Carbohydrates

State the general formula of carbohydrates

1. Cx(H2O)y.

State the general formula of monosaccharides

1. (CH2O)n.

Explain why all monosaccharides are reducing sugars

1. All monosaccharides are reducing sugars.

2. They have a free aldehyde or ketone group that can be oxidised.

3. Hence, they are capable of reducing other substances.

State the chemical formula of glucose

1. C6H12O6.

State the difference between alpha glucose and beta glucose

1. In alpha glucose, the hydroxyl group projects below the ring while in beta glucose, the hydroxyl group projects above the ring.

Describe the various functions of monosaccharides

1. Trioses such as glyceraldehyde and dihydroxyacetone are intermediates in respiration and photosynthesis.

2. Pentoses such as deoxyribose and ribose are building blocks for the synthesis of nucleic acids. Ribose is part of RNA. Deoxyribose is part of DNA. Ribulose biphosphate can be synthesised and used in the fixation of carbon during the light-independent stage of photosynthesis.

3. Hexoses such as glucose and fructose provide a source of energy. Glucose yields ATP when oxidised during respiration. Energy via the hydrolysis of ATP is used to drive biochemical processes in the cell. Disaccharides can be synthesised. Glucose is used in the synthesis of polysaccharides.

Describe the various functions of sucrose

1. Sucrose is the most abundant disaccharide in nature.

2. Sucrose is made of glucose and fructose.

3. It is commonly found in plants, where it is transported in large quantities through phleom tissues.

4. It is a good transport sugar because it is very soluble and can moved efficiently in high concentrations.

5. It is relatively unreactive.

6. It is non-reducing.

Describe the various functions of maltose

1. Maltose occurs as a product during the digestion of starch by enzymes called amylases.

2. Maltose is made of glucose and glucose.

3. It is commonly found in animals and germinating seeds.

4. It is a reducing sugar.

Describe the various functions of lactose

1. Lactose is found exclusively in milk.

2. Lactose is made of glucose and galactose.

3. It is an important source of energy for young mammals.

4. It is a reducing sugar.

Describe how glycosidic bonds are formed

1. A glycosidic bond is formed via a condensation reaction between two monosaccharides with the removal of one molecule of water.

2. This reaction is catalysed by enzyme.

Describe how glycosidic bonds are broken

1. A glycosidic bond is broken via a hydrolysis reaction with the addition of one molecule of water.

State the common types of glycosidic bonds

1. Alpha (1-4) glycosidic bond.

2. Alpha (1-6) glycosidic bond.

3. Alpha (1-2) glycosidic bond.

4. Beta (1-4) glycosidic bond.

Describe the various functions of starch

1. Starch is the main carbohydrate in plant cells.

2. It is a major fuel store in plants but absent from animals.

3. Starch molecules accumulate to form starch grains in many plant cells, chloroplasts of leaves, storage organs and seeds of cereals and legumes.

4. Most animals, including humans have enzymes amylase and glycoamylase that easily hydrolyse plant starch to monosaccharides when required hence making glucose available as a nutrient for cells.

Describe the structure of amylose

1. Amylose has an unbranched chain structure.

2. It consists of several thousand alpha glucose residues joined by alpha (1-4) glycosidic bonds.

3. The alpha (1-4) glycosidic bonds cause the amylose chain to coil helically into a more compact shape where most hydroxyl groups project into the interior of the helix to form hydrogen bonds within the chain.

4. There are no hydrogen bonds between the chains.

5. There are six alpha glucose residues per complete turn of the helix.

Describe the structure of amylopectin

1. Amylopectin has a branched chain structure where the branch points occur at every 24-30 alpha glucose residues.

2. The branches are formed by alpha (1-6) glycosidic bonds.

3. It consists of twice as many alpha glucose residues as amylose joined by alpha (1-4) glycosidic bonds.

4. The alpha (1-4) glycosidic bonds cause the amylopectin chain to coil helically into a more compact shape where most of the hydroxyl groups project into the interior of the helix to form hydrogen bonds within the chain.

