Carbohydrates 2

Question 1

What carbohydrates do we get from our diets?

Answer 1

Starch, Glycogen, cellulose and hemicellulose, oligosaccharides, disaccharides and monosaccharides

Question 2

Describe the absorption of glucose

Answer 2

Glucose and Na+ from the intestinal lumen passes through a sodium-glucose transporter protein on the apical surface of the epithelial cell.

Question 3

How is a low [Na+] maintained in the epithelial cells of the intestinal lumen?

Answer 3

A sodium-potassium pump on the basal surface of the epithelial cells transports more Na+ out than goes in.

Question 4

Which other monosaccharide is absorbed in a similar way to glucose?

Answer 4

Galactose

Question 5

What is the name of the transporter protein responsible for the passive diffusion of fructose into the wall of the intestine

Answer 5

GLUT5

Question 6

What is the function of cellulose/hemicellulose if it can’t be digested?

Answer 6

Increases faecal bulk and decreases transit time

Question 7

Many western diets are deficient in what type of carbohydrate?

Answer 7

Oligosaccharides (small polymers typically between 2 and 10 subunits large)

Question 8

Disaccharide deficiencies can be genetic or result from what?

Answer 8

1. Severe intestinal infection
2. Other inflammation of the gut lining
3. Drugs injuring the gut wall
4. Surgical removal of the intestine

Question 9

Disaccharide deficiencies are characterised by what symptoms?

Answer 9

Abdominal distention (swelling) and cramps

Question 10

How would a doctor diagnose a disaccharide deficiency?

Answer 10

Testing enzyme activity of lactase, maltase and sucrase

Question 11

What happens to glucose once its absorbed into the blood?

Answer 11

Goes through hepatic portal vein and into liver where it is phosphorylated into G6P. This stops it from leaving the cell as it isn’t recognised by transporters.

Question 12

Which enzymes are involved in phosphorylation of glucose?

Answer 12

Glucokinase (liver) and hexokinase (other tissues)

Question 13

Whats does it mean for an enzyme to have a high VMAX?

Answer 13

Its an efficient enzyme

Question 14

What does it mean for an enzyme to have a low KM?

Answer 14

High affinity for a substrate

Question 15

What happens when glycogen is mobilised in the liver?

Answer 15

Glycogen is converted to G6P which then is converted into glucose by the action of Glucose-6-phosphotase to be released into the blood

Question 16

What key difference is there in mobilisation of glycogen in skeletal muscles compared to into the liver?

Answer 16

Skeletal muscle has no Glucose-6-phosphatase therefore G6P is ‘trapped’ in cell and is respired

Question 17

Describe the first step in the synthesis of glycogen

Answer 17

1. Glycogenin covalently binds glucose from uracil-diphosphate (UDP-glucose) to form chains of approx. 8 glucose monomers
2. Glycogen synthase then extends the Glucose chains

Question 18

Describe step two of glycogen synthesis

Answer 18

The long chains formed in step one are broken by glycogen-branching enzyme and reattached at the 6th carbon position forming 1,6-glycosidic bonds (branches)

Question 19

Describe step one of glycogen mobilisation

Answer 19

Glucose monomers are removed one at a time from the non-reducing ends - relinquishing Glucose-1-phosphate (G1P) - catalysed by phosphorylase

Question 20

Describe step two of glycogen mobilisation

Answer 20

Transferase enzynes remove sets of three glucose residues from branches and adds it to the nearest non-reducing end, producing a linear chain.

Question 21

Describe step 3 of glycogen mobilisation

Answer 21

Glucosidase activity then removes the 1,6-glycosidic bond of the branching glucose residue - releasing a free glucose molecule

Question 22

What is Von Gierke’s disease?

Answer 22

Liver (and kidney, intestine) glucose-6-phophatase deficiency

Question 23

What are the symptoms of Von Gierke’s disease?

Answer 23

High [liver glycogen]

Low [blood glucose] - fasting hypoglycaemia

Question 24

What is the treatment for Von Gierke’s disease?

Answer 24

Regular carbohydrate feeding - little and often every 3-4 hours

Question 25

What is McArdle’s Disease?

Answer 25

Skeletal muscle phosphorylase deficiency

Question 26

What are the symptoms of McArdle’s Disease?

Answer 26

High [muscle glycogen]
Weakness and cramps after exercise
No increase in [blood glucose] after exercise

Question 27

What is the treatment for McArdle’s disease?

Answer 27

Avoid strenuous activity or make use of second wind (brief anaerobic exercise followed by aerobic exercise fuelled by oxidative phosphorylation of fatty acids)