AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Amino acids are the monomers from which proteins are made. The general…
Full specification wording
General properties of proteins: Amino acids are the monomers from which proteins are made. The general structure of an amino acid as: where NH represents an amine group, COOH represents a carboxyl group and R represents a side chain. The twenty amino acids that are common in all organisms differ only in their side group.
Amino acids are the monomers from which proteins are made.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 1 of 9
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Amino acids are the monomers from which proteins are made. Each amino acid has a central carbon atom bonded to four groups: an amine group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain called the R group. The amine and carboxyl groups must be drawn on the same central carbon. The twenty amino acids common to all organisms differ only in their R group. For example, glycine has a hydrogen atom as its R group, whereas cysteine has a side chain containing sulfur. The sequence of R groups influences a protein's properties because R groups vary in size, charge and polarity, so they determine the bonds that form when the protein folds.
What earns the marks
- Drawing the general structure of an amino acid with the amine group (-NH2), carboxyl group (-COOH), hydrogen atom and R group all bonded to the same central carbon.
- Labelling the amine group, the carboxyl group and the R group correctly on the general structure.
- Stating that the twenty amino acids common to all organisms differ only in their R group or side chain.
- Stating that amino acids are the monomers from which proteins are made.
- Explaining that R groups vary in size, charge and polarity, so they determine the bonds that form when the protein folds.
Common mistakes and how to put them right
Drawing the amine and carboxyl groups on different carbon atoms.
Put it right: Both groups, plus the hydrogen atom and R group, must be bonded to the same central carbon.
Saying the twenty amino acids differ in their amine or carboxyl groups.
Put it right: They differ only in their R group or side chain.
Writing carbonyl instead of carboxyl for the -COOH group.
Put it right: The carboxyl group is -COOH, and the amine group is -NH2.
Describing the R group vaguely as 'the rest of the molecule' with no reference to it being variable.
Put it right: State that the R group is the variable side chain that differs between amino acids.
Exam tips
- Learn the general structure well enough to draw it from memory, labelling the amine group, carboxyl group, hydrogen atom and R group.
- When discussing protein structure, remember that the R groups are the variable part that determines specific bonding interactions, such as ionic bonds, hydrogen bonds and disulfide bridges.
Try the lesson
The start of the General properties of proteins: Amino acids are the monomers from which proteins are made. The general structure of an amino acid as: where NH represents an amine group, COOH represents a carboxyl group and R represents a side chain. The twenty amino acids that are common in all organisms differ only in their side group. lesson, made from checked teaching notes for this specification point.
Have a go first
Two different amino acids are compared. Which statement explains why they are different amino acids rather than the same one?
Listen
Amino acids are the monomers from which proteins are made. Each amino acid has a central carbon atom bonded to four groups: an amine group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain called the R group. The amine and carboxyl groups must be drawn on the same central carbon. The twenty amino acids common to all organisms differ only in their R group. For example, glycine has a hydrogen atom as its R group, whereas cysteine has a side chain containing sulfur. The sequence of R groups influences a protein's properties because R groups vary in size, charge and polarity, so they determine the bonds that form when the protein folds. A strong answer earns marks for these points. drawing the general structure of an amino acid with the amine group (-NH2), carboxyl group (-COOH), hydrogen atom and R group all bonded to the same central carbon. labelling the amine group, the carboxyl group and the R group correctly on the general structure. stating that the twenty amino acids common to all organisms differ only in their R group or side chain. stating that amino acids are the monomers from which proteins are made. A common slip: drawing the amine and carboxyl groups on different carbon atoms. Instead, both groups, plus the hydrogen atom and R group, must be bonded to the same central carbon. Exam tip: Learn the general structure well enough to draw it from memory, labelling the amine group, carboxyl group, hydrogen atom and R group.
Key words
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amino acids
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general structure of an amino acid
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R group
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twenty common amino acids
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See every definition
- amino acids
- The monomers from which proteins are made.
- general structure of an amino acid
- A central carbon atom bonded to an amine group, a carboxyl group, a hydrogen atom and a variable R group.
- R group
- The variable side chain that differs between amino acids.
- twenty common amino acids
- The amino acids common to all organisms, which differ only in their R group or side chain.
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