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PolysaccharidesAQA A-Level Biology: Revision notes

Section 1

What is a polysaccharide?

Polysaccharides are formed by the condensation of many glucose units. Each glycosidic bond forms by condensation, and each can be broken by hydrolysis.

Three polysaccharides are in the specification:

  • glycogen and starch, made from α-glucose
  • cellulose, made from β-glucose.
Key termspolysaccharide

Section 2

Glycogen

Glycogen is the main energy store in animal cells (and in some fungi), found in the liver and muscles. It is made from α-glucose with α-1,4 glycosidic bonds and many α-1,6 bonds at the branch points.

  • Highly branched: many ends, so enzymes release glucose quickly.
  • Compact: a lot of energy stored in a small space.
  • Insoluble: does not change the water potential of the cell or diffuse out.
  • Large: cannot leave the cell.
Key termsglycogenbranched

Section 3

Starch

Starch is the energy store in plant cells, for example in potato tubers and seeds. It is a mixture of two polysaccharides of α-glucose:

  • Amylose: an unbranched chain (α-1,4 bonds) coiled into a helix, so it is compact.
  • Amylopectin: a branched chain with α-1,4 and α-1,6 bonds, so enzymes can hydrolyse it quickly from many ends.

Starch is large and insoluble, so it does not affect the water potential of the cell.

Key termsstarchamyloseamylopectin

Section 4

Cellulose

Cellulose is made from β-glucose. Alternate glucose molecules are inverted (rotated 180°) so that the glycosidic bonds can form, giving long, straight, unbranched chains.

Many hydrogen bonds form between the parallel chains. These bundle together as microfibrils, then fibres, with a high tensile strength. In plant cell walls this strength resists the pressure of osmotic water entry, so the cell does not burst.

Key termscellulosemicrofibril
Common mistake

Cellulose is not an energy store for the plant and it is not branched. Do not mix up its structure with starch.

Section 5

Linking structure to function

  • Glycogen (animals): branched, compact, insoluble. Energy store with rapid release.
  • Starch (plants): helical amylose and branched amylopectin, insoluble. Energy store.
  • Cellulose (plants): straight chains, hydrogen-bonded microfibrils. Strength and support.

In an exam always state the structural feature, then the property it gives, then the function.

Key termsstructure and function

Section 6

Iodine test for starch

  1. Add iodine in potassium iodide solution to the sample.
  2. If starch is present the colour changes from orange-brown to blue-black.
  3. If starch is absent it stays orange-brown.

No heating is needed. Do not confuse this with Benedict's reagent, which tests for reducing sugars and needs heating.

Key termsiodine test

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Polysaccharides

  1. A student compares three polysaccharides: glycogen from liver tissue, starch from a potato and cellulose from plant cell walls.
    Explain how the structure of glycogen makes it suitable as an energy store in liver cells.2 marks
  2. The muscle cells of a sprinter must release glucose for respiration very rapidly during a 100 m race. The cells of a potato tuber, by contrast, store starch for use over several weeks.
    Explain why glycogen is better than unbranched amylose as a store of glucose for rapid release.2 marks
  3. Plant cell walls contain cellulose. A plant cell placed in pure water takes in water by osmosis, but it does not burst.
    Explain how cellulose molecules are arranged to give the cell wall strength.3 marks
See the full worksheet

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).