Human health and physiology - Digestion

Human health and physiology - Digestion

1. Taking in food

   • Humans take food into their digestive system through the mouth and esophagus.
   • However, this food is not truly inside the body until it has passed through a layer of cells into the body's tissues.
   • This happens in the small intestine and is called absorption.
   • Small finger like projection from the wall of the small intestine called villi are specially adapted to absorb food molecules.
   • After food has been absorbed it is assimilated - it becomes part of the tissues of the body.

2. The need for digestion.

   • The food that humans eat contains substances made by other organisms, many of which are not suitable for human tissues.
     • They must therefore be broken down and reassembled in a form that is suitable.
   • 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.
     • These large molecules have to be broken down into small molecules that can then be absorbed by diffusion, facilitated diffusion or active transport.
   • The three main types of food molecule that need to be digested are starch, protein and triglycerides (fats and oils).
   • 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.
     • Enzymes of digestion:
       • Amylase
         • Example - Salivary Amylase
           • Source - Salivary glands
             • Substrate - Starch
               • Products - Maltose
                 • Optimum pH - pH 7
       • Protease
         • Example - Pepsin
           • Source - Wall of stomach
             • Substrate - Proteins
               • Products - Small polypeptides
                 • Optimum pH - pH 1.5
       • Lipase
         • Example - Pancreatic Lipase
           • Source - Pancreas
             • Substrate - Triglycerides (fats or oils)
               • Products - Fatty acids and glycerol
                 • Optimum pH - pH 7

3. Relationship between structure of villius and its function

   • Increase the surface area over which food is absorbed
   • An epithelium, consisting of only one thin layer of cells, is all that foods have to pass through to be absorbed.
   • Protrusions of the exposed part of the plasma membrane of the epithelium cells increase the surface area for absorption. These projections are called microvilli.
   • Protein channels in the microvilli membranes allow rapid absorption of foods by facilitated diffusion and pumps allow rapid absorption by active transport.
   • Mitochondrion in the epithelium cells provide the ATP needed for active transport.
   • Blood capillaries inside the villius are very close to the epithelium so the distance for diffusion of foods is very small.
   • A lacteal (a branch of the lymphatic system) in the centre of the villius carries away fats after absorption.

4. Functions of the stomach and intestines

   • Digestion of proteins begins in the stomach, catalysed by pepsin.
   • Bacteria which could cause food poisoning, are mostly killed by the acid conditions of the stomach.
     • The acidity also provides optimum conditions for pepsin to work.
   • Enzymes secreted by the wall of the small intestine complete the process of digestion.
     • The end products of digestion are absorbed by the villi protruding from the wall of the small intestine.
   • The indigestible parts of the food, together with a large volume of water, pass on into the large intestine.
     • Water is absorbed here leaving solid feces, which are eventually egested through the anus.