Your revision route
What you’ll learn
- Compare eukaryotic and prokaryotic cells and relate sub-cellular structures to their functions.
- Explain how specialised cells and differentiation support functions in multicellular organisms.
- Compare light and electron microscopes and calculate magnification, image size and real size.
- Describe the cell cycle, mitosis, chromosomes and the importance of identical daughter cells.
- Evaluate medical and plant uses of stem cells, including therapeutic cloning, risks and ethical issues.
- Explain diffusion, osmosis and active transport using concentration gradients and energy requirements.
- Relate surface area to volume ratio and exchange-surface adaptations to transport needs.
- Plan and evaluate microscopy and osmosis practicals safely.
- For separate Biology, explain microbial culturing, aseptic technique and bacterial-population calculations.
Build the big picture
Key ideas
Two cell plans, one enormous scale gap
A eukaryotic cell stores DNA inside a nucleus; a much smaller prokaryotic cell keeps its DNA loose in the cytoplasm.
- Animal, plant, algal, fungal and protist cells are eukaryotic: they have a cell membrane, cytoplasm and genetic material enclosed in a nucleus.
- A bacterial cell is prokaryotic: it has cytoplasm and a membrane inside a cell wall, but no nucleus.
- Bacterial DNA forms one main loop, and plasmids are smaller DNA rings that may carry extra genes.
- Typical eukaryotic cells are about 10–100 µm across; a bacterium is roughly 1 µm, so compare sizes with ratios, orders of magnitude and standard form.
The punchline: Classify the cell by where its DNA sits, then compare its size numerically.
A cell is a workshop, not a bag of jelly
Each sub-cellular structure handles a particular piece of the cell's work, from controlling entry to building proteins.
- The nucleus contains DNA and controls activities; cytoplasm hosts most chemical reactions; the cell membrane controls movement into and out of the cell.
- Mitochondria are the main site of aerobic respiration, while ribosomes make proteins.
- Plant cells often also have chloroplasts for photosynthesis, a cellulose wall for strength and a permanent vacuole of cell sap for support.
- Algal cells are eukaryotic and also have a cellulose cell wall; estimate relative sizes or areas when an exact measurement is unavailable.
The punchline: For every structure, write its precise function—not merely its name.
Specialised cells wear the right equipment
A cell's shape and sub-cellular structures are tuned to the job it performs in a tissue, organ or whole organism.
- Sperm cells have a tail for swimming, many mitochondria for respiration and an acrosome containing enzymes that help penetrate the egg.
- Nerve cells have a long fibre and branched endings for rapid connections; muscle cells contain contractile fibres and many mitochondria.
- Root hair cells have a long extension for absorption; xylem cells form hollow lignified tubes, while phloem cells form living transport tubes.
- An adaptation earns the mark only when you link structure → mechanism → function.
The punchline: Do not list features: explain how each feature improves the cell's job.
Differentiation turns options into expertise
Cells begin less specialised, then develop different structures so a multicellular organism can divide up its work.
- As organisms develop, cells differentiate and acquire the sub-cellular structures needed for particular functions.
- Most animal cells differentiate early; in mature animals, cell division is mainly used for repair and replacement.
- Many plant cells keep the ability to differentiate throughout life, especially cells in meristems.
The punchline: Differentiation changes cell structure and therefore changes cell function.
Three cells, two fundamental plans
Start with the DNA, then inspect the equipment. Eukaryotic cells use a nucleus; a much smaller bacterium does not.
- Animal cellEukaryotic: nucleus, cytoplasm, membrane, mitochondria and ribosomes; no cellulose wall or chloroplasts.
- Plant or algal cellEukaryotic: the shared cell parts plus a cellulose wall; plant cells often also have chloroplasts and a permanent vacuole.
- Bacterial cellProkaryotic: no nucleus; one main DNA loop and possible plasmids lie in cytoplasm inside a membrane and cell wall.
Magnification makes bigger; resolution reveals more
A microscope can enlarge a cell, but resolving power decides whether two nearby details can be seen separately.
- Light microscopes reveal cells and larger structures; electron microscopes have much greater magnification and resolving power.
- Higher resolving power has revealed smaller sub-cellular structures in finer detail, improving cell models over time.
- Use magnification = image size ÷ real size, convert both sizes into the same units first, and use standard form where appropriate.
- A biological drawing should use clear single lines, labels and a title, with a scale or calculated magnification.
The punchline: Convert units before calculating, and never attach a unit to magnification.
Separate Biology: grow one microbe, not a mystery crowd
Microbiology depends on aseptic technique: unwanted organisms would spoil the result and may create a hazard.
- Bacteria reproduce by binary fission, sometimes about every 20 minutes when nutrients and temperature are suitable; grow them in broth or as colonies on agar.
- Sterilise media and dishes, sterilise the inoculating loop, open the lid briefly, and secure it with tape without sealing all the way around.
- Incubate school cultures upside down at no more than 25 °C, then keep plates closed; these controls reduce contamination and pathogen risk.
- Calculate population after whole divisions and clear-zone area with πr²; expressing bacterial populations in standard form is Higher Tier.
The punchline: Separate Biology only: aseptic technique protects both validity and safety.
