Cell Structure

Why this comes first

Before you can calculate a magnification or name an organelle, you have to be able to get a specimen onto a slide and record what you see. Paper 2 tests this directly — "make a temporary preparation", "draw and label this cell" — and the drawing conventions it rewards are strict enough that careless candidates lose marks on drawings that look perfectly good. This section builds those two skills; the sections that follow turn what you see into numbers.

Making a temporary preparation

A temporary preparation is a fresh specimen mounted in a drop of liquid on a slide, ready to view straight away — as opposed to a permanent slide, which has been dehydrated and sealed to last years. The exam wants the method in the right order, with the reasons attached.

Biological drawing: plan and detail

Paper 2 asks for two different kinds of drawing, and the conventions differ slightly:

• A low-power plan shows the distribution of tissues across a whole section — drawn as outlines only, no individual cells.
• A high-power detail drawing shows a few individual cells accurately — this is where labels earn their marks.

Both follow the same non-negotiable conventions, and mark schemes award (and withhold) marks for them specifically:

• a sharp pencil with single, clear, continuous lines — never sketchy or broken;
• no shading of any kind — if a structure looks dark, label its name instead of filling it in;
• drawing only what you actually see, in the correct proportions;
• labels written horizontally, with straight lines drawn with a ruler that touch the structure — never crossing each other, never with arrowheads;
• a title, and for drawings from micrographs a stated magnification.

What magnification actually is

Every drawing, photomicrograph and electron micrograph in this course shows a specimen larger than it really is. The magnification states exactly how many times larger, and calculating it — in both directions — is the most examined single skill in this topic (69 marks across Papers 1+2 in 2021–2025, more than any other calculation in AS Biology).

The definition is a division:

where image size is the length you measure on the drawing or micrograph (with a ruler), and actual size is the true length of the specimen in real life. Students often write the fraction upside down under pressure — anchor it with the common-sense check that a drawing $10$ times wider than the cell it shows is a magnification of $\times 10$, not $\times \frac{1}{10}$.

The unit ladder: mm, µm, nm

The arithmetic of this topic is rarely hard — what loses marks is units that do not match. Biology measures lengths on a three-rung ladder, and every rung is a factor of $1000$:

• $1$ millimetre (mm) $= 1000$ micrometres (µm) — about the width of a sharp pencil line; a eukaryotic cell is typically $10$–$50$ µm across;
• $1$ micrometre (µm) $= 1000$ nanometres (nm) — a bacterium is $1$–$5$ µm, a virus $20$–$400$ nm, a ribosome about $25$ nm.

Going down the ladder (mm → µm → nm) you multiply by 1000 at each step; going up, you divide. The rule that drives every worked example in this section: before you divide image size by actual size, convert both into the same units — and the safest habit is to convert the measured image length (in mm) into the units the actual size is given in.

A clean demonstration with invented numbers

A textbook drawing of a mitochondrion is measured to be $48$ mm long. The same organelle's actual length is $4.0$ µm. Find the magnification of the drawing.

Step 1 — measure the image. Already given: $48$ mm.

Step 2 — convert to matching units. The actual size is in µm, so convert the image measurement to µm — one rung down, multiply by 1000:

Step 3 — write the formula and substitute.

Step 4 — evaluate.

Notice the units cancelled — µm ÷ µm leaves a pure number, which is why the answer is written $\times 12\,000$ and nothing else.

The third direction: how big should the image be?

The triangle has one more corner — image size = magnification × actual size. It is the direction examiners use when they tell you a drawing must be drawn at a stated magnification, and it is the one students most often attack backwards. A clean demonstration with invented numbers:

A chloroplast is actually $8.0$ µm long. How long should its drawing be if it is drawn at $\times 2500$?

Step 1 — write the formula, covering I on the triangle:

Step 2 — match units by converting the actual size into the units a drawing is measured in — millimetres, because rulers measure in mm:

Step 3 — substitute and evaluate:

So the drawing's chloroplast should measure $20$ mm on the paper — about two ruler-widths, which passes the sanity check that $\times 2500$ of something microscopic fills a sensible chunk of page. Notice this answer must carry units: it is a real length on paper, not a ratio. The sanity check generalises — whatever direction you are working in, ask whether your answer is bigger than what you started from when magnification is large, and smaller when you divided by it.

