Week 3 Carbohydrate Metabolism Part A

Question 1

Key carbohydrates supplied in diet

Answer 1

Monosaccharides
Disaccharides
Polysaccharides

Question 2

Naturally occurring food monosaccharides

Answer 2

Glucose- main CHO in blood
0-3% in veg, 1-8% in fresh fruit, dried fruit up to 35%
Not tolerated by diabetics

Fructose- only common ketose
0-3% in veg, 1-10% in fresh fruit, dried fruit up to 35%
Sweetest natural sugar

Galactose = rare in food free from ( instant coffee 2%) found in milk products after lactose hydrolysis

Ribose- commonest pentose
Found in meat,fish and poultry ( flavoured precursor)

Question 3

Disaccharides

Answer 3

Sucrose
Glucose + fructose= majority of free sugars in foods

Maltose
Glucose + Glucose
Produced in brewing process
Metabolised to ethanol by yeast
Lactose
Glucose + galactose
Found in milk products after lactose hydrolysis

Question 4

Polysaccharides starch

Answer 4

Starch contains 2 polysaccharides amylose and amylopectin

Amylose(15-30%) long, largely unbranched, chains of D-Glucose connected y a1-4 linkages

Amylopectin (70-85%) also composed of D-glose residues with (a1-4) linked chains like amylose with (a1-6) linkages

Question 5

GLYCOGEN

Answer 5

Mammalian storage polysaccharide of glucose

Question 6

NON STARCH POLYSACCHARIDES (NSPS) - CELLULOSE

Answer 6

Cellulose is the main polysaccharide found in plant cell walls
•Major component of dietary fibre

•Individual chains bou

Individual chains bound to each other through hydrogen bonding to form cellulose microfibrils
•Provide strength to cell walls in all directions, adjacent layers of microfibrils laid down in different directions

β-1,4-D-glucose polymer

Question 7

OTHER NSPS IN FOOD

Answer 7

Alginate (brown seaweed)
•Linear copolymer of mannuronate and guluronate
monomers, used as a thickener/gelling agent
•Arabinoxylan (bread)
•Xylose polymer backbone with arabinose substituents
•β-glucan (oats and barley)
•Soluble glucose polymer with β-1,3 and β-1,4-glycosidic
bonds, which helps to reduce cholesterol
•Glucomannan (konjac)
•Soluble polymer of glucose and mannose, with galactose substituents, useful for treating constipation
•Pectin (jam, fruit)

Question 8

CARBOHYDRATE METABOLISM IS ALMOST EXCLUSIVELY GLUCOSE METABOLISM - SOME QUICK PHYSIOLOGY

Answer 8

NSPs
= not easily digested or absorbed. May be fermented by gut microbiome

Glucose
Fructose
Sucrose
Lactose
Maltose
Amylose
Amylopectin

Question 9

DIFFERENT WAYS OF REPRESENTING D-GLUCOSE

Answer 9

Fisher projection
(last OH on the right  D enantiomer)

Haworth projection
(OH on anomeric carbon
below ring = α,
above ring = β)

Chair conformation - OH can be axial (vertical) or equatorial (at an angle)

Question 10

Take away

Answer 10

All carbohydrate is broken down and absorbed as monosaccharides
•Glucose
•Galactose
•Fructose
•All this drains into the hepatic portal vein – straight into the liver

•In the liver Fructose and Galactose can be utilised or converted into glucose for export out into the systemic circulation for use by all other tissues (e.g. muscle)

•THEREFORE: Carbohydrate metabolism is really Glucose metabolism