Digestion and absorption

Question 1

Digestion what it does

Answer 1

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

Large biological molecules in food eg starch/proteins are too big to be absorbed across cell membranes

Digestion breaks them down into smaller molecules eg glucose/amino acids which is then absorbed from the gut into the blood.

Question 2

Digestion of starch (polysaccharide)

Answer 2

Amylase hydrolyses starch to maltose (polysaccharide to disaccharide)

Amylase is produced by salivary glands and released into mouth and produced by the pancreas, released into small intestine

Membrane bound maltase (attached to epithelial cells lining the ileum of the small intestine) hydrolyses maltose to glucose (disaccharide to monosaccharide)

Hydrolysis of glycosidic bond

Question 3

Digestion of disaccharides

Answer 3

Membrane bound disaccharidases, eg maltase, sucrase, lactase (attached to epithelial cells lining the ileum of the small intestine) hydrolyse disaccharides to their constituent monosaccharides

eg maltase – maltose → glucose + glucose
eg sucrase – sucrose → fructose + glucose
eg lactase – lactose → galactose + glucose

Hydrolysis of glycosidic bond

Question 4

Digestion in mammals of lipids by lipase

Answer 4

Bile salts are produced by the liver, emulsify lipids to smaller lipid droplets which increases surface area (to volume ratio) of lipids, speeding up action of lipases

Lipase made in the pancreas, released into small intestine and hydrolyses lipids into monoglycerides and fatty acids

Breaking ester bond

Monoglycerides, fatty acids and bile salts stick together to form micelles

Question 5

Digestion in mammals of proteins by endopeptidases, exopeptidases and membrane-bound dipeptidases

Answer 5

Endopeptidases hydrolyse peptide bonds within a protein/between amino acids in the central region which break proteins into two or more smaller peptides

Exopeptidases hydrolyse peptide bonds at the ends of protein molecules, removing a single amino acid

Dipeptidases are often membrane bound in the ileum and hydrolyse the peptide bond between a dipeptide, resulting in two amino acids

Question 6

Co-transport mechanisms for the absorption of amino acids and of monosaccharides

Answer 6

Sodium ions actively transported out of epithelial cells lining the ileum into the blood via the sodium-potassium pump which creates a concentration gradient of sodium (higher concentration of sodium in the lumen than the epithelial cell)

Sodium ions and glucose move by facilitated diffusion into the epithelial cell from the lumen via a co-transporter protein which creates a concentration gradient of glucose (higher concentration of glucose in epithelial cell than blood)

Glucose moves out of the cell into the blood by facilitated diffusion via a protein channel.

Question 7

Role of micelles in the absorption of lipids

Answer 7

Monoglycerides and fatty acids diffuse out of micelles (in lumen) into the epithelial cells because they are lipid soluble.

Monoglycerides and fatty acids recombine to triglycerides which aggregate into globules

Globules are coated with proteins to form chylomicrons

Leave via exocytosis and enter lymphatic vessels, returning to blood circulation