Carbohydrates

Question 1

Monosaccharides

Answer 1

Simple sugar units (6C-hexoses), e.g. glucose and fructose found in fruit and honey.

Question 2

Disaccharides

Answer 2

Formed by 2 monomers linked by glycosidic bonds between the OH group and the anomeric C. E.g. maltose and lactose which are reducing sugars found in beer and milk. E.g. sucrose a table sugar making up 25% of our intake. There is no free anomeric C so is non-reducing.

Question 3

Polysaccharides

Answer 3

Polymers of medium to high molecular weight. They are distinguished by the identity of repeating units, length, type of bonding amd amount of branching exhibited.
Homopolysacchrarides; single monomeroc species.
e.g. starch which contains 2 types of glucose (amylose and amylopectin) with many non-reducing ends branching every 24-30 residues, found in cereals, potatos, rice. E.g glycogen formed by many glucose sub-units with branching every 8-1w residues found in meat. 90% in liver and skeletal muscle.
Heteropolysaccharides; 2+ monomer species

Question 4

Cellulose and hemicellulose

Answer 4

Found in plant cell walls and aren’t digested.

Question 5

Oligosaccharides

Answer 5

Alpha1-6 galactose found in peas, beans and lentils which are not digested.

Question 6

Digestion of carbohydrates

Answer 6

1. Mouth; amylose hydrolyses a1-4 bonds of starch.
2. Duodenum; pancreatic amylase hydrolyses a1-4 bonds of starch.
3. Jejunum; final digestion by mucosal cell-surface enzymes. Isomaltase hydrolyses a1-6 bonds, glucoamylase removed glucose sequentially from non-reducing ends, sucrase hydrolyses sucrose and lactase hydrolyses lactose. The main products are glucose, galactose and fructose.

Question 7

Absorption of carbohydrates

Answer 7

Monosaccharides;

1. Glucose is absorbed through an indirect ATP-powered process. ATP driven Na+ pump maintains low cellular [Na+] so glucose can continually be moved into epithelial cells even against a concentration gradient.
2. Galactose is similarly absorbed to glucose utilising gradients to facilitate transport.
3. Fructose binds to channel protein GLUT5 and moves down it’s concentration gradient.

Cellulose and hemicellulose; cannot be digested by the gut but it increases faecal bulk and decreases transit time for food to pass through the gut .

Question 8

Hexokinase

Answer 8

Found in other tissues. It has a low Km and a low Vmax.
Hexokinaseis the initial enzyme ofglycolysis, catalyzing the phosphorylation of glucose by ATP to glucose-6-P. It is one of the rate-limiting enzymes ofglycolysis.
Due to it’s high affinity even at low levels of [glc] it can grab it effectively and due it’s low Vmax tissues are easily satisfied.

Question 9

Glucokinase

Answer 9

Found in the liver, it has a high Km and a low Vmax. Glucokinase(EC 2.7.1.2) is an enzyme that facilitates phosphorylation of glucose to glucose-6-phosphate. Due to its low affinity to glucose when blood [Glc] is normal then the liver won’t grab it all leaving it for other cells. If blood [Glc] is high the liver will phosphorylate it quickly and traps it in the liver.

Question 10

The synthesis of glycogen

Answer 10

Step 1.
UDP (carrier molecule) glucose&raquo_space;glycogenin» Glucose&raquo_space;glycogen synthase» extended glucose chains

Step 2
Glc chains&raquo_space;glycogen branching enzyme» glycogen. This enzyme breaks the chains and re-attaches them via (a1-6) bonds to give branch points.

Question 11

Degradation of glycogen

Answer 11

Glycose monomers are removed one at a time from the non-reducing ends as G-1-P. As glycogen is heavily branched this happens rapidly.
Glycogen&raquo_space;glycogen phosporylase» G-1-P»de-branching transferase» removed 3Glc residues and non-reducing end (a1-4)
G-1-P»glucosidase» glc by breaking (a1-6). Leaving an unbranched chain which can be further degraded or built upon. We very rarely fully degrade glycogen as it is so big and we are constantly adding to it as well as degrading it.

Question 12

Function of glycogen in the liver

Answer 12

A decrease in [Glc] causes glycogen to be broken down to form glucose which is release into the blood.

Question 13

Function of glycogen in skeletal muscle

Answer 13

A decrease in [blood Glc] causes glycogen to be broken down via glycolysis to form lactate produces ATP. This is substrate-level phosphorylation.

Question 14

Function of glycolysis

Answer 14

The cellular degredation of glucose to yield ATP as an energy source through substrate level phosphorylation. Anaerobically is creates lactic acid to produce ATP. Aerobically it forms pyruvate.

Question 15

Lactate dehydrogenase and pyruvate dehydrogenase

Answer 15

Lactate degydrogenase catalyses pyruvate to lactate. Pyruvate dehydrogenase catalyses pyruvate to acety CoA. These enzymes are required to re-generate NAD+ maintaining redox balance.

Question 16

Descrive the fate of blood lactate

Answer 16

Lactate is produced in the absence of O2 in working skeletal muscle or in RBCs that don’t contain mitochodria. Lactate is converted back to glc in the liver by gluconeogenesis whoch repays the O2 debt run up by muscles.

Question 17

Precursors for gluconeogenesis

Answer 17

Lactate, pyruvate, glucerol (TAG), alanine, glutamine, CAC intermediates.

Question 18

Function of gluconeogenesis

Answer 18

The process of synthesising glucose from non-carbohydrate sources. It produces glucose after fasting or lots of exercise.

Question 19

Location of gluconeogenesis

Answer 19

Liver

Question 20

Gluconeogeneis steps

Answer 20

1. Conversion of pyruvate or lactate to phosphoenolpyruvic acid (PEP) via oxaloacetate.
2. Hydrolysis to cleave F-1,6-bisP into F-6-P. (Cytoplasm)
3. Dephosphorylation/hydrolysis of G-6-P to Glc (lumer on ER)

Question 21

Fate of absorbed galactose and fructose

Answer 21

Both are common dietary carbohydrates however the body does not have catabolic pathways for them. Most fructose is metabolised by the liver and they can both enter glycolysis at various points.

Question 22

Function of the pentose-phosphate pathway

Answer 22

• It produces NADPH (captures electrons) for all organisms.
• In the liver it is used for FA synthesis, steroid synthesis and drug metabolism.
• mammary gland: FA synthesis
• Adrenal cortex: steroid synthesis
• RBC: antioxidant -It produces pentose (5C sugar)
• is a precursor of ATP, RNA and DNA. It metabolises small amount of dietary pentoses.
• digestion of nucleotides.

The NADPH links the pentose-phosphate pathway (catabolic) with other anabolic processes.