The big idea: Twenty amino acids means twenty different R-groups. Sort them by how they behave with water and by charge, and you can predict what they do in a protein.
The points to remember
- The R-group is the only part that differs between amino acids, and R-groups differ in their chemistry.
- Hydrophobic R-groups are non-polar: water does not attract them.
- Hydrophilic R-groups are attracted to water. They are either polar (no charge) or charged.
- Charged R-groups are acidic (lose H⁺, become negative) or basic (gain H⁺, become positive).
Remember it as: Phobic or philic; polar or charged; acid minus, base plus.
Real example: sickle-cell haemoglobin: In sickle-cell anaemia, one charged R-group in each beta chain of haemoglobin is replaced by a hydrophobic one. The haemoglobin molecules stick together and the red blood cells bend into a sickle shape.
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The big idea: The protein's backbone is the same in every protein. Its R-groups are what make each protein different in shape and behaviour.
The points to remember
- Hydrophobic R-groups cluster together, away from water.
- Polar R-groups form hydrogen bonds with water and with each other.
- Charged R-groups form ionic bonds with R-groups of the opposite charge.
- So the order of R-groups decides how the chain folds, whether it dissolves, and what it can bind.
Remember it as: R-groups decide the fold, the fit and whether it dissolves.
| R-group | Water | Bonds it can form |
|---|---|---|
| Hydrophobic (non-polar) | repelled | hydrophobic interactions |
| Polar (uncharged) | attracted | hydrogen bonds |
| Acidic (negative) | attracted | ionic bonds, hydrogen bonds |
| Basic (positive) | attracted | ionic bonds, hydrogen bonds |
Hydrophilic is not the same as charged: Every charged R-group is hydrophilic, but not every hydrophilic R-group is charged: polar R-groups are hydrophilic with no overall charge.
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How it is asked: A two-mark Describe on how amino acid diversity gives proteins many forms, and questions that ask you to sort an R-group by its chemistry.
IB-style questionDescribe[2 marks]
Describe how the diversity of amino acids gives proteins a broad range of forms.
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See the answer broken into clear points, with guidance on how each point earns marks.
And a two-mark Deduce on a charged R-group:
IB-style questionDeduce[2 marks]
An R-group contains a carboxyl group, COOH. Deduce the charge it carries in a cell and the type of bond it can form.
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See the answer broken into clear points, with guidance on how each point earns marks.