What does Unit 1 of WJEC A-Level Biology cover?

Unit 1, Basic Biochemistry and Cell Organisation, is the foundation of the AS year. It covers the chemical elements and biological molecules (water, carbohydrates, lipids and proteins), cell structure and organisation, cell membranes and transport, biological reactions and enzymes, nucleic acids and replication, and cell division by mitosis and meiosis. Everything in the later units builds on these ideas.

How is Unit 1 examined in WJEC A-Level Biology?

Unit 1 is an AS written paper of structured short-answer questions and at least one extended response, drawing directly on the specification statements. Expect biochemical test recall, organelle functions, membrane transport explanations, enzyme graph interpretation, and a magnification or replication calculation. The content is also synoptic, so it reappears in A2 questions.

What are the most common mistakes in Unit 1?

The big ones are confusing condensation with hydrolysis, mixing up rough and smooth endoplasmic reticulum, defining osmosis by concentration instead of water potential, saying enzymes are killed rather than denatured above the optimum, and pairing DNA bases incorrectly. Precise wording earns the marks, so learn the exact distinctions.

How should I revise the biological molecules content?

Learn each molecule as a monomer, its polymer, the bond that joins them and the reaction involved (condensation), then the food test that detects it. Make a single table for Benedict's, iodine, biuret and the emulsion test with the colour change and conditions. Practise drawing condensation and hydrolysis as the reverse of each other.

Why is enzyme work so important in this unit?

Enzymes link every topic in the course, from digestion and respiration to replication and the kidney. Unit 1 sets up the induced-fit model, activation energy, and the effects of temperature, pH, substrate concentration and inhibitors. Mastering the graphs and the denaturation explanation here pays off across both years.

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WJEC A-Level Biology Unit 1 Basic Biochemistry and Cell Organisation: a deep dive on molecules, cells, membranes, enzymes, DNA and division

A deep-dive WJEC A-Level Biology guide to Unit 1, Basic Biochemistry and Cell Organisation. Covers biological molecules and water, cell structure, membranes and transport, enzymes, nucleic acids and replication, and cell division, with the exam patterns WJEC repeats.

Generated by Claude Opus 4.820 min readWJECUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What Unit 1 actually demands
Chemical elements and biological molecules
Cell structure and organisation
Cell membranes and transport
Biological reactions and enzymes
Nucleic acids and replication
Cell division
How Unit 1 is examined
Check your knowledge

What Unit 1 actually demands

Basic Biochemistry and Cell Organisation is the structural and chemical foundation of WJEC A-Level Biology. The unit runs from the molecules that build living things, through the cells they make up, how substances cross membranes, how enzymes control reactions, how DNA stores and copies information, and how cells divide. Examiners test two linked skills: precise recall of structures, molecules and processes, and the application of those facts to data, calculations and unfamiliar contexts.

This guide walks through all six clusters of the unit in a sensible build order, then sets out the exam patterns WJEC repeats. Each cluster has a matching dot-point page with practice questions; this overview ties them together.

Chemical elements and biological molecules

Living things are built mainly from carbon, hydrogen, oxygen and nitrogen, assembled into water, carbohydrates, lipids, proteins and nucleic acids. Monomers join into polymers by condensation (forming a bond and releasing water) and split by hydrolysis (adding water). Learn each molecule as a story: monosaccharides join by glycosidic bonds into polysaccharides (starch, glycogen, cellulose); glycerol and three fatty acids join by ester bonds into a triglyceride; amino acids join by peptide bonds into proteins with primary, secondary, tertiary and quaternary structure.

Water deserves its own attention. Its polarity gives it a role as a solvent, transport medium and reactant, plus a high specific heat capacity and latent heat of vaporisation. Finish with the biochemical tests: Benedict's for reducing sugars, iodine for starch, the biuret test for proteins, and the emulsion test for lipids. The food tests are almost guaranteed marks.

Cell structure and organisation

A eukaryotic cell is defined by a membrane-bound nucleus and membrane-bound organelles. Know the function of each: the nucleus, mitochondria, ribosomes (80S), rough and smooth endoplasmic reticulum, the Golgi body, lysosomes, and the plant extras of cellulose cell wall, chloroplasts and permanent vacuole. Prokaryotic cells are the contrast: no nucleus, no membrane-bound organelles, circular DNA, 70S ribosomes and a peptidoglycan wall.

The maths here is magnification. Magnification equals image size divided by actual size, and the commonest error is forgetting to convert units first. Microscopy also distinguishes the light microscope (resolves to about 200 nanometres) from the electron microscope, which reveals ultrastructure.

