Membranes, Proteins, DNA and Gene Expression
Edexcel International A-Level Biology· 2018 Specification Statements 2.1–2.18· 45 min read
1. Cell Membranes and Transport Mechanisms★★☆☆☆⏱ 10 min
Fluid Mosaic Model
Model of the cell membrane consisting of a flexible phospholipid bilayer with scattered embedded proteins, glycoproteins and cholesterol that can move laterally within the layer.
Gas exchange surfaces are adapted for efficient diffusion via high surface area to volume ratio, thin exchange surface, and maintained concentration gradient of gases, as described by Fick's Law. Mammalian lungs show all these adaptations, with millions of small alveoli providing large surface area, thin epithelial walls, and a constant blood flow and ventilation to maintain concentration gradients.
A student investigates the effect of increasing ethanol concentration on beetroot cell membrane permeability. Explain why the absorbance of purple pigment in the surrounding solution increases as ethanol concentration rises.
- 1
Ethanol is a non-polar solvent that disrupts the phospholipid bilayer of cell membranes, as phospholipids dissolve in ethanol.
- 2
As ethanol concentration increases, more of the bilayer breaks down, creating gaps that allow the large purple betalain pigment to leak out of the vacuole and cell into the surrounding solution.
- 3
Higher ethanol concentrations also denature membrane transport proteins, further compromising membrane integrity, leading to higher pigment concentration and higher absorbance readings.
Exam tip:
Always explicitly state that facilitated diffusion is passive (no ATP, down concentration gradient) to avoid losing marks in transport questions.
2. Protein Structure and Enzyme Function★★★☆☆⏱ 12 min
Amino Acid
Monomer of proteins, consisting of an amine group, carboxyl group, hydrogen atom and variable R group bonded to a central carbon atom. R groups do not need to be memorised for IAL Unit 1.
Amino acids join via condensation reactions to form peptide bonds, creating polypeptide chains. Protein structure has four levels: primary (amino acid sequence), secondary (alpha helix or beta pleated sheet held by hydrogen bonds), tertiary (3D folded shape), and quaternary (multiple polypeptide chains bound together). Globular proteins are soluble (e.g. haemoglobin, enzymes) while fibrous proteins are insoluble and structural (e.g. collagen). Enzymes are globular proteins with a specific 3D active site that binds complementary substrates, lowering activation energy to speed up reactions, functioning either inside (intracellular) or outside (extracellular) cells.
Compare the structure and function of haemoglobin (globular) and collagen (fibrous) proteins.
- 1
Haemoglobin is a compact, spherical globular protein, soluble in water, made of four polypeptide chains each bound to an iron-containing haem group.
- 2
Its structure is adapted to transport oxygen in red blood cells: its solubility allows it to move freely in blood plasma, and haem groups reversibly bind oxygen.
- 3
Collagen is a long, linear fibrous protein, insoluble in water, made of three polypeptide chains twisted into a strong triple helix held by cross-links between chains.
- 4
Its structure is adapted for structural support: the triple helix gives high tensile strength, making it ideal for tendons, bone and skin tissue.
Exam tip:
Name exact bonds for each level of protein structure: peptide bonds for primary, hydrogen bonds for secondary, and hydrogen, ionic, disulfide bridges and hydrophobic interactions for tertiary/quaternary structure.
3. DNA Structure and Replication★★★☆☆⏱ 10 min
Mononucleotide
Monomer of nucleic acids, consisting of a pentose sugar (deoxyribose for DNA, ribose for RNA), phosphate group and nitrogenous base. DNA uses A, T, C, G bases; RNA replaces T with uracil (U).
Nucleotides join via phosphodiester bonds to form polynucleotide strands. DNA forms a double helix held by hydrogen bonds between complementary base pairs: A pairs with T via 2 hydrogen bonds, C pairs with G via 3 hydrogen bonds. DNA replicates via the semi-conservative mechanism, where each new DNA molecule contains one original parent strand and one new strand, catalysed by DNA polymerase. The Meselson-Stahl experiment using nitrogen isotopes provides experimental evidence for semi-conservative replication.
Explain how the Meselson-Stahl experiment supports the semi-conservative model of DNA replication.
