3.3.3 Digestion and absorption (3.3 Organisms exchange substances with their enviornment)

Question 1

Explain what happens in digestion

Answer 1

• Large biological molecules hydrolysed to smaller 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 hydrolyses starch to maltose
• Membrane bound maltase hydrolyses maltose to glucose
• Hydrolysis of glycosidic bond

Question 3

Describe the digestion of disaccharides in mammals

Answer 3

Membrane bound disaccharidases hydrolyse disaccharide to 2 monosaccharides:
• Maltase - maltose => glucose + glucose
• Sucrase - sucrose => glucose + fructose
• lactase - lactose => glucose + galactose
- Hydrolysis of glycosidic bond

Question 4

Describe the digestion of lipids in mammals including action of bile salts

Answer 4

• Bile salts emulsify lipids causing them to form smaller lipid droplets
• This increases surface area of lipids for faster lipase activity
• Lipase hydrolysis lipids => monoglycerides + fatty acids
• hydrolysis of ester bond

Question 5

Describe the digestion of proteins by a mammal

Answer 5

• endopeptidases => hydrolyse internal bonds within a polypeptide => smaller peptides
- so more ends / surface area for exopeptidases
• Exopeptidases - hydrolyse terminal bonds at ends of polypeptide => single amino acids
• Membrane bound dipeptidases => hydrolyse bond between a dipeptide => 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 membrane of epithelial cells lining ileum
• maintain conc gradient for absorption

Question 7

Describe the pathway for absorption of produces of digestion in mammals

Answer 7

lumen of ileum => cell lining ileum => 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 to blood by Na+/K+ pump
• Establishing a conc gradient of Na+
2) • Na+ enters epithelial cell down its conc gradient with monosaccharides or amino acids against its conc gradient
• Via a co transporter protein
3) Monosaccharides or amino acid moves down a conc gradient into blood via facilitated diffusion

Question 9

Describe the absorption of lipids by a mammal , including the role of micelles

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 in 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 by diffusion
• Triglycerides reformed in 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