Cell Biology: Exam Practice

This lesson brings together all the key concepts from the AQA GCSE Biology Cell Biology topic (4.1) and provides guidance on how to tackle exam questions effectively. It covers common question types, command words, required practical skills, and a comprehensive review of the most frequently tested content.

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Command Words in Biology Exams

Understanding command words is essential for knowing what the examiner expects in your answer:

Command Word	What It Means	How to Answer
State	Give a brief, factual answer	One word, phrase, or short sentence. No explanation needed.
Describe	Say what happens or what you observe	Give an account of the process, pattern, or feature. Do not explain why.
Explain	Say why something happens	Give reasons using scientific knowledge. Link cause and effect.
Compare	Identify similarities AND differences	You must mention BOTH similarities and differences for full marks.
Suggest	Apply your knowledge to an unfamiliar situation	Use what you know to propose a reasonable answer, even if you have not been taught the specific context.
Evaluate	Give arguments for and against, then reach a conclusion	Weigh up the evidence or arguments on both sides. State which side you support and why.
Calculate	Work out a numerical answer	Show your working clearly. Include units.
Draw	Produce a diagram or graph	For biological drawings: use pencil, clear lines, labels with ruler. For graphs: label axes, plot accurately, draw a line of best fit.
Justify	Give reasons for your answer	Support your conclusion with evidence or scientific reasoning.

Exam Tip: The command word tells you exactly what to do. "Describe" does NOT require an explanation. "Explain" DOES require reasons. "Compare" MUST include both similarities and differences. Read the command word carefully before writing your answer.

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Key Definitions to Memorise

The following definitions are commonly tested and must be learned precisely:

Term	Definition
Eukaryotic cell	A cell with a nucleus and membrane-bound organelles (e.g. animal cells, plant cells).
Prokaryotic cell	A cell without a true nucleus; DNA is a single circular loop in the cytoplasm (e.g. bacteria).
Diffusion	The net movement of particles from a region of higher concentration to a region of lower concentration.
Osmosis	The diffusion of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane.
Active transport	The movement of substances from a region of lower concentration to a region of higher concentration, against the concentration gradient, using energy from respiration.
Mitosis	Cell division that produces two genetically identical diploid daughter cells. Used for growth, repair, and asexual reproduction.
Meiosis [H]	Cell division that produces four genetically different haploid daughter cells (gametes). Used for sexual reproduction.
Cell differentiation	The process by which a cell becomes specialised for a particular function.
Stem cell	An undifferentiated cell that can divide and differentiate into specialised cell types.
Magnification	How many times larger the image appears compared to the actual object.
Resolution	The ability to distinguish between two points that are very close together.

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Comparison Tables for Revision

Mitosis vs Meiosis [H]

Feature	Mitosis	Meiosis
Number of divisions	1	2
Daughter cells produced	2	4
Chromosome number	Diploid (2n) — same as parent	Haploid (n) — half of parent
Genetically identical?	Yes	No — all different
Where it occurs	Body cells (somatic cells)	Reproductive organs only
Purpose	Growth, repair, asexual reproduction	Gamete production
Crossing over?	No	Yes

Diffusion vs Osmosis vs Active Transport

Feature	Diffusion	Osmosis	Active Transport
What moves	Any small molecules/ions	Water only	Specific molecules/ions
Direction	High to low concentration	Dilute to concentrated solution	Low to high concentration
Energy needed?	No	No	Yes (from respiration)
Membrane needed?	No	Yes (partially permeable)	Yes (carrier proteins)
Example	O2 into blood at lungs	Water into root hair cells	Mineral ions into roots

Animal vs Plant Cells

Structure	Animal Cell	Plant Cell
Nucleus	Yes	Yes
Cell membrane	Yes	Yes
Cytoplasm	Yes	Yes
Mitochondria	Yes	Yes
Ribosomes	Yes	Yes
Cell wall	No	Yes (cellulose)
Chloroplasts	No	Yes (green parts)
Permanent vacuole	No	Yes

Prokaryotic vs Eukaryotic Cells

Feature	Prokaryotic	Eukaryotic
Nucleus	No	Yes
DNA	Circular chromosome + plasmids	Linear chromosomes in nucleus
Typical size	0.2–5 um	10–100 um
Membrane-bound organelles	No	Yes
Cell wall	Yes (peptidoglycan)	Plants: yes (cellulose). Animals: no

Exam Tip: These comparison tables cover the most commonly tested material in the Cell Biology topic. Practise reproducing them from memory. Many 4- and 6-mark questions can be answered well by recalling the information in these tables and applying it to the specific question.

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Required Practical Skills

Using a Light Microscope

You must be able to:

• Describe how to prepare a slide (add specimen, add stain, lower coverslip at an angle).
• Use the microscope (start on lowest power, use coarse then fine focus).
• Calculate total magnification (eyepiece x objective).
• Make a biological drawing (large, clear lines, labels with ruler, title, magnification stated).
• Use the magnification formula: magnification = image size / actual size.

Osmosis Investigation

You must be able to:

• Describe the method (potato cylinders in different sucrose concentrations).
• Identify the independent variable (sucrose concentration), dependent variable (change in mass), and control variables (volume of solution, size of potato, temperature, time).
• Calculate percentage change in mass.
• Plot a graph and identify the isotonic point (where the line crosses the x-axis).
• Explain the results using osmosis (water moves from dilute to concentrated).

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Common Calculation Types

1. Magnification Calculations

magnification = image size / actual size

actual size = image size / magnification

image size = actual size x magnification

Always convert to the same units first (usually micrometres).

2. Percentage Change

percentage change = ((final value - initial value) / initial value) x 100

3. Standard Form Conversions

Measurement	Standard Form
0.005 mm	5.0 x 10^-3 mm
50 um	5.0 x 10^1 um
2500 nm	2.5 x 10^3 nm

4. Unit Conversions

• mm to um: multiply by 1000
• um to mm: divide by 1000
• um to nm: multiply by 1000
• nm to um: divide by 1000

Exam Tip: Always show your working in calculation questions, even if you can do it in your head. If you make an arithmetic error but your method is correct, you will still gain marks for the working. Also, always include the correct units in your final answer.

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How to Answer 6-Mark Extended Response Questions

The AQA exam includes 6-mark questions that require an extended written response. These are marked on the quality of your scientific reasoning, not just facts. To score full marks:

1. Plan your answer — spend 30 seconds thinking before writing.
2. Structure your response logically — use a clear sequence.
3. Use scientific terminology — use the correct key terms (not vague language).
4. Explain, don't just list — link each point to the next.
5. Include specific examples — refer to named cells, organelles, or processes.
6. Answer the question — stay on topic and address the command word.