How does amylose differ from amylopectin?

Amylose is unbranched and helical, with 1,4 bonds only. Amylopectin is branched, with 1,4 bonds and 1,6 bonds at branch points.

Why is starch a good energy store?

It is compact, insoluble so it has no osmotic effect, and the branch ends are hydrolysed quickly to release glucose.

Why must alternate β-glucose units in cellulose be inverted by 180°?

So that the –OH groups on carbons 1 and 4 are next to each other and a 1,4 glycosidic bond can form.

What holds cellulose chains together in a microfibril?

Many hydrogen bonds between the hydroxyl groups of adjacent parallel chains.

Why is a microfibril so strong when each hydrogen bond is weak?

There are very many hydrogen bonds along the length of the parallel chains, which are collectively strong.

What is secondary thickening?

Extra layers of cellulose are added inside the primary wall and lignin is deposited in them.

State two roles of lignin in xylem vessels.

It makes the wall waterproof and rigid, so the vessel does not collapse under the low pressure in transpiration.

What type of cell is a sclerenchyma fibre?

A long, dead cell with a thick lignified wall that supports the plant.

How is tensile strength calculated?

Force (mass × 9.81) divided by cross-sectional area.

Name two controls in Core Practical 8.

Length, moisture content and age of the fibres, and the method of clamping.

Give two advantages of plant fibres and starch plastics over oil-based plastics.

They are renewable and biodegradable, and plants absorb carbon dioxide as they grow.

Give one disadvantage of using plant crops to replace oil-based plastics.

Land, water and fertiliser are needed and the crops may compete with food production.

Starch, cellulose and plant fibresEdexcel International A Level Biology: Flashcards

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Which monomer makes up starch?

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Which monomer makes up starch?: α-glucose.
Which monomer makes up cellulose?: β-glucose.
How does amylose differ from amylopectin?: Amylose is unbranched and helical, with 1,4 bonds only. Amylopectin is branched, with 1,4 bonds and 1,6 bonds at branch points.
Why is starch a good energy store?: It is compact, insoluble so it has no osmotic effect, and the branch ends are hydrolysed quickly to release glucose.
Why must alternate β-glucose units in cellulose be inverted by 180°?: So that the –OH groups on carbons 1 and 4 are next to each other and a 1,4 glycosidic bond can form.
What holds cellulose chains together in a microfibril?: Many hydrogen bonds between the hydroxyl groups of adjacent parallel chains.
Why is a microfibril so strong when each hydrogen bond is weak?: There are very many hydrogen bonds along the length of the parallel chains, which are collectively strong.
What is secondary thickening?: Extra layers of cellulose are added inside the primary wall and lignin is deposited in them.
State two roles of lignin in xylem vessels.: It makes the wall waterproof and rigid, so the vessel does not collapse under the low pressure in transpiration.
What type of cell is a sclerenchyma fibre?: A long, dead cell with a thick lignified wall that supports the plant.
How is tensile strength calculated?: Force (mass × 9.81) divided by cross-sectional area.
Name two controls in Core Practical 8.: Length, moisture content and age of the fibres, and the method of clamping.
Give two advantages of plant fibres and starch plastics over oil-based plastics.: They are renewable and biodegradable, and plants absorb carbon dioxide as they grow.
Give one disadvantage of using plant crops to replace oil-based plastics.: Land, water and fertiliser are needed and the crops may compete with food production.

Exam questions on Starch, cellulose and plant fibres

A food technology student extracts two polysaccharides from plant material. Substance P is extracted from potato tuber cells, where the plant uses it as an energy store. Substance Q is extracted from the cell walls of flax stems, where it gives the stems their strength. Both substances are polymers of glucose.
Explain two ways in which the structure of starch makes it suitable as an energy store in potato tuber cells.2 marks
Hemp stems contain bundles of long sclerenchyma fibres that are used to make rope. The wall of each fibre cell is made of cellulose microfibrils, and lignin is deposited in the wall during secondary thickening.
Explain why sclerenchyma fibres have a high tensile strength.2 marks
A student compared the tensile strength of fibres from two plants, X and Y. A fibre sample was clamped at its upper end, masses were hung from its lower end, and more mass was added until the sample snapped. Sample X had a diameter of 0.60 mm and snapped at a mean mass of 2.5 kg. Sample Y had a diameter of 0.40 mm and snapped at a mean mass of 1.9 kg. Assume that each sample has a circular cross-section and use a gravitational field strength of 9.81 N kg⁻¹.
Describe how the student should carry out this investigation so that the comparison between X and Y is valid and the results are reliable.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).