Nutrient Digestion I (Carbohydrates and Proteins)

Question 1

Describe monosaccharides.

Answer 1

• Hexose sugars (6C) e.g. glucose, galactose, fructose.

- Breakdown products of complex CHOs, which are absorbed by the small intestine.

Question 2

Describe disaccharides.

Answer 2

• 2 monosaccharides linked together by glycosidic bond.
• Broken down to constituent monomers by brush border enzymes in small intestine e.g.
  Lactose = glucose + galactose (broken down by lactase)
  Sucrose = glucose + fructose (sucrase)
  Maltose = glucose + glucose (maltase)

Question 3

Describe polysaccharides.

Answer 3

Starch and cellulose in plants.

Glycogen;

• Animal storage form of glucose.
• Glucose monomers linked by alpha-1, 4 glycosidic bonds (same as starch).

Question 4

Describe the enzymatic digestion of polysaccharides.

Answer 4

Alpha-amylase breaks down both starch and glycogen however there is no mechanism in humans for breaking down cellulose.

Question 5

Describe transcellular transport.

Answer 5

Substances travel through the cell, passing through both the apical and basolateral membrane.

Question 6

Describe paracellular transport.

Answer 6

Substances travel across an epithelium by passing through the intracellular space between cells.

Question 7

Describe vectorial transport.

Answer 7

Transport of a substance one direction across an epithelium. It requires transport proteins to be non-randomly distributed between the apical and basolateral membrane.

Question 8

Describe the mechanisms whereby glucose is absorbed across intestinal epithelial cells.

Answer 8

1. Glucose and Na bind to SGLT1.
2. SGLT2 undergoes conformational change, which flips the membrane.
3. SGLT1 loses its affinity for glucose and Na.
4. Na-K-ATPase pump in basolateral membrane pumps Na out of cell and K into cell.
5. GLUT2 transports glucose out of cell.

Question 9

Describe the mechanisms whereby fructose is absorbed across intestinal epithelial cells.

Answer 9

1. GLUT5 transports fructose through apical membrane.

2. GLUT2 transports fructose out of cell.

Question 10

Describe proteins.

Answer 10

• Polymers of amino acids linked together by peptide bonds.
• Large variations in chain length.
• Proteins often undergo post-translational modification.

Question 11

Describe peptides.

Answer 11

Small proteins, 3-10 amino acids in length.

Question 12

Describe the digestive processes for converting proteins to small peptides and amino acids.

Answer 12

Enzymes that hydrolyse protein bonds and reduce proteins or peptides to amino acids are called proteases and peptidases.
Endopeptidases - cleaves the proteins into 2 parts.
Exopeptidases - act only on terminal amino acid (amino- at H2N end, carboxy- at COOH end).

Question 13

Describe the mechanisms of absorption for amino acids.

Answer 13

(Same mechanism as glucose and galactose.)

1. Amino acid and Na bind to SAAT1.
2. SAAT1 undergoes conformational change, which flips the membrane.
3. SAAT1 loses its affinity for amino acid and Na.
4. Na-K-ATPase pump in basolateral membrane pumps Na out of cell and K into cell.
5. Specific transporter takes amino acid out of cell.

Question 14

Describe the mechanisms of absorption for small peptides.

Answer 14

1. Di-/tripeptide and H bind to PepT1.
2. NHE3 pumps Na into cell and H out of cell (creating acid microclimate along villi).
3. Na-K-ATPase pump in basolateral membrane pumps Na out of cell and K into cell.
4. Specific transporter takes di-/tripeptide out of cell.