Cell membranes and transport

The fluid-mosaic model describes the membrane as a fluid phospholipid bilayer studded with proteins. Five processes move substances across: simple diffusion and facilitated diffusion (both passive, down the gradient), osmosis (water from higher to lower water potential), active transport (against the gradient, using carrier proteins and ATP), and bulk transport by endocytosis and exocytosis. The reliable trap is defining osmosis by concentration rather than water potential, and forgetting that facilitated diffusion needs no ATP.

Biological reactions and enzymes

Enzymes lower activation energy by providing a complementary active site. The induced-fit model refines the lock-and-key idea: the active site moulds around the substrate to form an enzyme-substrate complex. Master the four rate graphs (temperature, pH, substrate concentration, enzyme concentration) and be able to explain denaturation above the optimum in terms of broken bonds and a changed active-site shape. Distinguish competitive inhibitors (bind the active site, reversed by more substrate) from non-competitive inhibitors (bind elsewhere, not reversed).

Nucleic acids and replication

DNA and RNA are polymers of nucleotides, each made of a pentose sugar, a phosphate and a base. Learn the base pairing precisely: adenine with thymine (two hydrogen bonds), cytosine with guanine (three). ATP is the energy currency, releasing usable energy when hydrolysed to ADP and phosphate. DNA copies itself by semi-conservative replication: helicase unwinds and separates the strands, each acts as a template, and DNA polymerase joins free nucleotides into the new strand, so each daughter molecule keeps one original strand.

Cell division

The cell cycle is interphase (G1, S, G2) followed by mitosis and cytokinesis, with DNA replicating in the S phase, not in mitosis. Mitosis (prophase, metaphase, anaphase, telophase) gives two identical diploid cells for growth and repair. Meiosis gives four genetically different haploid gametes through two divisions, generating variation by crossing over and independent assortment, with random fertilisation adding more.

How Unit 1 is examined

A typical WJEC profile for this unit:

Recall and short answer. Naming bonds and reactions, stating organelle functions, listing the food tests and colour changes, and pairing DNA bases.
Maths. A magnification or actual-size calculation (watch the unit conversion).
Graph and data questions. Interpreting enzyme rate graphs and explaining the shape, or comparing transport processes.
Extended answers. Triglyceride formation, semi-conservative replication, the structure-function of membranes, and the sources of variation in meiosis are all predictable.

Check your knowledge

A mix of recall and application questions covering the whole of Unit 1. Attempt them under timed conditions, then check against the solutions.

Describe how a triglyceride is formed from glycerol and fatty acids, naming the bond and reaction. (3 marks)
Give three differences between a prokaryotic and a eukaryotic cell. (3 marks)
State the colour change and conditions for the Benedict's test for reducing sugars. (2 marks)
A cell image is 40 mm long at a magnification of times 800. Calculate the actual length in micrometres. (2 marks)
Explain the effect of increasing substrate concentration on the rate of an enzyme-controlled reaction. (3 marks)
Explain why DNA replication is described as semi-conservative. (3 marks)
Describe two ways in which meiosis produces genetic variation. (4 marks)
Explain the difference between facilitated diffusion and active transport. (3 marks)

Solutions

Q1: One glycerol joins to three fatty acids by three condensation reactions. Each fatty acid forms an ester bond between its carboxyl group and a hydroxyl group of glycerol, releasing one water molecule, so three water molecules are released in total.
Q2: Any three: prokaryotes have circular DNA free in the cytoplasm whereas eukaryotes have a membrane-bound nucleus; prokaryotes lack membrane-bound organelles; prokaryotic ribosomes are 70S versus 80S; prokaryotic walls are peptidoglycan; prokaryotes may have plasmids.
Q3: Add Benedict's reagent and heat in a water bath. A positive result changes from blue to brick-red (orange/green to red depending on amount).
Q4: Convert: 40 mm equals 40 000 micrometres. Actual size equals image divided by magnification equals 40 000 divided by 800 equals 50 micrometres.
Q5: As substrate concentration rises, more enzyme-substrate complexes form, so the rate increases. Once all active sites are occupied (saturation), adding more substrate has no further effect and the rate plateaus.
Q6: Each new DNA molecule contains one original parental strand and one newly synthesised strand. The strands separate, each acts as a template, and complementary base pairing builds the new strand, so half of each molecule is conserved.
Q7: Crossing over in prophase I exchanges sections of chromatids between homologous chromosomes, making new allele combinations. Independent assortment in metaphase I lines up homologous pairs randomly, so gametes get different mixes of maternal and paternal chromosomes.
Q8: Both use transport proteins. Facilitated diffusion is passive and moves substances down the concentration gradient with no ATP. Active transport moves substances against the gradient using carrier proteins and ATP from respiration.