Revise Cell structure for IB Biology (A2.2)

Cell theory is the founding generalisation of biology, built up from centuries of microscope observations. It has three tenets: all living organisms are composed of one or more cells; the cell is the smallest unit of life; and all cells arise from pre-existing cells by division. The theory is supported by the observation that no smaller unit than a cell shows all the functions of life, and that spontaneous generation was disproved — cells only ever come from other cells. A few atypical cases stretch the definition rather than break it: skeletal muscle fibres are single cells with many nuclei, giant algae can be enormous, and fungal hyphae can have continuous cytoplasm — useful examples to know, but the three tenets remain the answer examiners expect.

All cells share four features: a plasma membrane, cytoplasm, ribosomes and DNA. The great divide is what a cell does with that DNA and whether it compartmentalises its interior. Prokaryotic cells (bacteria and archaea) are small and simple: their single circular chromosome of 'naked' DNA lies free in the cytoplasm in a region called the nucleoid, and they have NO membrane-bound organelles at all. Their ribosomes are the smaller 70S type, and outside the plasma membrane they have a cell wall (peptidoglycan in bacteria), sometimes a protective capsule, hair-like pili for attachment, and perhaps a flagellum for movement. Eukaryotic cells (plants, animals, fungi and protists) are larger and defined by compartmentalisation: their DNA is enclosed in a membrane-bound nucleus, and the cytoplasm is divided by internal membranes into organelles, each providing specialised conditions for a particular job. This division of labour lets incompatible reactions run at once and raises overall efficiency.

The single most useful habit in this topic is to link each organelle to its job, because exam questions rarely ask 'what is this?' — they ask 'what does it do?' or 'how is its structure adapted to its function?'. Work through the organelles below as a set of structure-function pairs.

Plant and animal cells are both eukaryotic and share the nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus and plasma membrane. What sets them apart is a small, specific list. Plant cells have a cellulose cell wall, chloroplasts and a large permanent central vacuole; animal cells have none of these, but often contain centrioles (used in cell division) and only small, temporary vacuoles. The trap to avoid is over-generalising from one difference: an animal cell lacks a cell WALL, but it still has a plasma membrane — losing that distinction is a classic dropped mark.

Micrographs come with either a stated magnification or, more often, a scale bar — a short line labelled with the real length it represents. You must be able to move between three quantities: the image size (what you measure with a ruler), the actual size (the real size of the structure) and the magnification (how many times larger the image is). They are linked by a single formula, and the one rule that decides most marks is that image size and actual size must be in the same unit before you divide.

$\text{magnification} = \frac{\text{image size}}{\text{actual size}}$

Rearranged, actual size = image size ÷ magnification. Magnification is a ratio and has no units. Remember the unit ladder: 1 mm = 1000 μm and 1 μm = 1000 nm, so it is usually easiest to convert every length to micrometres (μm) before substituting.

Everything a cell needs — oxygen, nutrients, and the removal of wastes and heat — must cross its surface, the plasma membrane. But the demand for those exchanges depends on how much living material there is, which is set by the cell's volume. The problem is geometric: as a cell grows, its volume increases faster than its surface area, so the surface-area-to-volume ratio (SA:V) falls. Beyond a certain size the membrane simply cannot supply the interior fast enough, and the cell cannot survive. This is why cells stay small and divide rather than swell, and why exchange surfaces in organisms are folded and flattened to keep SA:V high. The quickest way to see it is with cubes.

$\text{SA:V ratio} = \frac{\text{surface area}}{\text{volume}}$

A2.2 explanation questions are marked analytically: each distinct valid biological point is worth one mark, up to the maximum available. Method-style reasoning points are credited as they appear, an accuracy/answer mark (A) rewards the correct conclusion, and error-carried-forward (ECF) means an early slip does not cost you the marks that follow, as long as your reasoning is written down. Notice below how every mark is pinned to a specific, named idea — not to loose phrasing.

The ideas in A2.2 seed much of the course ahead. The structure-function pairing of organelles sets up membrane transport and the endomembrane pathway of protein secretion; the prokaryote-eukaryote divide underpins classification and the endosymbiotic origin of mitochondria and chloroplasts; and the surface-area-to-volume argument reappears wherever biology needs an efficient exchange surface — the folded villi of the gut, the branching alveoli of the lungs, the flattened red blood cell. Master the habit of reading structure as a clue to function, and of checking whether a size problem is really a surface-area-to-volume problem, and you have a template that keeps paying off across the whole syllabus.