Carbohydrates

Question 1

Describe the significance of molecular structure of monosaccharides.

Answer 1

1. Small in size and has many hydroxyl groups that can form hydrogen bonds with water
   -readily soluble in water
   -transported easily in water in animal and plant transport systems
2. Linear form possesses a free carbonyl group (C=O)
   -gives them reducing ability, hence called reducing sugars
3. Pentoses and hexoses can exist as rings
   -rings are stable building blocks for larger molecules
4. Ring structure exhibits a- and b- isomerism
   -same chemical formula gives rise to structurally different molecules
   -increases diversity of monosaccharides which can become building blocks for different molecules

Question 2

How do the structures of starch and glycogen make them good energy STORAGE molecules?

Answer 2

1. They are helical molecules (each alpha-glucose monomer is bent in one direction with respect to an adjacent monomer), allowing many a-glucose monomers to be packed per unit volume –> compact energy store.’
2. Most OH groups are involved in intramolecular hydrogen bonding within the helix; few OH groups available for hydrogen bonding with water –> insoluble, does not affect water potential of cells
3. Branched, have multiple branch ends/sites which hydrolytic enzymes can work on. More glucose molecules released rapidly at same time, more ATP generated by respiration per unit TIME (not mass).
4. Large molecules –> insoluble.

Question 3

How does structure of cellulose make it a good STRUCTURAL molecule?

Answer 3

1. Alternate glucose monomers rotated 180* wrt each other, forming long straight molecule with free OH groups projecting out in both directions, allowing interchain H bonding between cellulose molecules parallel to each other -> forming microfibrils -> high tensile strength
2. Has few OH groups available to H bond with water, most are involved in interchain H bonding. Only surface of microfibirl has OH groups for H bonding -> insoluble in water, water potential of cells unaffected by its presence
3. The meshwork of microfibrils that form the cell wall
   a. have a porous structure –> cell wall freely permeable to water and solutes, allows movement of substances across the cell wall
   b. are strong and rigid, distributes stress in all directions to prevent plant cells from bursting due to osmotic stress.
4. Cellulases that hydrolyse cellulose are found in very few organisms. Cellulose cannot be hydrolysed by most organisms. Good structural molecule.