3.3 Digestion and Absorption

Question 1

What is digestion?

Answer 1

The process by which large insoluble molecules are hydrolysed into smaller, soluble molecules that can be absorbed across the cell membranes into the bloodstream and delivered to cells in the body.

Question 2

Describe the human digestive system.

Answer 2

1. Mouth - chewing, mechanical digestion
2. Salivary glands - produces salivary amylase to break down the food
3. Oesophagus - peristalsis moves food down to the stomach
4. Stomach - digestion begins here, churning of food (mixed with enzymes)
5. Small intestine - site of food absorption
6. Large intestine - site of water reabsorption

• Pancreas (produces and secretes enzymes → amylase, protease, lipase)
• Liver (produces bile)
• Bile (neutralises stomach acid and emulsifies fats)

Question 3

Outline the digestion of carbohydrates.

Answer 3

Takes place in the mouth and small intestine
- amylase hydrolyses starch into maltose (small intestine, pancreas, mouth)
- the disaccharidase maltase catalyses the hydrolysis of maltose into glucose (cell-surface membranes of the epithelial cells lining the small intestine)

Question 4

Outline the digestion of proteins.

Answer 4

1. Protein digestion begins at the lumen of the stomach.
2. Endopeptidase hydrolyses the peptide bonds in the middle of the polypeptide to make smaller protein “chunks”.
3. Partially digested food passes through to the small intestine.
4. Pancreas secretes both endopeptidases and exopeptidases.
5. Finally there are dipeptidase enzymes found within the cell surface membrane of the epithelial cells in the small intestine which hydrolyse dipeptides into amino acids which are released into the cell’s cytoplasm.

Question 5

Outline the digestion of lipids.

Answer 5

Emulsification:
- bile salts emulsify the fatty liquid to increase its surface area
Digestion:
- takes place solely in the lumen of the small intestine
- lipase breaks down lipids into fatty acids and glycerol

Question 6

Describe the co-transport mechanisms for the absorption of amino acids and monosaccharides.

Answer 6

1. Na+ leaves the epithelium by active transport
2. Maintains a concentration gradient for Na+
3. Na+ ions co-transported into the epithelium with amino acids or glucose.

Question 7

Outline the role of micelles in the digestion of lipids.

Answer 7

1. Micelles contain bile salts and fatty acids
2. Make fatty acids more soluble in water
3. Fatty acids absorbed by diffusion
4. Triglycerides are reformed inside the cells
5. Vesicles move to the cell membrane and leave by exocytosis