Carbohydrates

Question 1

A few facts about carbohydrates

Answer 1

1. They are made up of the elements carbon, hydrogen and oxygen
2. Carbohydrate has a carbonyl group (C=O) (which can exist in either the form of a ketone* or an aldehyde*) and multiple hydroxyl groups (OH).

Question 2

Characteristics of monosaccharides:

Answer 2

Monosaccharides are:

• colourless,
• crystalline solids,
• that are freely soluble in water
• but insoluble in nonpolar solvents.

Question 3

What are the two forms of glucose?

Answer 3

Monosaccharides may exist as linear or ring structures.

In the ring structure, the anomeric carbon (i.e. carbon that is bonded to TWO oxygen
atoms), is formed.

Question 4

ABBA

Answer 4

Alpha is Below, Beta is Above

a. form of carbohydrate molecule - Lying below the plane of the ring
b. form of carbohydrate molecule - Lying above the plane of the ring

Question 5

How is a disaccharide formed?

Answer 5

Two monosaccharides form a disaccharide molecule, involving the loss of a water molecule. Thus, it is a condensation reaction.

+ A disaccharide molecule can be broken down into its constituent monosaccharides during a hydrolysis reaction. (Water is added during the process via: an enzyme/ acid hydrolysis)

Question 6

Define a glycosidic bond:

Answer 6

A glycosidic bond is defined as the bond formed between the anomeric carbon of one sugar unit and another carbon on the other sugar unit.

tLdr: A glycosidic bond is the covalent bond formed between two monosaccharides in a disaccharide molecule.

Question 7

a. (1,4) glycosidic bonds vs a. (1,6) glycosidic bonds

Answer 7

(1,4) - linear, unbranched chains. They are found in unbranched amylose, as well as amylopectin and glycogen

(1,6) - gives rise to branches in the chain. Found where branches occur in amylopectin and glycogen

Question 8

What is the structure of cellulose?

Answer 8

Cellulose is made of beta-glucose molecules.

In order to obtain b. (1,4) glycosidic bonds in cellulose, alternate monomers are inverted.

As such, cellulose is a polymer of up to 10,000 b-glucose molecules forming long unbranched chains. Many chains run parallel to each other and their hydroxyl groups (OH) project outwards from each chain.

Extensive hydrogen bonds form between the protruding OH groups of neighbouring chains allowing the establishment of rigid cross-links between chains.

Question 9

Relate the structure of starch to its storage function:

Answer 9

Starch is a large molecule - It is insoluble in water and thus does not affect the water potential in cells and living organisms

Starch is composed of several hundreds to thousands of glucose monomers - It acts as a large store of carbon (respiratory substrate > carbon sink)

Glucose monomers are linked by a. (1,4) glycosidic bonds - Starch may be easily hydrolysed by enzymes resent in plants and most organisms

Amylose molecules are helical in shape - Amylose is compact

Amylopectin molecules are highly branched due to the presence of a. (1,6) glycosidic bonds - Amylopectin is compact, and a large number of free ends are available for hydrolysis by amylase at any one time.

Anomeric carbon is involved in glycosidic bond formation, leaving few free hydroxyl groups - Starch is unreactive and chemically stable

Question 10

Compare glycogen vs amylopectin

Answer 10

It has similar structure to amylopectin but it is more extensively branched.

• The a. (1,6) glycosidic bonds occur every 8 - 12 glucose units
• It is therefore more compact than amylopectin

Question 11

Relate the structure of glycogen to its storage function:

Answer 11

Glycogen is a large molecule - It is insoluble in water and thus does not affect the water potential in cells and living organisms

Glycogen is composed of several hundreds to thousands of glucose monomers - It acts as a large store of carbon (respiratory substrate > carbon sink)

Glucose monomers are linked by a. (1,4) glycosidic bonds - Glycogen may be easily hydrolysed by enzymes resent in plants and most organisms

Glycogen molecules are highly branched due to the presence of a. (1,6) glycosidic bonds - Amylopectin is compact, and a large number of free ends are available for hydrolysis by amylase at any one time.

Anomeric carbon is involved in glycosidic bond formation, leaving few free hydroxyl groups - Glycogen is unreactive and chemically stable

Question 12

Describe how cellulose makes up the cell wall in plants

Answer 12

Cross-linked cellulose chains associate together to form microfibrils.

These microfibrils associate with other non-cellulose polysaccharides (?) and are arranged in bundles to form macrofibrils

Question 13

Characteristics of the cellulose cell wall

Answer 13

High tensile strength that confers cellulose considerable stability.
- Cellulose fibres are laid down in different directions in the different layers of the plant cell wall, permitting the cell wall to withstand forces exerted in all directions

Full permeability to water and solutes

Question 14

Relate the structure of cellulose to its support function:

Answer 14

Cellulose is a large molecule - It is insoluble in water, making it a good structural material

b-glucose units are linked by b. (1,4) glycosidic bonds which has a different molecular shape from a. (1,4) glycosidic bonds found in amylose or amylopectin

• only a few organisms produce cellulase
• and amylase can only hydrolyse a. (1,4) glycosidic bonds found in starch
• Cellulose is stable

Alternate inverted b-glucose units linked by b.(1,4) glycosidic bonds allow cellulose to form long, unbranched and straight chains. Extensive hydrogen bonds form between parallel chains. Straight parallel chains can be grouped into microfibrils, which eventually cluster into macrofibrils.
- Cellulose provides high tensile strength for structural support.