5. There are no hydrogen bonds between the chains.

6. It is compact due to its highly branched structure.

Compare between amylose and amylopectin

Similarities:

1. Both amylose and amylopectin chains are coiled helically into a more compact shape where most of the hydroxyl groups project into the interior of the helix to form to form hydrogen bonds.

2. Both amylose and amylopectin do not have hydrogen cross-links between the chains.

Differences:

1. Amylose has an unbranched chain structure while amylopectin has a branched chain structure.

2. Amylose gives a dark-blue colour with iodine in potassium iodide solution while amylopectin gives a red-violet colour with iodide in potassium iodide solution.

Describe how the structure of starch contributes to its function

1. Starch is a large molecule made up of many alpha glucose residues. Therefore, it is insoluble in water and hence prevented from diffusing out of cells. It can stored in large amounts without having any great effect on the water potential of cells.

2. Helical structure of amylose and amylopectin form compact structures. Therefore, large amounts of glucose can be stored within a fixed volume. Starch molecules accumulate to form starch grains.

3. Starch has a highly branched structure. Therefore, there are many free ends for enzymes to bind and hydrolyse starch hence more glucose can be released at a faster rate.

Describe the various functions of glycogen

1. Glycogen is a storage carbohydrate found in liver and muscle cells of vertebrate animals.

Describe the structure of glycogen

1. Glycogen has a highly branched structure where the branch points occur at every 8-12 glucose residues.

2. It consists of many alpha glucose residues joined by alpha (1-4) glycosidic bonds.

3. The branch points are formed by alpha (1-6) glycosidic bonds.

Describe how the structure of glycogen contributes to its function

1. Glycogen is a large molecule made up of many alpha glucose resides. Therefore, it is insoluble in water and prevented from diffusing out of cells. It can be stored in large amounts without having any great effect on the water potential of cells.

2. Helical chains form a compact structure. Therefore, large amounts of glucose can be stored within a fixed volume.

3. Glycogen has a highly branched structure. Therefore, there are many free ends for enzymes to bind and hydrolyse glycogen hence more glucose can formed at a faster rate.

Explain why glycogen is hydrolysed faster than starch

1. Glycogen has a greater number of branch points than amylopectin.

2. There are more free ends for enzymes to bind and hydrolyse glycogen than amylopectin.

3. Hence, glycogen is hydrolysed faster than starch.

Describe the various functions of cellulose

1. Cellulose is found in all plant cells.

2. It provides structural support for the plant.

3. Cellulose can only be hydrolysed to glucose by enzyme cellulase.

Describe the structure of cellulose

1. Cellulose has an unbranched chain structure.

2. It consists of many beta glucose residues joined by beta (1-4) glycosidic bonds.

3. Alternate beta glucose molecules must be rotated 180 degrees to each other for the hydroxyl groups to be in close proximity for condensation reaction to occur to form a straight glucose chain in cellulose.

4. The straight glucose chain allows hydroxyl groups to project outwards from each chain in all directions to form hydrogen bond cross-links between neighbouring chains.

5. Hydrogen bond cross-links allows cellulose chains to associate into large bundles called microfibrils which combine to form macrofibrils.

Describe how the structure of cellulose contributes to its function

1. Alternate beta glucose residues are rotated 180 degrees to each other to form beta (1-4) glycosidic bonds which result in a straight glucose chain where hydroxyl groups project outwards in all directions. Therefore, many hydrogen bond cross-links are formed between neighbouring cellulose chains to confer high tensile strength. Cellulose acts as a good structural polysaccharide and is also good cell wall material.

2. Cellulose chains associated into bundles called microfibrils which combine to form macrofibrils. Therefore, this contributes to high tensile strength. The cellulose cell wall prevents cells from bursting when there is a net movement of water molecules into the cell via osmosis when placed in a medium of higher water potential.

3. Presence of large inter-molecular spaces between macrofibrils. Therefore, cellulose layers are permable to water and solutes.

4. Cellulose is a large molecule. Therefore, it is insoluble in water which makes it a suitable structural polysaccharide.