Digestion and absorption and Mobilisation Flashcards
Where does most of the digestion occur?
In the small intestine
What is the difference between amylose and amylopectin?
Amylose= Unbranched glucose subunits
Amylopectin= branched
What is the main enzyme in carbohydrate digestion and where is it found?
What bond does it break?
alpha-amylase
Saliva and pancreatic secretions
Breaks alpha 1,4 glycosidic links. This means that 1,6 linkages are not broken so it yields branched dextrins from amylopectin.
What does isomaltase do?
It is a brush border enzyme in the small intestine that works on the alpha 1,6 linkages in the dextrins to degrade amylopectin to maltose and glucose
What are different types of transporter in the brush border for monosaccharide absorption?
SGLT1- glucose and galactose
GLUT1- glucose
GLUT2- glucose, galactose and fructose
GLUT5- fructose
What are the 2 main enzymes involved in the digestion of proteins?
Endopeptidases- break bonds in the centre of chains
Exopeptidases- Break bonds near the ends of chains
Where does digestion of proteins take place?
In the stomach (pepsin which is an endopeptidase) and the small intestine (enzymes from the pancreas and brush border)
How are amino acids absorbed?
Can occur passively or via facilitated diffusion and active transport mechanisms via Na+ dependent co-transporters
How can small peptides be abdorbed?
Small peptides can be transported into the enterocytes by active transport with H+ ions which are then hydrolysed by intrinsic tripeptides and dipeptidases and then amino acids are then absorbed into the blood
Where is calcium absorbed and how?
Absorbed along the whole length of the small intestine by both passive and active transport.
Calcium binds to a binder protein in the brush border which facilitates diffusion into the enterocyte.
Calcium can then bind to a protein in the cell called calbindin which maintains the concentration gradient and allows high amounts of calcium to be taken up into the cell as insoluble Ca2+ salts are formed in the cell.
Calcium is then pumped across the basolateral membrane by Ca2+ ATPase pump
How does most iron absorption occur?
Through a haem membrane carrier. Fe2+ diffuses into the enterocyte with a haem molecule and then separates inside the enterocyte through an enzyme controlled reaction.
Transport into the blood is then facilitated by ferroportin on the basolateral membrane
(Absorption is mostly in the form of Fe2+ not Fe3+)
What form does iron move around the blood in?
Fe2+ is oxidised to Fe3+ and is then transported in the blood bound to transferrin
Where is vitamin B12 absorbed?
In the terminal ileum
What must happen to avoid B12 being destroyed in the stomach?
It must bind to haptocorrin (R-protein) in saliva to protect it in the stomach
Where is major lipase, cholesterol esterase and phospholipase secreted?
Are they water soluble? What is the significance of this?
In pancreatic juice from the pancreas.
Yes they are water soluble which means that they only work on the surface of fat droplet so if it is large they can’t access fats in the middle of the fat droplets.
What process occurs to overcome the problem of large lipid droplets?
Emulsification- bile salts coat lipid droplets and stop them stick together (coalescing). This keeps fat droplets small and therefore provide a larger surface area for enzymes such as lipase to work.
How are most fatty acids absorbed into the blood?
Most fatty acids are insoluble so are taken up in the form of micelles
What happens to micelles in the enterocytes?
They are repackaged as triglycerides and then combined with proteins in the Golgi to form ‘chylomicrons’. They are then transported to lacteals
What is the unstirred layer? How does this aid fat absorption?
This is a layer of fluid in contact with the epithelial surface that does not mix with the rest of the chyme (gastric juices and partly digested food).
Micelles have to diffuse through this layer. The unstirred layer is more acid closer to the epithelium which promotes a reduction in the ionisation (less charged) of fatty acids making them more easy to absorb across the lipid membrane (as they are uncharged)
Where are bile salts reabsorbed? How?
In the ileum
Ionised bile acids are absorbed by active transport
Unionised acids are fat soluble and therefore can be absorbed passively.
Why can’t fatty acids be used in gluconeogensis?
This is because acetyl coA cannot be converted back into pyruvate as it is a one way reaction. Beta oxidation produces acetyl coA.
You can’t go round the cycle either like you can with amino acids as you aren’t putting excess carbon atoms into the cycle like you are amino acids.
Why are ketones produced?
Ketones are produced when acetyl coA conc is less in need in the Krebs cycle so acetyl coA is essentially in excess.
Ketones are water soluble so they can then be used as a fuel source is the brain (they can cross the blood brain barrier) which means other energy sources (e.g. glucose) are no longer required which preserves them.
