Carbohydrates Flashcards
What are the major carbohydrates in the diet
monosaccharides
disaccharides
polysaccharides
List some important monosaccharides
glucose
galactose
fructose
List some important disaccharides
maltose
lactose
sucrose
List some important polysaccharides
starch
glycogen
outline the digestion of carbohydrates
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
Outline the absorption of carbohydrates
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
Action and function of hexokinase
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
Action and function of glucokinase
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
Synthesis of glycogen
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
Degradation of glycogen
glucose monomers are removed one at a time from the non-reducing ends as G-1-P
function of glycolysis
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
Process of glycolysis
- phosphorylation of glucose - first rate-limiting step, catalyst = hexokinase
- Conversion of G-6-P to F-6-P,
- phosphorylation of F-6-P to F-1,6-bisP - second rate-limiting step, catalyst = phosphofructokinase -1
- cleavage of F-1,6-bisP
- interconversion of trios sugars
- oxidation of G-3-P to 1,3-bisPG
- transfer from 1,3-bisPG to ADP
- conversion of 3-PG to 2-PG
- dehydration of 2-PG to PEP
- transfer of p from PEP to ADP - third rate-limiting step, catalyst = pyruvate kinase
function of glycogen in skeletal muscle and liver
skeletal muscle - form of energy storage for the muscle
liver - as a store of glucose for use throughout the body
Function of lactate dehydrogenase
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
Function of pyruvate dehydrogenase
is a complex of three enzymes that converts pyruvate into acetyl-CoA by a process called pyruvate decarboxylation
What is the fate of blood lactate
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.
function of gluconeogenesis
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
Process of gluconeogenesis
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
Describe features of carbohydrates
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
Why store glucose in polymers
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
Describe glycoproteins
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
