3.3.3 Digestion and Absorption

digestion

the hydrolysis of large molecules into smaller molecules that can be absorbed and assimilated across cell membranes

CARBOHYDRATES -starch digestion

-in small intestine there is amylase and maltose

starch → maltose

-catalysed by amylase = hydrolyses glycosidic bonds to produce a disaccharide, made in saliva

CARBOHYDRATES -membrane-bound disaccharidases

-enzymes that are attached to the cell membrane of epithelial cells lining the ileum
-break down disaccharides into monosaccharides

maltose → glucose + glucose - catalysed by maltase

sucrose → glucose + frutose - catalysed by sucrase

lactose → glucose + galactose - catalysed by lactase

LIPIDS -lipase

-enzyme that catalyses lipid digestion

-lipids digested in small intestine

-produced in pancreas

-hydrolyses the ester bond found in triglycerides to form fatty acids and monoglycerides

LIPIDS -emulsification

lipids must be emulsified by bile salts produced by liver

-this breaks down large, fat molecules into smaller molecules which are lipid droplets where the monoglycerides and fatty acids stick with bile salts to form micelles

-lipid droplets - increase surface area so faster hydrolysis by lipases

-micelles - releases the monoglycerides and fatty acids closer to the surface of epithelial cell for easier absorption

PROTEINS -different peptidases

proteins - large, complex molecules that are hydrolysed by peptidases

-endopeptidases: hydrolyse peptide bonds between amino acids in central region of protein forming peptide molecules

-exopeptidases: hydrolyse peptide bonds between the amino acids on terminal/end of polypeptide

-dipeptidases: hydrolyse dipeptides into single amino acids

ABSORPTION -monosaccharides + amino acids

through co-transport

-sodium ions actively transported out of ileum epithelial cells, creates sodium ion concentration gradient for diffusion, sodium ions co-transported with glucose/ galactose or amino acids via co-transporter protein so enter blood by facilitated diffusion

ABSORPTION -lipids

-micelles contain monoglycerides and fatty acids and make them more soluble in water then release them close to lining of ileum

-this maintains a high concentration of fatty acids close to lining so are absorbed by diffusion

-triglycerides are formed and vesicles move to the cell membrane

‘Explain why lactose intolerant people suffer with watery faeces and bloating if they drink dairy products’

-do not have the lactase enzyme in small intestine so lactose is not hydrolysed and passes through large intestine

-the gut bacteria then multiplies due to this which produces gas (bloating)

-lactose remaining in large intestine lowers water potential so water stays there causing watery faeces