3.3 Digestion And Absorption

Question 1

Explain what happens in digestion

Answer 1

• large insoluble biological molecules hydrolysed to smaller soluble molecules
• that are small enough to be absorbed across cell membranes into blood

Question 2

Describe the digestion of starch in mammals

Answer 2

• amylase (produced by salivary glands / pancreas) hydrolyses starch into maltose
• membrane - bound maltose (attached to cells lining ileum) hydrolyses maltose to glucose
• hydrolysis of glycosidic bond

Question 3

Describe the digestion of disaccharides in mammals

Answer 3

• membrane-bound disaccharidases hydrolyse disaccharides to 2 monosaccharides:
  
  • maltase - maltose -> glucose + glucose
  • sucrase - sucrose -> glucose + fructose
  • lactase - lactose -> glucose + galactose

Question 4

Describe the digestion of lipids in mammals

Answer 4

• bile salts (produced by liver) emulsify lipids causing them to form smaller lipid droplets
• this increases surface area of lipids for increased / faster lipase activity
• lipase made in the pancreas hydrolyses lipids (e.g. triglycerides) -> monoglycerides + fatty acids
• hydrolysis of ester bonds

Question 5

Describe the digestion of proteins by a mammal

Answer 5

• endopeptidases-> hydrolyse internal peptide bonds within a polypeptide into smaller peptides
  
  • so more ends/ surface area for Exopeptidases
• Exopeptidases- hydrolyse terminal peptide bonds at end of polypeptides into single amino acids
• membrane-bound Dipeptidases - hydrolyse peptide bonds between a dipeptides into 2 amino acids
• hydrolysis of peptide bond

Question 6

Suggest why membrane-bound enzymes are important in digestion

Answer 6

• membrane-bound enzymes are located in cell membranes of epithelial cells lining ileum
• by hydrolysing molecules at the site of absorption they maintain concentration gradients for absorption

Question 7

Describe the pathway for absorption of products of digestion in mammals

Answer 7

Lumen inside of ileum -> cells lining ileum(part of small intestine) -> blood

Question 8

Describe the absorption of amino acids and monosaccharides in mammals

Answer 8

Co-transport
1:
- Na+ actively transported from epithelial cells lining ileum blood by a sodium,potassium pump
- establishing a concentration gradient of sodium which is higher in lumen than epithelial cell
2:
- sodium enters epithelial cells down its concentration gradient with monosaccharide of amino acids against it concentration gradient
- via a co-transporter protein
3:
- monosaccharide or amino acid moves down a concentration gradient into blood via facilitated diffusion

Question 9

Describe the absorption of lipids by a mammal

Answer 9

• bile salts combine with monoglycerides and fatty acids to form, micelles
  
  • micelles make monoglycerides and fatty acids more soluble in water
  • micelles carry fatty acids and monoglycerides to cells lining the ileum, where they break down to release them
  • this maintains a high concentration of fatty acids and monoglycerides near cells lining the ileum
• monoglycerides/fatty acids are absorbed into epithelial cells by diffusion as they’re lipid soluble
• triglycerides reformed in epithelial cells and aggregate into globules
• globules coated with proteins forming chylomicrons which are then packaged into vesicles
• vesicles move to cell membrane and fuse with it, releasing chylomicrons via exocytosis
  
  • chylomicrons enter lymphatic vessels and eventually return to blood circulation