The big idea: Proteins come in two broad shapes. Compact, rounded globular proteins do precise chemical jobs; long, fibrous proteins give strength.
The points to remember
- Globular proteins fold into a compact, rounded shape.
- They are usually soluble, with polar R-groups on the surface, and have a precise shape that binds specific molecules.
- Fibrous proteins are long and narrow: chains lie side by side, often in repeating patterns.
- They are usually insoluble and strong, so they give support and strength.
Remember it as: Globular: a ball that dissolves. Fibrous: a rope that holds.
| Feature | Globular (insulin) | Fibrous (collagen) |
|---|---|---|
| Shape | compact, rounded | long, narrow fibres |
| In water | soluble | insoluble |
| Sequence | varied, no simple repeats | repeating sequence |
| Job | signalling, catalysis, transport | strength and support |
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The big idea: Insulin and collagen show how shape suits function: one must dissolve and fit a receptor, the other must resist being pulled.
The points to remember
- Insulin is globular: small and soluble, so it travels in blood plasma.
- Its precise shape fits the insulin receptor on target cells, so it can signal them to take up glucose.
- Collagen is fibrous: three chains wind into a triple helix, and many helices pack side by side.
- That makes it strong under tension and insoluble: ideal for tendons, ligaments, skin and bone.
Remember it as: Insulin fits; collagen holds.
Insulin (globular)
- Compact, soluble
- Carried in blood plasma
- Precise shape binds its receptor
Collagen (fibrous)
- Long triple helices
- Insoluble, strong under tension
- Tendons, ligaments, skin, bone
Real example: scurvy: Without vitamin C, the body cannot make stable collagen. In scurvy, gums bleed and wounds fail to heal, because the tissues lose the strength that collagen fibres give them.
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How it is asked: Paper 1A shows diagrams of proteins and asks you to describe each (globular, fibrous, conjugated, a level of structure). Paper 2 can ask you to compare insulin and collagen.
Three protein diagrams: one is four subunits each with a small non-protein group, one is a long triple helix, one is a compact folded chain. Which row describes them?
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And a three-mark Compare and contrast:
Compare and contrast the structure of insulin and collagen.
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