PoH: Carbohydrates

Question 1

Name the 6 major carbs in our diet and where we find them

Answer 1

Starch – in cereals, potatoes, rice

Glycogen – in meat (however when animal dies, enzyme activity degrades much glycogen)

Cellulose and hemicellulose – plant wall cells (indigestible)

Oligosaccharides containing (a1=>6) linked galactose – peas, beans, lentils (indigestible)

Monosaccharides – glucose and fructose (not galactose) in fruit, honey

Disaccharides – lactose in dairy, sucrose in table sugar and maltose in beer (we don’t eat much maltose)

Question 2

How do we absorb monosaccharides?

Answer 2

Glucose and galactose are absorbed through indirect ATP-powered process. ATP-driven Na+ pump maintains low cellular 0Na+] so glucose can continually be moved in to the epithelial cells. This works even if glucose has to be moved into epithelial cells against its concentration gradient (ie if blood glucose is high).

Question 3

Which carbohydrate has a similar absorption pathway to glucose?

Answer 3

Galactose

Question 4

How do we absorb fructose?

Answer 4

It binds to the channel protein GLUTS and follows the concentration - high in gut lumen, low in blood

Question 5

What happens to cellulose and hemicellulose

Answer 5

It can’t be digested by the gut. They increase faecal bulk and decrease transit time. Polymers are broken down by gut bacteria, yielding CH4 and H2

Question 6

What 5 things can cause a disaccharide deficiency?

Answer 6

Genetics
Severe intestinal infection
Inflammation of gut lining
Drugs injected in gut wall
Surgical removal of intestine

Question 7

How can we diagnose a disaccharide deficiency

Answer 7

Enzyme tests to check for lactase, maltase or sucrase

Question 8

Intolerance to what is a disaccharide deficiency and what two things could cause it?

Answer 8

Lactose

It can be caused by undigested lactose being broken down by gut bacteria, causing gas build up

It can be caused by lactose drawing water from the gut into the lumen causing diarrhoea

Question 9

What 4 enzymes in the jejunum metabolise carbohydrates?

Answer 9

Isomaltase – hydrolyses (a1=>6) bonds
Glucoamylase – removes GIc sequentially from non-reducing ends
Sucrase – hydrolyses sucrose
Lactase – hydrolyses lactose

Question 9

What do hexokinase and glucokinase catalyse glucose into?

Answer 9

Glucose 6-phosphate

Question 9

Briefly outline the digestion and absorption of carbs

Answer 9

Mouth - salivary amylase hydrolises in starch
Stomach - no carb metabolism
Duodenum – pancreatic amylase works, just like in mouth
Jejunum – final digestion by mucosal cell-surface enzymes

Question 9

What 2 enzymes catalyse glucose, and where does it occur?

Answer 9

Glucokinase in the liver
Hexokinase in other tissues

Question 10

Contrast glucokinase and hexokinase in terms of affinity for glucose and rate of reaction

Answer 10

Glucokinase
- Low affinity for glucose
- Rapid rate of reaction

Hexokinase
- High affinity for glucose
- Slow rate of reaction

Question 10

Under what circumstances dos the liver “grab” glucose?

Answer 10

When blood glucose is high

Question 10

Describe the 3 fates of G6P

Answer 10

Undergo glycolysis
The pentose pathway
Converted to glycogen for skeletal muscle

Question 11

Synthesis of glycogen

Answer 11

Glycogenin takes glucose from uracil-diphosphate glucose (UDP) to form chains of approx. 8 glucose residues. Glycogen synthase takes over and extends the glucose chains

The chains formed by glycogen synthase are broken by glycogen-branching enzyme and reattached via (a1=>6) bonds to give branch points

Question 12

Degradation of glycogen

Answer 12

Glucose monomers are removed one at a time from non-reducing ends as glucose-1-phosphate (G1P)
A de-branching enzyme removes branches

In the cell, the G1P is easily converted to glucose, so it’s released in liver or used by muscles as ATP

Cannot diffuse as GLUT transporters won’t recognise it.

When blood glucose is normal, the liver doesn’t “grab” all glucose, so other tissues have it. When blood glucose is high, the liver “grabs” glucose as it has a rapid rate of reaction.

Question 13

Describe the function of glycogen in the liver

Answer 13

If blood glucose falls
Glycogen is converted to G6P
G6P is converted to glucose via enzyme G6Pase
Glucose then diffuses into the blood

Question 14

Describe the function of glycogen in skeletal muscle

Answer 14

Glycogen is converted to G6P
Glycolysis/substrate level phosphorylation occurs
ATP and lactate is generated

ATP is used by muscle contraction
Lactate is waste product