
3. The three main types of food molecule that need to be digested are starch, protein and triglycerides (fats and oils).
4. Digestion of these large molecules happens naturally at body temperature but only at a very slow rate. Enzymes are essential to speed up the process.
  1. Enzymes of digestion:
    1. Amylase
      1. Example - Salivary Amylase
        Source - Salivary glands
        Substrate - Starch
        Products - Maltose
        Optimum pH - pH 7
    2. Protease
      1. Example - Pepsin
        Source - Wall of stomach
        Substrate - Proteins
        Products - Small polypeptides
        Optimum pH - pH 1.5
    3. Lipase
      1. Example - Pancreatic Lipase
        Source - Pancreas
        Substrate - Triglycerides (fats or oils)
        Products - Fatty acids and glycerol
        Optimum pH - pH 7
Relationship between structure of villius and its function
1. Increase the surface area over which food is absorbed
2. An epithelium, consisting of only one thin layer of cells, is all that foods have to pass through to be absorbed.
3. Protrusions of the exposed part of the plasma membrane of the epithelium cells increase the surface area for absorption. These projections are called microvilli.
4. Protein channels in the microvilli membranes allow rapid absorption of foods by facilitated diffusion and pumps allow rapid absorption by active transport.
5. Mitochondrion in the epithelium cells provide the ATP needed for active transport.
6. Blood capillaries inside the villius are very close to the epithelium so the distance for diffusion of foods is very small.
7. A lacteal (a branch of the lymphatic system) in the centre of the villius carries away fats after absorption.
Functions of the stomach and intestines
1. Digestion of proteins begins in the stomach, catalysed by pepsin.
2. Bacteria which could cause food poisoning, are mostly killed by the acid conditions of the stomach.
  1. The acidity also provides optimum conditions for pepsin to work.
3. Enzymes secreted by the wall of the small intestine complete the process of digestion.
  1. The end products of digestion are absorbed by the villi protruding from the wall of the small intestine.
4. The indigestible parts of the food, together with a large volume of water, pass on into the large intestine.
  1. Water is absorbed here leaving solid feces, which are eventually egested through the anus.

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Human health and physiology - Digestion

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