OCR A Level Biology A: Cell structure, microscopy, prokaryotes and eukaryotes (H420)

This guide teaches section 2.1.1 (a) to (k), Cell structure, in the OCR A Level Biology A specification (H420, Version 4.1, April 2026, first assessment 2025). It follows the specification numbering. For a shorter overview of the whole of Module 2 see Foundations in Biology. Condensed recall is in the revision notes and exam-style work is in the practice questions. Course pages: the OCR A Level Biology hub, the printable checklist and the free diagnostics.

What this unit covers

Reference	What you need to be able to do
2.1.1(a)	Use microscopy to observe cells and structures in eukaryotes; appreciate images from light, TEM and SEM
2.1.1(b)	Prepare and examine slides for light microscopy, using an eyepiece graticule and stage micrometer
2.1.1(c)	Use staining, including differential staining
2.1.1(d)	Draw and annotate cells or cells in sections of tissue
2.1.1(e)	Use and rearrange the magnification formula
2.1.1(f)	Distinguish magnification from resolution for the three microscopes
2.1.1(g)	Ultrastructure and functions of eukaryotic components
2.1.1(h)	Interpret photomicrographs, including TEM and SEM images
2.1.1(i)	Link the organelles that make and secrete proteins
2.1.1(j)	Importance of the cytoskeleton
2.1.1(k)	Compare prokaryotic and eukaryotic cells

Microscopes and what they show (a, f)

A light microscope shines light through a thin specimen. It can show living cells, colour and moving material, and stains add contrast. Its detail is limited because light has a relatively long wavelength.

In a transmission electron microscope (TEM), a beam of electrons goes through a very thin slice of the specimen. Electrons are absorbed more by dense regions, so the image is a flat, black, white and grey picture of the inside of the cell. It shows ultrastructure such as membranes, ribosomes and the folds of a mitochondrion.

A scanning electron microscope (SEM) moves a beam over the surface of a specimen and collects electrons coming from it. The result is a three-dimensional-looking image of the outside of a cell or tissue. The specimen need not be sectioned, but you see the surface only.

Both electron microscopes keep the specimen in a vacuum, which kills living material, and the preparation can create artefacts, which are features in the image that were not in the living cell. You are not required to recall exact resolutions or magnifications for each microscope. You must be able to say which gives more detail and why.

Magnification and resolution (f)

Magnification is the factor by which the image exceeds the real size of the specimen. Resolution is the minimum gap that lets two neighbouring points be seen apart. An electron beam has a far shorter wavelength than visible light, so electron microscopes resolve finer detail. Enlarging an image beyond the resolution of the instrument only gives a bigger, blurrier picture. This is called empty magnification.

Feature	Light	TEM	SEM
Radiation	Light	Electron beam through specimen	Electron beam scanned over surface
Specimen	Thin; can be living	Ultra-thin section; dead	Whole or coated; dead
Image	2D, colour possible	2D, monochrome, internal detail	3D appearance, surface detail
Resolution	Lowest of the three	Highest of the three	Better than light, usually below TEM

Slides, stains and drawings (b, c, d)

Making a temporary slide (b)

1. Cut or tear a very thin piece of tissue so light can pass through, for example onion epidermis.
2. Place it flat on a clean slide in a drop of water or stain.
3. Hold a cover slip at an angle with a mounted needle and lower it slowly onto the specimen. This pushes air out and prevents bubbles.
4. Blot spare liquid with filter paper, then start on low power and move to higher power only once the specimen is in focus.

Use the coarse focus only at low power, and never move the objective towards the slide while looking down the eyepiece.

Eyepiece graticule and stage micrometer (b)

An eyepiece graticule is a ruler fitted in the eyepiece whose divisions have no fixed size. Each objective lens needs its own calibration against a stage micrometer, a slide ruled in known lengths. Line up the two scales and work out how much real length one graticule division represents. Then remove the micrometer and measure the specimen in graticule divisions.

Worked example 1. A stage micrometer line of 100 µm lines up with 40 graticule divisions at the ×10 objective.

• One division = 100 ÷ 40 = 2.5 µm.
• An epidermal cell spans 14 divisions, so its length is 14 × 2.5 = 35 µm.
• At the ×40 objective, with the same eyepiece, the image is four times bigger, so one division = 2.5 ÷ 4 = 0.625 µm. The calibration has to be repeated, or scaled, for each objective.

Staining (c)

Most cell structures are transparent, so stains add contrast. Iodine in potassium iodide solution stains starch blue-black. Methylene blue and eosin are examples of stains for general detail, and acetic orcein stains chromosomes.

Differential staining uses more than one stain, or a stain that behaves differently with different structures, so that components or cell types can be told apart. In haematoxylin and eosin staining, nuclei stain blue-purple and cytoplasm pink. The Gram stain divides bacteria into two groups: Gram-positive cells keep the purple crystal violet and look purple, while Gram-negative cells lose it and take up the red counterstain. This difference is linked to the structure of the cell wall.

Drawings (d)

A good biological drawing:

• uses a sharp pencil and clear continuous lines, with no shading and no colouring;
• is large and in proportion, showing only what you can see;
• has ruled label lines that do not cross and do not end in arrowheads;
• has a title and a magnification or scale bar;
• for tissue, draws the outlines of the cells in a region, not every cell, and never draws a cell that is not there.

