Carbohydrates 1-2

Question 1

Name the types of carbohydrates in the diet.

Answer 1

Monosaccharides
Disaccharides
Polysaccharides

Question 2

Describe starch as a carbohydrate within the body.

Answer 2

Starch is a polysaccharide composed of 2 glucose monomers; amylose, amylopectin. It has many non-reducing ends which are acted on when starch is broken down.

Question 3

Describe glycogen as a carbohydrate within the body.

Answer 3

Glycogen is a polymer of glucose (a1-4) linked subunits with branches of (a1-6). This means it ultimately has more glucose molecules than starch, and is also more extensively branched.

Question 4

Describe the reasoning behind the storage of glucose in polymers.

Answer 4

Compactness
Amylopectin and glycogen have many non-reducing ends, allows them to be readily synthesised or degraded to and from monomers.
Polymers are OSMOTICALLY INACTIVE. If free glucose was in the cell, Glc (inside)&raquo_space; Glc (outside). The glucose would move out of the cell down the concentration gradient, or the cell would use lots of energy trying to keep within the cell.

Question 5

Describe glycoproteins generally.

Answer 5

Proteins that have carbohydrates covalently bonded.
Most extracellular eukaryotic proteins (membrane proteins or secreted proteins) have associated carbohydrate molecules.

Question 6

Describe GAGs (glycosaminoglycans).

Answer 6

Found in mucus and synovial fluids. These are unbranched polymers that are made from repeating units of hexuronic acid and an amino acid sugar, which alternate through the chains. The fact that they are unbranched allows them to be “slippery” meaning that they are responsible for the lubrication of mucus and synovial fluid

Question 7

Describe proteoglycans.

Answer 7

The carbohydrate content is more than protein content.
Formed from GAGs covalently bonding to their “core” protein. They are macromolecules found on the surface of cells or in between cells in the EC matrix.
Possess a disorganised structure, which prevents them from meshing together, which allows them to form connective tissue.

Question 8

Give a general description of mucopolysaccharidoses.

Answer 8

Mucopolysaccharidoses = genetic conditions caused by absence or malfunction of enzymes that are required for breakdown GAGs.
Causes a build up of GAGs in connective tissue, blood or other cells of the body.
The build up damages cellular architecture and function.
This can cause inflammation of joints, or stunt bone growth.

Question 9

Name the main carbohydrates in our diet.

Answer 9

Starch
Glycogen
Cellulose (don’t digest)
Hemicellulose (don’t digest)
Oligosaccharides containing (a1-6) linked galactose (don’t digest)
Lactose
Sucrose
Maltose
Glucose
Fructose

Question 10

Briefly describe the digestion of carbohydrates, (until the jejunum).

Answer 10

Food enters the mouth, and salivary amylase hydrolases the (a1-4) bonds of starch.
There is no carbohydrate digestion in the stomach.
In the 1st of the 3 parts of the small intestine (Duodenum), pancreatic amylase works as in the mouth.
In the Jejunum, this is where final digestion by mucosal cell-surface enzymes.

Question 11

Describe the absorption of glucose into epithelial cells.

Answer 11

In the fed state (just after a meal), there will be high dietary glucose in the gut, which means there is high Na+(Cl+). So the sodium-glucose symport allows 2 sodium to pass down its concentration gradient from the intestinal lumen to the epithelial cell, carrying one glucose with it.
ATP-driven Na+ pump maintains the low cellular sodium, so that glucose can continually be moved into the epithelial cells.

Question 12

Describe the absorption of fructose.

Answer 12

Fructose is different and binds to channel proteins GLUT5, they simply move down their concentration gradient, (high in gut lumen, low in blood).

Question 13

Describe how the indigestible carbohydrates are absorbed.

Answer 13

Cellulose and hemicellulose.
Both are not digested, they increase faecal bulk and decrease transit time.
Oligosaccharides are also not digested, lack of them in the diet can lead to poor health, due to food not being digested and lingering in the body for some time - this has been proved to cause some cancer.
These polymers are broken down by gut bacteria; yielding methane and hydrogen.

