Polysaccharides
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires prior knowledge of carbohydrates. You can test your knowledge on these below.
What is a polysaccharide?
A polysaccharide is a complex carbohydrate formed from the condensation reactions of many, repeated monosaccharides.
What are the main types of polysaccharides found in living organisms?
Glycogen, starch, and cellulose.
From what monosaccharides is sucrose (a disaccharide) made?
Glucose and fructose
Topic Explainer Video
Check out this @JoeDoesBiology video that explains polysaccharides or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Definition of Polysaccharides
- Polysaccharides are large, complex carbohydrates composed of many repeated, monosaccharides joined by glycosidic bonds through condensation reactions.
- They are insoluble, making them suitable for storage.
Types of Polysaccharides
1. Glycogen (Storage in Animals)
- Made of many α-glucose monomers.
- Highly branched structure with 1,4 and 1,6 glycosidic bonds, allowing enzymes to rapidly hydrolyse glycogen into glucose (to be used in respiration)..
- Found in animal cells, particularly in liver and muscle cells.
- Insoluble, preventing osmotic effects in animal cells as it does not affect the water potential.
- Large molecule meaning it cannot diffuse out of cells.
2. Starch (Storage in Plants)
- Made of many α-glucose molecules.
- Exists in two forms:
- Amylose: A helical, unbranched structure with 1,4 glycosidic bonds, making it compact. This means lots can be stored in a small space.
- Amylopectin: Branched, with 1,4 and 1,6 glycosidic bonds, allowing rapid hydrolysis by enzymes to release glucose for respiration.
- Insoluble, preventing osmotic effects in plant cells as it does not affect the water potential.
- Large molecule meaning it cannot diffuse out of cells.
3. Cellulose (Structural in Plants)
- Made of many β-glucose molecules.
- Long, straight, unbranched chains with 1,4 glycosidic bonds.
- Straight chains are held together by many hydrogen bonds to form microfibrils.
- Microfibrils are joined together to make macrofibrils.
- Many hydrogen bonds help give structural strength to cellulose and plant cell walls, preventing plant cells from bursting under osmotic pressure.
Summary
Key Terms
- Polysaccharide: A complex carbohydrate formed from many, repeated monosaccharides
- Glycosidic Bond: A covalent bond formed between monosaccharides in a condensation reaction.
- Hydrolysis: The breakdown of a molecule using addition of water.
- Microfibrils: Bundles of cellulose molecules providing strength to plant cell walls.
Exam Tip
When discussing the structure and function of polysaccharides, always link structural features to function.
No answer provided.
Describe and explain how the structure of starch and glycogen make them suitable for their function in storage. (4 marks)
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Starch and glycogen are insoluble, so they do not affect water potential in cells.
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Amylose has a helical structure, making it compact, meaning a lot can be stored in a small space.
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Amylopectin is branched, allowing enzymes to rapidly hydrolyse starch into glucose for respiration.
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Glycogen is highly branched, enabling rapid glucose release to meet high metabolic demands in animals.
- Starch and glycogen are large molecules meaning they cannot diffuse out of cells.