Chemistry of Digestion

1. Chemistry of Digestion To be able to identify the sites of production and action of: • amylases; • endopeptidases; • exopeptidases; • lipase; • maltase; • bile. And describe mechanisms for the absorption of food by the ileum, including the roles of diffusion, facilitated diffusion and active transport.

2. Testing enzyme activity • If a source of enzyme is placed in an agar plate, the enzyme will diffuse out through the agar, turning the substrate into product as it goes. • There must be a way to distinguish the substrate from the product, and the reaction will then show up as a ring around the enzyme source. • The higher the concentration of enzyme, the higher the diffusion gradient, so the faster the enzyme diffuses through the agar, so the larger the ring in a given time. • The diameter of the ring is therefore proportional to the enzyme concentration. • This can be done for many enzymes, e.g. a protein agar plate can be used for a protease enzyme, or a starch agar plate can be used for the enzyme amylase.2

4. Digestion of Carbohydrates • By far the most abundant carbohydrate in the human diet is starch (in bread, potatoes, cereal, rice, pasta, biscuits, cake, etc) • There may also be a lot of sugar (mainly sucrose) and some glycogen (in meat).

5. Salivary amylase • Starts the digestion of starch. • Very little digestion actually takes place, since amylase is quickly denatured in the stomach • It does help to clean mouth and reduce bacterial infection.

6. Pancreatic amylase • Digests all the remaining starch in the duodenum. • Amylase digests starch molecules from the ends of the chains in two-glucose units, forming the disaccharide maltose. • Glycogen is also digested here.

7. Disaccharidases • In the membrane of the ileum epithelial cells complete the digestion of disaccharides to monosaccharides. • This includes maltose from starch digestion as well as any sucrose and lactose in the diet. • There are three important disaccharidase enzymes: maltase glucose maltose sucrase glucose + fructose sucrose lactase glucose + galactose lactose

8. Monsaccharides • The monsaccharides (glucose, fructose and galactose) are absorbed by active transport into the epithelial cells of the ileum, whence they diffuse into the blood capillaries of the villi. • Active transport requires energy in the form of ATP, but it allows very rapid absorption, even against a concentration gradient. • The membrane-bound disaccharidases and the monosaccharide pumps are often closely associated:

9. The carbohydrates that make up plant fibres (cellulose, hemicellulose, lignin, etc) cannot be digested, so pass through the digestive system as fibre.

10. Digestion of Proteins • Protease enzymes are potentially dangerous because they can break down other enzymes (including themselves!) and other proteins in cells. • To prevent this they are synthesised in the RER of their secretory cells as inactive forms, called zymogens. • These are quite safe inside cells, and the enzymes are only activated in the lumen of the intestine when they are required.

11. Rennin (in gastric juice) • This converts the soluble milk protein caesin into its insoluble calcium salt. • This keeps it in the stomach longer so that pepsin can digest it. • Rennin is normally only produced by infant mammals. • It is used commercially to make cheese.

12. Pepsin (in gastric juice) • This digests proteins to peptides, 6-12 amino acids long. • Pepsin is an endopeptidase, which means it hydrolyses peptide bonds in the middle of a polypeptide chain. • It is unusual in that it has an optimum pH of about 2 and stops working at neutral pH.

13. Pancreatic endopeptidases • These continue to digest proteins and peptides to short peptides in the duodenum. • Different endopeptidase enzymes cut at different places on a peptide chain because they have different target amino acid sequences • This is an efficient way to cut a long chain up into many short fragments, and it provides many free ends for the next enzymes to work on.

14. Exopeptidases • These are found in the membrane of the ileum epithelial cells and complete the digestion of the short peptides to individual amino acids. • Exopeptidases remove amino acids one by one from the ends of peptide chains. • Carboxypeptidases work from the C-terminal end • Aminopeptidases work from the N-terminal end, • Dipeptidases cut dipeptides in half.

15. The amino acids are absorbed by active transport into the epithelial cells of the ileum. • From here they diffuse into the blood capillaries of the villi. • The membrane-bound peptidases and the amino acid transporters are closely associated.

16. Inactive  active • Pepsin is synthesised as inactive pepsinogen, and activated by the acid in the stomach • Rennin is synthesised as inactive prorennin, and activated by pepsin in the stomach • The pancreatic endopeptidases are activated by specific enzymes in the duodenum • The membrane-bound peptidase enzymes do not have this problem since they are fixed, so cannot come into contact with cell proteins. • The lining of mucus between the stomach wall and the food also protects the cells from the protease enzymes once they are activated.

17. Digestion of Triglycerides • Fats are emulsified by bile salts to form small oil droplets called micelles, which have a large surface area. • Pancreatic lipase enzymes digest triglycerides to fatty acids and glycerol in the duodenum. • Fatty acids and glycerol are lipid soluble and diffuse across the membrane (by lipid diffusion) into the epithelial cells of the villi in the ileum.

18. In the epithelial cells of the ileum triglycerides are re-synthesised and combine with proteins to form tiny lipoprotein particles called chylomicrons. The chylomicrons diffuse into the lacteal - the lymph vessel inside each villus. The emulsified fatty droplets give lymph its milky colour, hence name lacteal. The chylomicrons are carried through the lymphatic system to enter the bloodstream at the vena cava, and are then carried in the blood to all parts of the body. They are stored as triglycerides in adipose (fat) tissue. Fats are not properly broken down until they are used for respiration in liver or muscle cells.

19. Digestion of Nucleic acids • Pancreatic nuclease enzymes digest nucleic acids (DNA and RNA) to nucleotides in the duodenum. • Membrane-bound nucleotidase enzymes in the epithelial cells of the ileum digest the nucleotides to sugar, base and phosphate, which are absorbed.

20. Other substances • Many substances in the diet are composed of small molecules that need little or no digestion. • These include sugars, mineral ions, vitamins and water. These are absorbed by different transport mechanisms: • Cholesterol and the fat-soluble vitamins (A, D, E, K) are absorbed into the epithelial cells of the ileum by lipid diffusion • Mineral ions and water-soluble vitamins are absorbed by passive transport in the ileum • Dietary monosaccharides are absorbed by active transport in the ileum • Water is absorbed by osmosis in the ileum and colon.

21. Digestion in Fungi • Fungi are not consumers like animals, but are either saprophytes (decomposers), or pathogens. • They therefore use saprophytic nutrition,(as opposed to holozoic) which means they do not ingest their food, but use extracellular digestion. • Fungi secrete digestive enzymes (carbohydrases, proteases and lipases) into the material that surrounds them and then absorb the soluble products (sugars, amino acids, etc).

22. Fungi are usually composed of long thin threads called hyphae. These grow quickly, penetrating dead material such as leaves, as well as growing underground throughout soil. The cotton wool appearance of bread mould growing on decaying bread is typical of a mass of hyphae, called a fungal mycelium. These thin hyphae give fungi a large surface area to volume ratio. They contain many nuclei, since they are formed from the fusion of many cells.