B5e - Digestion

B5e - Digestion

1. Physical digestion

   • Physical digestion involves breaking food down into smaller pieces without making any chemical changes. Physical digestion happens in: the mouth when food is chewed the stomach when food is squeezed and moved around Physical digestion is important because it lets the food pass through the digestive system more easily, and it provides a larger surface area for enzymes to work on.

2. Chemical digestion

   • Chemical digestion involves breaking large food molecules into smaller molecules using chemical reactions. These smaller molecules can then be absorbed into the blood. Enzymes speed up chemical digestion. These are natural catalysts made of proteins. Different enzymes catalyse different digestion reactions. Stomach acid provides the correct pH for stomach protease to function properly. Different enzymes are produced in different parts of the gut. carbohydrase enzymes catalyse the breakdown of starch into sugars in the mouth and small intestine protease enzymes catalyse the breakdown of proteins into amino acids in the stomach and small intestine lipase enzymes catalyse the breakdown of fats and oils into fatty acids and glycerol in the small intestine

3. Absorbtion

   • The products of chemical digestion are absorbed into the body in the small intestine: sugars and amino acids pass into the bloodstream by diffusion fatty acids and glycerol pass into the lymph The bloodstream and lymph carry them to the body’s tissues.
   • The intestine has several adaptations for the efficient absorption of small molecules from digestion: it has a thin lining it has a good blood supply it is very long and has a large surface area Tiny finger-like projections called villi provide a large surface area for absorption to take place. They also have a rich supply of blood vessels to carry away absorbed molecules.

4. Parts of the digestive system

   • Salivary glands
     • produce saliva that moistens food and contains carbohydrase enzymes
   • Stomach
     • produces hydrochloric acid and protease enzymes
   • Pancreas
     • produces carbohydrase, protease and lipase enzymes
   • Liver
     • produces bile
   • Gall bladder
     • stores bile
   • Small intestine
     • produces carbohydrase, protease and lipase enzymes, and absorbs digested food
   • Large intestine
     • absorbs water

5. Starch digestion

   • Starch is a complex carbohydrate. Each molecule consists of very many glucose molecules joined together. The digestion of starch is a two-stage process: it is first digested to form maltose (each maltose molecule consists of two glucose molecules joined together) the maltose is then digested to form glucose

6. Changing pH

   • The pH is at alkaline levels in the mouth and small intestine, but the pH in the stomach is at acidic levels. The enzymes there work at different optimum pH levels. For example: salivary amylase works best just below pH 7 stomach protease works best at around pH 1.6 pancreatic lipase works best at around pH 8

7. Bile

   • As food passes from the stomach into the small intestine, the pH must be raised. Bile - an alkaline substance - is released into the small intestine to achieve this. Bile also emulsifies (breaks down) fats in the small intestine. This is important, because it provides a larger surface area in which the lipases can work.