3 Substance exchange- Digestion & Absorption Flashcards
What happens during digestion?
Large biological molecules are hydrolysed into smaller ones that can be absorbed across cell membranes, used for a number of functions
Why can’t biological molecules ingested be transported into the cells?
They’re too large, must first be broken down by hydrolysis
How are carbohydrates digested?
-Begins in the mouth
-Amylase (enzyme produced in salivary glands + pancreas) catalyses hydrolysis of starch into maltose. This breaks a glycosidic bond.
-Membrane bound disaccharidases (enzymes in cell membranes of epithelial cells in ileum; small intestine) catalyse hydrolysation of disaccharides into monosaccharides
-Monosaccharides are transported into epithelial cells in ileum using transporter proteins; glucose plus galactose by co-transporter proteins and fructose by facilitated diffusion
-Once inside, can be used for respiration
How are lipids digested?
Mostly takes place in small intestine
-Lipase (enzyme made by pancreas, released in small intestine) hydrolyses lipids into fatty acids and monoglycerides. This breaks an ester bond
-Bile salts made by the liver, help digest lipids by making small droplets; micelles (emulsification). These have a large surface area, allowing quicker hydrolysis
-Micelles allow monoglycerides and fatty acids to be absorbed by epithelial cells in ileum. They diffuse through the cell surface membrane into the cell cytoplasm
How are proteins digested?
Mostly in small intestine
-Endopeptidases (enzymes that catalyse hydrolysis of bonds) hydrolyse peptide bonds between amino acids in centre of protein, to make polypeptides
-Exopeptidases catalyse hydrolysis of peptide bonds at ends of polypeptide, making dipeptides and amino acids
-Dipeptidases hydrolyse dipeptides into amino acids
-Amino acids are then transported into epithelial cells in ileum by co-transporter proteins
How are amino acids/monosaccharides absorbed?
-Sodium-dependent co-transporter proteins are located in epithelial cell membranes and they actively transport sodium ions into the blood; causing the concentration of sodium ions in epithelial cells to decrease
-This causes sodium ions in the ileum to diffuse down their concentration gradient through a co-transporter protein
-When sodium binds to a co-transporter protein, amino acids/monosaccharides also bind; causing it to undergo a conformational change
-These are all transported into the epithelial cell cytoplasm
-Can now be used inside epithelial cells or absorbed into bloodstream
How are lipids absorbed?
-Micelles move to surface of epithelial cells, break down to release monoglycerides and fatty acids
-Diffuse into epithelial cell (as non-polar)
-Endoplasmic reticulum reforms them into triglycerides, travel to golgi apparatus; packaged into lipoproteins
-Move into lymphatic capillary by exocytosis, then to bloodstream where they’re hydrolysed into triglycerides ready for absorption
