A Level Biology: Cell Structure (Cambridge 9700)

This guide covers Topic 1 Cell structure, an AS Level topic for Cambridge International AS & A Level Biology 9700, 2025–2027 series. AS Level candidates study Topics 1–11 plus practical skills; A Level candidates study all 19 topics plus practical skills.

Where this fits in 9700

Cell structure is the first of nineteen topics in 9700, and it is the one every other topic ultimately depends on — biological molecules, enzymes, cell membranes and transport, and the mitotic cell cycle (the topics immediately following it) all assume a working model of what a cell actually contains and how that differs between organism types.

Syllabus coverage

CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — TOPIC 1 CELL STRUCTURE

• 1.1 The microscope in cell studies — how microscopy (light and electron) is used to study cell structure, including calculating magnification and actual size from a given image; using an eyepiece graticule calibrated against a stage micrometre to measure specimen size, converting between millimetres, micrometres and nanometres; and making and annotating temporary preparations and scientific drawings of cells and tissues
• 1.2 Cells as the basic units of living organisms — the structure and function of organelles in eukaryotic cells (cell surface membrane, nucleus with its nuclear envelope and nucleolus, rough and smooth endoplasmic reticulum, Golgi body, mitochondria, ribosomes, lysosomes, centrioles, cilia and microvilli, and, in plant cells, the cell wall, chloroplasts, the vacuole and its surrounding tonoplast, and plasmodesmata); the structural and functional differences between plant and animal cells; how prokaryotic cells differ from eukaryotic cells; and the structure of viruses as acellular infective particles

How to approach it

Magnification calculations in 1.1 are graded almost purely on procedural accuracy (magnification = image size ÷ actual size, with consistent units), so timed practice converting between millimetres and micrometres and rearranging the formula for any of its three variables closes most of the gap here — a wrong unit conversion is one of the most common, and most avoidable, sources of lost marks in this sub-topic. Electron versus light microscopy is also worth comparing directly: know why electron microscopes achieve much higher resolution (shorter wavelength) and what that trade-off costs in practice (samples must be dead and processed, not living and in real time).

For 1.2, the strongest answers compare rather than simply list — being able to state precisely how a named organelle’s structure relates to its function (for instance, why mitochondria have a highly folded inner membrane, or why ribosomes are free or bound depending on the protein’s destination) is tested more often than simple identification. The eukaryotic–prokaryotic comparison is worth deliberate practice as a two-column table (presence or absence of a nucleus, membrane-bound organelles, cell diameter, cell wall composition), since exam questions frequently ask candidates to distinguish the two cell types directly rather than describe each in isolation. Because this topic underpins so much of what follows, treating it as a quick warm-up rather than genuinely secure foundational knowledge is a common mistake — gaps here tend to resurface as confusion in later topics on transport and cell division.

Worked example: magnification calculation

The core formula, magnification = image size ÷ actual size, can be rearranged for whichever variable a question asks for, and most lost marks come from unit conversion rather than the algebra itself. If a cell’s image is measured at 45 mm across on a micrograph labelled ×3,000 magnification, the actual size is found by rearranging to actual size = image size ÷ magnification: 45 mm ÷ 3,000 = 0.015 mm, which converts to 15 micrometres (since 1 mm = 1,000 micrometres). Working consistently in one unit throughout the calculation, and only converting to the unit the question asks for at the very end, avoids the mid-calculation conversion errors that are the most common source of lost marks in this sub-topic.

Electron versus light microscopy in more depth

Electron microscopes achieve far higher resolution than light microscopes because they use a beam of electrons, which has a much shorter effective wavelength than visible light, and resolution is fundamentally limited by wavelength – a shorter wavelength allows two points that are closer together to still be distinguished as separate. This higher resolution comes at a practical cost: electron microscopy requires samples to be dead, dehydrated and often coated or stained, and viewed in a vacuum, so it cannot show living processes happening in real time the way light microscopy can. Knowing this trade-off precisely – higher resolution against the loss of the ability to observe living, moving specimens – is what exam questions comparing the two techniques typically test, rather than simply knowing that electron microscopy has “better resolution”.

