Carbohydrates 2

Question 1

What carbohydrates are there in our diets?

Answer 1

• Starch
• Glycogen
• Cellulose and hemicellulose
• Ogliosaccharides containing (a1-6) linked galactose
• Lactose, sucrose, maltose
• Glucose, fructose

Question 2

Where in the body does carbohydrate digestion take place?

Answer 2

• Mouth
• Duodenum
• Jejunum

Question 3

Describe the digestion of carbohydrates in the mouth

Answer 3

Salivary amylase hydrolyses (a1-4) bonds of starch

Question 4

Describe the digestion of carbohydrates in the duodenum.

Answer 4

Pancreatic amylase works as in the mouth

Question 5

Describe the digestion of carbohydrates in the jejunum.

Answer 5

Final digestion by mucosal cell-surface enzymes:

• Isomaltase: hydrolyses (a1-6) bonds
• Glucoamylase: removes Glc sequentially from non-reducing end
• Sucrase: hydrolyses sucrose
• Lactase: hydrolyses lactose

Question 6

What are the main products of carbohydrate digestion?

Answer 6

• Glucose
• Galactose
• Fructose

Question 7

How is glucose absorbed?

Answer 7

• 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 its concentration gradient

Question 8

How is galactose absorbed?

Answer 8

Similar mode of absorption to glucose utilising gradients to facilitate its transport

Question 9

How is fructose absorbed?

Answer 9

• Binds to the channel protein GLUT5

- Simply moves down its concentration gradient

Question 10

What is the function of cellulose and hemicellulose?

Answer 10

-They cannot be digested by the gut so instead they increase faecal bulk and decrease transient time

Question 11

When the polymers of cellulose/hemicellulose are broken down, what is yielded?

Answer 11

• CH4

- H2

Question 12

What can disaccharidase deficiencies result fro?

Answer 12

• Genetic
• Severe intestinal infection
• Other inflammation of the gut lining
• Drugs injuring the gut wall
• Surgical removal of the intestine

Question 13

What are disaccharidase deficiencies characterised by?

Answer 13

Abdominal distension and cramps

Question 14

How are disaccharidase deficiencies diagnosed?

Answer 14

• Enzyme tests of intestinal secretion

- Usually checking for lactase, maltase or sucrose activity

Question 15

What is lactose intolerance?

Answer 15

• Most common disaccharide defiency

- If lactase is lacking, then ingestion of milk will give disaccharidase deficiency symptoms

Question 16

Why do symptoms of disaccharidase symmptoms occur in lactose intoerance

Answer 16

• 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 17

How can the symptoms of lactose intolerance be avoided?

Answer 17

• Avoiding milk products
• Using milk products treated with fungal lactase
• Supplementing diet with lactase

Question 18

What happens to glucose after it is absorbed?

Answer 18

• Glc diffuses through the intestinal epithelium cells into the portal blood and on to the liver
• Glc is immediately phosphorylated into G-6-P by the hepatocytes
• G-6-P cannot diffuse out of the cell because GLUT transporters won’t recognis eit

Question 19

What are the enzymes catalyst involved in the phosphorylation of glucose to G-6-P?

Answer 19

• Gluokinase (liver)

- Hexokinase (other tissues

Question 20

What does high Vmax mean?

Answer 20

Efficient enzyme

Question 21

What does low Km mean?

Answer 21

High affinity for substrate

Question 22

What happens when blood Glc is normal?

Answer 22

The liver doesn’t (grab) all of the glucose so other tissue have it

Question 23

What happens when blood Glc is high?

Answer 23

-Liver ‘grabs’ the Glc

Question 24

What does high glucokinase Vmax mean?

Answer 24

It can phosphorylate all the Glc quickly thus most absorbed Glc is trapped by the liver

Question 25

What does hexokinase low Km mean?

Answer 25

Even al tow [Glc] tissues can grab Glc effectively

Question 26

What does hexokinase low Vmax mean?

Answer 26

Tissues are easily satisfied so don't keep grabbing Glc

Question 27

What are the fates of G-6-P?

Answer 27

-Converted to Glc in the liver then transported through the blood to other tissues -Glc acted on by hexokinase ot form G-6-P G-6-P can then enter glycolysis, pentose phosphate pathway or be stored as glycogen

Question 28

Where is glycogen mainly found?

Answer 28

90% is in the liver and skeletal muscle

Question 29

What happens in the liver if [blood Glc] falls?

Answer 29

Glycogen is converted to G-6-P then transported as glucose in the blood

Question 30

What happens in skeletal muscle to glycogen?

Answer 30

G-1-P is converted to G-6-P which is acted on by substrate level phosphorylation in glycolysis to form lactate and ATP for muscle contraction

Question 31

How is glycogen synthesise?

Answer 31

- Glycogenin begins the process by covalently binding Glc from uracil diphosphate glucose to form chains of approx. 8 Glc residues. - Then glycogen synthase takes over and extends the Glc chains - The chains formed by glycogen synthase are then broken by glycogen-branching enzyme and re-attached via (a1-6) bonds to give branch points

Question 32

How does the degradation of glycogen begin?

Answer 32

Glc monomers are removed one at a time from the non-reducing ends as G-1-P

Question 33

What happens after terminal Glc residues are removed to release G-1-P by glycogen phosphorylase from glycogen?

Answer 33

Glc near the branch is removed in a 2 step process by debranching enzyme.

Question 34

What roles does transferase play in the degradation of glycogen?

Answer 34

Transferase activity of de-branching enzyme removes a set of 3 Glc residues and attaches them to the nearest non-reducing end via a (a1-4) bond

Question 35

What role does glucosidase play in the degradation of glycogen?

Answer 35

Glucosidase activity then removes the final Gls by breaking a (a1-6) linkage to release free Glc.

Question 36

What is the end product of the degradation of glycogen?

Answer 36

An unbranched chain which can be further degraded or built upon as needed

Question 37

What is Von Gierke's disease?

Answer 37

Liver (kidney and intestine) Glucose-6-Phosphatase deficiency

Question 38

What are the symptoms of Von Gierkes's disease?

Answer 38

- High [liver glycogen]- maintains its normal structure - Low [blood Glc] - fasting hypoglycaemia because glycogen cannot be used as an energy source all Glc must come from dietary carbohydrate - High [blood lactate]- lacticacidaemia bacaue the lactate produced be skeletal muscle cannot be reconverted to Glc in the liver

Question 39

What is the treatmernt for Von Gierke's disease?

Answer 39

Regular carbohydrate feeding- little and often

Question 40

What is McArdle's disease?

Answer 40

Skeletal muscle phosphorylase deficiency

Question 41

What are the symptoms of McArdle's disease?

Answer 41

-High [muscle glycogen]-maintains its correct structure -Weakness and cramps after exercise No increase in [blood Glc] after excercise

Question 42

What is the treatment for McArdle's dsease?

Answer 42

- Avoid strenuous activity | - Exercise briefly wait for the pain to subside, continue to excercise