AS 1 Cell structure | A Level Biology 9700

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🎯What you need to be able to do

Make a temporary preparation of cellular material and draw cells from slides and photomicrographs.
Calculate magnifications and actual sizes from drawings, photomicrographs and electron micrographs.
Calibrate an eyepiece graticule against a stage micrometer and measure in mm, µm and nm.
Define resolution and magnification, and explain the difference with reference to light and electron microscopy.
Recognise eukaryotic organelles and outline the structure and function of each.
Outline the key features of a prokaryotic cell, compare it with eukaryotic cells, and state what a virus is.

📚The biology

Magnification and resolution are not the same thing

Magnification is how many times larger the image is than the object. Resolution is the smallest distance between two points that can still be seen as two points rather than one blur. Magnifying beyond the resolution limit gives you a bigger blur, not more detail — this is the distinction the syllabus asks you to explain, and it is worth two marks almost every year.

Resolution is limited by the wavelength of the radiation used. Light has a wavelength of roughly 400–700 nm, which caps a light microscope at about 200 nm resolution and about ×1500 useful magnification. An electron beam has a far shorter wavelength, so an electron microscope resolves to about 0.5 nm and magnifies usefully to ×500 000 or more.

Light microscope — living material, colour, cheap, but only organelles larger than about 200 nm (nucleus, chloroplasts, large mitochondria).
Transmission electron microscope (TEM) — electrons pass through an ultra-thin section, giving a 2-D image of internal ultrastructure.
Scanning electron microscope (SEM) — electrons are reflected off a surface, giving a 3-D surface image at lower resolution than a TEM.

Both electron microscopes need a vacuum, so specimens must be dead and are chemically fixed and stained with heavy metals. Any artefact question hinges on that.

The magnification calculation

\[ \text{magnification} = \frac{\text{image size}}{\text{actual size}} \]

Rearranged, actual size = image size ÷ magnification. The whole difficulty in these questions is units, so fix the conversions in your head now:

1 mm = 1000 µm

1 µm = 1000 nm

1 mm = 10⁶ nm

Measure the image in millimetres with a ruler, convert to micrometres by multiplying by 1000, and only then divide by the magnification. Doing it in that fixed order removes almost every error on this topic.

Where a micrograph carries a scale bar instead of a stated magnification, measure the bar itself: magnification = measured length of bar ÷ the length it represents.

Eyepiece graticule and stage micrometer

An eyepiece graticule is a scale in the eyepiece. Its divisions are arbitrary — they mean nothing until you calibrate them, and they mean something different at each objective lens. A stage micrometer is a slide with a known scale, typically 1 mm divided into 100 divisions of 10 µm each.

To calibrate: superimpose the two scales, count how many eyepiece divisions line up with a known length on the stage micrometer, and divide. That gives you the value of one eyepiece division in micrometres at that objective. Then swap the micrometer for your specimen and count divisions.

Recalibrate whenever you change objective. The eyepiece graticule does not move when you change lens, but the image behind it does, so one division covers a different real distance at ×10 and at ×40. Carrying a ×10 calibration into a ×40 measurement gives an answer four times too big, and the number still looks plausible.

Drawing cells for marks

Paper 3 asks for two kinds of drawing and marks them differently.

Plan diagram — the distribution of tissues, with the correct proportions of each layer and no individual cells drawn. Drawing cells into a plan diagram loses the mark.
High-power drawing — a few cells showing correct shapes, relative sizes, cell walls drawn as two lines (three where two cells touch), and only observable contents.

In both cases: sharp pencil, single clear unbroken lines, no shading, use most of the space, and label with straight ruled lines that touch the structure named.

Eukaryotic organelles

Learn each as structure → function, because that is how the marks are written.

