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Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next. (2026)
Question
A diagram shows a cell measuring 2 µm long with a single circular loop of DNA in the cytoplasm and two small DNA rings nearby. Is this cell eukaryotic or prokaryotic? Give two reasons.
Solution
Check for a true nucleus. The DNA here is free in cytoplasm (no membrane around it) — that is a prokaryote signature.
Check the size. 2 µm fits the prokaryote range (1–10 µm), not the typical eukaryote range (10–100 µm).
The two small DNA rings are plasmids — a feature unique to prokaryotes.
Answer
Prokaryotic. Reasons: (1) DNA is free in cytoplasm (no membrane-bound nucleus); (2) small size of 2 µm, in the bacterial range; (3) presence of plasmids. (Any two for the two marks.)
Examiner note
AQA mark scheme accepts any two distinct reasons. Linking 'plasmid → bacterium' is a higher-mark response than 'looks small'.
Question
A plant cell is 0.000045 m wide. Express this width in micrometres, and in standard form in metres.
Solution
Convert to µm by multiplying by (since 1 m = µm).
Now write 0.000045 m in standard form. Move the decimal 5 places to the right.
Answer
45 µm, or .
Question
An animal cell is m wide. A bacterium is m wide. How many of these bacteria would fit end-to-end across one animal cell?
Solution
Divide the animal-cell width by the bacterium width.
Simplify the power of 10.
Answer
20 bacteria fit end-to-end across the animal cell.
Examiner note
When dividing standard-form numbers, subtract the powers of 10. AQA examiner reports note many candidates lose marks here by forgetting the sign change on the negative exponent.
Question
A student looks at a cell under a microscope. The cell has a regular rectangular shape, a large central sac and green organelles. Which type of cell is it? Give two reasons.
Solution
Identify the key plant features visible. The rectangular shape is given by a cell wall. The large central sac is the permanent vacuole. The green organelles are chloroplasts.
All three of these are plant-only features (cell wall, permanent vacuole, chloroplasts).
Answer
Plant cell. Reasons (any two): presence of cell wall (regular rectangular shape), permanent vacuole (large central sac), chloroplasts (green organelles).
Question
Muscle cells contain large numbers of mitochondria. Explain how this is related to the function of muscle cells. (3 marks)
Solution
Identify the function of a muscle cell.
Link mitochondria to the process they perform (aerobic respiration).
Connect aerobic respiration to the function.
Answer
Muscle cells contract (1) and need a lot of energy to do this. Mitochondria are the site of aerobic respiration (1). More mitochondria means more respiration so more energy is released for contraction (1).
Examiner note
Three-mark structure: function → organelle activity → energy link. Don't write 'mitochondria produce energy' — say 'release' or 'transfer'.
Question
Describe TWO ways a root-hair cell is adapted for absorbing water and ions from soil. (2 marks)
Solution
Long thin extension provides large surface area for absorption.
Many mitochondria release energy for active transport of mineral ions against concentration gradient.
Answer
Question
Compare the structure and function of xylem and phloem cells. (4 marks)
Solution
Xylem cells are DEAD, hollow with lignified walls; phloem cells are ALIVE.
Xylem has no end walls (continuous tube); phloem has sieve plates (perforated).
Xylem carries water + minerals UP only; phloem carries dissolved sugars BOTH ways (translocation).
Phloem has companion cells alongside (with mitochondria); xylem does not.
Answer
Xylem cells are dead and hollow with lignified walls (1) and carry water + minerals up only (1). Phloem cells are alive with sieve plates between them and companion cells alongside (1) and carry sugars up and down (translocation) (1).
Examiner note
Top-band answers use 'whereas' or 'in contrast' to make the comparison explicit.
Question
What is meant by 'cell differentiation'? (1 mark)
Solution
Give the AQA mark scheme definition.
Answer
The process by which a cell becomes specialised for a particular function.
Question
Explain ONE difference between when most plant cells differentiate and when most animal cells differentiate. (2 marks)
Solution
State the timing in each.
Link to the location/structure (meristem for plants; embryonic stage for animals).
Answer
Most animal cells differentiate at an early embryonic stage and lose the ability afterwards (1). Most plant cells can differentiate throughout the plant's life, in meristems at root and shoot tips (1).
Examiner note
Mark scheme requires BOTH the timing AND the link to the structure for the second mark.
Question
An image of a plant cell measures 30 mm across. The actual cell is 60 µm across. What is the magnification?
Solution
Convert mm to µm so both are in same units.
Apply the equation.
