3.3 Digestion and absorption

Question 1

What happens during digestion?

Answer 1

Large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes

• Large biological molecules in food eg. starch/proteins too big to be absorbed across cell membranes
• Digestion breaks them into smaller molecules eg. glucose/amino acids -> absorbed from the gut to the blood

Question 2

Describe the process of starch (polysaccharide) digestion:

Answer 2

Saliva enters the mouth from the salivary glands, which is physical digestion. Starch gets broken down into maltose, by amylase hydrolysing the glycosidic bonds. Food is then swallowed, the stomach acid destroys the enzyme, so no more starch is digested. Food then passes on into the small intestine where it mixes with pancreatic juice, which contains pancreatic amylase that hydrolyses the remaining starch. The epithelial cell lining of the intestine produces maltase. So maltose breaks down into glucose, by maltase.

Question 3

Describe the process of the digestion of disaccharides?

Answer 3

Membrane bound disaccharidases, e.g. maltase, sucrose, lactase (attached to epithelial cells lining the ileum of the small intestine)→hydrolyse disaccharide to x2 named monosaccharides

• Maltase which catalyses the hydrolysis of maltose to glucose
• Sucrase which catalyses the hydrolysis of sucrose to glucose and fructose
• Lactase which catalyses the hydrolysis of lactose to glucose and galactose

Hydrolysis of glycosidic bond

Question 4

Describe the action of bile salts:

Answer 4

• Bile salts produced by the liver
• Bile salts emulsify lipid to smaller lipid droplets
• Increasing SA:V of lipids speeds up action of lipases

Question 5

Describe how the digestion in mammals take place by using lipase:

Answer 5

• Lipase made in the pancreas, released to small intestines
• Lipase hydrolyses lipids -> monoglycerides + fatty acids
• Breaking ester bond
• Monoglycerides, fatty acids and bile salts stick together to form micelles

Question 6

What do endopeptidases do?

Answer 6

• Hydrolyse peptide bonds within a protein / between amino acids in the central region
• Breaking protein into two or more smaller peptides

Question 7

What do exopeptidases do?

Answer 7

• Hydrolyse peptide bonds at the ends of protein molecules
• Removing a single amino acid

Question 8

What do dipeptidases do?

Answer 8

• Often membrane bound in ileum
• Hydrolyse peptide bond between a dipeptide
  = 2 amino acids

Question 9

Describe the mechanisms for the absorption of the products of digestion (include co-transport mechanisms):

Answer 9

1. Sodium ions actively transported out of epithelial cells lining the ileum, into the blood, by the sodium-potassium pump. Creating a concentration gradient of sodium (higher conc. of sodium in lumen than epithelial cell)
2. Sodium ions and glucose move by facilitated diffusion into the epithelial cell from the lumen, via a co-transporter protein
3. Creating a concentration gradient of glucose – higher conc. of glucose in epithelial cell than blood
4. Glucose moves out of cell into blood by facilitated diffusion through a protein channel

Question 10

What do micelles contain?

Answer 10

• Bile salts
• Monoglycerides
• Fatty acids

Question 11

Describe the role of micelles in the absorption of fats into the cells lining the ileum

Answer 11

• Micelles have bile salts, fatty acids and monoglycerides
• Therefore makes the micelles more soluble in water when compared to fatty acid alone
• Micelle carries fatty acid to the ileum
• There is a higher concentration of fatty acids in the ileum, which maintains a high concentration gradient
• Fatty acids get absorbed by diffusion to the inside of the epithelial cell