[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 (10)

Answer 4

1. bile emulsifies the fat, increasing its surface area and allowing lipase to hydrolyse faster
2. triglycerides are too large to be absorbed in the current form
3. lipase hydrolyses the triglyceride into monoglyceride and 2 fatty acids
4. monoglycerides, fatty acids and bile salts now form a micelle
5. micelle transports them to a cell membrane, breaks apart and releases them there
6. monoglyceride and 2 fatty acids diffuse through membrane as they are smaller
7. monoglyceride and 2 fatty acids reassemble into triglyceride
8. newly formed triglyceride then has proteins added to it in the golgi
9. newly formed chylomicron is transported to the cell membrane in a vesicle
10. it leaves the cell and enters the 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