Answer
×500
Question
A bacterium appears 8 mm long under ×4000 magnification. What is its actual length in µm?
Solution
Rearrange the magnification equation.
Substitute (keep image in mm for now).
Convert to µm.
Answer
2 µm.
Question
A mitochondrion is 1.5 µm long. In an electron micrograph it appears 12 cm long. Calculate the magnification.
Solution
Convert both to same units (µm).
Apply the equation.
Answer
×80,000.
Question
A culture starts with 100 bacteria. They divide every 30 minutes. Calculate the number of bacteria after 2 hours.
Solution
Find number of divisions.
Apply growth equation.
Answer
1600 bacteria.
Question
An inhibition zone has a diameter of 14 mm. Calculate its area in mm² to 2 s.f. (Use .)
Solution
Halve the diameter to get the radius.
Apply .
Round to 2 s.f.
Answer
≈ 150 mm².
Examiner note
Common slip: using 14 (diameter) in πr² gives 615 mm² — wrong by a factor of 4. Always halve first.
Standard form
When to use
Use to express very small (or very large) numbers compactly. Compulsory for cell-size answers on AQA papers.
Example
A bacterium 2 µm wide is .
Unit conversions (length)
When to use
Convert measurements between units before substituting into a formula. Always use SI units (m) for calculations.
Magnification equation
When to use
Use whenever a microscope question gives you two of the three quantities. Always convert image and actual to the same units first. On the AQA equation sheet.
Bacterial growth (binary fission)
When to use
Use whenever bacteria are dividing at a regular rate. Calculate from total time ÷ division time.
Area of a circle
When to use
Use to calculate the area of an inhibition zone. Always halve the diameter first.
A cell with a true (membrane-bound) nucleus and other membrane-bound organelles such as mitochondria.
Animal cells, plant cells, fungal cells, protist cells.
A small cell with no nucleus; its DNA sits free in the cytoplasm as a single loop, often with smaller rings called plasmids.
All bacteria (e.g. E. coli, Salmonella).
A small, circular piece of DNA found in prokaryotic cells, additional to the main DNA loop. Plasmids often carry useful genes such as antibiotic resistance.
A specialised sub-cellular structure that carries out a particular function inside a cell (e.g. nucleus, mitochondrion, chloroplast, ribosome).
A way of writing numbers as , with and an integer. Used for very small or very large quantities.
Membrane-bound organelle that contains the cell's chromosomes (DNA) and controls all the cell's activities.
Jelly-like substance inside the cell membrane where chemical reactions take place and most organelles are located.
Partially permeable boundary controlling what enters and leaves the cell.
Organelles that are the site of aerobic respiration; energy is released from glucose here.
Related: Aerobic respiration
Tiny organelle that is the site of protein synthesis — amino acids are joined into proteins here.
Tough outer layer made of cellulose; provides strength and prevents the cell from bursting.
Plant organelle containing chlorophyll; absorbs light energy for photosynthesis.
Large sap-filled sac in a plant cell; helps keep the cell turgid (firm).
A cell that has developed specific features adapting it for one particular function. Once fully specialised, most cells cannot divide.
The long, thin extension of a neurone that carries electrical impulses away from the cell body.
Enzyme-containing cap at the tip of a sperm cell that digests the outer layer of an egg to allow fertilisation.
Hard, waterproof substance deposited in xylem walls; provides strength and prevents the tube from collapsing.
Movement of dissolved sugars (e.g. sucrose) through phloem in a plant. Can move both up and down the plant.
The process by which a cell becomes specialised for a particular function. Different genes are switched on, producing the proteins that give the cell its features.
A region of a plant (root tip and shoot tip) containing unspecialised cells that keep dividing and can differentiate throughout the plant's life.
When different cells (or tissues, organs) perform different specialised functions; allows multicellular organisms to do much more than a single cell could.
How many times larger an image is than the real object. Has no units — written as ×N.
The shortest distance between two points that can still be seen as separate. Lower number = better detail.
A dye added to a specimen to make cell structures visible under a microscope. Iodine for plant tissue; methylene blue for animal tissue.
A set of procedures used to prevent contamination of a microbial culture by unwanted microorganisms.
A clear area on a bacterial lawn where bacteria have not grown because of the antibiotic or antiseptic diffusing from a disc.
Asexual reproduction in bacteria — one cell divides into two genetically identical cells.
Mistake
Saying 'prokaryotes have a small nucleus'.
Why it happens
Students learn that prokaryotes are small and assume their nucleus must be small too.
How to avoid it
Memorise: prokaryotes have NO nucleus. The DNA is free in the cytoplasm. Always say 'no membrane-bound nucleus'.