Cell surface membrane — phospholipid bilayer; controls entry and exit, and carries receptors and antigens.
Nucleus, nuclear envelope, nucleolus — double membrane with pores; contains chromatin (DNA + histones); the nucleolus makes ribosomes.
Rough ER — flattened sacs studded with 80S ribosomes; transports proteins made by those ribosomes.
Smooth ER — no ribosomes; synthesises lipids and steroids.
Golgi body — stack of flattened sacs; modifies proteins, adds carbohydrate to make glycoproteins, packages them into vesicles, and makes lysosomes.
Mitochondria — double membrane, inner one folded into cristae; site of aerobic respiration; contain small circular DNA and 70S ribosomes.
Ribosomes — 80S in the cytoplasm, 70S in chloroplasts and mitochondria; site of translation.
Lysosomes — membrane-bound sacs of hydrolytic enzymes; digest worn-out organelles and material taken in by phagocytosis.
Centrioles and microtubules — hollow tubes of tubulin; form the spindle and the cytoskeleton.
Cilia — 9+2 microtubule core; beat to move fluid or the cell.
Microvilli — folds of the cell surface membrane that increase surface area for absorption.
Chloroplasts — double membrane, thylakoids stacked into grana, stroma; site of photosynthesis; circular DNA and 70S ribosomes.
Cell wall — cellulose in plants; gives support and prevents bursting.
Plasmodesmata — cytoplasmic channels through plant cell walls linking adjacent cells.
Large permanent vacuole and tonoplast — stores solutes and generates turgor pressure in plant cells.

Cells use ATP from respiration to drive every energy-requiring process, and the presence of small circular DNA and 70S ribosomes in mitochondria and chloroplasts is the observation the endosymbiont idea rests on.

Prokaryotic cells and viruses

A typical bacterium is unicellular, generally 1–5 µm across, with a peptidoglycan cell wall, circular DNA free in the cytoplasm, 70S ribosomes, and no organelles bounded by double membranes — so no nucleus, no mitochondria, no chloroplasts.

Viruses are non-cellular. Every virus has a nucleic acid core — DNA or RNA, never both — and a protein capsid; some also have an outer envelope of phospholipids taken from the host membrane. They have no cytoplasm, no ribosomes and no metabolism, which is why the question “is a virus alive?” is a legitimate one and why antibiotics do not touch them (see Topic 10).

✏️Worked example

A student views onion epidermis using a ×10 objective and a ×10 eyepiece. With a stage micrometer in place, 40 eyepiece divisions coincide with 0.6 mm on the micrometer. The student then measures a cell as 18 eyepiece divisions long. (a) Find the value of one eyepiece division in µm. (b) Find the actual length of the cell. (c) In a drawing, that cell is 90 mm long. Find the magnification of the drawing.

(a) Convert to micrometres first: 0.6 mm = 600 µm. Then

\[ \text{1 eyepiece division} = \frac{600}{40} = 15\ \mu\text{m} \]

(b) The cell spans 18 divisions, so its actual length is

\[ 18 \times 15 = 270\ \mu\text{m} \]

(c) Both measurements must be in the same unit. The drawing is 90 mm = 90 000 µm, and the object is 270 µm, so

\[ \text{magnification} = \frac{90\,000}{270} = \times 333 \]

Magnification has no units — it is a ratio — so write ×333, not 333 µm.

Check it. 270 µm is a plausible onion epidermal cell: these are large, and a typical plant cell sits in the 50–500 µm range. If part (b) had come out as 0.27 µm you would be claiming a cell smaller than a bacterium, and if it were 270 mm you would have one you could see across the room. Sanity-check the order of magnitude against something you know: a red blood cell is 7 µm, a bacterium 1–5 µm.

The total magnification is a red herring here. The ×10 eyepiece and ×10 objective give ×100 down the microscope, and candidates reflexively divide by it. But the graticule calibration already accounts for the optics — that is the entire point of calibrating. Dividing 270 by 100 gives 2.7 µm, which is bacterium-sized and wrong. Use the total magnification only in part (c), where you are asked about the drawing, and even there it is the drawing size over the actual size, not the microscope's own number.