- 1
E. coli were grown for many generations in heavy ¹⁵N medium, so all DNA contained ¹⁵N, then transferred to light ¹⁴N medium to replicate.
- 2
After 1 replication, DNA showed a single intermediate density band, ruling out the conservative model (which would produce one heavy and one light band).
- 3
After 2 replications, DNA showed one intermediate and one light band, ruling out the dispersive model (which would only produce intermediate bands), confirming semi-conservative replication.
Exam tip:
You only need to name DNA polymerase as an enzyme involved in DNA replication for Unit 1; helicase, ligase and other replication enzymes are not assessed here.
4. Gene Expression and Mutations★★★★☆⏱ 10 min
Gene
A sequence of DNA bases that codes for the amino acid sequence of a specific polypeptide, via the processes of transcription and translation.
The genetic code is triplet (3 bases per amino acid), non-overlapping, and degenerate (multiple codons code for the same amino acid). Protein synthesis occurs in two stages: transcription (RNA polymerase builds a complementary mRNA strand from the DNA template strand in the nucleus) and translation (mRNA binds to a ribosome, tRNA molecules carry complementary amino acids matching mRNA codons, forming a polypeptide chain). Mutations are changes to the DNA base sequence, including substitution, insertion and deletion; many are silent, some cause disorders like cystic fibrosis or increase cancer risk.
A substitution mutation changes the DNA triplet TTA to TTG. The mRNA codon for TTA is AAU (asparagine), and the codon for TTG is AAC (also asparagine). Explain the effect of this mutation on the final protein.
- 1
The genetic code is degenerate, meaning multiple codons can code for the same amino acid.
- 2
This mutation changes the DNA triplet, but the resulting mRNA codon still codes for asparagine, the same amino acid as the original sequence.
- 3
This is a silent mutation, so the amino acid sequence of the protein is unchanged, and protein structure and function remain normal.
Exam tip:
Insertion and deletion mutations cause a frameshift, changing all amino acids coded for after the mutation point, so they usually have a much larger effect than substitution mutations.
5. Inheritance, Cystic Fibrosis and Genetic Screening★★★★☆⏱ 13 min
Monohybrid Inheritance
Inheritance of a single characteristic controlled by one gene with two or more alleles, which can be autosomal or X-linked (sex-linked).
Cystic fibrosis is an autosomal recessive disorder caused by a mutation in the CFTR gene, which impairs chloride ion transport, leading to thick sticky mucus that reduces gas exchange efficiency, blocks digestive ducts and reduces fertility. Sex-linked traits like red-green colour blindness are carried on the X chromosome, so males (XY) only need one copy of the recessive allele to show the trait, while females (XX) need two copies. Genetic screening includes carrier testing, pre-implantation genetic diagnosis (PGD), CVS and amniocentesis, with associated ethical issues including informed consent, risk of miscarriage, and stigma around genetic conditions.
Cystic fibrosis is an autosomal recessive disorder. A couple are both heterozygous carriers of the CF mutation. Calculate the probability their child will have cystic fibrosis.
- 1
Define alleles: F = normal dominant allele, f = recessive cystic fibrosis allele. Parental genotypes are both Ff.
- 2
Each parent can produce gametes with F or f alleles. A Punnett square gives offspring genotypes: 25% FF, 50% Ff, 25% ff.
- 3
Only the ff genotype causes cystic fibrosis, so the probability of an affected child is 1 in 4 (25%).
Exam tip:
For X-linked inheritance questions, always write alleles as superscripts on the X chromosome (e.g. Xᵇ for colour blindness) and note that males cannot be carriers of X-linked recessive traits.
6. Common Pitfalls
Wrong move:
Describing facilitated diffusion as active transport.
Why:
Facilitated diffusion uses proteins but moves molecules down a gradient without ATP, so it is passive.
Correct move:
Explicitly label facilitated diffusion as passive, and reserve active transport for ATP-dependent processes moving molecules against their gradient.
Wrong move:
Stating uracil is present in DNA or thymine in RNA.
Why:
DNA uses A/T/C/G bases, RNA uses A/U/C/G bases; this is a common mark-losing error.
Correct move:
Always match bases to nucleic acid type: A-T for DNA, A-U for RNA.
