OCR A-Level Biology Module 2 Foundations in biology: cells, molecules, enzymes and division

What Module 2 actually demands

Foundations in biology is the structural and chemical bedrock of OCR A-Level Biology A. The module runs from the molecules of life and the cells they build, through how enzymes control metabolism and how DNA is copied and read, to how cells divide and organise into tissues. The examiners test two linked skills: precise recall of structures, molecules and processes, and the application of those facts to unfamiliar data, calculations and experimental contexts.

This guide walks through all six clusters of the module in the order most students find easiest to build on, then sets out the exam patterns OCR repeats. Each cluster has a matching dot-point page with practice questions; this overview ties them together.

Cell structure and microscopy

A eukaryotic cell is defined by a true membrane-bound nucleus and membrane-bound organelles; membranes compartmentalise it so incompatible reactions run side by side. You must identify and give the function of the nucleus (envelope, pores, chromatin and nucleolus), rough and smooth endoplasmic reticulum, the Golgi apparatus, mitochondria (cristae, the site of respiration), chloroplasts (thylakoids and stroma, the site of photosynthesis), ribosomes (80S, the site of translation), lysosomes, centrioles, the cell wall and the cytoskeleton.

The most examined idea is the protein production and secretion pathway: a gene is transcribed in the nucleus, ribosomes on the rough endoplasmic reticulum translate the mRNA, a vesicle carries the protein to the Golgi apparatus, which modifies and packages it into a secretory vesicle, and the vesicle fuses with the cell-surface membrane to release the protein by exocytosis, powered throughout by mitochondrial ATP.

Prokaryotic cells are smaller and simpler: no nucleus, no membrane-bound organelles, circular DNA free in the cytoplasm, plasmids, 70S ribosomes and a murein wall. For microscopy, distinguish magnification (how enlarged) from resolution (the smallest separable distance, limited by wavelength), know the light microscope, TEM and SEM, and calibrate an eyepiece graticule against a stage micrometer. The magnification equation is magnification equals image size divided by real object size, always in the same units.

Biological molecules

Water's polarity and hydrogen bonding give it the properties OCR asks about: a solvent for reactions and transport, a high specific heat capacity for thermal stability, a high latent heat of vaporisation for cooling, cohesion for the transpiration stream, and a lower density as ice that insulates water bodies.

Carbohydrates are built from monosaccharides joined by glycosidic bonds (condensation). Starch and glycogen (alpha-glucose) are compact, insoluble energy stores, glycogen being more branched for fast release, while cellulose (beta-glucose) forms straight, hydrogen-bonded microfibrils for strength. Lipids include triglycerides (glycerol plus three fatty acids by ester bonds, an energy store) and amphipathic phospholipids that form bilayers. Proteins have four levels of structure (primary sequence by peptide bonds; secondary alpha-helices and beta-sheets by hydrogen bonds; tertiary fold by hydrogen, ionic, disulfide and hydrophobic interactions; quaternary assembly), giving soluble globular proteins and strong fibrous proteins. Nucleic acids are polymers of nucleotides; DNA is a double helix held by complementary base pairing (A-T by 2 bonds, C-G by 3).

Know the tests cold: Benedict's (reducing sugars), the hydrolysis route for non-reducing sugars, iodine (starch), the emulsion test (lipids) and biuret (protein).

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. Rate rises with temperature (more kinetic energy and collisions) up to an optimum, then falls as the enzyme denatures; pH away from the optimum and extreme temperatures denature the enzyme by breaking the bonds that hold the tertiary structure. Rate also rises with substrate concentration until active sites are saturated (V max), and with enzyme concentration when substrate is in excess.

Competitive inhibitors resemble the substrate and block the active site (overcome by more substrate, V max unchanged); non-competitive inhibitors bind an allosteric site and change the active-site shape (not overcome by more substrate, V max reduced). Cofactors, coenzymes (mobile, for example NAD) and prosthetic groups (permanently bound) assist catalysis.

DNA replication and the genetic code

DNA copies itself semi-conservatively: helicase unwinds the helix and breaks the hydrogen bonds, each strand templates a new strand by complementary base pairing, and DNA polymerase joins nucleotides by phosphodiester bonds, giving molecules with one old and one new strand. Meselson and Stahl's heavy-nitrogen experiment is the classic evidence.

The genetic code is triplet (three bases per amino acid), degenerate (several triplets per amino acid), non-overlapping and universal. mRNA carries the code from the nucleus and tRNA anticodons deliver amino acids at the ribosome, where peptide bonds build the polypeptide.

The cell cycle and mitosis

The cell cycle is interphase (G1 growth, S replication, G2 growth and checking, regulated by checkpoints) followed by mitosis and cytokinesis. DNA replicates in S phase, not in mitosis. Mitosis runs prophase (chromosomes condense, spindle forms), metaphase (chromosomes on the equator), anaphase (centromeres divide, chromatids pulled to the poles using ATP) and telophase (nuclei reform), producing two genetically identical diploid nuclei for growth, repair and asexual reproduction. Loss of control by oncogenes or faulty tumour suppressor genes causes cancer. The mitotic index equals cells in mitosis divided by total cells.

Meiosis, stem cells and cell organisation

Meiosis is a two-division reduction division producing four genetically varied haploid gametes, with variation from crossing over (prophase I), independent assortment (metaphase I and II) and random fertilisation. Stem cells are unspecialised cells classed by potency: totipotent (any cell plus placenta), pluripotent (any body cell), multipotent and unipotent (limited), with medical uses and an embryo-based ethical debate that induced pluripotent stem cells help address. Differentiated cells express only part of the genome and are organised into tissues, organs and organ systems.

How Module 2 is examined

A typical OCR profile for Foundations in biology:

• Multiple choice and short answer. Identifying organelles, classifying a molecule or bond, ordering the stages of mitosis, and matching a stem-cell type to a source.
• Maths. A magnification or actual-size calculation (watch the unit conversion), graticule calibration, or a mitotic index.
• Applied and data questions. Interpreting a Meselson-Stahl result, an enzyme rate graph, or a biochemical-test outcome, and evaluating a microscopy method.
• Level-of-Response extended answers. The secretory pathway, the four levels of protein structure with their bonds, and how loss of cell-cycle control causes cancer are all predictable.

Check your knowledge

A mix of recall and application questions covering the whole of Module 2. Attempt them under timed conditions, then check against the solutions.

1. Describe the roles of the rough endoplasmic reticulum, the Golgi apparatus and vesicles in secreting a protein. (4 marks)
2. Explain how the structure of cellulose suits its function in plant cell walls. (3 marks)
3. Describe the four levels of protein structure and the main bond stabilising each. (4 marks)
4. Explain the effect of a non-competitive inhibitor on the rate of an enzyme reaction and on V max. (3 marks)
5. An image of a cell is 90 mm long at a magnification of times 1800. Calculate the actual length in micrometres. (2 marks)
6. Describe semi-conservative DNA replication, naming the enzymes involved. (4 marks)
7. Explain why DNA replicates during interphase rather than during mitosis. (2 marks)
8. Explain how meiosis produces genetically different gametes. (4 marks)