new carbohydrates

Question 1

Why are carbohydrates a major energy source?

Answer 1

They are highly oxidisable - high energy H atom-associated electrons

Question 2

Name 3 important monosaccharides.

Answer 2

Glucose, galactose and fructose.

Question 3

Name 3 important disaccharides.

Answer 3

Sucrose, lactose and maltose.

Question 4

Which disaccharides are reducing and which are non-reducing?

Answer 4

Maltose and lactose are reducing sugars. Sucrose is a non-reducing sugar.

Question 5

Which glucose polymers are found in starch (plants)?

Answer 5

25% amylose and 75% amylopectin. Amylose is unbranched (alpha 1,4), amylopectin is branched and unbranched (alpha 1,4 and 1,6)

Question 6

Where is glycogen in the body?

Answer 6

90% in the liver and skeletal muscle. Liver: blood glucose falls? G6P –> glucose released into the blood. Skeletal muscle: no G6P –> glycogen –> G6P by glycolysis –> lactate.

Question 7

What are GAG’s (mucopolysaccharides) and where are they found?

Answer 7

Un-branched polymers made from repeating units of hexuronic acid and an AA sugar. Found in mucus and in synovial fluid around joints.

Question 8

Give an example of a mucopolysaccaridoses.

Answer 8

Hurler syndrome.

Question 9

Define the 9 major carbohydrates in the diet.

Answer 9

Glucose, fructose, sucrose, maltose, lactose, starch, glycogen and undigested hemi-cellulose and cellulose.

Question 10

Describe the digestion of carbohydrates.

Answer 10

Mouth: salivary amylase hydrolyses alpha 1,4 bonds of starch. Stomach: no digestion. Duodenum: pancreatic amylase hydrolyses alpha 1,4 bonds. Jejunum: final digestion by mucosal cell-surface enzymes isomaltase, glucoamylase, sucrase and lactase.

Question 11

How are glucose and galactose absorbed?

Answer 11

Glucose via an ATP-driven Na pump that moves Na out and brings glucose in, even against its concentration gradient. Galactose also uses concentration gradients.

Question 12

How is fructose absorbed?

Answer 12

Binds to GLU5 and moves down its concentration gradient (high in gut lumen, low in blood).

Question 13

How are hemicellulose and cellulose absorbed?

Answer 13

They cannot be digested. They increase faecal bulk. Gut bacteria can break them down a bit and yield methane and H2.

Question 14

What is the most common disaccharidase deficiency?

Answer 14

Lactose intolerance. Lactase in lacking and so digestion of milk causes abdominal distension, cramps etc. Lactose in osmotically active - diarrhoea.

Question 15

What do glucokinase and hexokinase do?

Answer 15

Digested glucose diffuses through intestinal epithelial cells into the portal blood then onto the liver. Glucose is immediately phosphorylated into G6P in cells so it is trapped. Glucokinase does this in the liver and hexokinase in other tissues.

Question 16

Describe the Km and Vmax for glucokinase and hexokinase.

Answer 16

Glucokinase: high Km and high Vmax. Good enzyme but low affinity for glucose - most absorbed glucose trapped in liver. Hexokinase: low Km low Vmax. High affinity for glucose so when concentrations are low in the blood it can still find some. Not a good enzyme.

Question 17

How is glycogen synthesised?

Answer 17

Glycogenin binds to glucose via UDP. Glycogen synthase takes over and extends the chains. Chains are broken and reformed (alpha 1,6) by glycogen branching enzyme.

Question 18

How is glycogen degraded?

Answer 18

Glucose monomers removed 1 at a time from the non-reducing end as G1P (glycogen phosphorylase). G1P converted to G6P. Liver? G6-phosphatase –> glucose –> blood. Skeletal muscle? Substrate-level phosphorylation for muscle contraction to produce lactate.

Question 19

What is Von Gierke’s disease?

Answer 19

Liver, kidney and intestine G6-phosphatase deficiency. G6P cannot be broken down to glucose so all glucose must come from the diet. High lactate in blood and hypoglycaemia

Question 20

What is McArdle’s disease?