📝Practise

Work through these, then reveal the answer. Each question targets a different objective from the list above.

1. A structure measures 24 mm on an electron micrograph taken at ×12 000. What is its actual size in µm?

Actual size = image ÷ magnification = 24 mm ÷ 12 000 = 0.002 mm. Convert: 0.002 × 1000 = 2 µm. Alternatively convert first: 24 mm = 24 000 µm, then 24 000 ÷ 12 000 = 2 µm. Either order works provided you convert at only one point — converting twice is the usual slip.

2. Explain why a light microscope cannot resolve two ribosomes lying 30 nm apart, even at ×2000 magnification.

Resolution is limited by the wavelength of the radiation. Visible light has a wavelength of about 400–700 nm, so the smallest distance a light microscope can resolve is roughly 200 nm. Two objects 30 nm apart are far closer than that limit, so they are recorded as a single blur. Increasing magnification to ×2000 enlarges that blur but adds no new detail — this is empty magnification. Only radiation of much shorter wavelength, such as an electron beam, has the resolving power needed.

3. Give three structural features that distinguish a typical bacterial cell from a plant cell.

Any three of: (i) bacterial DNA is circular and free in the cytoplasm, whereas plant DNA is linear and enclosed in a nucleus with a double membrane; (ii) bacteria have 70S ribosomes, plant cytoplasm has 80S; (iii) the bacterial cell wall is peptidoglycan, the plant wall is cellulose; (iv) bacteria have no organelles surrounded by double membranes, so no mitochondria, chloroplasts or nuclear envelope; (v) bacteria are typically 1–5 µm across against tens to hundreds of µm for plant cells. Note that both have a cell wall, a cell surface membrane, cytoplasm and ribosomes — naming those as differences is a common way to score zero.

4. A secretory cell exports large amounts of a glycoprotein. Name the organelles involved, in order, and state the role of each.

Nucleus — the gene is transcribed to mRNA, which leaves through a nuclear pore. Ribosomes on the rough ER — translate the mRNA into a polypeptide, which enters the ER lumen. Rough ER — transports the protein and buds off vesicles carrying it. Golgi body — modifies the protein and adds the carbohydrate that makes it a glycoprotein, then packages it. Secretory vesicles — carry it to the cell surface membrane, where exocytosis releases it. Mitochondria supply the ATP for vesicle transport and exocytosis throughout. Marks are given for the sequence, so keep them in order.

5. A student calibrates an eyepiece graticule at the ×4 objective and finds one division equals 25 µm. Without recalibrating, they switch to the ×40 objective and measure a cell as 12 divisions. What answer will they get, what is the true value, and why?

They will report 12 × 25 = 300 µm. Switching from ×4 to ×40 magnifies the image ten times more, so each eyepiece division now covers one tenth of the real distance: 2.5 µm, not 25 µm. The true length is 12 × 2.5 = 30 µm. Their answer is ten times too large — exactly the factor by which the objective changed. The eyepiece graticule never moves; only the image behind it changes size, which is why calibration is per-objective.

6. State two features found in both chloroplasts and mitochondria but not in the rest of a eukaryotic cell’s cytoplasm, and suggest what they imply.

Both contain small circular DNA and 70S ribosomes, and both are bounded by a double membrane. Those are prokaryotic features — a bacterium has circular DNA and 70S ribosomes, while the surrounding eukaryotic cytoplasm has 80S ribosomes and linear nuclear DNA. This suggests that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by an early eukaryotic cell and retained, which is the endosymbiotic theory. The syllabus only requires you to state the features; the interpretation is worth knowing because it makes them memorable.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

Cell Image Library — a searchable archive of real light and electron micrographs, useful for practising identification before Paper 3
“Learn Genetics” (University of Utah) — the interactive cell size and scale slider, which fixes the mm / µm / nm hierarchy better than any table
Any A Level practical skills video showing eyepiece graticule calibration — watching the two scales superimpose once is worth more than reading the method three times