Digestion and Absorption

what happens during digestion?

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

carbohydrate digestion

• saliva is secreted from the salivary glands and enters the mouth, it contains salivary amylase which hydrolyses the glyccosidic bonds in starch to maltose

• food then enters the stomach where acid denatures the salivary amylase enzyme

• the food then enters the small intestine where it mixes with pancreatic juice containing pancreatic amylase which hydrolyses starch into maltose

• the epithelium lining contains membrane-bound disaccharidease maltase which hydrolyses maltose into alpha glucose

lipid digestion

• lipid digestion begins in the small intestine, the liver produces bile sallts which are stored in the gall bladder and secreted into the small intestine 

• bile salts are secreted into the small intestine where they emulsify lipid globules into micelles, increasung the surface area for lipase action

• lipase enzymes hydrolyse ester bonds in triglycerides producing monoglycerides and fatty acids

lipid absorption before entering the cell

• The products of triglyceride digestion (fatty acids, monoglycerides) remain associated with bile salts to form micelles

• The bile salts make the fatty acids and monoglycerides soluble in water

• The micelles carry the fatty acids and monoglycerides to the cell surface membrane of the ileum epithelial cells and break apart

• This releases the monoglycerides and fatty acids, which are non-polar. They diffuse straight through the phospholipid bilayer into the cell.

• Bile salts stay in the lumen of the ileum to be reused.

lipid absorption after entering the cell

• once inside the cell, the monoglycerides and fatty acids are recombined in the smooth endoplasmic reticulum  to make triglycerides.

• The triglycerides are then sent to the Golgi apparatus via vesicles.

• The Golgi apparatus processes triglycerides into chylomicrons, that also contain protein. Chylomicrons leave the Golgi apparatus via vesicles.

• the vesicles move to and fuse with the cell-surface membrane at the base of the cell, causing the chylomicrons to leave the cell, chylomicrons are taken up by nearby lymph vessels.

micelle function and adaptations

micelles are water soluble tiny droplets containing bile salts and fatty acids and monoglycerides

• They make the products of lipid digestion soluble

• transport these products to the lining of the ileum in epithelial cells

• increases surface area of lipids for lipase action

what are the three types of enzyme involved in protein digestion

• endopeptidases

• exopeptidases

• dipeptidases

how are these enzymes involved in digestion and absorption of proteins?

• hydrolysis of peptide bonds causes proteins to break into smaller chains

• endopeptidases hydrolyse peptide bonds within the polypeptide chain which makes shorter polypeptides

• exopeptidases hydrolyse peptide bonds on the ends of polypeptides producing dipeptides/amino acids

• dipeptidases in the cell surface membrane of ileum epithelial cells hydrolyse dipeptides into amino acids

• the amino acids are absorbed into the epithelial cells by co transport with sodium ions

endo= middle

exo= ends

di= two

describe the mechanism for the absorption of amino acids in the ileum

Amino acids are absorbed into epithelial cells in the ileum by co-transport with sodium ions.
Sodium ions are actively transported out of the epithelial cells into the blood by the sodium-potassium pump.
This maintains a low sodium ion concentration inside the cell, creating a diffusion gradient from the lumen into the epithelial cell.
Sodium ions then diffuse in through co-transport proteins, bringing amino acids with them, and the amino acids move into the blood by facilitated diffusion