Amino acids: core structure
A generalized amino acid has a central alpha (α) carbon bonded to H, an amine group (-NH₂), a carboxyl group (-COOH) and a variable R-group.
The R-group is the only part that differs between amino acids.
You should be able to draw and label a generalized amino acid.
Exam tip: if asked what makes amino acids different, the answer is the R-group.
A generalized α-amino acid is shown with the amine group, carboxyl group, hydrogen, alpha carbon and variable R-group. This is the exact model you need to reproduce in IB Biology diagrams. Source
Peptide bonds and condensation
Condensation reaction: two amino acids join and water is removed.
The bond formed is a peptide bond.
Dipeptide = 2 amino acids joined together.
Polypeptide = many amino acids joined together.
Word equation: amino acid + amino acid → dipeptide + water.
You should be able to draw a generalized dipeptide and identify the peptide bond.
The image is the main file preview near the top of the page. Look for the title/description “Formation of a peptide bond”.
This diagram shows two amino acids joining by condensation to form a peptide bond, with water released. It is useful for recognizing exactly where the bond forms between the carboxyl group of one amino acid and the amine group of another. Source
Dietary amino acids
Essential amino acids cannot be synthesized by the body and must be obtained from food.
Non-essential amino acids can be synthesized from other amino acids.
Vegan diets must be planned carefully so all essential amino acids are obtained.
You do not need to memorize specific examples of essential vs non-essential amino acids for this topic.
Protein diversity
There are 20 amino acids coded for in the genetic code.
Peptide chains can vary in length from a few amino acids to thousands.
Amino acids can be arranged in any order.
This gives an effectively enormous variety of possible polypeptides.
Exam link: different sequence → different shape → different function.
Effect of pH and temperature
Temperature and pH can alter protein structure.
Loss of normal protein shape is called denaturation.
Denatured proteins usually lose function because shape determines function.
Heat and extreme pH mainly disrupt the bonds/interactions maintaining higher levels of structure, not the primary peptide sequence in typical exam explanations.
This image shows a folded protein becoming uncoiled/unfolded when environmental conditions change. It helps explain why high temperature or extreme pH causes loss of shape and therefore loss of function. Source
HL only: R-groups and protein diversity
R-groups determine the properties of amino acids and therefore the properties of the finished polypeptide.
R-groups may be hydrophobic or hydrophilic.
Hydrophilic R-groups may be polar, charged, acidic or basic.
Do not spend time memorizing many specific side chains unless your teacher asks; focus on how R-group properties affect folding and function.
HL only: Primary structure
Primary structure = the sequence of amino acids in a polypeptide.
The exact order and position of amino acids determines the final 3D conformation.
Therefore proteins have precise, predictable and repeatable structures.
A change in primary structure can change all higher levels of structure.
HL only: Secondary structure
Secondary structure is local folding into alpha (α) helices and beta-pleated sheets.
It is stabilized by hydrogen bonds in regular positions along the backbone.
α-helix = coiled structure.
β-pleated sheet = folded/pleated structure.
This diagram summarizes the four levels of protein structure: primary, secondary, tertiary and quaternary. It is especially helpful for comparing α-helices, β-pleated sheets, full 3D folding, and multiple-subunit proteins in one place. Source
HL only: Tertiary structure
Tertiary structure = overall 3D shape of one polypeptide.
Stabilized by:
Hydrogen bonds
Ionic bonds
Disulfide bonds (covalent) between cysteines
Hydrophobic interactions
Some R-groups become positively or negatively charged and can form ionic bonds.
In water-soluble globular proteins, hydrophobic amino acids are usually folded into the core.
In integral membrane proteins, hydrophobic regions help the protein sit inside the membrane.
HL only: Quaternary structure
Quaternary structure = association of two or more polypeptide subunits.
Non-conjugated proteins named in the syllabus: insulin and collagen.
Conjugated protein named in the syllabus: haemoglobin.
A conjugated protein contains a non-polypeptide component as part of its functional structure.
This page shows haemoglobin, a protein made of four subunits, illustrating quaternary structure. It is a strong example for IB Biology because it links multiple polypeptide chains to a specific function: oxygen transport. Source
Globular vs fibrous proteins
Globular proteins are generally compact and often soluble.
Fibrous proteins are generally long, repetitive and used for structural roles.
Insulin is a good example of a relatively compact functional protein.
Collagen is a fibrous structural protein with high tensile strength.
Always link shape to job in exam answers.
Key examples to know
Insulin → example of a non-conjugated protein; useful for showing form related to function.
Collagen → example of a fibrous, non-conjugated structural protein.
Haemoglobin → example of a conjugated protein with quaternary structure.
Cysteine → important because two cysteines can form disulfide bonds.
Checklist: can you do this?
Draw and label a generalized amino acid and a generalized dipeptide.
Write the word equation for condensation forming a dipeptide.
Explain how primary structure determines higher levels of protein structure.
Compare α-helix, β-pleated sheet, tertiary and quaternary structure.
Predict how changes in pH or temperature can cause denaturation and loss of function.
Exam-ready links to remember
Amino acid sequence determines 3D shape.
R-group properties determine folding.
Bonding and interactions stabilize structure.
Shape determines function.
Heat / extreme pH can denature proteins.
Hydrophobic R-groups tend to be hidden inside globular proteins in aqueous environments.
Multiple subunits give rise to quaternary structure.
Strong answers always connect specific structural features to specific biological functions.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.