2.2.2Additional lessons
The Variety of Proteins — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 18 of 142
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.
Section 1 of 5Sources of Variety
Chain length
- Chain length Polypeptides range from a few amino acids to several thousand, so length is one source of variety.
Amino acid sequence
- Amino acid sequence At each position any of 20 amino acids can be added, so the order determines the identity of the protein.
Genetic control
- Genetic control The sequence of codons in mRNA, copied from a gene, dictates which amino acid is added at each position.
Number of combinations
- Number of combinations A chain of n amino acids has 20ⁿ possible sequences, which is more than the number of proteins that actually exist.
Structure follows sequence
- Structure follows sequence The sequence decides how the chain folds, and the shape of the protein decides its function.
Section 2 of 5Examples of Polypeptides
Examples of Polypeptides
Rubisco
- Rubisco Enzyme that fixes carbon dioxide in photosynthesis;
- a globular protein with 16 polypeptide subunits found in all leaves.
Insulin
- Insulin Hormone secreted by beta cells of the pancreas, made of two short polypeptide chains and acting on cell receptors.
Immunoglobulins
- Immunoglobulins Antibodies that bind a specific antigen, with shapes that match different pathogens and toxins.
Rhodopsin
- Rhodopsin Light-sensitive pigment in rod cells of the retina, held in the membrane.
Collagen
- Collagen Fibrous protein of three polypeptides wound together that strengthens skin, bones and teeth.
Spider silk
- Spider silk Fibrous protein that keeps its properties across wide temperature ranges and is very strong for its mass.
Section 3 of 5Roles of Proteins
Because proteins have many forms, they can carry out nearly every cell function.
1
Catalysis: enzymes bind substrates at the active site and speed up specific reactions.
2
Transport: carrier and channel proteins move ions and molecules across membranes, and haemoglobin carries oxygen.
3
Structure: fibrous proteins such as collagen and keratin give strength and support to tissues.
4
Signalling and defence: hormones, receptors and antibodies carry messages and protect against pathogens.
5
Movement and adhesion: actin and myosin allow muscle contraction, and adhesion proteins hold cells together.
Section 4 of 5Worked Example
Question How many different sequences are possible for a polypeptide that is 5 amino acids long, if each position can be any of the 20 amino acids?
- Each of the five positions has 20 options.
- Multiply the options for each position20 × 20 × 20 × 20 × 20.
- This equals 20⁵ = 3 200 000.
Answer 3 200 000 different sequences are possible for a pentapeptide, which shows how quickly variety grows with chain length.
Section 5 of 5Exam Checklist & Tips
2.2.2 – Key Points for the Exam
- Polypeptide A chain of amino acids joined by peptide bonds.
- Globular protein A protein folded into a compact, roughly spherical shape that is usually soluble.
- Fibrous protein A long, strand-like protein that is usually insoluble and structural.
- Codon A triplet of bases in mRNA that specifies one amino acid.
- Antibody An immunoglobulin that binds a specific antigen.
Common Mistakes
- Writing that there are 20 proteins when there are 20 amino acids but a vast number of proteins.
- Forgetting that the order of amino acids, not only their number, determines the identity of the protein.
- Treating all proteins as enzymes; many are structural, transport or signalling proteins.
- Saying codons code for proteins; each codon specifies a single amino acid.
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