Question 14

Describe the fate of absorbed glucose once it has moved into the epithelial cells.

Answer 14

Glucose diffuses through the intestinal epithelium cells into the portal blood and on to the liver.
Glucose is then immediately phosphorylated into glucose-6-phosphate by the hepatocytes of the liver.
Glucose-6-phosphate cannot diffuse out of the cell because GLUT transporters wont recognise it.
The glucose is now essentially trapped within the cell.

Question 15

Describe the actions and functions of glucokinase.

Answer 15

Glucokinase phosphorylate glucose to produce glucose-6-phosphate.
Glucokinase has a high Km, which means it has a low affinity for glucose, so it doesn’t “grab” glucose when the blood glucose levels are normal.
However glucokinase has a high Vmax which indicates it is an efficient enzyme and can rapidly change glucose to G-6-P.
Primarily found in the liver.

Question 16

Describe the actions and functions of hexokinase.

Answer 16

Hexokinase (tissues)
Has a high affinity for glucose, and is an inefficient enzyme.
Even at low glucose levels, tissues can grab glucose effectively, the low Vmax means tissues are ‘easily satisfied’ so don’t keep grabbing for glucose.

Question 17

Describe the 1st step of the synthesis of glycogen.

Answer 17

Glycogen does not form from glucose monomers.
Glycogenin (the central enzyme) begins the process by covalently binding the glucose from uracil-diphosphate (UDP)-glucose to form chains of approximately 8 glucose residues, these are primers.
Then glycogen synthase takes over and extends glucose to the growing chain.

Question 18

Describe the 2nd step of the synthesis of glycogen (after glycogen synthase has taken over).

Answer 18

The chains formed by glycogen synthase of approximately 8 glucose residues, are then broken down by glycogen-branching enzyme and reattached via (a1-6) bonds to give branch points.

Question 19

Describe the degradation of glycogen.

Answer 19

The degradation or mobilisation of glycogen:
The enzyme glycogen phosphorylase catalyses the transfer of phosphate groups from glucose residues.

The glucose near the branch is removed in a 2-step process by de-branching enzyme.

Transferase activity of de-branching enzyme removes a set of 3 glucose residues and attached them to the nearest non-reducing end via (a1-4) bond, this is straightening out the chain.

Glucosidase activity then removes the final glucose by breaking a (a1-6) linkage to release free glucose.
This leaves an unbranched chain, which can be further degraded or built upon as needed.

Question 20

Compare the functions of glycogen in skeletal muscle and the liver.

Answer 20

In skeletal muscle there is no glucose-6-phosphatase, so the G-6-P progresses to lactate via glycolysis. The production of lactate also generates ATP, which is utilised for muscle contraction.
THIS IS SUBSTRATE LEVEL PHOSPHORYLATION.
In the liver the blood glucose falls and the glycogen is converted to G-6-P. The G-6-P is then converted to glucose by Glucose - 6 -Phosphatase. The glucose is excreted into the blood. Ultimately raising blood glucose levels.

Question 21

Describe the function of glycolysis.

Answer 21

Main function is to split glucose into two, generating the only energy available to generate when they lack O2 (exercising muscle) or mitochondria (RBC).

Question 22

Describe the phases of the process of glycolysis.

Answer 22

Phase 1 = preparatory phase (step 1 -4).
Phase 2 = pay-off phase (step 5-10).
For each glucose passing through the preparatory phase, 2 molecules of G-3-P are formed, and 2 ATP are used, with 4 ATP gained in pay-off stage.
=energy generation, net gain of 2 ATP.

Question 23

Describe the preparatory phase of glycolysis.

