Digestion

Question 1

How are carbohydrates digested? (From mouth to intestine)

Answer 1

1. Mouth -amylase
2. Upper stomach - gastric acid
   (low pH inactivates amylase)
3. Small intestine - pancreatic amylase (95%)
   (pancreatic amylase then converts carbs to disaccharides, maltose and a mix of short, branches chains of glucose molecules)
   3b. enzymes of BB of small intestine break products down into monosaccharides. then transported across intestinal epithelium into blood. (fructose is transported via GLUT molcule, glucose and glactose couple of Na+)
4. They enter interstitial fluid via facilitated diffusion with GLUT proteins
5. They then diffuse into the blood through capillary pores

Question 2

Describe the 3 mains structures of carbs

Answer 2

1. Monosaccharides. single sugars, glucose, fructose, galactose
2. Oligosaccharides. several sugars, lactose, sucrose
3. Polysccharides, starch glycogen

Question 3

What glycosidic links does glycogen have?

Answer 3

alpha 1-4 glycosidic links

branching alpha 1-6 glycosidic links

Question 4

How are proteins digested? (From mouth to intestine)

Answer 4

-broken into dipeptides, tripeptides and amino acids, absorbed by small intestine
(N group in amino acids = urea)
1. Stomach, pepsin, Hcl, renin, gelainase
2. Small intestine, trypsin & chymotrypsin
-peptideases breakdown peptides into amino acids in intestinal epithelial cells
-small peptides can be broken into amino acids by BB ezymes in lumen
3. most dipeptides and tripeptides are absorbed via secondary transport coupled to H+ grad
4. free amino acids enter via secondary amino acid transporters and allow amino acids to diffuse into interstitial fluid through capillary pores
5. energy produced by Na+/K+ pump
6. a few proteins travel via endo and exocytosis

(infants can absorb more proteins than adults - they must absorb antibodies)

Question 5

How are fats digested? (From mouth to intestine)

Answer 5

1. Duodenum
   - pancreatic lipase - hydrolyses 1 and 3 bonds with ease but 2 bonds at a slower rate
   - Lipase acts on fats that have been emulsified but cannot act without the protein colipase (this is secreted by the pancreas, it binds lipase to the surface of the droplet)
   - Pancreatic esterase hydrolyses cholesterol esters
2. Intestinal lumen

A. fats are finely emulsified by detergent action of BILE SALTS lecithin and monoglycerides

• EMULSIFICATION - large droplets broken into small droplets to increase SA and rate
  i) motility of GI tract = mechanical disruption
  ii) emulsifying agent, phospholipids and bile salts in bile-

B. lipids and bile salts act spontaneously to form MICELLES, these generally contain free fatty acids, monoglycerides and cholesterol (they act as a ‘holding station’)
-micellar formation further solubilizes the lipids and provides a mechanism for their transport to the BB by diffusion

C. during their passage into the epithelial cell they are resynesized into triglycerides is maintains conc grad for molecules into cells
-veciles pinch off the endoplasmic recticulum fusing with the plasma membrane releasing fat molecules

D. Pass into LACTEALS and eventually make it to the systemic veins