Digestion

Question 1

Digestion of STARCH

Answer 1

1. Food enters the mouth and is broken up by teeth, then mixed with saliva.
2. Salivary amylase starts to hydrolyse glycosidic bonds in the starch producing maltose.
3. In the stomach this salivary amylase is denatured due to acidic pH
4. In the small intestine, pancreatic amylase continues to hydrolyse the starch to maltose.
5. Maltose is then hydrolysed to glucose by maltase enzymes in the membrane of the epithelial cells. The glucose can then be absorbed.

In mammals, cellulose is NOT hydrolysed, as the enzyme cellulase is not produced.

Question 2

Describe the role of the enzymes of the digestive system in the complete breakdown of starch

Answer 2

1. Salivary / Pancreatic amylase hydrolyses starch into maltose;
2. By hydrolysing glycosidic bonds;
3. Maltose is hydrolysed into glucose;
4. By maltase;

Question 3

Describe how glucose is absorbed from the ilium into the blood (co-transport)

Answer 3

1. Na+ are actively transported out of epithelial cell into the blood (by sodium potassium pump);
2. This creates a concentration gradient of Na+ (between lumen of the ilium and the epithelial cell)
3. Na+ and glucose enter by facilitated diffusion using cotransporter proteins.
4. Na+ diffuse into the cell down its concentration gradient.
5. Glucose moves into the cell against its concentration gradient
6. Glucose moves into the blood by facilitated diffusion

Question 4

Protein digestion

Answer 4

1. Proteins / polypeptides are hydrolysed by enzymes called proteases. This process begins in the stomach.
2. Endopeptidases hydrolyse the peptide bonds within the polypeptide chain.
3. This produces many smaller polypeptide chains and increases the surface area for the next enzyme. This makes digestion faster and more efficient.
4. Exopeptidases hydrolyse the peptide bonds at the terminal ends of the protein, removing one amino acid at a time.
5. Exopeptidases are specific: one group are complementary to the N terminal end and another group are complementary to the C terminal end. Amino acids and dipeptides are produced.
6. The dipeptidases - enzymes embedded in the cell surface membrane of the epithelial cells. These hydrolyse dipeptides into amino acids which can then be absorbed by facilitated diffusion, active transport and cotransport.

Question 5

Describe how amino acids are absorbed from the ilium into the blood (co-transport)

Answer 5

1. Na+ (sodium ions) actively transported out of epithelial cell into the blood (by sodium potassium pump);
2. This creates a concentration gradient of Na+ (between lumen of the ilium and the epithelial cell);
3. Na+ and amino acid enter by facilitated diffusion;
4. Na+ moves into the cell down its concentration gradient;
5. Amino acids move into the cell against their concentration gradient / down an electrochemical gradient;
6. Amino acids move into the blood by facilitated diffusion.

Question 6

Lipid digestion

Answer 6

1. Lipid droplets are mixed with bile salts to form smaller droplets (emulsified);
2. (Smaller droplets) increase surface area for faster hydrolysis by lipase;
3. Triglycerides hydrolysed into glycerol, fatty acids and monoglycerides;
4. Glycerol and fatty acids form micelles;
5. Micelles enter the epithelial cell by simple diffusion.
6. At the S.E.R→Fatty acids, glycerol recombined to form tryglycerides.
7. At the golgi apparatus →Triglycerides are modified, proteins are added to form lipoproteins (called chylomicrons) and packaged into vesicles.
8. Chylomicrons are transported into a lymph vessel by exocytosis. They then enter the blood.

Question 7

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

Answer 7

1. Micelles include bile salts and fatty acids;
2. Make the fatty acids (more) soluble in water;
3. carry fatty acids to cell/lining (of the ileum);
4. Maintain high(er) concentration of fatty acids to cell/lining (of the ileum);
5. Fatty acids (absorbed) by diffusion;