[6.10] absorption of the products of digestion

Question 1

how are the products of carbohydrate and protein digestion initially absorbed?

Answer 1

• polysaccharides are digested into monosaccharides and polypeptides are digested into amino acids, which are small enough to be absorbed
• glucose and amino acids are initially absorbed by facilitated diffusion as there is a higher concentration of them in the small intestine lumen than in the blood

Question 2

what happens when facilitated diffusion of glucose or amino acids cannot happen?

Answer 2

• facilitated diffusion cannot happen when there is a higher concentration of glucose / amino acids in blood than in the small intestine lumen
• the remaining molecules are then absorbed into the blood by co-transport

Question 3

describe the co-transport of glucose and amino acids (5)

Answer 3

1. Na+ ions are actively pumped out of the epithelial cell into the blood through the sodium potassium pump. this requires ATP
2. now there is a higher Na+ concentration in the small intestine lumen compared to the epithelial cells
3. Na+ ions do facilitated diffusion through a sodium-glucose co-transport protein into the epithelial cell, and carry a glucose / AA molecule in, against their concentration gradient. no energy is needed
4. now there is a higher concentration of glucose / AAs in the epithelial cell than the blood
5. glucose / AAs now do facilitated diffusion down the concentration gradient into the blood, through a protein channel

Question 4

absorption of lipids from the small intestine (9)

Answer 4

1. lipase hydrolyses triglyceride into MG + 2 FAs
2. monoglycerides + FAs + bile salts form a micelle
3. micelle transports them to a cell membrane, breaks apart and releases them there
4. MG + 2 FAs diffuse through membrane
5. monoglyceride + 2 FAs reassemble into a triglyceride
6. proteins are added to triglyceride in the golgi
7. this forms a chylomicron
8. chylomicron is transported to cell membrane in a vesicle
9. chylomicron leaves cell, enters lymphatic system by exocytosis

Question 5

what happens to the lipids after it enters the lymphatic system?

Answer 5

• lymphatic system drains fat into bloodstream then capillaries
• lipase enzyme embedded in cell membrane of endothelial cells hydrolyse triglyceride into monoglyceride + fatty acids
• these monoglycerides + fatty acids diffuse via simple diffusion into tissues and body cells

Question 6

what does a micelle consist of?

Answer 6

• monoglycerides
• fatty acids
• bile salts

Question 7

what is the role of a chylomicron?

Answer 7

to transport lipids in the lymphatic system by holding the non-polar triglycerides inside them

• contained within a vesicle
• made from phospholipids and lipoproteins

Question 8

brief summary of absorption of products of lipid digestion

Answer 8

• intestine
• epithelial cell
• lacteal
• lymphatic vessel
• drain into blood stream
• capillaries