digestion and absorption Flashcards
Explain what happens in digestion
Large (insoluble) biological molecules hydrolysed to smaller (soluble) molecules
● That are small enough be absorbed across cell membranes into blood
Describe the digestion of starch in mammals
● Amylase (produced by salivary glands / pancreas) hydrolyses starch to maltose
● Membrane-bound maltase (attached to cells lining ileum) hydrolyses maltose to glucose
● Hydrolysis of glycosidic bond
Describe the digestion of disaccharides in mammals
● Membrane-bound disaccharidases hydrolyse disaccharides to 2 monosaccharides:
○ Maltase - maltose → glucose + glucose
○ Sucrase - sucrose → fructose + glucose
○ Lactase - lactose → galactose + glucose
● Hydrolysis of glycosidic bond
Describe the digestion of lipids in mammals, including action of bile salts
● Bile salts (produced by liver) emulsify lipids causing them to form smaller lipid droplets
● This increases surface area of lipids for increased / faster lipase activity
● Lipase (made in pancreas) hydrolyses lipids (eg. triglycerides) → monoglycerides + fatty acids
● Hydrolysis of ester bond
Describe the digestion of proteins by a mammal
● Endopeptidases - hydrolyse internal (peptide) bonds within a polypeptide → smaller peptides
○ So more ends / surface area for exopeptidases
● Exopeptidases - hydrolyse terminal (peptide) bonds at
ends of polypeptide → single amino acids
● Membrane-bound dipeptidases - hydrolyse (peptide)
bond between a dipeptide → 2 amino acids
● Hydrolysis of peptide bond
Suggest why membrane-bound enzymes are important in digestion
Membrane-bound enzymes are located on cell membranes of epithelial cells lining ileum
● (By hydrolysing molecules at the site of absorption they) maintain concentration gradients for absorption
Describe the pathway for absorption of products of digestion in mammals
Lumen (inside) of ileum → cells lining ileum (part of small intestine) → blood
Describe the absorption of amino acids and monosaccharides in mammals
Co-transport:
● Na+ actively transported from
epithelial cells lining ileum to
blood (by Na+/K+ pump)
● Establishing a concentration gradient of Na+
(higher in lumen than epithelial cell)
● Na + enters epithelial cell down
its concentration gradient with
monosaccharide or amino
acid against its concentration
gradient
● Via a co-transporter protein
● Monosaccharide or amino
acid moves down a
concentration gradient into
blood via facilitated diffusion
Describe the absorption of lipids by a mammal, including the role of micelles
● Bile salts combine with monoglycerides and fatty acids to form micelles
○ Micelles make monoglycerides and fatty acids (more) soluble in water
○ Micelles carry fatty acids and monoglycerides to cells lining the ileum, where they break down to release them
○ This maintains a high concentration of fatty acids and monoglycerides near cells lining the ileum
*fatty acids are absorbed (into epithelial cell) by diffusion (as they’re lipid soluble)
● Triglycerides reformed in (epithelial) cells and turn into globules
● Globules coated with proteins forming chylomicrons which are then packaged 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
Describe the process of starch digestion [5
(salivary/pancreatic) Amylase;
Starch to Maltose;
Maltase;
Maltose to glucose;
Hydrolysis;
Glycosidic bonds
Describe the processes involved in the absorption and transport of digested lipid molecules from the ileum into lymph vessels. [5]
*Micelles contain bile salts and fatty acids/monoglycerides;
*Make fatty acids/monoglycerides more soluble (in water) OR Bring/release/carry fatty acids/monoglycerides to cell/lining (of the iluem)
*Fatty acids/monoglycerides absorbed by diffusion; REJECT Micelle enters cell
*Triglycerides (re)formed (in cells); (Smooth Endoplasmic Reticulum)
*(Golgi) Vesicles move to cell membrane;
Describe the role of micelles in the absorption of fats into the cells of the ileum. [5]
*Micelles include bile salts and fatty acids;
*Make the fatty acids (more) soluble in water;
*Bring/release/carry fatty acids to cell/lining (of the ileum);
*Maintain high(er) concentration of fatty acids to cell/lining (of the ileum);
*Fatty acids (absorbed) by diffusion;
Describe the role of enzymes in the digestion of proteins in a mammal. [4]
(Reference to) hydrolysis of peptide bonds;
*Endopeptidase act in the internal of protein/polypeptide.
*Exopeptidases act at end of protein/polypeptide
*Dipeptidase acts on dipeptide/between two amino acids produces (single) amino acids;
The action of endopeptidases and exopeptidases can increase the rate of protein digestion. Describe how. [2]
Exopeptidases hydrolyse peptide bonds at the ends of a polypeptide/protein AND endopeptidases hydrolyse internal peptide bonds within a polypeptide/protein;
More ‘ends’ OR More surface area;
Describe and explain two features you would expect to find in a cell specialised for absorption. [4]
*Folded membrane/microvilli so large surface area (for absorption);
Accept ‘brush border’ for ‘microvilli’.
*Large number of co-transport /carrier/channel proteins so fast rate (of absorption)
*Large number of mitochondria so make (more) ATP (by respiration) OR
Large number of mitochondria to release energy for active transport;
*Membrane-bound (digestive) enzymes so maintains concentration gradient (for fast absorption);
