Carbohydrates

Question 1

Bonds in disaccharides (and polysaccharides)

Answer 1

Monomers linked by glycosidic bonds

Maltose (glucose + glucose)
Sucrose (glucose + fructose)
Lactose (glucose + galactose)

Question 2

What is an (alpha1->4) link?

Answer 2

Glycosidic bond within subunits in chains

Question 3

What is an (alpha1->6) link?

Answer 3

Glycosidic bond of branches onto chain

Question 4

What are carbohydrates with carbohydrate attached and what is the benefit?

Answer 4

Glycoproteins

• Increase protein solubility
• Protect from degradation
• Communication between cells

Question 5

What is mucopolysaccharidoses?

Answer 5

Defective enzyme this should break down gylcosaminogylcans (GAGs) causing it to build up in tissues, blood and damaging structure and function

• Hurler syndorme

Question 6

Outline digestion

Answer 6

1. Chewing and salivary enzymes (lipase, amylase)
2. Travel to stomach and HCl breaks it down further
3. Bile and pancreatic enzymes released to duodenum
4. Large particles –> glucose/fructose/galactose in jejunum
5. Absorbed to microvilli by Na/Glu symporter

Question 7

How is fructose absorbed to microvilli?

Answer 7

Channel protein GLUT 5 into cells then diffuse into blood

Question 8

Outline absorption of cellulose and hemicellulose

Answer 8

Not digested by gut, like oligosaccharides - polymers broken down by gut bacteria (form CH4 and H2 gas)

Question 9

Describe lactose intolerance

Answer 9

No lactase enzyme = lactose broken down by gut bacteria so gas build up and irritant acid damaging cell well and draws water from gut to lumen causing diarrhoea

Question 10

Why is glucose phosphorylated to G-6-P in the liver?

Answer 10

As G-6-P is not able to diffuse out of cells by GLUT transporters, trapping it in cell

Question 11

Difference between hexokinase and glucokinase

Answer 11

Glucokinase in liver as has high Vmax so phosphorylates Glu rapidly so most Glu->G-6-P trapped in liver

Hexokinase: low Km = HIGH affinity so at low [Glu] it can be converted to G-6-P in tissues and low Vmax means tissues are easily satisfied

Question 12

Outline synthesis of glycogen (glycogenesis)

Answer 12

1. Glucose from UDP-Glucose to form an 8 Glc chain ((alpha1–>4) links)
2. Glycogen synthase extends Glc chain
3. Glycogen-branching enzyme break Glc chain and re-attach by (alpha1-6) links as a branch

Question 13

Outline degradation of glycogen

Answer 13

1. Glc removed one at a time from non-reducing end by GLYCOGEN PHOSPHATASE - and they’re then called G-1-P
2. Debranching enzymes transfers 3 GLC near branch to non-reducing end with (alpha1->4) links
3. GLUCOSIDASE removes final Glc on branch breaking (alpha1->6) link

Question 14

Glucose 6-phosphatase deficiency

Answer 14

• High [liver glycogen]
• Low [blood Glc] - glycogen can’t be used as energy source
• High [blood lactate] in skeletal muscle

Question 15

What reaction does lactate dehydrogenase catalyse?

Answer 15

Pyruvate –> lactate (produces NAD+)

Question 16

What reaction does pyruvate dehydrogenase catalyse?

Answer 16

Pyruvate –> acetyl CoA

Question 17

What tissues are reliant on glucose for energy?

Answer 17

• Brain
• Nervous system
• RBC
• Testes
• Embryonic tissue

Question 18

How does fructose enter gluconeogenesis?

Answer 18

Two mechanisms

• hexokinase converts it to F-6-P
• Fructose -> F 1-P -> DHAP

Question 19

How does galactose enter gluconeogenesis?

Answer 19

Converted to G 6-P and enters glycolysis

Question 20

Function of pentose phosphate pathway

Answer 20

Produce NADPH and pentoses (5C sugars) for ATP, RNA and DNA

Question 21

Outline catabolic mechanism of PPP

Answer 21

G 6-P –> Ribulose 5-P –> nucleotides, coenzymes, DNA, RNA

produces NADPH

Question 22

Outline anabolic mechanism of PPP

Answer 22

Ribulose 5-P –> 6-G-P

Question 23

What is the NADPH created in the PPP used for?

Answer 23

Lipid synthesis (FA, sterol etc) and NADP+ used is recycled back into PPP

Question 24

Affect of alcohol on gluconeogenesis

Answer 24

NAD+ used for ethanol -> acetaldehyde -> acetate - so NAD+ not available to gluconeogenesis

Lacticacidaemia and hypoglycaemia