Carbohydrates Flashcards
What are the main functions of carbohydrates in the human body?
- Store of Potential energy
- Structural and protective
- Contribute to cell-cell communication
What are the three main monosaccharides in our diet?
Glucose
Fructose
Galactose
What are the three main disaccharides in our diet?
Lactose
Maltose
Sucrose
What are disaccharides?
Two sugar monomers linked by a glycosidic bond.
What two glucose polymers are constituents of starch?
Amylose and Amylopectin
What is the difference between amylose and amylopectin?
Amylose is a straight chain molecule with only alpha 1-4 glycosidic bonds
Amylopectin is branched and so contains alpha 1-4 and alpha 1-6 glycosidic bonds. It has a more compact structure
How is glucose absorbed into the blood?
Na-Glucose transporter is used to transport 2 sodium ions and 1 glucose molecule from the extracellular space into the epithelial cell. This is driven by high extracellular [Na].
Na+/K+ ATPase is used to pump 3 NA+ out of the cell for every 2 K+ pumped in. This maintains high extracellular [Na]
Glucose uniporter allows downhill efflux of glucose into the blood stream.
Describe the structure of glycogen?
Glucose monomers linked by alpha 1-4 or alpha 1-6 glycosidic bonds to create a more extensively branched structure than starch.
Why is glucose stored as polymers in the body?
- It is more compact
- Non-reducing ends can be readily synthesised or degraded to be used as an energy source on immediate demand.
- Starch is not soluble and so does not upset the osmotic balance
What is the difference between proteoglycans and glycoproteins?
Proteoglycans have a greater carbohydrate than protein content.
Glycoproteins have a greater protein than carbohydrate content.
What two enzymes are used to catalyse the phosphorylation of glucose to glucose-6-phosphate?
Hexokinase and Glucokinase
How do glucokinase and hexokinase differ in their activities?
Hexokinase operates in the cells of the body. It has a low Km and Vmax and so will operate even at very low [Glc]
Glucokinase operates in the cells of the liver. It has a high Vmax and Km and so has very fast activity but will only operate when [glc] is high.
How is glycogen synthesised?
- Glucose from uracil diphosphate-glucose is bound by glycogenin
- Glycogen synthase takes over and extends the glucose chain
- Glycogen branching enzymes break bits off the chain and reattach them as branches using alpha 1-6 glycosidic bonds.
How is glycogen degraded?
- Glycogen phosphorylase removes glucose monomers one at a time to form glucose-1-phosphate
- Transferase activity of debranching enzymes removes branches and reattches them by 1-4 glycosidic bonding to the nearest non-reducing end.
- Leaves unbranched chain for further degradation.
Which three glycolysis reactions are non-reversible?
1) Phosphorylation of glucose (1)
2) Phosphorylation of fructose-6-phosphate (3)
3) Conversion of PEP to Pyruvate (creates ATP) (10)
What catalyst is used in the conversion of glucose-6-phosphate to fructose-6-phosphate?
Phosphohexose isomerase
What reaction does phosphofructokinase catalyse?
Conversion of Fructose-6-phosphate to fructose-1,6-bisphosphate
What catalyst is used to cleave the fructose-1,6-bisphosphate in glycolysis?
Fructose-1,6-bisphosphate aldolase
What are the products of cleavage of fructose-1,6-bisphosphate in glycolysis, and which product is useful?
Dihydroxyacetone phosphate
Glyceraldehyde-3-phosphate (useful product)
What catalyst is used to convert dihydroxyacetone phosphate into glyceraldehyde-3-phosphate in glycolysis?
Triosephosphate isomerase
What is the product of oxidation of glyceraldehyde-3-phosphate in glycolysis?
1,3-bisphosphoglycerate
What catalyst is used in the substrate level phosphorylation using the substrate 1,3-bisphosphoglycerate and what is the product?