Answer 20

Skeletal muscle phosphorylase deficiency. High blood glycogen, weakness and cramps after exercise.

Question 21

How is energy made when cells lack energy or don’t have mitochondria?

Answer 21

Glycolysis - substrate-level phosphorylation.

Question 22

Where does glycolysis occur?

Answer 22

The cytosol.

Question 23

What are the 2 phases of glycolysis?

Answer 23

Preparatory and payoff

Question 24

What is the net gain of ATP and NADH per glucose molecule in glycolysis?

Answer 24

2 ATP and 2 NADH.

Question 25

What are the first 3 steps of glycolysis?

Answer 25

1) Glucose –> G6P by hexokinase, 2) G6P –> F6P by phosphokinase isomerase, 3) F6P –> F-1,6-bisP by PFK1 - first committed step of glycolysis

Question 26

What are the 2 splitting reactions of glycolysis (steps 4 and 5)?

Answer 26

4) Cleavage of F-1,6-bisP into 2 x 3C sugars, 5) interconversion so both sugars are G3P

Question 27

Which reactions produce NADH and ATP in glycolysis?

Answer 27

6) oxidation of G3P (G3P dehydrogenase) to 1,3-bisPG, 7) P transfer from 1,3-bisPG to ADP (PG kinase). These reactions are energy-coupled.

Question 28

What is the difference between substrate-level and respiration-linked phosphorylation?

Answer 28

Substrate level: soluble enzymes and chemical intermediates. Respiration-linked: membrane bound enzymes and gradients of protons.

Question 29

What are the final 3 steps of glycolysis?

Answer 29

8) conversion of 3PG –> 2PG by PGmutase, 9) dehydration by endolase of 2PG –> PEP, 10) transfer of P from PEP to ADP by pyruvate kinase. Pyruvate and 2 ATP are produced.

Question 30

How can NAD+ be regenerated?

Answer 30

Niacin (vitamin) and lactate (made from pyruvate). NAD+ is essential for glycolysis.

Question 31

What happens to excess pyruvate in humans?

Answer 31

Can be reduced by lactate dehydrogenase into lactate.

Question 32

What is the cori cycle?

Answer 32

During strenuous exercise, ATP is made via substrate-level phosphorylation. Lactate made then converted to glucose in the liver by gluconeogenesis.

Question 33

What happens to pyruvate in cells with excess O2?

Answer 33

Pyruvate is oxidised to acetyl CoA in the mitochondria.

Question 34

Which tissues rely completely on glucose as their source of energy?

Answer 34

Brain, NS, RBC’s, testes and embryonic tissues.

Question 35

Which steps in glycolysis are irreversible?

Answer 35

1, 3 and 10.

Question 36

What is the final step of gluconeogenesis that makes free glucose?

Answer 36

G6Phosphatase converts G6P to glucose in the ER.

Question 37

How many bypass reactions are there in gluconeogenesis and what is their function?

Answer 37

There are 4 bypass reactions. They sidestep the 3 irreversible reactions of glycolysis. They utilise the cytosol and mitochondria.

Question 38

How are fructose and galactose metabolised?

Answer 38

The body doesn’t have distinct pathways for them. Fructose is mostly metabolised by the liver. Fructose is broken down via the F1P pathway to F1P then G3P. Galactose is converted to G1P.

Question 39

What is the function of the pentose-phosphate pathway?

Answer 39

Produces NADPH and pentoses - 5C sugars which are precursors for ATP, DNA and RNA.

Question 40

What are the 2 phases of the pentose-phosphate pathway?

Answer 40

1) oxidative/irreversible: generates NADPH (lots of NADPH required), 2) reversible/non-oxidative: interconverts G6P and PP to makes lots of different sugars (lots of nucleotides required)

Question 41

What happens when alcohol is ingested?

Answer 41

NAD+ is reduced to NADH in the liver. Gluconeogenesis is inhibited –> lacticacidaemia and hypoglycaemia.

Question 42

What is black water fever?

Answer 42

G6P dehydrogenase deficiency. Causes haemolytic anaemia.