Two scales, two different jobs

You cannot hold a ruler up to the image inside a microscope — the image exists only in your eye (or on a camera). To measure a specimen down the microscope you need two scales working together:

• The eyepiece graticule is a transparent plastic disc etched with a fine scale, sitting inside the eyepiece. It is fixed in the optics, so it stays put whatever you do to the slide — but its divisions are arbitrary: nobody knows what one division is worth until you calibrate it. We call one division an eyepiece unit (epu).
• The stage micrometer is a special slide carrying a scale of precisely known length — typically $1$ mm divided into $100$ parts (each division $0.01$ mm $= 10$ µm), or with divisions $0.1$ mm apart. It is the ruler; it just cannot be used to measure your specimen, because its markings are far too coarse for cells.

The measurement strategy is therefore: use the stage micrometer to find what one eyepiece unit is worth, then use the graticule to measure the specimen.

A clean demonstration with invented numbers

At a certain objective, $8$ epu are found to span exactly $200$ µm of the stage micrometer.

Calibration. $1\ \text{epu} = 200\ \mu\text{m} \div 8 = 25$ µm.

Measuring a specimen. A cell is seen to stretch across $12$ epu:

Two habits to carry into every graticule question: convert the stage-micrometer length into µm before dividing (a $0.1$ mm division is $100$ µm, not $0.1$ µm), and keep asking "which objective was this calibration made at?"

Two words that are not synonyms

Magnifying something and seeing it clearly are different achievements, and examiners probe the difference relentlessly — Resolution vs magnification; electron microscopy carried 53 marks across Papers 1+2 in 2021–2025. You met magnification in §02: how many times larger the image is than the specimen. Resolution (also called resolving power) is a different property entirely: it is the minimum distance two objects can be apart and still be seen as two separate objects rather than one blurred blob. A poster-size blow-up of a blurry photograph has enormous magnification and terrible resolution — everything is bigger, nothing is clearer.

Why the light microscope hits a wall at about 0.25 µm

Resolution is limited by the wavelength of whatever is being used to form the image. Light travels as waves, and waves cannot turn sharply around objects much smaller than themselves — light passing beside a tiny object diffracts around it, smearing the image. As a working rule, two points closer together than roughly half the wavelength of the illuminating light blur into one.

Visible light has wavelengths of about $400$–$700$ nm, so the finest detail a light microscope can resolve is around half of that: in practice about $0.25$ µm (= 250 nm). Quote this figure in the exam and attach the reason — because it is set by the wavelength of light — because that pairing is what mark schemes pay for.

This wall explains exactly which structures a light microscope can and cannot show:

• visible: anything larger than about 250 nm — nuclei, chloroplasts, mitochondria (as small ovals), whole bacteria, whole cells;
• invisible: anything smaller — ribosomes (~25 nm), cell surface membranes (~7 nm thick), ER membranes, the details inside a mitochondrion. These exist, but light cannot resolve them, so the cytoplasm looks featureless where they sit.

Transmission and scanning electron microscopes

Electron microscopes fire a beam of electrons instead of light. Accelerated electrons have wavelengths thousands of times shorter than visible light, so the resolution wall moves from $\sim 0.25$ µm down to fractions of a nanometre — membranes and even ribosomes become visible. The two types point the beam at the specimen in different ways, and Paper 1 expects you to tell their products apart:

Both need elaborate preparation — specimens must be dried, fixed and placed in a vacuum, because air scatters electrons — so both work on dead material, unlike the light microscope.

A clean demonstration with invented numbers

A light microscope resolves $0.25$ µm. Inside a bacterium $2$ µm long, two DNA loops lie $0.1$ µm apart. What can be seen?

Convert so both lengths share units (they already do): the bacterium is $2$ µm across, comfortably larger than $0.25$ µm, so it is detected. The two loops are only $0.1$ µm apart — less than the resolution — so however much you magnify, they show as one merged smear, not two loops. Detected but not resolved: magnifying harder never fixes resolution, because resolution belongs to the instrument, not to the enlargement.

The heart of the topic

This is the single heaviest sub-topic in AS Biology — 290 marks across Papers 1+2 in 2021–2025, more than magnification and resolution combined. The electron microscope (§04) is what made it examinable: once resolution dropped below $0.25$ µm, the "featureless cytoplasm" of light microscopy resolved into membranes, stacks, tubules and granules, each with a shape that betrays its job.

The skill being tested has two halves, and outline questions want both:

• recognise — name the structure from what an electron micrograph or drawing shows (a stack of flattened sacs with vesicles = Golgi body);
• link structure to function — say what it does in terms of what it looks like (its folded inner membrane gives a large surface area for respiration enzymes).

