AQA GCSE Biology 8461: Cell structure – Revision Notes

These are condensed recall notes for section 4.1.1 Cell structure (4.1.1.1 to 4.1.1.6) of the AQA GCSE Biology (8461) specification, for first teaching 2016 with exams from 2018 (version 1.0). Cell biology is examined on Paper 1, set at Foundation and Higher Tier. The one (HT only) point in this section is labelled Higher tier only. For full explanations and worked examples, use the Cell structure study guide.

When you are ready, test yourself with the Cell structure practice questions. For cell division and transport, use the topic-level Cell biology revision notes. The course hub is AQA GCSE Biology, and the printable checklist lists every topic. To check where you stand, take a free diagnostic.

4.1.1.1 Eukaryotes and prokaryotes

	Eukaryotic (plant, animal)	Prokaryotic (bacteria)
Size	Larger; can be up to about 100 times wider	Much smaller, a few μm or less
Genetic material	Enclosed in a nucleus	Single DNA loop, not in a nucleus
Plasmids	No	May have one or more small rings of DNA
Cell membrane and cytoplasm	Yes	Yes
Cell wall	Plants and algae (cellulose)	Yes, surrounding the membrane

Prefixes and standard form

Prefix	Symbol	Value
centi	c	10⁻²
milli	m	10⁻³
micro	μ	10⁻⁶
nano	n	10⁻⁹

mm  --×1000-->  μm  --×1000-->  nm
mm  <--÷1000--  μm  <--÷1000--  nm

Order of magnitude = a factor of 10. Divide one size by the other (same units), then write the answer as a power of ten. 10³ means three orders of magnitude.

Worked reminder: 30 μm = 30 × 10⁻⁶ m = 3 × 10⁻⁵ m. Check: the number in front must be at least 1 and less than 10.

4.1.1.2 Animal and plant cells

Animal cells (most): nucleus, cytoplasm, cell membrane, mitochondria, ribosomes.

Plant cells (often) add: chloroplasts, permanent vacuole filled with cell sap.

Plant and algal cells also have: a cellulose cell wall, which strengthens the cell.

Structure	Function
Nucleus	Genetic material that controls the cell’s activities
Cytoplasm	Most chemical reactions happen here
Cell membrane	Controls what enters and leaves
Mitochondria	Aerobic respiration, releasing energy
Ribosomes	Protein synthesis
Chloroplasts	Absorb light for photosynthesis
Permanent vacuole	Holds cell sap
Cell wall (cellulose)	Strengthens the cell
Plasmids (bacteria)	Extra small rings of DNA

Estimation

Use estimation to judge the relative size or area of sub-cellular structures when exact measurement is not possible or not needed, such as irregular shapes or unclear edges.

Method in steps – estimating size or area

1. Find a known length (a scale bar or a given cell size).
2. Count how many times the structure fits along it, or treat it as a rectangle or circle.
3. Divide or multiply to get the estimate.
4. Write “about” or “approximately” and give a sensible number of figures.

Required practical 1 – method in steps

1. Thin layer of cells (e.g. onion epidermis) on a slide.
2. Add a stain (iodine solution for plant cells) so structures show up.
3. Lower a coverslip at an angle to avoid air bubbles.
4. Lowest-power objective first; coarse focus, then fine focus.
5. Move to a higher-power objective; refocus with fine focus only.
6. Pencil drawing, clear lines, no shading, labels with ruled lines.
7. Add a magnification scale (scale bar).

Total magnification = eyepiece × objective. ×10 eyepiece and ×10 objective gives ×100.

4.1.1.3 Cell specialisation

Cell	Feature → what it allows
Sperm	Tail → swims; many mitochondria → energy for swimming
Nerve	Long → carries impulses a long way; branched endings → links to many cells
Muscle	Protein fibres → contract; many mitochondria → energy
Root hair	Long projection → large surface area to absorb water and mineral ions
Xylem	Hollow tubes of dead cells, lignin → water flows freely; tubes are strong
Phloem	End walls with pores → dissolved sugars pass from cell to cell

If the exam describes a cell you have not met, use the same “feature → what it allows” pattern with the information given.

4.1.1.4 Cell differentiation

• As an organism develops, cells differentiate into different types.
• A differentiating cell gains the sub-cellular structures it needs, and becomes specialised.
• Most animal cells differentiate early. Many plant cells keep the ability throughout life.
• In mature animals, cell division is mainly for repair and replacement.
• Importance: it produces the different cell types that form tissues and organs.

4.1.1.5 Microscopy

	Light microscope	Electron microscope
Developed	First	Later
Magnification	Lower	Much higher
Resolution (resolving power)	Lower; cannot resolve below about 200 nm	Much higher
Result	Cells, nucleus, chloroplasts	Many more sub-cellular structures in fine detail

Magnification = how many times bigger the image is. Resolution = ability to distinguish two close points as separate.

magnification = size of image / size of real object

Method in steps – magnification questions

1. Write the equation.
2. Convert both sizes to the same unit (usually μm).
3. Substitute and calculate.
4. Check the answer is sensible (a cell is not 3 m wide).
5. Give the unit for a size; no unit for magnification. Use standard form if asked.

