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4.1.3Unit 4: Energy, Environment, Microbiology and Immunity

Starch and Cellulose: Structure and Function — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 4 parts Lesson 10 of 28 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Section 1 of 4Starch Structure

4.1.3 Starch and Cellulose: Structure and Function

Starch is the storage polysaccharide of plants and is made from two different polymers of α-glucose.

Amylose
  • An unbranched helical chain of α-glucose joined by 1,4 glycosidic bonds, which makes up roughly 10 to 30 per cent of starch.
Amylopectin
  • Has 1,4 bonds plus 1,6 glycosidic bonds that form branches, and makes up roughly 70 to 90 per cent of starch.
Helix shape
  • The coiled amylose chain is compact, which makes it more resistant to digestion than glucose.
Branching
  • Many branch ends give many terminal glucose units that can be hydrolysed quickly for respiration or added to for storage.
Storage sites
  • Starch is held as granules in plastids such as chloroplasts and amyloplasts.

Section 2 of 4Why Starch Stores Energy

Why Starch Stores Energy
Compact
  • The coiled and branched shape packs a large amount of glucose into a small space.
Insoluble
  • Starch does not dissolve, so it has no osmotic effect and does not draw water into the cell.
Glucose contrast
  • Dissolved glucose would lower water potential in the cell, so water would enter and walls would need to be stronger.
Slow digestion
  • Large polymers of thousands of glucose units take longer to digest than free glucose.

Section 3 of 4Cellulose

Cellulose is a structural polysaccharide built from β-glucose.

1
Long chains are made of β-glucose joined by 1,4 glycosidic bonds, with alternate molecules rotated by 180° to each other.
2
This inversion places the chains in straight, unbranched lines instead of coils.
3
Many hydrogen bonds form between neighbouring chains and bundle them into strong microfibrils.
4
Microfibrils have high tensile strength, so the wall can resist the turgor pressure of the cell without breaking.
5
Cellulose fibres combine with other molecules such as lignin to form a matrix that strengthens the wall and supports the plant.

Section 4 of 4Exam Checklist & Tips

4.1.3 – Key Points for the Exam

  • Amylose An unbranched, helical polymer of α-glucose in starch with 1,4 bonds only.
  • Amylopectin A branched polymer of α-glucose in starch with 1,4 and 1,6 bonds.
  • Glycosidic bond A covalent bond between two monosaccharides formed by condensation.
  • Microfibril A bundle of cellulose chains held together by hydrogen bonds.
  • Turgor pressure The pressure exerted by cell contents on the cell wall.

Common Mistakes

  • Saying that cellulose is made of α-glucose, when it is made of β-glucose with alternate units inverted.
  • Stating that the individual hydrogen bonds in cellulose are strong;
    • it is the large number of them that gives strength.
  • Forgetting that amylopectin has 1,6 branch points as well as 1,4 bonds.
  • Claiming starch is soluble, when its insolubility is the reason it does not affect water potential.

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Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.