An organelle is a membrane-bound structure inside a cell with a specific function. Compartmentalisation is the point: membranes let incompatible reactions run side by side in the same cell, each in its own optimised space.

Three details the examiner always digs at

The catalogue above is the pass mark. The distinctions below are where the actual marks concentrate, because they separate candidates who memorised names from candidates who know the architecture.

1. Counting membranes. "Which organelles are enclosed by a double membrane?" is among the most repeated Paper 1 stems there is. Only three structures have envelopes of two membranes: the nucleus (the nuclear envelope — count its inner and outer membranes, pores notwithstanding), the mitochondrion and the chloroplast. Everything else membrane-bound is single: rER, sER, Golgi, lysosomes, the tonoplast, secretory vesicles. And some structures have no membrane at all: ribosomes, centrioles, microtubules, cilia and microvilli (they are built from membrane and protein, but they do not enclose anything).

2. Ribosomes come in two sizes — and the small ones hide inside organelles. Cytoplasmic ribosomes of eukaryotes are 80S. But the ribosomes inside your own mitochondria and chloroplasts are 70S — the same size as bacterial ribosomes, and no coincidence (that story arrives in §07). A comparison-table question loves this row: bacteria 70S, eukaryotic cytoplasm 80S, mitochondria and chloroplasts 70S despite living inside a eukaryote.

3. Small circular DNA lives inside mitochondria and chloroplasts. Both organelles carry their own few genes as a small circle of naked DNA, quite separate from the linear chromosomes in the nucleus — another echo of bacteria, and the reason these organelles can synthesise some of their own enzymes using their own 70S ribosomes. Never call this DNA linear, and never say it is surrounded by a nuclear envelope.

The plant additions

Plants build four structures animal cells never carry: a cellulose cell wall, plasmodesmata threading through it, a large permanent vacuole with its tonoplast, and chloroplasts. Animal-only structures are just as short a list: centrioles (plants grow spindles without them) and, among the surface specialisations, cilia are typical of animal epithelia rather than plants. Everything else in the catalogue — nucleus, mitochondria, ER, Golgi, ribosomes, lysosomes, cell surface membrane — is shared.

Reading a cell's job from its organelle profile

Flip the catalogue around and it becomes a detective tool: a cell's function predicts which organelles it stocks heavily. Secretory cells (making and exporting protein, like the mucus-making goblet cells of the airways) need extensive rER to make the protein, an extensive Golgi body to process and package it, and crowds of secretory vesicles — which is why goblet cells dominate the answer to any "most Golgi / most single-membrane structures" question. Absorbing cells (small-intestine epithelium) pack microvilli for area and mitochondria to drive active transport. Muscle cells are stuffed with mitochondria for contraction. Paper 1 tests exactly this inference, usually as "which cell type…?" — reason from the job to the organelles, then match the options.

A clean demonstration before the past-paper questions

An electron micrograph of a secretory cell shows three unlabelled structures: (i) curved stacks of flattened membrane sacs with small round sacs budding off their edges; (ii) oval organelles with an inner membrane folded back on itself; (iii) sheets of membrane dotted with tiny dark granules.

Work each one through the catalogue: (i) stack + budding vesicles = Golgi body, packaging proteins for export; (ii) double membrane + folded cristae = mitochondrion, supplying the ATP that secretion costs; (iii) ribosome-studded sheets = rough ER, making the protein being exported. Membrane counts: single, double, single respectively — and the trio tells you the cell's trade before you read a word of stem: a busy protein-exporting cell. That two-move habit — recognise from structure, then link to function — is everything this section teaches.

Sorting the catalogue into three lists

Everything in §05's catalogue belongs to exactly one of three groups, and compare questions are just those groups written down quickly:

• shared — nucleus with nuclear envelope and nucleolus, both ER types, Golgi body, mitochondria, ribosomes, lysosomes, cell surface membrane;
• plant only — cellulose cell wall, chloroplasts, large permanent vacuole with tonoplast, plasmodesmata;
• animal only — centrioles (and cilia as a surface specialisation of animal epithelia).

That is the whole comparison. The examiner's version adds two finer rows worth memorising: the carbohydrate store (starch grains in plants, glycogen granules in animals — visible in electron micrographs as dense particles), and the fixed, regular box-like shape a wall imposes on plant cells versus the irregular, flexible outline of animal cells.