Scale bar: magnification = measured length of bar / length written on bar (same units).

4.1.1.6 Culturing microorganisms

• Bacteria divide by binary fission, as often as once every 20 minutes with enough nutrients and a suitable temperature.
• Grow them in nutrient broth or as colonies on agar gel.
• Uncontaminated cultures are needed to test disinfectants and antibiotics.

Aseptic step	Why
Sterilise Petri dishes and media	Kill microorganisms already present
Flame the inoculating loop	Kill microorganisms on the loop
Tape the lid	Stop airborne microorganisms getting in
Store upside down	Condensation does not drip onto the agar
Incubate at 25°C (school labs)	Reduces the chance of growing human pathogens

Method in steps – bacterial numbers

1. Convert the time to the same unit as the division time.
2. Divisions = time / mean division time.
3. Final number = starting number × 2^(divisions).
4. Higher tier only: write the answer in standard form.

Method in steps – area of a colony or clear zone

1. Halve the diameter to get r.
2. Area = πr².
3. Give the unit (mm² or cm²).

Required practical 2 – method in steps

1. Spread bacteria evenly on an agar plate using aseptic technique.
2. Add discs soaked in different antiseptics or antibiotics, plus a control disc (sterile water).
3. Tape the lid, incubate at 25°C.
4. Measure the diameter of each clear zone of inhibition.
5. Calculate each area with πr². Larger area = more effective.

Must-know distinctions

• Nucleus vs DNA loop: bacteria have DNA but no nucleus.
• Cell membrane vs cell wall: the membrane controls what passes in and out; the wall strengthens.
• Magnification vs resolution: bigger image vs finer detail.
• Image size vs real size: image size is the one measured on the drawing or micrograph.
• Radius vs diameter: πr² needs the radius.
• Differentiation vs division: differentiation changes what a cell is; division makes more cells.

Quick self-test

1. Convert 0.004 mm to micrometres.
2. Name the two kinds of DNA a bacterial cell may contain.
3. Name three structures found in plant cells but not animal cells.
4. An image of a cell is 12 mm wide. The real cell is 6 μm wide. Calculate the magnification.
5. A cell’s image is 30 mm long at ×1500. Find its real length in μm.
6. A cell is 2 × 10⁻⁵ m long and a bacterium is 2 × 10⁻⁶ m. How many times longer is the cell?
7. Why is a root hair cell long and thin?
8. Why are school cultures incubated at 25°C?
9. A bacterium divides every 25 minutes. Starting with one, how many bacteria are there after 100 minutes?
10. A clear zone has a diameter of 14 mm. Calculate its area to 3 significant figures.
11. Higher tier only: a culture of 40 bacteria divides every 30 minutes. How many are there after 4 hours? Give your answer in standard form.
12. State the difference between magnification and resolution.

Answers

1. 0.004 × 1000 = 4 μm.
2. A single DNA loop and plasmids.
3. Chloroplasts, permanent vacuole, cellulose cell wall.
4. 12 mm = 12 000 μm; 12 000 / 6 = ×2000.
5. 30 / 1500 = 0.02 mm = 20 μm.
6. (2 × 10⁻⁵) / (2 × 10⁻⁶) = 10 times (one order of magnitude).
7. It gives a large surface area for absorbing water and mineral ions.
8. It reduces the chance of growing pathogens harmful to humans.
9. 100 / 25 = 4 divisions; 2⁴ = 16 bacteria.
10. r = 7 mm; π × 7² = 154 mm².
11. 4 h = 240 min; 240 / 30 = 8 divisions; 40 × 2⁸ = 10 240 = 1.024 × 10⁴ bacteria.
12. Magnification is how many times bigger the image is; resolution is the ability to distinguish two close points as separate.

Where marks are usually lost

• Writing “bacteria have no DNA” instead of “no nucleus”.
• Leaving out “permanent” when naming the plant cell vacuole.
• Mixing units in the magnification equation, for example dividing mm by μm.
• Giving a unit for magnification, or leaving the unit off a real size.
• Using diameter in πr², which makes the area four times too big.
• Counting the wrong number of divisions when the time is in hours and the division time in minutes.
• Giving “best temperature for bacteria” as the reason for 25°C instead of safety.
• Naming a feature of a specialised cell without saying what it allows the cell to do.
• In the required practical, not including a magnification scale on the drawing.
• Saying electron microscopes are better because of “magnification” alone, without resolution.

Official syllabus

AQA GCSE Biology (8461) specification, for first teaching 2016, exams from 2018, version 1.0, published by AQA – section 4.1.1 Cell structure (4.1.1.1 to 4.1.1.6), with required practical activities 1 and 2.