digestion and absorption

Question 1

1. What digestion?

Answer 1

The hydrolysis of large biological molecules into smaller molecules that can be absorbed across the cell membrane of epithelial cells in the ileum (small intestine).

Question 2

2. Describe and explain how carbohydrate
   digestion occurs in mammals.

Answer 2

Starch digestion begins in the mouth when salivary amylase from saliva mixes with food in the mouth. Salivary amylase hydrolyses glycosidic bonds turning some starch into maltose. Food is swallowed and enters the stomach where salivary amylase is denatured due to the low pH. Pancreatic amylase is released into the small intestine along with alkaline salts from the liver to neutralise the acid. Pancreatic amylase hydrolyses the remaining starch into maltose. In the cell membranes of the epithelial cells lining the ileum (small intestine) are membrane bound disaccharidases, maltase, lactase, sucrase. These hydrolyse disaccharides into monosaccharides for absorption.

Question 3

3. What is the advantage of membrane-bound
   enzymes?

Answer 3

They do not need to be constantly replenished as they are not removed with waste from the small intestine because they are bound to the cell surface membrane of the epithelial cells. The product (monosaccharides) are at the epithelial cell membrane, so have a very short diffusion distance into the cell.

Question 4

4. What are the membrane bound enzymes
   and what are their substrates and products?

Answer 4

Sucrase- hydrolyses sucrose into fructose and glucose Maltase- hydrolyses maltose into glucose Lactase-hydrolyses lactose into galactose and glucose Dipeptidase- hydrolyses dipeptides into amino acid

Question 5

5. Describe how proteins are digested in
   human gut.

Answer 5

Endopeptidases produced by the stomach. Endopeptidases are also made by the pancreas and are released into the small intestine. Endopeptidases hydrolyse internal peptide bonds which breaks polypeptides into smaller polypeptide chains. Exopeptidases are made by the pancreas and released into the small intestine. They hydrolyse peptide bonds at the ends of the polypeptide chain releasing amino acids or dipeptides. Dipeptidases hydrolyse dipeptides into amino acids.

Question 6

6. What are the advantages of using
   endopeptidases and exopeptidases

Answer 6

Endopeptidases hydrolyse internal peptide bonds therefore there are more ends or increase in surface area for exopeptidases which hydrolyse bonds at ends of chains.

Question 7

7. Describe how glucose/amino acids are
   absorbed by cotransport.

Answer 7

Sodium ions actively transported from ileum/epithelial cell to blood; This forms a concentration gradient for sodium to enter cells from gut lumen and with it, glucose/amino acids enters by facilitated diffusion with sodium ions against the glucose/amino acid concentration gradient; Glucose/amino acid diffuses into the blood, down the concentration gradient, through another protein channel.

Question 8

8. Explain why cotransport of glucose and amino acids is called indirect active transport.

Answer 8

Energy is used to actively transport the sodium ions out of the epithelial cell into the blood, glucose/amino acids are transported passively at the other end of the cell.

Question 9

9. Describe and explain how lipids are digested in the human gut.

Answer 9

Bile salts emulsify lipids from large droplets to small droplets, small lipid droplets have a large surface area for lipase to hydrolyse ester bonds forming monoglycerides and fatty acids. The monoglycerides and fatty acids remain bound to the bile salts in small droplets called micelles.

Question 10

10. Explain why the pH decreases when lipids
    are hydrolysed.

Answer 10

Fatty acids are produced, which are acidic and so lower the pH.

Question 11

11. Describe and explain how lipids are
    absorbed.

Answer 11

Micelles interact with the phospholipid bilayer of the epithelial cells and the fatty acids/monoglycerides enter by simple

diffusion as they are lipid soluble. Once in the cell triglycerides are reformed by the smooth endoplasmic reticulum, then the golgi modifies the triglyceride but forming structures called chylomicrons (triglycerides, cholesterol, protein and phospholipids) the golgi packages chylomicrons into vesicles. Vesicles perform exocytosis releasing chylomicrons into lacteals (lymph system). The lymph system then carries the chylomicrons to the cardiovascular system.

Question 12

11. What are the benefits of emulsifying lipids
    to smaller droplets?

Answer 12

Droplets increase surface areas for lipase action; So faster hydrolysis of lipids.

Question 13

12. What are micelles and what is their
    function?

Answer 13

Micelles contain bile salts bound to fatty acids and monoglycerides. Micelles make the fatty acids and monoglycerides more soluble in water Micelles carry fatty acids and monoglycerides to the cell membrane of intestinal epithelial cell Micelles breakdown close to the cell surface membrane of the epithelial ells of the ilium to release fatty acids and monoglycerides to the site of absorption

Question 14

13. What is the difference between villi and
    microvilli?

Answer 14

Villi are projections of the wall of the ilium (folded wall) Microvilli are projections of the cell surface membrane of the epithelial cells that form the outer layer of the wall of the ilium.