3.3.3 - digestion and absorption

Question 1

what happens during digestion?

Answer 1

large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes

Question 2

what are the different types of carbohydrase enzymes?

Answer 2

amylase, maltase, sucrase and lactase

Question 3

carbohydrases - amylase?

Answer 3

produced in the mouth and pancreas (either salivary or pancreatic)
it hydrolyses the alternate glycosidic bonds of a starch molecule to produce the disaccharide maltose

Question 4

carbohydrases - maltase?

Answer 4

produced by the lining of the ileum
hydrolyses maltose into the monosaccharide alpha glucose

Question 5

carbohydrases - sucrase?

Answer 5

produced by the lining of the ileum
hydrolyses the single glycosidic bond in a sucrose molecule to produce alpha glucose and fructose

Question 6

carbohydrases - lactase?

Answer 6

produced by the lining of the ileum
hydrolyses the single glycosidic bond in a lactose molecule to produce alpha glucose and galactose

Question 7

what is a membrane-bound disaccharidase, and what are examples?

Answer 7

a carbohydrase which digests disaccharides and is part of the cell-surface membranes of the epithelial cells which line the ileum (but not released into the lumen)
eg. maltase, sucrase, lactase

Question 8

what are the different types of protease/petidase enzymes?

Answer 8

endopeptidase, exopeptidase, dipeptidase

Question 8

what happens during the process of carbohydrates being digested?

Answer 8

1. saliva enters the mouth from the salivary glands and is mixed with food in chewing
2. salivary amylase in the saliva begins hydrolysis starch to maltose, and contains mineral salts which keep the pH at around neutral (optimum)
3. food is swallowed and enters the stomach - the acid in it denatures the amylase to prevent further hydrolysis of starch
4. food passes into the small intestine and is mixed with pancreatic juice
5. pancreatic amylase in the pancreatic juice hydrolyses any remaining starch to maltose, and alkaline salts maintain the pH at neutral
6. the food is pushed along the ileum by muscles in the intestine wall, and maltase in the epithelial cells hydrolyses the maltose to alpha glucose

Question 9

peptidases - endopeptidase?

Answer 9

hydrolyses the peptide bonds between amino acids in the central region of a protein molecule, forming a series of peptide molecules

Question 10

peptidases - exopeptidase?

Answer 10

hydrolyses the peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidase, which progressively releases dipeptides and single amino acids

Question 11

peptidases - dipeptidase?

Answer 11

hydrolyses the bond between the two amino acids of a dipeptide
dipeptidases are membrane-bound (part of the cell-surface membrane of the epithelial cells lining the ileum)

Question 12

what are lipases?

Answer 12

enzymes produced in the pancreas which hydrolyse the ester bonds in triglycerides to form fatty acids and monoglycerides

Question 13

what is a monoglyceride?

Answer 13

a glycerol molecule with a single fatty acid molecule attached

Question 14

what is the role of bile salts in lipid digestion?

Answer 14

produced in the liver, they split lipids up into tiny droplets called micelles in the emulsification process
this increases the surface area of the lipids so the action of lipases is sped up
(they do this before lipases digest the molecule further)

Question 15

how is the ileum of the small intestine structured?

Answer 15

the wall is folded and has villi (projections of about 1mm long)
the villi have thin walls, and inside each one there is a thin epithelium, then blood capillaries, then the lacteal

Question 16

how do the properties of the villi increase the efficiency of the absorption of the products of digestion?

Answer 16

increase SA for diffusion
have thin walls, reducing diffusion distance
contain muscle allowing them to move - maintains diffusion gradient because as they absorb projects of digestion from food adjacent to them, new food with more products replaces them
have good blood supply so blood can carry away absorbed molecules, maintaining the concentration gradient
epithelial cells which line the villi have microvilli which further increase surface area

Question 17

how are amino acids and monosaccharides absorbed in the small intestine?

Answer 17

via co-transport
amino acids are absorbed in exactly the same process as glucose molecules - they can move instead of glucose molecules through the sodium-glucose co-transporter from the lumen into the epithelial cells

Question 18

what are micelles?

Answer 18

tiny structures around 4-7nm diameter formed when the monoglycerides and fatty acids produced during digestion remain in association with the bile salts which initially emulsified the lipid droplets

Question 19

what are chylomicrons?

Answer 19

special particles adapted for the transport of lipids

Question 20

how are triglycerides absorbed in the small intestine?

Answer 20

1. micelles come into contact with the epithelial cells lining the villi of the ileum
2. they break down releasing the monoglycerides and fatty acids - these are non-polar molecules so they easily diffuse across the cell surface membrane into the cells
3. once in the cells, the monoglycerides and fatty acids are transported the ER and recombined to triglycerides
4. firstly in the ER and then the Golgi apparatus, the triglycerides associate with cholesterol and lipoproteins, forming chylomicrons
5. the chylomicrons move out of the epithelial cells via exocytosis and enter lymphatic capillaries called lacteals found at the centre of each villus
6. from the lacteals, the chylomicrons pass into the lymphatic vessels and then the bloodstream
7. the triglycerides in the chylomicrons are hydrolysed by an enzyme in the endothelial cells of blood capillaries, then diffuse into cells