Starch, cellulose and plant fibresEdexcel International A Level Biology: Revision notes

Starch is a polysaccharide made of α-glucose monomers joined by glycosidic bonds formed in condensation reactions. It is a mixture of two polymers.

• Amylose is an unbranched chain of α-glucose joined by 1,4 glycosidic bonds. It coils into a helix, so it is compact.
• Amylopectin has 1,4 bonds in its chains and 1,6 bonds at branch points. The many branch ends are hydrolysed quickly by enzymes to release glucose.

Starch is insoluble, so it does not change the water potential of the cell and does not diffuse out. It is stored in starch grains in chloroplasts and in storage organs such as potato tubers.

Cellulose is made of β-glucose monomers joined by 1,4 glycosidic bonds. In β-glucose the –OH on carbon 1 is above the ring, so each glucose unit must be inverted by 180° relative to its neighbour for the bond to form. The chains are therefore straight and unbranched.

Hydroxyl groups on adjacent parallel chains form many hydrogen bonds. Each bond is weak, but together they hold many chains side by side as a microfibril. Microfibrils have a very high tensile strength and are insoluble and chemically stable.

Microfibrils are laid down in the primary wall in layers that run in different directions, embedded in a matrix of other polysaccharides. This arrangement resists stretching in all directions.

Some cells then undergo secondary thickening: further layers of cellulose are added inside the primary wall and lignin is deposited in them. Lignin is a rigid, waterproof polymer. The cell contents die, leaving a thick, strong, impermeable wall.

• Sclerenchyma fibres are long, dead cells with thick lignified walls. They give support to the stem and have a high tensile strength.
• Xylem vessels are dead, hollow, lignified tubes. The lignin makes them waterproof and stops them collapsing inwards under the low pressure produced by transpiration.

The tensile strength of a fibre is the force it can withstand before it breaks.

1. Strip fibres from stems (for example, from nettle, flax or hemp) and cut samples to the same length.
2. Clamp the upper end and hang a mass holder from the lower end.
3. Add masses in small equal steps until the fibre snaps, and record the total mass. Catch the falling masses safely.
4. Repeat several times and calculate a mean.

Control length, moisture, age of the plant and the method of clamping. Force = mass × 9.81, and tensile strength = force ÷ cross-sectional area. Dividing by the area allows fibres of different thickness to be compared.

Plant fibres such as hemp, flax and jute are used to make ropes, fabrics and composites. Starch can be used to make biodegradable plastics for packaging and bags.

Advantages over oil-based products:

• renewable, because they can be regrown, whereas oil is finite
• plants absorb carbon dioxide while growing, giving a lower net carbon footprint
• biodegradable, so less persistent waste and pollution

Disadvantages:

• land, water, fertiliser and pesticide are needed, and crops may compete with food production
• starch plastics may be weaker, degrade when wet and be more costly