2.2.52 · Biological Molecules
Starch and Glycogen — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
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Lesson summary
2.2.5 Starch and Glycogen — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the structure and functions of starch and glycogen.Compact — many molecules fit in a small space, so large amounts can be stored.Insoluble — they have no osmotic effect, whereas solub…
- Chain shapes
- Composition
- Amylopectin
- Triglyceride
- Phospholipid
- Monosaccharide
Section 1 of 8Why they are storage polysaccharides
Compact
- Compact many molecules fit in a small space, so large amounts can be stored.
Insoluble
- Insoluble they have no osmotic effect, whereas soluble glucose would lower water potential and draw in water.
Chain shapes
- Chain shapes chains may be branched or unbranched, and straight or coiled.
Section 2 of 8Starch
Source
- Source the storage polysaccharide of plants, stored as granules in plastids such as chloroplasts.
Digestion
- Digestion it takes longer to digest than glucose.
Composition
- Composition amylose and amylopectin, both made of alpha-glucose.
Section 3 of 8Amylose and amylopectin
Amylose
- 10–30% of starch.
- It is an unbranched helix with alpha-1,4 glycosidic bonds.
- The helix makes it compact and resistant to digestion.
Amylopectin
- 70–90% of starch.
- It has alpha-1,4 bonds and alpha-1,6 bonds that create branches.
- Branches give many terminal glucose ends that are easily hydrolysed for respiration or added to for storage.
Section 4 of 8Glycogen
Glycogen
Source
- Source the storage polysaccharide of animals and fungi.
Structure
- Structure highly branched and not coiled, with alpha-1,4 and alpha-1,6 bonds.
Location
- Location liver and muscle cells hold visible granules because respiration is fast there.
Function
- Function more branching than amylopectin makes it more compact and gives more free ends for rapid addition or removal of glucose.
Section 5 of 8Comparison of starch and glycogen
| Feature | Amylose | Amylopectin | Glycogen |
|---|---|---|---|
| Monomer | alpha-glucose | alpha-glucose | alpha-glucose |
| Branched | No | Yes | Yes, more highly |
| Helix | Yes | No | No |
| Bonds | alpha-1,4 | alpha-1,4 and 1,6 | alpha-1,4 and 1,6 |
| Source | Plants | Plants | Animals and fungi |
Section 6 of 8More from the specification
More from the specification
Polysaccharide Energy Stores
- Starch (in plants) and glycogen (in animals) are compact, insoluble polymers formed from α-glucose monosaccharides linked by covalent glycosidic bonds.
Structural Role of Cellulose
- Cellulose is an unbranched polymer of β-glucose where straight chains form hydrogen bonds to assemble into rigid microfibrils in plant cell walls.
Structure of Triglycerides
- Formed from one glycerol molecule condensed with three fatty acids via covalent ester bonds, serving as energy-dense, hydrophobic storage molecules.
Amphipathic Phospholipids
- Phospholipids consist of a hydrophilic phosphate head and two hydrophobic fatty acid tails,
- forming the stable structural matrix of biological membranes.
Condensation
- A reaction that joins molecules and removes water.
Lipids
- Concentrated energy, insulation, cell membranes.
Sucrose
- The sugar mainly transported in phloem.
Covalent bond
- A shared electron pair linking monomers.
Non-reducing sugar
- A sugar that cannot reduce Benedict's reagent until hydrolysed.
Condensation reaction
- A reaction joining molecules with the release of water.
Cellulose
- A structural polysaccharide of β-glucose.
Triglyceride
- A lipid made of glycerol and three fatty acids.
Polymer
- A large molecule made of many repeating monomers.
Phospholipid
- Glycerol, two fatty acids and a phosphate group, which is amphipathic.
Monosaccharide
- A single sugar unit such as glucose, fructose or galactose.
Hydrophobic
- Water-hating; non-polar and unable to dissolve in water.
Carbohydrate
- A molecule of carbon, hydrogen and oxygen, including sugars, starch, glycogen and cellulose.
Carbon
- Element with four valence electrons that forms the backbone of organic molecules.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Starch | The plant storage polysaccharide made of amylose and amylopectin. |
| Glycogen | The animal and fungal storage polysaccharide. |
| Amylose | An unbranched helical chain of alpha-glucose. |
| Amylopectin | A branched polymer of alpha-glucose. |
| Plastid | A plant organelle such as a chloroplast that can store starch. |
| Branching | Side chains that create many free ends for enzymes. |
Section 8 of 8Exam technique
1
Saying glycogen is the plant store — it is the animal and fungal store; starch is in plants.
2
Saying starch is one molecule — it is a mixture of amylose and amylopectin.
3
Saying amylose is branched — it is an unbranched helix.
4
Saying storage polysaccharides are soluble — they are insoluble so cause no osmotic effect.
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