Polysaccharides as energy storage compounds
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Question
Define a polysaccharide.
Answer
A large molecule (polymer) made of **many monosaccharides** joined together.
Question
What is the monomer of starch, glycogen and cellulose?
Answer
**Glucose** — a monosaccharide.
Question
Name the three polysaccharides you must know and their roles.
Answer
**Starch** (energy store in plants), **glycogen** (energy store in animals), **cellulose** (structural support in plant cell walls).
Question
Which form of glucose builds starch and glycogen?
Answer
**Alpha-glucose**.
Question
Which form of glucose builds cellulose?
Answer
**Beta-glucose**.
Question
Describe the structure of starch.
Answer
**Alpha-glucose** chains that are **coiled (helical) and lightly branched**.
Question
Describe the structure of glycogen.
Answer
**Alpha-glucose** chains that are **highly branched** (even more than starch).
Question
Describe the structure of cellulose.
Answer
**Beta-glucose** in **long, straight, unbranched chains** that hydrogen-bond into **fibres**.
Question
Give three features that make a polysaccharide a good energy store.
Answer
It is a **large/compact glucose polymer** (lots of energy), **insoluble** (no effect on osmosis), and **branched** (many ends for fast glucose release).
Question
Why is being insoluble useful for a storage polysaccharide?
Answer
It does **not dissolve**, so it does **not affect the cell's water balance** (osmosis) and stays as a store.
Question
Why does branching help a storage polysaccharide?
Answer
Branches give **many free ends**, so glucose can be **added or removed quickly** by hydrolysis when energy is needed.
Question
Explain how cellulose's structure suits its function.
Answer
Straight **beta-glucose** chains **hydrogen-bond** side by side into strong **fibres**, which support the **plant cell wall**.
Question
State the role of cellulose in plant cells.
Answer
It provides **structural support** — its fibres strengthen the **cell wall**.
Question
Why can humans not digest cellulose?
Answer
We lack the enzyme to break its **beta-glucose** links, so it passes through as dietary **fibre**.
Question
What reaction joins glucose units into a polysaccharide, and what is released?
Answer
**Condensation** — each link **releases one water molecule (H₂O)**.
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Topic 2.1 hub
Carbohydrates and lipids
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