Source: AQA Examiner Report Paper 1 2023.
Mistake
Writing that bacteria have a cellulose cell wall.
Why it happens
Plant cells have cellulose walls, and students transfer the idea across.
How to avoid it
Cellulose = plant cells only. Bacterial walls are peptidoglycan (you don't have to spell it, just say 'not cellulose').
Mistake
Writing standard form with , e.g. .
Why it happens
Students stop converting when they hit a power of ten that looks 'small enough'.
How to avoid it
Check is between 1 and 10. becomes .
Mistake
Confusing 'plasmid' with 'plasma membrane'.
Why it happens
The words look similar.
How to avoid it
Plasmid = small DNA loop in a bacterium. Plasma membrane = another name for cell membrane (any cell). Sound them out aloud.
Mistake
Writing that mitochondria 'produce' or 'make' energy.
Why it happens
Everyday language treats energy as something you can create.
How to avoid it
Use 'release' or 'transfer'. Energy is moved from the chemical store of glucose to other stores during respiration.
Source: AQA Examiner Report Paper 1 2022.
Mistake
Drawing chloroplasts in root cells.
Why it happens
Students assume all plant cells have chloroplasts.
How to avoid it
Chloroplasts need light. Roots are underground → no chloroplasts. Only green parts of the plant photosynthesise.
Mistake
Confusing cell wall with cell membrane.
Why it happens
Both are at the edge of the plant cell; students treat them as the same thing.
How to avoid it
Cell wall = OUTSIDE, made of cellulose, gives structure. Cell membrane = INSIDE the wall, partially permeable, controls what enters/leaves. Both are present in plant cells; only the membrane is in animal cells.
Mistake
Labelling the vacuole as 'empty space'.
Why it happens
It looks clear under a microscope.
How to avoid it
The vacuole is filled with cell sap (water, sugars, salts). The sap creates turgor pressure.
Mistake
Listing features of a specialised cell without linking each to a function.
Why it happens
Students think the feature alone earns the mark.
How to avoid it
Each feature needs a 'so that …' or 'because …' clause. AQA mark schemes credit the LINK, not the noun.
Source: AQA Examiner Report Paper 1 2023.
Mistake
Saying xylem cells are alive.
Why it happens
Students assume all cells in a living plant are alive.
How to avoid it
Xylem cells DIE during development — their cytoplasm and nucleus disappear, leaving an empty tube. Phloem cells stay alive.
Mistake
Saying phloem transports sugars only downwards.
Why it happens
Many revision sources draw arrows pointing down.
How to avoid it
Sugars made in leaves can travel UP to growing shoots/fruits or DOWN to roots/storage organs. AQA accepts 'both directions'.
Mistake
Saying differentiated cells have different DNA.
Why it happens
Students see different cell types and assume the genetic information must differ.
How to avoid it
All body cells contain the same DNA (the genome). It's gene EXPRESSION — which genes are active — that differs.
Mistake
Saying 'plant cells differentiate everywhere throughout life'.
Why it happens
Students simplify the spec point.
How to avoid it
Plant differentiation throughout life happens in specific REGIONS — meristems at the root tip and shoot tip. Most other plant cells are specialised.
Mistake
Mixing units (e.g. mm with µm) when applying the magnification equation.
Why it happens
The numbers seem to work out OK without converting.
How to avoid it
Always convert image and actual to the SAME unit first. 1 mm = 1000 µm.
Source: AQA Examiner Report Paper 1 2023.
Mistake
Confusing magnification and resolution.
Why it happens
Both relate to 'seeing things better'.
How to avoid it
Magnification = bigger. Resolution = clearer. You can magnify a blurry image; that doesn't improve resolution.
Mistake
Adding eyepiece and objective magnifications instead of multiplying.
Why it happens
Misremembering the formula.
How to avoid it
Multiply: ×10 eyepiece × ×40 objective = ×400 total.
Mistake
Using the diameter as r in .
Why it happens
Students measure the zone diameter and forget to halve it.
How to avoid it
Always note: r = diameter / 2 before substituting.
Mistake
Saying we 'fully seal the Petri dish to keep bacteria in'.
Why it happens
Containment seems sensible.
How to avoid it
Tape in 4 places only. Fully sealing causes anaerobic conditions which favour dangerous pathogens like Clostridium.
Mistake
Saying we incubate at 37 °C in schools.
Why it happens
Body temperature is the obvious answer.
How to avoid it
Schools incubate at 25 °C. 37 °C grows human pathogens efficiently, which is dangerous.