2.2.4Theme A — Unity and diversity
Amino Acid Diversity — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 14 of 14 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Classes of R Group
The 20 amino acids differ only in the R group, but this difference is enough to determine protein behaviour.
Non-polar
- Non-polar R groups such as those of valine and leucine contain carbon and hydrogen only and are hydrophobic.
Polar uncharged
- Polar uncharged R groups such as serine contain hydroxyl groups that form hydrogen bonds with water.
Acidic
- Acidic Aspartic acid has an R group that carries a negative charge at cell pH, so it can form ionic bonds.
Basic
- Basic Lysine has an R group that carries a positive charge at cell pH, attracting negative charges.
| Class | Example | Behaviour in water |
|---|---|---|
| Non-polar | Valine, leucine | Repelled by water |
| Polar | Serine | Dissolves, forms hydrogen bonds |
| Acidic | Aspartic acid | Negatively charged |
| Basic | Lysine | Positively charged |
Part 2 of 5Effects on Folding
Effects on Folding
Hydrophobic core
- Hydrophobic core In soluble proteins, non-polar R groups are buried inside, away from water.
Hydrophilic surface
- Hydrophilic surface Polar and charged R groups lie on the outside where they interact with water, helping the protein dissolve.
Ionic bonds
- Ionic bonds Oppositely charged R groups attract each other and hold folds in place.
Special cases
- Special cases Cysteine can form disulfide bridges, and proline creates a bend in the chain.
Part 3 of 5Diversity in Membrane Proteins
The same principle applies in membranes, where location decides which R groups are needed.
Transmembrane regions
- Transmembrane regions Parts that cross the bilayer are rich in non-polar amino acids that interact with fatty acid tails.
Exposed regions
- Exposed regions Parts projecting into water have polar or charged R groups.
Channel interiors
- Channel interiors Channels for ions have polar amino acids lining the pore so charged particles can pass.
Active sites
- Active sites Specific R groups in the site bind substrates and take part in catalysis.
Part 4 of 5Essential Amino Acids
1
Plants and many microorganisms can synthesise all 20 amino acids.
2
Humans can make some amino acids from other molecules.
3
Nine amino acids cannot be made by adult humans and must be obtained from the diet.
4
These are called essential amino acids, and include leucine, lysine and tryptophan.
5
A diet with too little of an essential amino acid limits protein synthesis, even if total protein is sufficient.
Part 5 of 5Exam Checklist & Tips
2.2.4 – Key Points for the Exam
- R group The variable side chain that gives each amino acid its identity.
- Hydrophobic Repelled by water; non-polar.
- Hydrophilic Attracted to water; polar or charged.
- Essential amino acid An amino acid that an organism cannot make and must obtain in its diet.
- Disulfide bridge A covalent bond between the sulfur atoms of two cysteine residues.
Common Mistakes
- Saying the amino and carboxyl groups differ between amino acids; only the R group differs.
- Placing hydrophobic residues on the outside of soluble proteins.
- Saying all 20 amino acids are essential; adults can make eleven of them.
- Stating that all R groups are charged.
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