3.3.3 Digestion & Absorption

Question 1

Digestion of carbohydrates

Answer 1

• amylase hydrolyses starch to maltose
  Produced in salivary glands
  Produced in pancreas (duodenum)
• maltase hydrolyses maltose to glucose
• sucrose hydrolyses sucrose to glucose and fructose
• lactase hydrolyses lactose to glucose and galactose

Question 2

Where are disaccharides produced

Answer 2

By the duodenum wall and bound to cell membranes

Question 3

Protein digestion process - role of exopeptidases

Answer 3

• hydrolyse peptide bonds at the end of chains, producing dipeptides
• remove single amino acids
• example is dipeptidases - work specifically on dipeptides - break peptide bond between 2 a.a
• found in the cell-surface membrane of epithelial cells in small intestine

Question 4

Digestion of protein - endopeptidases

Answer 4

Hydrolyse peptide bonds in the middle of chains, producing shorter polypeptides

Both found in the stomach wall and the pancreas (in duodenum)

• examples are trypsin, pepsin
• pepsin released in the stomach by cells in the stomach lining - works in acidic conditions (HCl in the stomach)

Question 5

Digestion of proteins - role of dipeptidase

Answer 5

Hydrolyses dipeptides to amino acids

• Produced by the duodenum wall and bound to cell membranes

Question 6

Digestion of lipids - role of lipase

Answer 6

Lipase hydrolyses triglycerides to monoglycerides and 2 fatty acids

• produced in the pancreas (acts in the duodenum

Question 7

Functions of bile

Answer 7

1. Emulsifies lipids to smaller droplets
   - increases S.A for lipase activity
   - increases rate of hydrolysis
2. Forms micelles
   - monoglyceride and fatty acids stick with the bile salts to form micelles. keep them dissolved in ileum (contents)
   - transports digested products to cell membrane for absorption

Question 8

Absorption of lipids in the ileum

Answer 8

• Movement of ileum contents brings micelles into contact with brush border and micelle breaks down
  
  • Monoglycerides & fatty acids diffuse across membrane into cell (simple diffusion)
  • Monoglycerides and fatty acids transported to smooth endoplasmic reticulum, where they are recombined to form triglycerides
  • Triglycerides are transported to Golgi, where the triglycerides associate with cholesterol and lipoproteins to form chylomicrons
  • Chylomicrons transported by vesicle to the cell membrane & released by exocytosis
  • Chylomicrons enter the lacteal. These transport the lipids via lymphatic vessels into the blood vessels

Question 9

How the products of digestion are absorbed across cell membranes - monosaccharides

Answer 9

• glucose is absorbed by Active T. With sodium ions via co-transporter proteins
• galactose is absorbed using same co-transporter protein
• fructose is absorbed via facilitated diffusion through different transporter protein

Question 10

Monoglycerides and fatty acids absorbed across the membrane

Answer 10

• micelles move monoglycerides + F.A towards the epithelium
  HOW? micelles constantly break up and reform so they can release monoglycerides + F.A allowing them to be absorbed
• whole micelles not taken up across the epithelium
• monoglycerides + F.A are lipid soluble, so diffuse directly across epithelial cell membrane

Question 11

Amino acids

Answer 11

• a.a absorbed via co-transport with Na ions
• sodium ions are actively T out the ileum epithelial cells -> blood
• creates sodium ion conc gradient
• na ions diffuse from lumen of ileum -> cells through sodium-dependent- transporter proteins + carry a.a with them.