Carbohydrate - digestion and storage

Question 1

What are the carbohydrates in our diet?

Answer 1

Starch

Glycogen
Meat (however when the animal dies enzyme activity in the tissue degrades much of the glycogen stores)

Cellulose and hemicellulose
Plant cell walls – we don’t digest this

Oligosaccharides containing (α1→6) linked galactose
Peas, beans, lentils – not digested

Lactose, sucrose, maltose
Milk, table sugar, beer

Glucose, fructose
Fruit, honey

Question 2

How are carbohydrates digested?

Answer 2

-mouth
salivary amylase hydrolyses
(alpha1 to 4) bonds of starch

stomach- no carb digestion

duodenum- pancreatic amylase works like mouth

small intestine-Final digestion by mucosal cell-surface enzymes

Question 3

What do the different mucosal cell surface enzymes do?

Answer 3

Isomaltase – hydrolyses (α1→6) bonds
Glucoamylase – removes Glc sequentially from non-reducing ends
Sucrase – hydrolyses sucrose
Lactase – hydrolyses lactose

Question 4

What are the main products that gut will actually absorb?

Answer 4

Glucose , galactose and fructose

Question 5

Absorption of glucose?

Answer 5

Glucose is absorbed through an indirect ATP-powered process

ATP-driven Na+ pump maintains low cellular [Na+], so glucose can continually be moved in to the epithelial cells

This system continues to work even if glucose has to be moved into the epithelial cells against it’s concentration gradient (i.e. When blood glucose is high)

Question 6

Absorption of galactose ?

Answer 6

Galactose has a similar mode of absorption as glucose, utilising gradients to facilitate it’s transport

Question 7

Absorption of fructose?

Answer 7

Fructose is slightly different,
Binds to the channel protein GLUT5
Simply moves down it’s concentration gradient (high in gut lumen, low in blood)

Question 8

What are the uses of cellulose and hemicellulose?

Answer 8

increase faecal bulk and decreases transit time
better gut health
cancer of gut if food left for too long in gut

Question 9

What breaks down polymers?

Answer 9

Gut bacteria and yields CH4 and H2

Question 10

Dissacharidase deficiencies causes?

Answer 10

Deficiencies may be genetic#

Or, they can result from,
Severe intestinal infection
Other inflammation of the gut lining
Drugs injuring the gut wall
Surgical removal of the intestine

Question 11

symptoms of dissacharidase deficiences?

Answer 11

abdominal distension and cramps

Question 12

what are the symptoms of someone with disaccharidase deficiency for e.g. lacking lactase activity?

Answer 12

Undigested lactose is broken down by gut bacteria causing gas build up and irritant acids

Lactose is osmotically active, thus drawing water from the gut into the lumen causing diarrhoea

Question 13

Fate of absorbed glucose?

Answer 13

Glc diffuses through the intestinal epithelium cells into the portal blood and on to the liver

Glc is immediately phosphorylated into glucose 6-phosphate by the hepatocytes (or any other cell glucose enters)

Glucose 6-phosphate cannot diffuse out of the cell because GLUT transporters won’t recognise it

This effectively traps the glucose in the cell

Question 14

Characteristics of glucokinase?

Answer 14

in liver
High K m for glucose (low affinity for glucose)
High V max (efficient enzyme)

Question 15

Characteristics of hexokinase?

Answer 15

in other tissues
Low K m (high affinity for glucose)
Low V max (not as efficient enzyme)

Question 16

Why does glucokinase in liver have low affinity for glucose?

Answer 16

Blood [Glc] normal – the liver doesn’t “grab” all of the glucose, so other tissues have it

Question 17

What happens when blood glucose is high after a meal?

Answer 17

Blood [Glc] high (after meal) - liver “grabs” the Glc as it has a high V max

Question 18

What does a high glucokinase V max mean for glucose?

Answer 18

High glucokinase Vmax means it can phosphorylate all that Glc quickly, thus most absorbed Glc is trapped in the liver

Question 19

Why does hexokinase have a high affinity for glucose?

Answer 19

Hexokinase low Km means even at low [Glc] tissues can “grab” Glc effectively

Question 20

Why does hexokinase have a low enzyme efficiency compared to glucokinase?

Answer 20

Hexokinase low Vmax means tissues are “easily satisfied”, so don’t keep “grabbing” Glc

Question 21

Where is most glycogen stored?

Answer 21

90% in liver and skeletal muscle

Question 22

What happens in liver - if blood glucose drops?

Answer 22

-Glycogen broken down into glucose 6 phosphate
- enzyme glucose 6 phosphatase removes phosphate
-now glucose and can enter blood stream

Question 23

What happens in skeletal muscle?

Answer 23

there is no glucose 6-phosphatase,
glycogen to G-6-P (Glycolosis) to lactate

Question 24

what is von Gierke’s disease?

Answer 24

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

High concentrations of glycogen in the liver
Low blood glucose levels
high blood lactate levels

Question 25

Treatment of von gierkes disease?

Answer 25

eating carbs little and often

Question 26

What is McArdle’s disease?

Answer 26

skeletal muscle glycogen phosphorylase problem
high muscle glycogen conc
after exercise- cramps
-no increase in blood glucose after exercise

symptoms dont appear in resting state as muscles can use other energy sources (Glc and fatty acids from blood)

Question 27

Treatment of McArdle’s disease?

Answer 27

Treatment:
Avoid strenuous activity
exercise briefly and then repeat