Carbohydrate and Protein Digestion Flashcards
What form of carbohydrates are able to be digested?
Monosaccharide hexose sugars (6C) - glucose, galactose and fructose
What are disaccharides?
Two monosaccharides linked together by a glycosidic bond which are broken down by enzymes to be absorbed
Give 3 examples of disaccharides
Lactose
Sucrose
Maltose
What enzyme breaks down lactose and what does it become?
Lactase: lactose -> glucose + galactose
What enzyme breaks down sucrose and what does it become?
Sucrase: sucrose -> glucose and fructose
What enzyme breaks down maltose and what does it become?
Maltase: maltose -> glucose + glucose
Give 3 examples of a polysaccharide
Starch
Cellulose
Glycogen
What is starch?
Plant storage form of glucose, with glucose monomers being linked by alpha-1-4 glycosidic bonds
Can starch be digested by humans?
Yes, bonds hydrolysed by amylase in saliva and pancreas
What is cellulose?
Constituent of plant cell walls, with glucose monomers are linked by beta-1-4 glycosidic bonds
Can cellulose by digested by human?
No, as we don’t produce enzyme cellulase
What is glycogen?
Animal storage form of glucose, with glucose monomers linked by alpha-1-4 glycosidic bonds
What are the two membranes across the epithelium of the small intestine?
Apical (brush border) and bass-lateral membrane
What occurs to nutrients after passing through the wall of the small intestine?
Most move into capillaries, and as all gut tube venous blood passes through the hepatic portal system, absorbed material travels through liver for filtration
Fats pass into the lacteal and through the lymphatic systems which drains into the venous system
Describe process of monosaccharides (and galactose) absorption across intestinal epithelial cells
- Apical Na-glucose symporter transports the two molecules into the cell, but only if both are present in lumen
- Na is pumped straight out of the cell via Na-K – ATPase transporter
- Once the concentration of glucose in the cell exceeds the blood, glucose diffuses into the blood via the GLUT-2 transporter
- The uptake of nutrients into the bloodstream increases the osmostic gradient, so water moves thought the highly water permeable tight junctions and into the bloodstream
Where does most absorption occur and what shape are the epithelial cells?
Small intestine - cuboidal epithelial cells with villi
How is fructose absorbed?
It is not Na-dependent, and moves across the apical membrane by facilitated diffusion via the GLUT5 transporter, and into the blood by GLUT2 transporter on the basolateral membrane
No water drawn in by osmosis as it is immediately broken down in the blood and therefore no gradient formed
How are proteins broken down?
Enzymes hydrolyse the peptide bonds between the amino acids
Enzymes: proteases and peptidases
What are the two groups of enzymes for protein digestion?
Endopeptidases and exopeptidases
What are endopeptidases?
Protease enzymes: breaks down peptide bond in the interior of the AA chain and break a long peptide chain into smaller fragments
What are exopeptidases?
Peptidase enzymes: acts on terminal amino acids on either end and removes one AA at a time
Aminopeptidases: works on amino-terminal end
Carboxypeptidases: works on carboxylate-terminal end
How are amino acids absorbed?
Most free AA absorbed via Na-dependent co-transport, but some transporters are H dependent
How are AA absorbed via H dependent transporters?
Some epithelium exists in an acid micromlimate where H ions to maintain a low pH
- Dipeptides and tripeptides are transported via PepT1 which uses H-dependent co-transport.
- The H- ion taken up is then used in the NHE3 transporter also on the apical membrane, to pump Na into the cell
- Na is moved out of the cell via the Na-K ATPase
- AA moved out of cell into blood
