CCEA A-Level Biology Cells, Viruses and Reproduction: a complete overview of cells, membranes, viruses and cell division

What this module demands

Cells, Viruses and Reproduction is where the chemistry of the first module becomes living structure. It covers how cells are organised, how the membrane controls what enters and leaves, what viruses are and how they replicate, how cells divide, and how organisms exchange and transport gases. CCEA tests precise structure recall and the ability to explain how structure suits function.

This guide walks through the five dot points of the module, then sets out the exam patterns CCEA repeats. Each topic has a matching dot-point page with practice questions; this overview ties them together.

Cell structure and membranes

Eukaryotic cells have a nucleus and membrane-bound organelles, each with a role: mitochondria for respiration, ribosomes and rough ER for protein synthesis, the Golgi apparatus for modifying and packaging, and lysosomes for digestion. Prokaryotic cells are smaller, with no nucleus, no membrane-bound organelles, 70S ribosomes and a peptidoglycan wall. The membrane follows the fluid-mosaic model: a phospholipid bilayer with proteins, cholesterol and glycoproteins.

Transport across membranes

Substances cross the membrane by diffusion and facilitated diffusion (passive, down a gradient), osmosis (water down a water potential gradient), active transport (against the gradient, using ATP and carrier proteins), and bulk transport by endocytosis and exocytosis. The rate depends on the concentration gradient, surface area, diffusion distance and temperature, as summarised by Fick's law.

Viruses

A virus is a non-cellular particle of nucleic acid in a protein capsid, sometimes enveloped. Viruses are not living cells and replicate only inside hosts: the lytic cycle bursts the cell to release new viruses, while the lysogenic cycle integrates the viral DNA dormantly. HIV is a retrovirus that uses reverse transcriptase to make DNA from its RNA, destroying helper T cells and leading to AIDS.

Cell division and gas exchange

Mitosis (prophase, metaphase, anaphase, telophase) produces two identical diploid cells, while meiosis produces four genetically different haploid gametes, with variation from crossing over and independent assortment. Gas exchange surfaces such as alveoli, leaves and insect tracheae are large, thin and moist with a maintained gradient; oxygen is carried by haemoglobin (Bohr effect) and carbon dioxide mostly as hydrogencarbonate, transported by the circulatory and plant transport systems.

How this module is examined

A typical CCEA profile for Cells, Viruses and Reproduction:

• Recall and structure. Labelling organelles and membranes, comparing cell types, and stating viral structure.
• Definitions and explanation. Distinguishing the transport methods, explaining osmosis with water potential, and explaining viral replication.
• Sequencing. Ordering the stages of mitosis and the products of meiosis, and the events of synaptic and gas transport.
• Application and data. Magnification calculations, oxygen dissociation curves and exchange-surface features.

Check your knowledge

A mix of recall and explanation questions covering the module. Attempt them under timed conditions, then check against the solutions.

1. State two structural differences between a prokaryotic and a eukaryotic cell. (2 marks)
2. Explain why the cell membrane is described as a fluid mosaic. (2 marks)
3. Define osmosis in terms of water potential. (2 marks)
4. State two ways active transport differs from facilitated diffusion. (2 marks)
5. Give two reasons why viruses are not classed as living cells. (2 marks)
6. State two differences between mitosis and meiosis. (2 marks)
7. State three features of an efficient gas exchange surface. (3 marks)
8. Explain how the Bohr effect helps deliver oxygen to tissues. (2 marks)