Catalyst: Phosphoglycerate kinase
Product: 3-Phosphoglycerate
What catalyst is used to convert 3-PG to 2-PG in glycolysis?
Phosphoglycerate mutase
In what glycolysis reaction is the catalyst enolase used?
Dehydration of 2-PG to for PEP (Phosphoenolpyruvate)
What catalyst is used in the substrate level phosphorylation reaction which sees the conversion of PEP to pyruvate?
Pyruvate kinase
What is the Cori Cycle?
Process whereby lactate, produced by anaerobically respiring muscle, is transported to the liver and converted to glucose. This glucose can then return to the muscle and be reconverted to pyruvate.
What happens to the pyruvate produced by glycolysis in anaerobic conditions?
It is converted to lactate, which enters the Cori cycle. This process allows the oxidation of NADH back to NAD+, allowing further glycolysis to continue.
What happens to the pyruvate produced by glycolysis in aerobic conditions?
Converted to acetyl CoA (and CO2) within the mitochondria of the cell.
What is gluconeogenesis?
Method of glucose synthesis from non-carbohydrate molecules to meet the energy demands of respiring tissue.
What are bypass reactions in gluconeogenesis?
Reactions required to overcome the non-reversible steps of glycolysis.
Where does the final step of gluconeogenesis (to create free glucose) occur?
In the lumen of the ER
What catalyst is required for the conversion of galactose to galactose-1-phosphate?
Galactokinase
What reaction does the enzyme fructokinase catalyse?
Conversion of fructose to fructose-1-phosphate
Where does the citric acid cycle occur?
Mitochondrial matrix
Where do the glycolysis reactions occur?
Cytosol of cell
How is acetyl CoA made?
Pyruvate is broken up into 2C fragments.
These 2C fragments (acetyl groups) combine with Coenzyme A
How does acetyl CoA enter the citric acid cycle?
Acetyl CoA enters the mitochondrial matrix. The Coenzyme A drops the 2C fragment, which combines with 4C oxaloacetate, creating 6C Citrate.
How is oxaloacetate regenerated in the citric acid cycle?
A series of decarboxylase reactions (to create CO2) and dehydrogenase reactions (to form NADH or FADH2)
How many moles of ATP would anaerobic respiration of one mole of glucose yield?
2
How many moles of ATP would full aerobic respiration of one mole of glucose yield?
36
How many moles of ATP are produced per mole of reduced NAD?
2.5/3
How many moles of ATP are produced per mole of reduced FAD?
1.5/2
What are the 5 protein complexes in the electron transport chain?
- NADH Q Oxioreductase
- Succinate Q reductase
- Cytochrome C oxioreducatse
- Cytochrome C oxidase
- ATP synthase
How does the electron transport chain allow storage of energy?
As electrons are passed along the series of protein complexes, protons are pumped from the mitochondrial matrix to the intermembrane space.
This establishes a proton concentration gradient which acts as a store of potential energy.
What is the role of ATPase in the electron transport chain?
To harvest the potential energy which can be stored due to the proton concentration gradient.
NADH cannot cross the mitochondrial membrane. How are the electrons from reduced NAD transported to the protein complexes of the electron transport chain?
Using glycerol phosphate shuttle
How does the glycerol phosphate shuttle operate?
Electrons from reduced NAD are passed to dihydroxyacetone phosphate, creating Glycerol-3-phosphate.
Glycerol-3-phosphate can cross the mitochondrial membrane to pass these electrons to FAD to create FADH2
Name the mobile carriers used in the electron transport chain.
Ubiquinone and Cytochrome C
What is chemiosmosis?
The pumping of electrons form the mitochondrial matrix to the intermembrane space during electron transport along the series of protein complexes.
What is the only site of oxidative phosphorylation in eukaryotes?
Mitochondria
By what mechanism is ATP yielded in the electron transport chain?
Oxidative phosphorylation
What are the functions of the pentose phosphate pathway?
Metabolises the small amount of pentose sugars in the diet
Creates NADPH
