AS Booklet 6- Digestion and Absorption

Question 1

How are carbohydrates digested?

Answer 1

• Salivary amylase in mouth. Starch->maltose (disaccharide). Only small volume of starch broken down bc short time in mouth, chewing increases surface area so hydrolysis is quicker.
• Small intestine- pancreas secretes pancreatic juiced into duodenum (small intestine), contains pancreatic amylase. All starch->maltose.
• Complete hydrolysis of Starch->Alpha glucose occurs in ileum. Maltase in the cell-surface membrane of the epithelial cells lining the ileum hydrolyses maltose to alpha glucose. - Glucose is absorbed by cells forming microvilli using cotransport.

Question 2

How are lipids digested?

Answer 2

Lipids->glycerol, fatty acids and monoglycerides by lipaseZ

• Starts in duodenum, lipase secreted by pancreas. Hydrolyses Ester bonds.
• Rate of hydrolysis of lipids is increased by bile salts (alkaline fluid made by liver, stored in gall bladder, released into duodenum via bile duct.) as they emulsify lipids so increase in surface area but no ester bonds broken.

Question 3

How are proteins digested?

Answer 3

• Begins in stomach. Endopeptidase (peptin, optimum pH of 1-2, HCl secreted by cells of stomach lining) hydrolyses internal peptide bonds to form smaller polypeptides and peptides.
• Pancreas secretes exopeptidase. Hydrolyses terminal peptide bond at either end so individual amino acids can be removed. Eventually = dipeptide.
• Final stage in ileum. Didpeptidases (membrane-bound) in the cell-surface membrane of the cells lining the ileum (microvilli) break down the dipeptides into single peptides (amino acids), which can then be absorbed.

Question 4

What is the structure of the ileum?

Answer 4

Ileum adapted for absorption of digested products:
• Large SA due to long length and villi and microvilli.
• Single layer of epithelial cells = short diffusion pathway.
• Villi contain blood capillaries, absorbs monosaccharides and amino acids = maintains high diffusion gradient.
• Lacteals (lymph vessels) in villi absorb digested lipids = maintains high diffusion gradient.
• Large number of mitochondria present to supply ATP for active transport of digested molecules.
• Carrier and channel proteins for absorption of specific molecules by active transport and facilitated diffusion.

Question 5

What can be found in the cell-surface membrane of the cells lining the inside of the ileum?

Answer 5

Membrane-bound enzymes, specifically disaccharides such as maltase, sucrase and galactose and dipeptidases.

Question 6

How are digested lipids absorbed?

Answer 6

• Monoglycerides and fatty acids associate with bile salts and phospholipids, forming micelles.
• Micelles are smaller than emulsion droplets, and transport poorly soluble monoglycerides and fatty acids to surface of epithelial cell where they can be absorbed.
• Micelles break down next to epithelial cell to release fatty acids and monoglycerides. Non-polar molecules, do can diffuse though phospholipid bilayer of cell-surface membrane (microvilli).
• Short-chained fatty acids diffuse directly into blood from surface.
• Longer fatty acids, monoglycerides and glycerol recombine in epithelial cell to form triglycerides (lipids).
• They’re packaged in protein forming chylomicrons.
• Chylomicrons absorbed into lacteals in villi which eventually drain into blood.

Question 7

How are monosaccharides and amino acids absorbed?

Answer 7

• Initially, high conc. of monosaccharides and amino acids in lumen of ileum and low conc. in epithelial cells so they get absorbed down a concentration gradient by facilitated diffusion involving specific channel or carrier proteins.
• Capillaries next to epithelial cell allow monosaccharides to diffuse into blood and then be carried away, so concentration gradient is maintained and equilibrium is not reached.

Question 8

How does co-transport work?

Answer 8

• When conc. of monosaccharides OR amino acids in epithelial cells increase above that in lumen, cotransport has to move monosaccharides into the cells because the concentration gradient is now the opposite (high conc. in cells to low conc. in lumen of ileum).
• Contransport works against conc. gradient to absorb molecules into cells.
• Monosaccharides or amino acids taken into cells along with NA+.
• High conc. of NA+ in lumen, low in cell due to capillaries taking NA+ away from cell constantly so concentration gradient.
• NA+ moves along conc. gradient into cell via carrier protein and monosaccharide or amino acid follows.
• Na+ actively transported through carrier proteins into capillaries to maintain conc. gradient, using a sodium-potassium pump which used ATP.
• Monosaccharide or amino acid passes into blood capillary by facilitated diffusion.