A clean demonstration before the past-paper question

An electron micrograph shows a cell with an angular outline, a thin layer of cytoplasm crowded with organelles around a huge empty-looking central sac, and several green-black oval bodies containing stacked internal membranes. Judge it. Wall → plant. Central sac bounded by a single membrane → large permanent vacuole (tonoplast). Ovals with stacks → chloroplasts (grana). Verdict: a photosynthesising plant cell, probably leaf mesophyll — you could also predict plenty of mitochondria for active transport even if none happen to lie in this particular slice. One pass through the three-lists table reads the entire cell.

A whole organism in one cell

Bacteria are single cells that must do everything — feeding, growing, responding, reproducing — with none of the compartmental equipment §05 gave eukaryotic cells. That austerity is the answer to every prokaryote question: unicellular, generally $1$–$5$ µm across (a tenth of a typical eukaryotic cell or less), with no internal membranes worth the name. The syllabus names six features, and mark schemes quote them almost verbatim:

• unicellular — one cell carries out all the functions of life;
• generally 1–5 µm in diameter;
• a cell wall of peptidoglycan (never call it cellulose — cellulose is the plant wall material);
• circular DNA — a single loop of naked DNA, free in the cytoplasm with no nuclear envelope around it;
• 70S ribosomes (smaller than the 80S of eukaryotic cytoplasm);
• an absence of organelles surrounded by double membranes — no nuclear envelope, no mitochondria, no chloroplasts.

Beneath the wall sits a normal cell surface membrane, and everything happens directly in the cytoplasm: respiration enzymes are not housed in mitochondria because there are no mitochondria — the reactions simply run loose in the cell.

The full three-way comparison

With §05's catalogue and this section's six features, you can complete any plant–animal–bacterium grid. The rows that earn marks:

The genetic-material row is the most examined line in the whole topic: it is not enough to know both groups have DNA — where it sits (envelope or not) and what shape it is (linear or circular) is exactly what the examiner asks you to distinguish.

A clean demonstration before the past-paper questions

Sort these four claims into the right circle: (a) respires aerobically; (b) mRNA binds 80S ribosomes; (c) contains circular DNA; (d) surrounded by a peptidoglycan wall.

(a) both circles — bacteria respire, eukaryotic cells respire (even though eukaryotes do it in mitochondria). (b) eukaryote circle alone — bacterial ribosomes are 70S, so their mRNA binds 70S. (c) both circles — bacterial chromosome is circular, and eukaryotic mitochondria/chloroplasts carry small circular DNA too. (d) prokaryote circle alone (plants have walls, but of cellulose). Every claim lands by asking two questions: do both groups have it? and if so, in the same form?

Not cells, and not quite alive

Seven sections in, every structure you have met has passed at least some of the tests that define a cell. The topic ends with the structures that fail all of them: all viruses are non-cellular. A virus has no cytoplasm, no ribosomes, no cell surface membrane of its own, and carries out no metabolism — it does not feed, respire, excrete or grow. It cannot even reproduce alone: it can only replicate by hijacking a host cell's machinery, which is precisely what makes it a parasite of cells and the bridge into this course's infectious-disease topics.

What a virus does have is a short parts list, and every mark-scheme answer on this sub-topic is assembled from exactly three items:

• a nucleic acid core — either DNA or RNA, never both (every true cell carries both; each virus commits to one);
• a capsid — a coat of protein enclosing the core;
• in some viruses, an outer envelope of phospholipids, stolen from the host cell's membranes as the virus exits.

Why so few drugs have any effect on viruses

Designing a drug means finding a target the pathogen has and the patient lacks. Bacteria offer plenty — peptidoglycan walls, bacterial ribosomes, bacterial enzymes — which is why antibiotics exist. Viruses offer almost nothing:

• few targets: no cell wall, no cell surface membrane of their own (any envelope is host-derived phospholipid), no ribosomes;
• no or very few enzymes of their own to inhibit;
• no metabolism: antibiotics act against growing, living cells, and a virus outside its host does nothing at all;
• and even when it is replicating, the virus sits inside host cells, physically out of reach of most drugs — attacking it there would mean poisoning the patient's own tissue.

So the same austerity that defines a virus structurally explains why it is so hard to treat.

A clean demonstration before the past-paper questions

Suppose an electron micrograph shows an $80$ nm particle: a dense thread coiled inside a geometric shell, itself wrapped in a bubble-like outer layer. Read it through the census: the shell = capsid (protein); the thread inside = nucleic acid core — call it DNA or RNA, whichever evidence establishes, never both; the bubble layer = phospholipid envelope — present here, but remember it is a some-virus feature, not part of the definition. If asked for key features of this virus you could add its envelope; if asked for key features of viruses generally, stick to non-cellular + core + capsid.