Wrong move:
Writing X-linked male genotypes as 'Bb' instead of XᴮY/XᵇY.
Why:
X-linked genes are only present on the X chromosome; the Y chromosome has no corresponding allele.
Correct move:
Write sex-linked alleles as superscripts on the X chromosome, with the Y chromosome for males carrying no allele.
Wrong move:
Claiming all mutations change protein structure.
Why:
The genetic code is degenerate, so many substitution mutations are silent, coding for the same amino acid.
Correct move:
Distinguish between silent, missense and nonsense mutations, and note most mutations have no phenotypic effect.
Wrong move:
Mixing up the timing and risk of CVS and amniocentesis.
Why:
Examiners regularly test knowledge of these screening procedures, and mixing details loses marks.
Correct move:
Remember: CVS = 10–13 weeks, 1% miscarriage risk; amniocentesis = 15–20 weeks, 0.5% risk.
Wrong move:
Calling phosphodiester bonds 'phosphate bonds' or 'sugar bonds'.
Why:
The exact bond name between nucleotides is required for marks in exam answers.
Correct move:
Always use the full term 'phosphodiester bond' when describing bonds in polynucleotide strands.
7. Quick Reference Cheatsheet
Key Concept | Core Details | Exam Must-Know |
|---|---|---|
Fluid Mosaic Model | Phospholipid bilayer with embedded proteins, glycoproteins, cholesterol; components move laterally | Cholesterol regulates membrane fluidity; glycoproteins for cell recognition |
Membrane Transport | Passive: diffusion, facilitated diffusion; Active: ATP-dependent transport, endo/exocytosis | Osmosis = free water movement down water potential gradient |
Protein Structure | Primary (peptide) → Secondary (H bonds) → Tertiary (mixed bonds) → Quaternary (multiple chains) | Globular = soluble (enzymes, haemoglobin); Fibrous = structural (collagen) |
DNA Replication | Semi-conservative, catalysed by DNA polymerase | Meselson-Stahl ¹⁵N/¹⁴N experiment proves semi-conservative replication |
Protein Synthesis | Transcription (DNA→mRNA, RNA polymerase) → Translation (mRNA→protein, ribosome/tRNA) | Genetic code: triplet, non-overlapping, degenerate |
Inheritance | Autosomal recessive (CF) needs 2 recessive alleles; X-linked recessive (colour blindness) needs 1 allele in males | X-linked alleles written as Xⁿ superscripts; males cannot be carriers |
Genetic Screening | Carrier test, PGD, CVS (early, higher risk), amniocentesis (late, lower risk) | Ethics: informed consent, miscarriage risk, stigma, selection debates |
8. Frequently Asked
What is the difference between carrier and channel proteins?
Channel proteins form hydrophilic pores across the membrane for passive diffusion of small charged/polar molecules. Carrier proteins bind to specific molecules, change shape, and can transport molecules passively (facilitated diffusion) or actively (using ATP).
Why are most substitution mutations silent?
The genetic code is degenerate, meaning multiple codons code for the same amino acid. Many substitution mutations change a codon to another that codes for the same amino acid, so the protein sequence is unchanged.
What are the key differences between CVS and amniocentesis?
CVS is performed at 10–13 weeks of pregnancy, has a ~1% risk of miscarriage, and gives results earlier. Amniocentesis is performed at 15–20 weeks, has a ~0.5% risk of miscarriage, and is slightly more accurate.
Going deeper
- syllabus_documentEdexcel IAL Biology 2018 SpecificationOfficial specification for Unit 1 Topic 2
- practical_guideCore Practicals 3 & 4 Step-by-Step Guide
What's Next
Now that you have mastered Unit 1 Topic 2, you are ready to progress to the remaining Unit 1 content on the cardiovascular system, diet, and risk of non-communicable diseases. This topic forms the foundation for higher-level concepts across the IAL Biology specification, including genetic engineering in Unit 5 and gene expression regulation in Unit 2. Make sure you are confident with Core Practicals 3 and 4, as they are frequently assessed in structured questions, and practice drawing genetic diagrams for monohybrid and sex-linked inheritance to avoid common mark-losing errors. Review past paper questions on the Meselson-Stahl experiment and fluid mosaic model, as these are common high-mark question topics.