Eduqas A-Level Biology Core Concepts: a deep dive on molecules, cells, membranes, enzymes, nucleic acids and division

What the Core Concepts actually demand

The Core Concepts are the structural and chemical foundation of Eduqas A-Level Biology, and Eduqas samples them in every paper. They run from the molecules that build living things, through the cells they make up, how substances cross membranes, how enzymes control reactions, how DNA stores, copies and expresses 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 in a sensible build order, then sets out the exam patterns Eduqas repeats. Each cluster has a matching dot-point page with practice questions; this overview ties them together.

Biological compounds

Living things are built mainly from carbon, hydrogen, oxygen and nitrogen, assembled into water, carbohydrates, lipids and proteins. 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, latent heat of vaporisation and cohesion. 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 magnification from resolution, and the light microscope from the electron microscope, which reveals ultrastructure because of its far shorter wavelength.

Cell membranes and transport

The fluid-mosaic model describes the membrane as a fluid phospholipid bilayer studded with proteins, cholesterol and glycoproteins. 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.

Enzymes

Enzymes are globular protein catalysts that lower activation energy by forming enzyme-substrate complexes; the induced-fit model explains specificity and catalysis better than lock-and-key. Master the four rate factors (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, overcome by more substrate, maximum rate unchanged) from non-competitive inhibitors (bind elsewhere, not overcome, maximum rate reduced).

Nucleic acids and protein synthesis

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). DNA copies itself by semi-conservative replication: helicase unwinds and separates the strands, each acts as a template, and DNA polymerase joins free nucleotides, so each daughter molecule keeps one original strand. Protein synthesis is transcription (mRNA copied from the template in the nucleus) then translation (ribosomes reading codons, tRNA bringing amino acids), with the code triplet, degenerate, non-overlapping and universal.

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. The mitotic index is the proportion of cells in mitosis.

How the Core Concepts are examined

A typical Eduqas profile for this content:

• 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) or a mitotic index.
• Graph and data questions. Interpreting enzyme rate graphs and explaining the shape, or comparing transport processes.
• Extended answers. Triglyceride formation, semi-conservative replication, the secretory pathway, the structure-function of membranes, and the sources of variation in meiosis are all predictable, and any of them can become the levels-of-response QER question.

Check your knowledge

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

1. Describe how a triglyceride is formed from glycerol and fatty acids, naming the bond and reaction. (3 marks)
2. State two structural differences between a prokaryotic and a eukaryotic cell. (2 marks)
3. State the colour change and conditions for the Benedict's test for reducing sugars. (2 marks)
4. A cell image is 40 mm long at a magnification of times 800. Calculate the actual length in micrometres. (2 marks)
5. Explain the effect of a non-competitive inhibitor on the maximum rate of an enzyme reaction. (3 marks)
6. Explain why DNA replication is described as semi-conservative. (2 marks)
7. Describe two ways in which meiosis produces genetic variation. (4 marks)
8. Define osmosis in terms of water potential. (2 marks)