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
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Starch Structure
Starch is the storage polysaccharide of plants and is made from two different polymers of α-glucose.
Amylose
- 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
- 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
- Helix shape The coiled amylose chain is compact, which makes it more resistant to digestion than glucose.
Branching
- Branching Many branch ends give many terminal glucose units that can be hydrolysed quickly for respiration or added to for storage.
Storage sites
- Storage sites Starch is held as granules in plastids such as chloroplasts and amyloplasts.
Part 2 of 4Why Starch Stores Energy
Why Starch Stores Energy
Compact
- Compact The coiled and branched shape packs a large amount of glucose into a small space.
Insoluble
- Insoluble Starch does not dissolve, so it has no osmotic effect and does not draw water into the cell.
Glucose contrast
- Glucose contrast Dissolved glucose would lower water potential in the cell, so water would enter and walls would need to be stronger.
Slow digestion
- Slow digestion Large polymers of thousands of glucose units take longer to digest than free glucose.
Part 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.
Part 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.