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31 Cards in this Set

  • Front
  • Back

Which form of carbs is the natural form?

D

What are the sources of carbs?

60% plant storage - starch


small amounts of glycogen - <1% from animal muscle and liver


about 25% in western diets comes from sucrose

Glycogen and starch comparison

Glycogen


- storage in mammals


- highly branched




Starch


- storage in plants


- 2 forms amylose and amylopectin


- amylose is straight and forms coils


- amylopectin is more branched

Amylose and cellulose comparison

Amylose


- Poly 1,4- alpha D glucopyranoside


- broken down in mouth and intestine


- glucose generated (also maltose)




Cellulose


- poly 1,4- beta D glucopyranoside (beta bond if the only difference)


- principle component in dietary fibre


- limited digestion in lower intestines, only possible due to bacterial enzymes

What is the difference between alpha and beta linkages?

Alpha = away so the 6th carbon and the OH group on the first carbon point in opposite directions




Beta = they point in the same direction

Uses of mono and disaccharides

- energy


- glycosylation of proteins (proteoglycans and glycoproteins)


- structural components (GAGs, glycolipids)


- carbon skeletons for biosynthesis of amino acid

Lipids - biological role (uses)

Membranes - phospholipids


energy reserves - TAGs


Vitamins and hormones (steroids)


Lipophillic bile


Protection of organisms



FAs state at physiological pH

FAs are readlily ionised at physiological pH and therefore give negative charge to molecules and fluids.

Lipid digestion

Stomach - lingual lipaseand gastric lipase attack TAGs and hydrolyse small amount of FAs




Small intestine - acid chyme (stomach contents) stimulates mucosa cells to release hormone which stimulates the pancreas and gall bladder to release digestive enzymes and bile respectively.




Other mucosal cells release secretin which causes the pancreas to release bicarbonate rich fluid to neutralise the chyme.



Reactions of lipid digestion

TAG


- pancreatic lipase


2 FAs + Mono acylglycerol




Cholesteryl ester


- cholesteryl ester hydrolase


Cholesterol + FA




Phospholipids


- Phospholipase A2


lysophospholipid + FA


- lysophospholipase


Glycerol Phosphoryl Lipase +FA

Lipid absorption

Enzymic digestion creates polar products that form micelles of FAs, monoacylglycerol and cholesterol. These are absorbed.

Lipid transportation in circulation

Lipids are insoluble in plasma. In order to be transported they form balls called chylomicrons with specific proteins (apoproteins).




Chylomicrons have the hydrophilic areas on the outside and contain hydrophobic proteins and lipids on the inside (TAGs and cholesterol).

Function of apoproteins

Other than forming chylomicrons apoproteins also have:


- structural role


- binding site for receptors


- activators and co-enzymes for enzymes involved in lipid metabolism

Classes of lipoproteins

Chylomicron


- transport of dietary TAGs




Very low density lipoprotein


- transport of biosynthesised TAGs




Low density lipoproteins


-Delivers cholesterol to peripheral tissue




High density lipoproteins


- removes used cholesterol and returns it to the liver

What are chylomicrons?

Assembled in intestinal mucosal cells


Enter lymphatic system and then blood


Contain mostly TAG but pick up apoproteins from HDL in circulation




Function is to transport dietary TAG to adipose or muscle for storage or energy



Breakdown of chylomicrons

Chylomicron circulates until they associates with lipoprotein lipase (enzyme on surface of cell). This is mediated by apoprotein C2 which is picked up from HDL in circulation.




Lipoprotein lipase hydrolyses the TAG and the FAs diffuse into the local tissues.




With the TAG gone the chylomicron shrinks and forms a chylomicron remnant. It dissociates from the lipoprotein protease and returns the C2 to the HDL.




The remnant is taken up by the liver.

What are VLDLs?

Synthesised in the liver. Contains mostly TAG but also a significant amount of cholesterol and cholesterol ester.




Function is to transport endogenously synthesised TAG to extra hepatic (liver) tissue where it can be stored as fat or to muscle for energy. The cholesterol is delivered to extrahepatic tissues once the VLDL has been metabolised to LDL.




Metabolism is essentially the same as chylomicrons

What are LDLs?

Formed by metabolism of VLDLs




Transports cholesterol to peripheral tissue

Role of LDL in atherosclerosis

Endothelium is damaged


LDLs penetrate vascular wall, deposit in the wall and are damaged by oxidation.


Oxidised LDLs attract macrophages which ingest LDL.


Macrophages become overloaded with lipid and become "foam" cells which die and release lipid in the vessel wall (forming plaques).


Plaque matures to atherosclerotic plaque.


Grows and constricts vessel.


ETC ETC

What are HDLs?

Act as a reservoir for apoproteins which can be donated or received by other lipoproteins


Also play a vital role in scavenging "used" cholesterol




HDLs are taken up by the liver and degraded, the cholesterol is secreted as bile salts or repackaged in VLDLs for redistribution.

What are bile salts?

Excess cholesterol in liver is convert to bile salts. They are the polar constitute of the major excretory pathway of cholesterol.




They are stored in the gall bladder and secreted into the small intestine to emulsify lipids.




Almost all is reabsorbed and transported back to the liver. 1-2g is lost each day.




Fibre can bind to bile salts and prevent i's reabsorption, this is one of the health benefits of fibre.





Where does carb digestion take place?

Oral cavity


- mechanical


- chemical - salivary amylase (from the serous glands)




Digestion in GI tract


- chemical - enzymatic

Features of salivary amylase

Optimal pH6.7


Active in oral cavity and in stomach for about 2 hours (until pH drops too far)


Hydrolyses alpha 1,4 linkages of starch to oligosaccharides

Features of pancreatic amylase

Secreted from exocrine acini into duodenum


Optimal pH 6.7-7


Hydrolysis of 1,4 alpha bonds


Responsible for digestion of more complex carbohydrate that take longer to breakdown to di or oligosaccharides

What are brush border enzymes?

Enzymes found on surface of villi


Maltase, sucrase, lactase


optimal pH 7-8


Hydrolyse disaccharides to monosaccharides

Carbohydrate absorption

Occurs in the small intestine (mainly the middle section)


Villi and micro villi increase surface area to facilitate absorption


Glucose and galactose enter the epithelial cells of the intestine via active transport using sodium dependent co-transport (it is the sodium potassium pump that requires energy)


Glucose and galactose move into the blood stream via facilitated diffusion through GLUT2.

How is fructose absorbed?

Facilitated diffusion in and out of epithelium


GLUT5 and GLUT2

Factors affecting carb absorption

State of mucosa - faster through intact mucosa


Thyroid hormones - increase absorption


Mineralocorticoid increases rate of absorption


Sodium concentration - increase absorption

Where are insulin and glucagon secreted from?

Insulin - beta cells of Islets of Langerhans


Glucagon - alpha cells

Structure and synthesis of insulin (needs checking)

Small protein


2 polypeptide chains (A and B) linked by disulphide bond




Synthesis:


Proproinsulin ---RER----> Proinsulin ---TGN---> Insulin




Stored as an inactive hexamer (monomer in active form)

Effects of diabetes

Intracellular


- glucogenesis and gluconeogenesis


- breakdown of fats


- ketone body production - diabetic ketoacidosis


- decrease protein synthesis = body wastage, lethargy, increased appetite


- susceptibility to infection, impaired wound healing




Extracellular


- changes to osmotic gradient = dehydration of cells = hyperglycaemic coma


- blood glucose higher than renal threshold = urine has high sugar


- increased urination


- increased thirst