Theme A: Unity and Diversity

A2.2 Cell structure

SL & HL 8 min read

The discovery that all living things are built from cells is one of the great unifying ideas in biology, and A2.2 is where you assemble the toolkit for the rest of the course. Two themes run through it: unity — every organism shares a common cellular basis — and diversity — cells vary enormously in size, shape and internal organisation. The section also rewards practical understanding: you are expected to interpret micrographs, use a microscope, and reason about why cells are the size they are. Keep asking what does this structure do and the long lists of organelles become a logical set of solutions to a cell’s problems.

Cell theory and the functions of life

The cell theory states that all living organisms are composed of cells, that the cell is the smallest unit of life, and that cells arise only from pre-existing cells. It is a theory in the strong scientific sense: supported by vast evidence and not seriously contradicted.

A living cell, even a single-celled organism, must carry out the functions of life: nutrition, metabolism, growth, response to stimuli, excretion, homeostasis and reproduction. Unicellular organisms such as Paramecium or Chlamydomonas perform all of these within one cell, which makes them favourite exam examples for demonstrating that a single cell can be a complete organism.

Be ready to discuss atypical cells that appear to challenge the theory: striated muscle fibres contain many nuclei, fungal hyphae form continuous multinucleate threads, and red blood cells lose their nucleus. These are explained as specialised exceptions rather than refutations.

Microscopy and measuring cells

Cells were discovered through microscopy, and the syllabus expects you to understand and use it. Light microscopes let you view living specimens in colour but have limited resolution (the ability to distinguish two close points as separate). Electron microscopes use beams of electrons, giving far higher resolution and revealing organelle ultrastructure, but specimens must be dead and treated.

Distinguish magnification (how many times larger the image is) from resolution (level of detail). You should be able to calculate magnification with the formula magnification = image size ÷ actual size, rearranging it to find any unknown, and to work in consistent units (1 mm = 1000 µm). Modern techniques such as fluorescent staining and freeze-fracture, mentioned in the syllabus, have extended what microscopy can reveal about cell components.

Prokaryotic and eukaryotic cells

All cells fall into two broad types. Prokaryotic cells (bacteria and archaea) are small and simple: they have no membrane-bound nucleus or organelles. Their DNA is a single circular molecule in a region called the nucleoid, and they may carry small extra DNA loops called plasmids. They have a cell wall, ribosomes (smaller, 70S), and divide by binary fission.

Eukaryotic cells (animals, plants, fungi, protists) are larger and compartmentalised: their DNA is enclosed in a true nucleus, and they contain membrane-bound organelles. Compartmentalisation is a key advantage — it allows incompatible reactions to occur simultaneously in separate spaces and concentrates enzymes and substrates where they are needed.

You should also compare animal and plant cells: plant cells additionally have a cellulose cell wall, a large permanent vacuole and chloroplasts, which animal cells lack; animal cells may contain centrioles.

Organelles and the surface-area-to-volume ratio

Each organelle is a solution to a functional need. Know these well:

A recurring exam idea is the surface-area-to-volume (SA:V) ratio. As a cell grows, its volume increases faster than its surface area, so a large cell cannot exchange materials or lose heat across its membrane fast enough for its needs. This limits cell size and explains why cells are small, why some are flattened or elongated to increase surface area, and why exchange surfaces such as villi are folded. Compartmentalisation in eukaryotes also increases the internal membrane area available for reactions.

Key terms

Cell theory
The principle that all organisms are made of cells, the cell is the smallest unit of life, and cells come only from existing cells.
Prokaryote
A cell without a membrane-bound nucleus or organelles, with DNA in a nucleoid; bacteria and archaea.
Eukaryote
A cell with a true membrane-bound nucleus and membrane-bound organelles; animals, plants, fungi and protists.
Organelle
A specialised structure within a cell that performs a particular function.
Compartmentalisation
The separation of cell functions into membrane-bound regions, allowing different conditions and reactions side by side.
Resolution
The ability of a microscope to distinguish two close points as separate; higher in electron microscopes.
Magnification
The number of times larger an image appears than the actual object; image size divided by actual size.
Surface-area-to-volume ratio
The ratio of a cell’s surface area to its volume; it falls as size increases, limiting exchange and cell size.
Nucleoid
The region of a prokaryotic cell containing its single circular DNA molecule, not bounded by a membrane.

Exam technique

Quick check
As a cube-shaped cell doubles in length along each side, what happens to its surface-area-to-volume ratio?
  1. It increases, improving exchange across the membrane
  2. It decreases, making exchange across the membrane harder
  3. It stays the same because both increase equally
  4. It becomes zero
Show answer
Answer: B. Volume increases with the cube of length while surface area increases with the square, so the ratio falls and exchange becomes more difficult.

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