3. Digestion & Absorption

Question 1

Suggest why the stomach does not have villi or microvilli.

Answer 1

• food in stomach x yet hydrolysed into soluble molecules
• x be absorbed
• villi would be futile

Question 2

List 3 enzymes produced by thr epithelium of the ileum.

Answer 2

1. maltase
2. sucrase
3. lactase

Question 3

1. Suggest the process by which microorganisms produce ‘a large volume of gas’ in lactose intolerant individuals.
   /
2. Suggest a reason why this gas is unlikely to be CO2.

Answer 3

1. Respiration
2. CO2 formed in aerobic respiration vs conditions in colon = anaerobic

Question 4

Explain how lowering the water potential in the colon can cause diarrhoea.

Answer 4

• water moves from epithelial cells into lumen of colon
• water faeces

Question 5

Describe digestion in the mouth.

Answer 5

• Mechanical: chewing- ↑ s.a. for stomach to work on in c. digestion –> quicker digestion
• Chemical: Saliva- 1. amylase hydrolyses starch 2. mineral salts maintains ~ neutral pH (optimum)

Question 6

Why are polysaccharidases (eg. amylase) produced in the mouth?

Answer 6

• polysaccharides take longer to digest
• start earlier in mouth
• single molecules ready to be absorbed in the S.I.

Question 7

Features of the oesophagus.

Answer 7

• muscular –> peristalsis
• rings of cartilage –> flexibility + protection

Question 8

Digestion in the stomach.

Answer 8

C:
- HCl- kills pathogens in food
- releases protein (peptin- pH2)
- mucus- line stomach wall for protection

M:
- muscular- churns food–> ↑ s.a.

Question 9

Digestion in the pancreas.

Answer 9

Pancreatic juice
- pancreatic amylase- cont. hydrolysis of starch (after stop in stomach ∵ denaturation of salivary amylase)
- alkaline salts (bicarbonates)- neutralises acid–> optimum pH for amylase (+enzymes) in the S.I. + x erode intestinal wall

Question 10

Processes in the large intestine

Answer 10

• absorbs excess water
• bacteria w/ enzymes- break down complex carbs (eg. plant-based cellulose)

Question 11

Describe the action of amylase

Answer 11

• produced in mouth & pancreas
• hydrolyses alternate g. bonds of starch molecule
• produce maltose (disaccharide)

Question 12

Explain the importance of maltase being membrane-bound.

Answer 12

• in epithelial lining of ileum
• broken down –> directly absorbed after broken down
• ↓ diffusion distance for monos

Question 13

Explain the importance of pancreatic amylase.

Answer 13

• ensures complete breakdown of polysaccharides

:)
* salivary amylase denatured in stomach
* broken down into smaller chains
* ↑ s.a. for disaccharidases to work on
* broken down into** monos for absorption**

Question 14

Explain how a co-transporter of sodium and glucose works. (7)

Answer 14

1. transporter faces into lumen- ✔️bind w/ Na+ ❌glucose
2. Na+ binds –> CONFORMATIONAL CHANGE –> opens glucose binding site
3. glucose binds
4. protein reorients in mem –> sites holding glucose & Na+ face into epithelial cell
5. Na+ dissociates –> cytoplasm ==> glucose binding destabilises
6. glucose dissociates –> cytoplasm
7. protein reorients back –> original position

Question 15

Explain the action of endopeptidases using an example.

Answer 15

• hydrolyse internal peptide bonds in polypeptide chains
• to ↑ end points & ↑ s.a. for exopeptidases
  Example:
• pepsin made as pepsinogen (inactive form ∵ would damage stomach lining before release)
• acid (~pH2) –> CONFORMATIONAL CHANGE –> acitvated
• goblet cells- mucus lining stomach wall –> protects against HCl
• trpsin & chymotrypsin- in pancreatic juice

Question 16

Describe the action of exopeptidases.

Answer 16

• hydrolyse terminal peptide bonds in a polypeptide chain (remove single a.a.)

Question 17

Describe the action of dipeptidases.

Answer 17

• act on dipeptides
• hydrolyse peptide bond between 2 a.a.
• in cell-surface mem on epithelial cells in s.i.

Question 18

Describe how lipids are digested into its component molecules.

Answer 18

• hydrolysed by lipases (produced in pancreas)
• only hydrolyse 2 ester bonds
• triglycerides –> 2x fatty acids + monoglyceride

Question 19

Describe the actions of bile salts.

Answer 19

• emulsify lipids into smaller droplets (splitting x broken down ∵ x bonds broken)
• ↑ s.a.

Question 20

Describe what micelles are and their functions.

Answer 20

• small droplets of monog & fatty acids w/ bile salts

Functions:
1. ↑ s.a. for lipase to work on –> broken down into monog + fatty acids (non-polar- easily diffuse)
2. transport products to mem of epithelium of s.i.

Question 21

List 3 organelles that you would expect to be numerous &/ well-developed in an epithelial cell of the ileum, explain why.

Answer 21

1. ER- resynthesise triglycerides from monog & fatty acids
2. Mitochondria- ATP for co-transport of glucose & a.a.
3. Golgi- form chylomicrons from triglycerides, cholesterol, lipoproteins

Question 22

In addition to having microvilli, state one other feature of the epithelial cells of the ileum that would increase the rate of absorption of amino acids.

Answer 22

↑ no. of protein channels + carriers

Question 23

Describe the path taken by monoglycerides and fatty acids after being absorbed into the epithelial cell.

Answer 23

• ER: reformed into triglycerides
• Golgi: + cholesterol + lipoproteins (+phospholipids) –> chylomicron (vesicle of triglycerides)
• chylomicron released by exocytosis (chylomicron fuses w/ mem)
• enter lacteal (lymphatic vessel) –> bloodstream

Question 24

Explain how bile salts are able to split lipids into smaller droplets.

Answer 24

• 2 ends
• lipophilic (hydrophobic- soluble in fat)- arrange in fat droplets
• lipophobic (hydrophilic- soluble in water)- sticking out
• prevents fat droplets from sticking to each other to form a large droplet

Question 25

Why is the importance that the blood is moving for the uptake of glucose?

Answer 25

• maintains conc. gradient
• glucose moves into blood by f. diffusion