The magnification formula (e)

magnification = size of image ÷ size of real object

Rearranged: size of image = magnification × real size, and real size = size of image ÷ magnification. Use the same units for the image and the object, and convert only at the end. Useful conversions: 1 mm = 1000 µm and 1 µm = 1000 nm. A scale bar gives magnification directly: measure the bar and divide by the length it represents.

Worked example 2. An SEM image of a pollen grain is 62 mm across. The grain’s real diameter is 31 µm.

• Convert the real size: 31 µm = 0.031 mm.
• Magnification = 62 ÷ 0.031 = ×2000.

Worked example 3. In a TEM micrograph a scale bar is 25 mm long and is labelled 500 nm. A mitochondrion is 68 mm long in the image.

• Magnification = 25 mm ÷ 500 nm = 25 000 000 nm ÷ 500 nm = ×50 000.
• Real length = 68 mm ÷ 50 000 = 0.00136 mm = 1.36 µm, or 1360 nm.
• Check by proportion: 68 ÷ 25 × 500 nm = 1360 nm.

Order of magnitude. A cell about 40 µm across is about 20 times wider than a bacterium of 2 µm, which is just over one order of magnitude. A quick estimate like this lets you test whether an answer is sensible.

Ultrastructure of eukaryotic cells (g)

Component	Structure	Function
Plasma membrane	Phospholipid bilayer with proteins	Partially permeable boundary; controls exchange
Nucleus	Surrounded by a nuclear envelope (double membrane with pores); contains chromatin	Holds DNA; controls the activity of the cell; mRNA leaves through the pores
Nucleolus	Dense region inside the nucleus	Site of ribosomal RNA synthesis; ribosome subunits are put together here
Rough ER	Flattened sacs (cisternae) with ribosomes on the outer surface	Folds and processes proteins made on its ribosomes
Smooth ER	Membrane tubes without ribosomes	Makes and processes lipids
Golgi apparatus	Stack of flattened membrane sacs with vesicles	Modifies, sorts and packages proteins; makes lysosomes
Ribosomes	Two subunits of RNA and protein; no membrane	Site of protein synthesis
Mitochondrion	Double membrane, inner membrane folded into cristae, matrix	Aerobic respiration; makes ATP
Lysosome	Single membrane around hydrolytic enzymes	Digests worn-out organelles and material taken in
Chloroplast	Double membrane, stacks of thylakoids (grana) in stroma	Photosynthesis
Cell wall	Plant: cellulose fibres	Strength; resists bursting
Centrioles	Pair of short tubes of microtubules, at right angles, in animal cells	Involved in forming the spindle in cell division
Cilia and flagella	Projections from the surface, with microtubules inside	Cilia move fluid or particles over a surface; flagella move a cell

Mitochondria and chloroplasts contain their own DNA and ribosomes. Chloroplasts are found in photosynthetic cells but not in all plant cells.

Photomicrographs (h)

Work through a micrograph in this order: look for the scale bar, decide whether it is TEM (flat, internal) or SEM (surface, 3D), then identify structures by their features.

• Double membrane with inner folds: mitochondrion. A double membrane with flattened stacks: chloroplast.
• Stack of curved flattened sacs with small vesicles: Golgi apparatus.
• Flattened sacs studded with dots: rough ER. Dots are ribosomes.
• Dark-staining round region with pores in its double boundary: nucleus.
• Cut surfaces appear as circles, ovals or long strips because a thin section can slice an organelle at any angle. A long tube may look like a circle. Say this in a “suggest why” answer.

Making and secreting a protein (i)

1. DNA in the nucleus is transcribed to mRNA, which leaves through a nuclear pore.
2. mRNA is translated by ribosomes on the rough ER. The growing polypeptide enters the cisternae of the rough ER, where it is folded and processed.
3. Vesicles bud off the rough ER and carry the protein to the Golgi apparatus.
4. The Golgi chemically alters the protein, for instance by attaching sugar groups, and sorts it into secretory vesicles.
5. Vesicles move to the plasma membrane and fuse with it, releasing the protein by exocytosis.

ATP from mitochondria supplies energy for this pathway.

The cytoskeleton (j)

The cytoskeleton is a network of protein fibres and tubules in the cytoplasm. It has three roles in the specification.

• Mechanical strength: it supports the shape of the cell and holds organelles in place.
• Transport within cells: vesicles and organelles move along tracks, for example vesicles on microtubules.
• Cell movement: it enables cilia and flagella to beat and cells to change shape or move.

Prokaryotes and eukaryotes (k)

Feature	Prokaryote	Eukaryote
Size	Usually smaller	Usually larger
Genetic material	Circular DNA free in cytoplasm; no nucleus	Linear DNA in chromosomes inside a nucleus
DNA packaging	Not wound round histones	Associated with histones
Ribosomes	Smaller (70S)	Larger (80S) in the cytoplasm
Organelles	No membrane-bound organelles	Membrane-bound organelles present
Cell wall	Peptidoglycan (bacteria)	Cellulose in plants; chitin in fungi; absent in animals
Extras	Plasmids; some have a capsule	No plasmids in most; none in animals

Similarities: both have a plasma membrane, cytoplasm, ribosomes and DNA as genetic material.

Common errors

• Writing “magnification” when you mean “resolution”, or the reverse.
• Mixing units in the magnification formula.
• Saying electron microscopes give a higher magnification only. The key advantage is resolution.
• Describing SEM as showing internal structure.
• Saying that prokaryotes have “no DNA”, rather than no nucleus.

Official syllabus

OCR, A Level in Biology A (H420), specification Version 4.1 (April 2026), section 2.1.1 Cell structure.