Answer 23

Once glucose is inside our cells, glycolysis occurs.
Glucose is phosphorylated by hexokinase. (use of 1 ATP) to give:
G-6-P, this is converted by phosphohexose isomerase to:
F-6-P, which is phosphorylated by phosphofructokinase-1 (PFK-1). (use of 1 ATP). The phosphorylation gives:
F-1,6bisP, which is then cleaved by aldolase to give 2 different 3C triose sugars:
G-3-P, at this stage there is an interconversion of these triose sugars by triose phosphate isomerase.
In the prep phase there was 2 ATP used and there were 2 irreversible steps.

Question 24

Describe the final digestion of carbohydrates in the jejunum.

Answer 24

The final digestion is carried out by mucosal cell-surface enzymes:
1 - the (a1-6) bonds are hydrolysed.
2 - Glc is removed sequentially from non-reducing ends.
3 - Sucrose is hydrolysed
4 - Lactose is hydrolysed.

Question 25

Describe the resulting products of the digestion of carbohydrates.

Answer 25

The main products are;
Glucose, galactose and fructose.
These are absorbed by the gut.

Question 26

Describe the absorption of glucose from epithelial cells to the blood.

Answer 26

Glucose is absorbed through an indirect ATP powered process.
GLUT 2 transporters transport glucose across the basal surface down its concentration gradient.

Question 27

Describe the absorption of galactose.

Answer 27

Galactose has a similar mode of absorption as glucose, utilising gradients to facilitate its transport.

Question 28

Describe the pay-off phase of glycolysis.

Answer 28

The first step of the pay-off phase(6).
NAD+ is used and NADH is produced to oxidise G-3-P with the enzyme glyceraldehyde-3-phosphate dehydrogenase to produce:
1,3-bisPG, which transfers its phosphate group by phosphoglycerate kinase to ADP, generating 1 (2) ATP and producing:
3-PG, which is converted by phosphoglycerate mutase to 2-PG by transferring the Pi from the 3rd to 2nd carbon.
2-PG is dehydrated by enolase to give:
PEP, which transfers its phosphate group to ADP, generating ATP. This final step produces pyruvate by the enzyme pyruvate kinase.

Question 29

Describe the function of lactate dehydrogenase.

Answer 29

At the end of glycolysis, pyruvate has many different fates, one of which being fermentation to lactate, this lactate is used in exercising muscle. (lactate dehydrogenase) Another fate of pyruvate is conversion to Acetyl CoA, which is then used in the citric acid cycle.
All of pyruvates fates will produce NAD+ to replenish the NAD+ required for the reduction of intermediate metabolites.

Question 30

Describe the fate of blood lactate.

Answer 30

Lactate is the end product of anaerobic metabolism and is disposed of by gluconeogenesis into the liver. Higher [blood lactate] leads to lactic acidaemia , this is because the lactate produced in skeletal muscle cannot be converted to glycogen in the liver.

Question 31

Describe the by-pass reactions involved in gluconeogenesis.

Answer 31

Reaction 1, 3 and 10 of glycolysis are irreversible due to their high -ve delta G.
These by-pass reactions allow for independent control of glycolysis and gluconeogenesis.

Question 32

Describe the A+B by-pass reactions involved in gluconeogenesis.

Answer 32

Pyruvate is inserted into the mitochondria, where it is able to convert itself to oxaloacetate, which then coverts to malate. Malate is then able to leave the mitochondria, where it converts back to oxaloacetate.
This oxaloacetate is then used in the next step with PEP.

Question 33

Describe by-pass reaction C in gluconeogenesis.

Answer 33

The phosphate group is removed from fructose - 1,6 - bisphosphate, to give fructose - 1 - phosphate and Pi.
There is no ATP produced.

Question 34

Describe the by-pass reaction D in gluconeogenesis.

Answer 34

This involves the dephosphorylation of G-6-P to glucose. The reaction is just the straightforward hydrolysis of G-6-P.

Question 35

Describe the fates of absorbed galactose and fructose.

Answer 35

Galactose and fructose can enter glycolysis at various points.
The body does not have pathways for the catabolism of these sugars, Most fructose and galactose is metabolised by the liver. (converted back to glucose as needed).