Carbohydrates

Question 1

What are the major carbohydrates in the diet

Answer 1

monosaccharides

disaccharides

polysaccharides

Question 2

List some important monosaccharides

Answer 2

glucose

galactose

fructose

Question 3

List some important disaccharides

Answer 3

maltose

lactose

sucrose

Question 4

List some important polysaccharides

Answer 4

starch

glycogen

Question 5

outline the digestion of carbohydrates

Answer 5

mouth - salivary amylase hydrolyses bonds of starch

stomach - no carbohydrate digestion

duodenum - pancreatic amylase works as in mouth

jejunum - final digestion by mucosal cell-surface enzymes
isomaltase- hydrolyse bonds
glucoamylase - removes glucose sequentially from non-reducing ends
sucrase- hydrolyse sucrose
Lactase - hydrolyse lactose

Question 6

Outline the absorption of carbohydrates

Answer 6

Glucose absorbed through indirect ATP powered process

Na+ ATP-driven pump maintains low cellular Na+, so glucose can continually move into epithelial cell - Glucose symport

Galactose - similar absorption to glucose

Fructose - binds to channel protein GLUTS, moves down concentration gradient

cellulose and hemicellulose - cannot be digested by gut but increase faecal bulk and decrease transit time, polymers broken down by gut bacteria

Question 7

Action and function of hexokinase

Answer 7

Hexokinase catalyzes the phosphorylation of glucose, the rate-limiting first step of glycolysis

A hexokinase is an enzyme that phosphorylates hexoses (six-carbon sugars), forming hexose phosphate

Question 8

Action and function of glucokinase

Answer 8

the glucose sensor in the beta cell by controlling the rate of entry of glucose into the glycolytic pathway (glucose phosphorylation) and its subsequent metabolism

Question 9

Synthesis of glycogen

Answer 9

begins from glycogen by binding covalently glucose from uracil diphosphate (UPD)- glucose to form chains of approx. 8 glucose residues

glycogen synthase takes over and extends the glucose chains

chains formed by glycogen synthase are then broken by glycogen - branching enzyme and re-attached via (a1-6) bonds to give branch points

Question 10

Degradation of glycogen

Answer 10

glucose monomers are removed one at a time from the non-reducing ends as G-1-P

Question 11

function of glycolysis

Answer 11

first metabolic pathway of cellular respiration to produce energy in the form of ATP

for 1 glucose passing through prepatory phase 2 G-3-P formed to enter pay-off phase

for each glucose, 2 ATP are used in prepatory phase and 4 ATP gained in pay-off phase

net 2 ATP and NADH per glucose

Question 12

Process of glycolysis

Answer 12

1. phosphorylation of glucose - first rate-limiting step, catalyst = hexokinase
2. Conversion of G-6-P to F-6-P,
3. phosphorylation of F-6-P to F-1,6-bisP - second rate-limiting step, catalyst = phosphofructokinase -1
4. cleavage of F-1,6-bisP
5. interconversion of trios sugars
6. oxidation of G-3-P to 1,3-bisPG
7. transfer from 1,3-bisPG to ADP
8. conversion of 3-PG to 2-PG
9. dehydration of 2-PG to PEP
10. transfer of p from PEP to ADP - third rate-limiting step, catalyst = pyruvate kinase

Question 13

function of glycogen in skeletal muscle and liver

Answer 13

skeletal muscle - form of energy storage for the muscle

liver - as a store of glucose for use throughout the body

Question 14

Function of lactate dehydrogenase

Answer 14

an enzyme that the body uses during the process of turning sugar into energy for your cells to use

catalyses equilibrium reaction of pyruvate to lactate

found in many of the body’s tissues and organs, including the muscles, liver, heart, pancreas, kidneys, brain and blood cells

Question 15

Function of pyruvate dehydrogenase

Answer 15

is a complex of three enzymes that converts pyruvate into acetyl-CoA by a process called pyruvate decarboxylation

Question 16

What is the fate of blood lactate

Answer 16

In each case the ultimate fate of the lactate is oxidation to ATP, CO2, and H2O by aerobic metabolism

Lactate circulating in the bloodstream can also be transported to the liver, where it is reconverted by the processes of gluconeogenesis/glyconeogenesis into glucose or glycogen, respectively.

Question 17

function of gluconeogenesis

Answer 17

provides glucose when dietary intake is insufficient, from other sources

7/10 glycolysis reactions are reversible

cells bypass these reactions with enzymes that catalyse a separate set of irreversible reactions

Question 18

Process of gluconeogenesis

Answer 18

Bypass reactions- 4 reactions that sidestep the 3 irreversible reactions of glycolysis

reactions A&B- in the mitochondria, pyruvate - oxaloacetate - PEP

reaction C - in cytosol, fructose 1,6-bisP catalyses this hydrolysis

reaction D - in cytosol, dephosphorylation of G-6-P to glucose, straight forward hydrolysis of G-6-P, glucose 6-phosphate is catalyst

Question 19

Describe features of carbohydrates

Answer 19

highly oxidisable - sugar and starch has “high energy” H atom, major source of energy, carbohydrate catabolism is major metabolic process

function to store potential energy - starch in plants, glycogen in animals

have structural and protective functions - extra-cellular matrices of animal cells

contribute to cell-cell communication - ABO blood groups

Question 20

Why store glucose in polymers

Answer 20

compactness

amylopectin and glycogen have many non-reducing ends - allows them to be readily synthesised and degraded to and from monomers respectively, speeds up formation or degredation

polymers from hydrated gels and are not really ‘in solution’ - osmotically active, if free glucose in cells will move down conc. gradient or use huge amount of energy keeping it in cell

Question 21

Describe glycoproteins

Answer 21

Glycosaminoglycans (GAGs) - in mucus and synovial fluid around joints, un-branched polymers made from repeating units of hexuronic acid and an amino-sugar, which attenuate through the chain

Proteoglycan - more carbohydrate the protein, formed from GAGs covalently attaching to proteins, macromolecules found on surface of cells or between cells in extracellular matrix, form part of many connective tissues in the body

Glycoproteins - more protein than carbohydrate, very similar to proteoglycans, found outer plasma membrane and extracellular membrane, also in blood within cells in secretory system