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

These notes condense section 2.1.1 (a) to (k) of the OCR A Level Biology A specification (H420, Version 4.1, April 2026). Use the full study guide for explanations and fully worked examples, then test yourself with the practice questions. Related pages: Foundations in Biology revision notes, the OCR A Level Biology hub, the printable checklist and the free diagnostics.

Key definitions

• Magnification: image size divided by real size.
• Resolution: the minimum gap that lets two neighbouring points be seen apart; a smaller gap means finer detail.
• Ultrastructure: the fine detail of a cell that only an electron microscope can show.
• Artefact: a feature of an image produced by preparation that was not in the living cell.
• Differential staining: using stains that are taken up differently by different structures or cell types.
• Calibration: finding what real length one eyepiece graticule division represents, using a stage micrometer.
• Cytoskeleton: a network of protein fibres and tubules in the cytoplasm.

Microscopes at a glance (a, f, h)

	Light	TEM	SEM
Beam	Light	Electrons through specimen	Electrons scanned over surface
Shows	Cells, large organelles	Internal ultrastructure	Surface shape
Image	Colour, 2D	Monochrome, 2D	Monochrome, 3D appearance
Living specimens	Possible	No (vacuum)	No (vacuum)
Preparation	Simple	Ultra-thin sections, heavy staining	Surface prepared, no sectioning
Resolution	Lowest	Highest	Between

Why electron microscopes resolve better: the beam has a shorter wavelength than visible light. Magnification beyond the resolution limit adds no detail.

You are not required to recall exact resolution or magnification values for each microscope.

Methods in steps

Temporary slide (b)

1. Thin specimen on a slide in a drop of water or stain.
2. Lower a cover slip at an angle with a mounted needle to avoid bubbles.
3. Blot excess liquid.
4. Focus on low power first, then change objective.

Calibrating a graticule (b)

1. Line up the stage micrometer and graticule scales.
2. Count the graticule divisions that match a known stage micrometer length.
3. Real length per division = known length ÷ graticule divisions.
4. Repeat for every objective; the value changes with magnification.

Magnification calculation (e)

1. Convert image and object to the same unit (1 mm = 1000 µm; 1 µm = 1000 nm).
2. Pick the form: M = I ÷ A, I = M × A or A = I ÷ M.
3. Give the answer with a unit, or as a multiplier such as ×2000 for magnification.
4. Check the size is sensible for the structure.

Reading a scale bar

1. Measure the bar with a ruler.
2. Magnification = measured bar length ÷ length it stands for (same units).
3. Use that magnification to find any real size.

Formulas

Quantity	Formula	Example
Magnification	image size ÷ real size	48 mm ÷ 0.12 mm = ×400
Real size	image size ÷ magnification	48 mm ÷ 400 = 0.12 mm = 120 µm
Image size	magnification × real size	400 × 0.12 mm = 48 mm
Graticule division	known length ÷ divisions	100 µm ÷ 40 = 2.5 µm

Worked reminder. A TEM bar of 20 mm represents 200 nm. A vesicle is 14 mm across. Magnification = 20 mm ÷ 200 nm = 20 000 000 ÷ 200 = ×100 000. Real width = 14 ÷ 20 × 200 = 140 nm.

Drawings (d)

Sharp pencil, continuous lines, no shading, no colour. Large, in proportion, title and magnification. Ruled label lines that do not cross, with no arrowheads. Draw only what is visible. Tissue drawings show cell outlines in a section, not every cell.

Stains (c)

Stain	Use
Iodine in potassium iodide	Starch, blue-black
Methylene blue	General stain; nuclei and bacteria
Eosin	Cytoplasm, pink
Acetic orcein	Chromosomes
Gram stain (differential)	Separates bacteria into Gram-positive (purple) and Gram-negative (red)

Eukaryotic organelles (g)

Organelle	Key structure	Key function
Nucleus	Double nuclear envelope with pores; chromatin	Stores DNA; controls the cell
Nucleolus	Dense area in the nucleus	Makes ribosomal RNA; builds ribosome subunits
Rough ER	Cisternae with ribosomes	Folds and processes proteins
Smooth ER	Tubes without ribosomes	Makes lipids
Golgi	Stacked flattened sacs, vesicles	Modifies, packages and sorts; makes lysosomes
Ribosome	Two subunits, no membrane	Protein synthesis
Mitochondrion	Double membrane, cristae, matrix	Aerobic respiration; ATP
Lysosome	Single membrane, hydrolytic enzymes	Digestion
Chloroplast	Double membrane, grana, stroma	Photosynthesis
Plasma membrane	Phospholipid bilayer	Controls entry and exit
Centrioles	Two short microtubule tubes at right angles	Spindle formation
Cell wall (plants)	Cellulose	Strength
Cilia and flagella	Projections with microtubules	Move fluid or the cell

Worked reminders

Graticule and a change of objective. One division is 2.5 µm at the ×10 objective. At ×40 the image is four times larger, so a division is 2.5 ÷ 4 = 0.625 µm. A nucleus that spans 16 divisions at ×40 is 16 × 0.625 = 10 µm across.

