3.3 digestion and absorption

What happens in digestion

• large insoluble biological molecules hydrolysed into smaller soluble molecules

• These are then small enough to be absorbed across cell membranes into blood

Describe the digestion of starch in mammals

• amylase hydrolyses starch to maltose

• Membrane bound maltose, which is attached to cells lining ileum, hydrolyses maltose to glucose

• Hydrolysis of glycosidic bond

Describe the digestion of disaccharides in mammals

• membrane-bound disaccharides hydrolyse disaccharides to 2 monosaccharides

• Hydrolysis of glycosidic bonds

Give examples of disaccharides and the monosaccharides they are made from

(And their name as membrane bound disaccharides)

Glucose+glucose = maltose (maltose)

Glucose+fructose = sucrose (sucrase)

Glucose+galactose = lactose (lactase)

Why are bile salts produced, where is lipase made

Bile salts= produced by liver

Lipase= made in pancreas

Describe the digestion of lipids, including bile salts

• bile salts emulsify lipids causing them to form smaller lipid droplets

• This increases surface area of lipids for increased/faster lipase activity

• Lipase hydrolyses lipids (triglycerides —> monoglycerides + fatty acids)

• Hydrolysis of ester bond

Describe the digestion of proteins in mammals

• endopeptidases hydrolyse internal peptide bonds, within a polypeptide, making peptides smaller

• So there are more ends/ surface area for exopeptidases

• Exopeptidases hydrolyse internal peptide bonds at the ends of polypeptides, turning them into single amino acids

• Membrane bound dipeptidases hydrolyse peptide bonds between a dipeptide

• Hydrolysis of peptide bond

Why are membrane-bound enzymes important in digestion

• membrane-bound enzymes are located on the cell membrane of epithelial cells lining ileum

• By hydrolysing molecules at the site of absorption they maintain a concentration gradient for absorption

Describe the pathway for absorption of products in digestion of mammals

• lumen/ inside of ileum

• Cells lining ileum (part of small intestine)

• Blood

How are amino acids and monosaccharides absorbed in mammals

Co transport

Describe the absorption of amino acids and monosaccharides by co-transport

• Na actively transported from epithelial cells lining the ileum to the blood by Na/K pump

• Establishing a concentration gradient of Na (higher in lumen than cell)

• Na enters epithelial cell down its concentration gradient, with monosaccharide/ amino acid going against its conc gradient

• Via a co-transport protein

• Monosaccharide/ amino acid moves down a conc gradient into blood via facilitated diffusion

Describe the absorption of lipids, including the role of micelles

• bile salts combine with monoglycerides and fatty acids to form micelles

• Monoglycerides/ fatty acids are absorbed into epithelial cell by diffusion, since they are lipid soluble

• Triglycerides reformed in epithelial cells and aggregate into globules

• Globules coated with proteins forming chlyomicrons, which are then packed into vesicles

• Vesicles move to cell membrane and fuse with it, releasing chylomicrons via exocytosis

• Chylomicrons enter lymphatic vessels and eventually return to blood circulation

What do micelles do

• make monoglycerides and fatty acids more soluble in water

• Carry fatty acids and monoglycerides to cells lining the ileum, where they break down and release them

• This maintains a high conc gradient of fatty acids and monoglycerides near the cells lining the ileum

Where is amylase produced

Salivary glands/ pancreas