3.3.3 digestion and absorption

Question 1

Large biological molecules

Answer 1

E.g. starch/proteins too big to be absorbed across cell membranes

Digestion breaks them into smaller molecules e.g. glucose/amino acids > absorbed from the gut to the blood

Question 2

Digestion of starch (polysaccharide)

Answer 2

Amylase hydrolyses starch to maltose

• amylase produced by salivary glands, released into mouth
• amylase produced by pancreas, released into small intestine

Membrane bound maltase > hydrolyse maltose to glucose

Hydrolysis of glycosidic bond

Question 3

Digestion of disaccharides

Answer 3

Membrane bound disaccharides > hydrolyse disaccharide to monosaccharides

Hydrolysis of glycosidic bond

Question 4

Digestion of lipids by lipase

Answer 4

Bile salts produced by liver

Bile salts emulsify lipid to smaller lipid droplets
- increasing surface area to speed up action of lipase

Lipase hydrolyses lipids > monoglycerides + fatty acid, breaking ester bond

Monoglycerides, fatty acids and bile salts stick together to form micelles

Lipase made in pancreas, released to small intestine

Question 5

Digestion of proteins using endopeptidases

Answer 5

hydrolyse peptide bonds within a protein in the central region,

breaking the protein into 2 or more smaller proteins

Question 6

Digestion of proteins using exopeptidase

Answer 6

Hydrolyse peptide bonds at the end of protein molecules

Removing a single amino acid

Question 7

Digestion of proteins using dipeptidase

Answer 7

Often membrane bound in ileum

Hydrolyse peptide bond between a dipeptide

= 2 amino acids

Question 8

Co-transport for absorption of amino acids and of monosaccharides

Answer 8

Sodium ions actively transported out of epithelial cells lining the ileum, into the blood, by the sodium-potassium pump, creating a concentration gradient of sodium

Sodium ions and glucose move out by facilitated diffusion into the epithelial cells from the lumen, via a co-transporter protein.

Creating a concentration gradient of glucose

Glucose moves out of the cell into blood by facilitated diffusion, through a protein channel

Question 9

Absorption of products of digestion by cells lining the ileum

Absorption of lipids

Answer 9

Monoglycerides and fatty acids diffuse out of micelles (in lumen) into epithelial cell
— as lipid soluble

Monoglycerides and triglycerides recombine to triglycerides which aggregate into globules

Globules coated with proteins to form chylomicrons

Leave via exocytosis and enter lymphatic vessels

Return to blood circulation

Question 10

Why might pH change (2)

Answer 10

Production of fatty acids

Fatty acids produced cause fall in pH

Question 11

Why might pH stop falling (1)

Answer 11

Substrate all used up

Equilibrium reached

PH denatures enzyme

Question 12

Why might pH fall at different rates (2)

Answer 12

Bile salts produce many small lipid droplets

Emulsifies lipids