Drawing magnification. A cell is 0.045 mm long and your drawing of it is 135 mm long. Magnification = 135 ÷ 0.045 = ×3000. A 10 µm scale bar on that drawing would be 10 × 3000 = 30 000 µm = 30 mm long.

Order of magnitude. A mitochondrion of about 1 µm in a cell of about 20 µm is a factor of about 20, so the cell is just over one order of magnitude wider.

Reading a photomicrograph (h)

1. Find the scale bar or stated magnification, and work out the real size of a feature.
2. Decide the type of image. Flat, with internal detail and a grey scale: TEM. Surface, with shadows and depth: SEM.
3. Name structures from their features: double membrane with inner folds is a mitochondrion; stacks of flattened sacs with small vesicles is the Golgi; dots on flattened sacs is rough ER; pores in a double boundary is the nucleus.
4. Remember that a section is a thin slice. An organelle cut across can look circular, and cut lengthways can look like a strip.
5. Comment on resolution only when asked; do not call a bigger image a better one.

Similarities of prokaryotes and eukaryotes

Both have a plasma membrane, cytoplasm, ribosomes and DNA. Both make proteins by transcription and translation. Questions often ask for a similarity as well as a difference, so learn one or two of each.

Protein secretion (i)

nucleus (DNA, mRNA) → nuclear pore → ribosome on rough ER → vesicle → Golgi → secretory vesicle → fuses with plasma membrane → exocytosis. Mitochondria supply ATP.

Cytoskeleton (j)

Three roles: mechanical strength, transport within the cell and cell movement.

Prokaryote or eukaryote (k)

Feature	Prokaryote	Eukaryote
Nucleus	None	Present
DNA	Circular, free, not with histones	Linear, with histones
Ribosomes	70S	80S
Organelles	No membrane-bound organelles	Present
Wall	Peptidoglycan (bacteria)	Cellulose or chitin or none
Plasmids	Often	Not usually

Must-know distinctions

• Magnification versus resolution.
• TEM (internal, 2D) versus SEM (surface, 3D).
• Rough ER (protein) versus smooth ER (lipid).
• Mitochondrion (cristae, respiration) versus chloroplast (grana, photosynthesis).
• Lysosome (digestion) versus Golgi (packaging).
• Cilia (move fluid) versus flagella (move the cell).

Quick self-test

1. State why an electron microscope has a higher resolution than a light microscope.
2. Why can a TEM not be used to look at living cells?
3. A stage micrometer length of 150 µm matches 50 graticule divisions. Calculate the length of one division.
4. A cell measures 12 graticule divisions at that calibration. State its length.
5. Calculate the magnification of an image 90 mm long of a structure 30 µm long.
6. A chloroplast is 5 µm long. Calculate its length in an image magnified ×8000, in mm.
7. Name the organelle that makes ribosomal RNA.
8. Give two ways a rough ER differs from a smooth ER.
9. Name the structure that releases a secreted protein from the cell.
10. State three roles of the cytoskeleton.
11. Which has 80S ribosomes, a bacterium or a yeast cell?
12. Why does a Gram-negative cell look red after Gram staining?

Answers

1. Electrons have a shorter wavelength than light.
2. The specimen is in a vacuum and is killed by the preparation.
3. 150 ÷ 50 = 3 µm.
4. 12 × 3 = 36 µm.
5. 30 µm = 0.030 mm; 90 ÷ 0.030 = ×3000.
6. 5 µm × 8000 = 40 000 µm = 40 mm.
7. The nucleolus.
8. Rough ER has ribosomes and makes and processes protein; smooth ER has none and makes lipids.
9. Exocytosis: a secretory vesicle fuses with the plasma membrane.
10. Mechanical strength, transport within the cell, cell movement.
11. Yeast (eukaryote).
12. It loses the crystal violet and takes up the red counterstain.

Where marks are usually lost

• Using the terms magnification and resolution interchangeably.
• Failing to convert units before dividing, so the answer is out by 1000.
• Quoting a magnification without checking that the answer is sensible for the structure.
• Saying “the nucleus is the control centre” without saying how, for example that it holds DNA coding for proteins.
• Calling the mitochondrion the “powerhouse” without naming ATP and aerobic respiration.
• Confusing cilia (move fluid over a surface) with flagella (move a cell), or mixing up centrioles with the nucleolus.
• Drawing shaded, coloured or sketchy lines and omitting a title or magnification.
• Saying prokaryotes have no genetic material rather than no nucleus.
• Not saying that a TEM section is a 2D slice, so one organelle can appear as a circle or a strip.

Official syllabus

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