2.2.2Inquiry Process
The Variety of Proteins — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 44 of 160
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 6Sequence Variety
Small changes in the order of units give completely different molecules, in the same way letters make many words.
Twenty amino acids
- Twenty amino acids Cells use 20 different amino acids, and any of them can occupy any position in a polypeptide chain.
Length
- Length Polypeptides vary from fewer than 50 to many thousands of amino acids.
Number of sequences
- Number of sequences For a chain of n amino acids there are 20ⁿ possible sequences, which becomes astronomically large.
Genetic control
- Genetic control Only a fraction of possible sequences are actually produced, as each sequence is coded for by a gene.
| Chain length | Possible sequences |
|---|---|
| 2 | 400 |
| 3 | 8 000 |
| 4 | 160 000 |
| 10 | About 1.0 × 10¹³ |
Part 2 of 6Functions of Proteins
Functions of Proteins
Enzymes
- Enzymes Biological catalysts such as amylase and DNA polymerase that speed up reactions by lowering activation energy.
Structure
- Structure Collagen and keratin give strength to tissues, hair and nails, while tubulin forms microtubules.
Transport
- Transport Haemoglobin carries oxygen, and membrane channel and pump proteins move substances across membranes.
Signalling and defence
- Signalling and defence Insulin acts as a hormone, receptors bind signals, and antibodies recognise pathogens.
Movement
- Movement Actin and myosin interact to cause muscle contraction.
Part 3 of 6Genome and Proteome
1
The genome is the complete set of genes of an organism.
2
The proteome is the full set of proteins expressed by a cell, tissue or organism at a given time.
3
Different cell types express different genes, so their proteomes differ even though the genome is the same.
4
A proteome can contain more different proteins than there are genes because of alternative splicing and chemical modification.
5
Comparing proteomes under different conditions shows how cells respond to disease or environment.
Part 4 of 6Why Variety Matters
Function from shape
- Function from shape Each sequence folds into a specific shape, and shape determines what the protein can bind or do.
Adaptability
- Adaptability A huge number of possible proteins allows evolution to produce new functions by mutation and selection.
Specificity
- Specificity Enzymes and receptors can recognise a single type of molecule among thousands.
Information link
- Information link Because the amino acid order comes from the DNA sequence, mutation can change the protein.
Part 5 of 6Worked Example
Question How many different sequences are possible for a polypeptide of 4 amino acids, using the 20 standard amino acids?
- Each of the 4 positions can hold any of 20 amino acids.
- Total = 20 × 20 × 20 × 20 = 20⁴.
- 20⁴ = 160 000.
Answer 160 000 possible sequences.
Part 6 of 6Exam Checklist & Tips
2.2.2 – Key Points for the Exam
- Proteome All the proteins expressed by a cell, tissue or organism at a given time.
- Genome The complete set of genetic information of an organism.
- Enzyme A protein that acts as a biological catalyst.
- Antibody A protein made by white blood cells that binds to a specific antigen.
Common Mistakes
- Using the number 20 × n instead of 20ⁿ when calculating possible sequences.
- Equating the genome with the proteome.
- Saying every protein is an enzyme; many have structural or transport roles.
- Assuming different cells have different DNA; they differ in gene expression.
Last updated:
\n