Organelle structure linked to function

Strong answers in 1.2 explain structure-function relationships precisely rather than describing an organelle’s appearance alone. Mitochondria have a highly folded inner membrane, the cristae, which increases the surface area available for the enzymes and electron carriers involved in aerobic respiration, directly supporting their role in producing ATP. Ribosomes exist in two locations depending on the destination of the protein being made: free ribosomes in the cytoplasm produce proteins used within the cell itself, while ribosomes bound to the rough endoplasmic reticulum produce proteins destined for secretion or for use in membranes, since the rough ER allows the protein to be processed and packaged for export. Being able to state this kind of precise structure-function link, rather than simply identifying an organelle by name, is what separates strong answers from adequate ones on this sub-topic.

Prokaryotic versus eukaryotic cells

The comparison between prokaryotic and eukaryotic cells is worth practising as a direct two-column contrast rather than two separate descriptions: eukaryotic cells have a nucleus enclosed by a nuclear envelope and a range of membrane-bound organelles, while prokaryotic cells lack a true nucleus (their DNA lies free in the cytoplasm) and lack membrane-bound organelles altogether. Prokaryotic cells are typically much smaller (usually in the range of 0.5-5 micrometres in diameter) and have a cell wall built from peptidoglycan (murein), which differs entirely in composition from the cellulose or chitin found in the cell walls of eukaryotic plants and fungi respectively. Plasmids are additional small circular DNA molecules that some prokaryotic cells carry, but they belong to the genetic-technology topic rather than to this comparison.

The eyepiece graticule, stage micrometre and units

Measuring a specimen’s actual size under a light microscope requires an eyepiece graticule — a scale fitted inside the eyepiece with arbitrary divisions — calibrated against a stage micrometre, a slide carrying a scale of known length, for each objective lens in turn, since the real-world value of one graticule division changes with magnification. Once the value of one division is known at a given magnification, the stage micrometre is removed and the graticule is used to measure specimens directly. Candidates should be fluent converting between the three units used throughout this sub-topic: 1 mm = 1,000 μm, and 1 μm = 1,000 nm.

Temporary preparations and drawing

Preparing a temporary mount (for example of onion epidermis or a cheek-cell smear) and producing a scientific drawing from it is practical, assessed content, not simply a laboratory exercise done once and forgotten. A specimen is placed in a drop of water or stain on a slide and covered with a coverslip, lowered at an angle to avoid trapping air bubbles. Scientific drawings must show clean, single, continuous lines with no shading, must be drawn to a scale stated or calculable from the drawing, must be proportionally accurate, and must carry a title, the magnification or scale, and clear label lines that do not cross.

Viruses

Viruses are acellular — they are not built from cells and carry out no metabolism of their own. A virus consists of nucleic acid (DNA or RNA) enclosed in a protein coat, the capsid; some viruses also have an outer lipid envelope derived from a host cell’s membrane. Because they have no ribosomes, no cytoplasm and no means of independent respiration or reproduction, viruses can only replicate by infecting a living host cell and redirecting its ribosomes and enzymes to make new viral components.

Plant and animal cell comparison

Plant and animal cells share the organelles common to all eukaryotic cells, but plant cells possess several structures animal cells lack: a cellulose cell wall external to the cell surface membrane, giving the cell a fixed shape and preventing it bursting through excess osmotic water uptake; chloroplasts, the site of photosynthesis, in photosynthetic tissues; a large, permanent, central vacuole bounded by a membrane called the tonoplast, which maintains turgor and stores solutes; and plasmodesmata, cytoplasmic connections that pass through gaps in adjacent cell walls linking neighbouring plant cells. Animal cells, by contrast, typically possess centrioles (which organise the spindle during division) and have a flexible, irregular shape without a cell wall.

The organelles in full

Beyond mitochondria and ribosomes, several other organelles carry named, examinable functions. The nucleus is bounded by a double membrane, the nuclear envelope, perforated by nuclear pores that allow mRNA and ribosomal subunits to pass out into the cytoplasm; within it, the nucleolus manufactures ribosomal RNA and assembles ribosomes. The Golgi body is a stack of flattened, membrane-bound sacs that modifies, sorts and packages proteins arriving from the rough endoplasmic reticulum, and buds off vesicles including lysosomes. Lysosomes are membrane-bound sacs of hydrolytic enzymes that digest worn organelles, foreign material and pathogens. Centrioles, found in animal cells, organise the spindle fibres used to separate chromosomes during division, and give rise to cilia and microvilli, which are finger-like or hair-like extensions of the cell surface membrane that respectively move fluid or particles across a surface (cilia) and increase surface area for absorption (microvilli).

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027 (Version 1, September 2022) — cambridgeinternational.org.