Chromosomes are organised DNA packages
Inside the nucleus, long DNA molecules are arranged as chromosomes carrying many genes.
- A gene is a section of DNA, and each chromosome carries many genes.
- In body cells, chromosomes are normally found in pairs.
- Before mitosis, the DNA is copied so each new nucleus can receive an identical chromosome set.
The punchline: Keep the hierarchy clear: nucleus contains chromosomes; chromosomes contain genes.
Make the model move
Interactive checkpoint
Touch the science. Change a state, build a route or test a relationship.
Turn the microscope dials
Make magnification respond
Change the image size and real size. Both sliders use micrometres, so the calculation needs no hidden unit conversion.
magnification = image size ÷ real size
Magnification600 times
Magnification = image size ÷ real size. The matching micrometre units cancel, so magnification has no unit; write a value such as ×600.
Choose the crossing
Which membrane route fits the evidence?
Explore each cell scenario. Use the substance and gradient—not a memorised cell name—to identify the mechanism.
When oxygen concentration is higher outside, oxygen diffuses through the membrane into the respiring cell.
1 of 4 states explored
Diffusion and osmosis move down a gradient without energy from respiration. Active transport spends transferred energy to move particles against one.
Cell division begins long before the split
The cell cycle is a preparation-and-division sequence, with mitosis forming only its nuclear-dividing stage.
- Before division, a cell grows, increases sub-cellular structures such as ribosomes and mitochondria, and replicates its DNA.
- During mitosis, one set of chromosomes is pulled to each end and the nucleus divides.
- The cytoplasm and cell membrane then divide, producing two genetically identical daughter cells.
- Mitosis supports growth and development, plus repair and replacement; identify it wherever identical body cells are being produced.
The punchline: Describe the full cell cycle in order; do not use ‘mitosis’ for every stage.
Stem cells hold biological options open
An undifferentiated stem cell can copy itself and produce cells that later specialise.
- Embryonic stem cells can form most human cell types; adult bone-marrow stem cells can form several types, including blood cells.
- Therapeutic cloning can make an embryo genetically matched to a patient, reducing rejection risk when its stem cells are used.
- Possible treatments for conditions such as diabetes or paralysis must be weighed against viral-infection risk and ethical or religious objections.
- Meristem stem cells can form any plant cell throughout life, helping clone rare plants or rapidly produce identical disease-resistant crops.
The punchline: Evaluate stem cells by balancing source, potential benefit, evidence, risk and ethics.
Diffusion follows the crowd gradient
Random particle motion produces a net movement from higher concentration to lower concentration.
- Diffusion moves dissolved particles or gases down a concentration gradient and requires no energy from respiration.
- Oxygen and carbon dioxide diffuse during gas exchange; urea diffuses from cells into blood plasma for excretion.
- A steeper concentration gradient, higher temperature and larger membrane surface area usually increase diffusion rate.
- A thin membrane shortens the diffusion path; an efficient blood supply and ventilation maintain steep gradients in animals.
The punchline: State the substance, direction and concentration gradient in every diffusion answer.
Three routes across a cell membrane
The substance, concentration gradient and energy requirement reveal which transport mechanism is operating.
- DiffusionDissolved particles or gases move down a concentration gradient; no energy from respiration is required.
- OsmosisWater diffuses from a dilute solution to a more concentrated one through a partially permeable membrane.
- Active transportParticles move against their concentration gradient using energy transferred by respiration.
Growth creates a transport problem
As an organism becomes larger, its surface area to volume ratio falls, so its outer surface can no longer supply every cell fast enough.
- A single-celled organism has a large surface area to volume ratio, so exchange across its surface can meet its needs.
- Multicellular organisms need specialised exchange surfaces and organ systems because they have longer transport distances and a smaller ratio.
- Alveoli and small-intestine villi combine large area, thin barriers and rich blood supplies; fish gills also maintain exchange with flowing water.
- Root hairs enlarge the absorbing surface, while broad, thin leaves and stomata support gas exchange in plants.
The punchline: Calculate the ratio, then connect organism size to the need for specialised exchange.
Osmosis is water's membrane crossing
Water diffuses through a partially permeable membrane from a dilute solution to a more concentrated one.
- Osmosis is passive: water moves down its water-concentration gradient and no energy from respiration is required.
- Plant tissue gains mass when net water enters and loses mass when net water leaves.
- Calculate percentage mass change from the initial mass; a negative answer represents a loss.
- Analyse water uptake with percentages or a compound measure such as change in mass per unit time, not description alone.
The punchline: A complete definition must name water, a partially permeable membrane and the correct direction.
Active transport climbs uphill
When a cell needs particles to move against their concentration gradient, energy from respiration pays the cost.
- Active transport moves a substance from a more dilute solution to a more concentrated solution and requires energy from respiration.
- Root hair cells use it to absorb mineral ions when the soil solution is more dilute than the cell contents.
- Cells in the small intestine use it to absorb sugar into blood even when the sugar concentration is already higher in the blood.
- Diffusion and osmosis move down a gradient; active transport is the mechanism that can move against one.
The punchline: Look at the gradient first: movement against it identifies active transport.
Words worth knowing
Key definitions
- eukaryotic cell
- A cell whose genetic material is enclosed within a nucleus. Animals, plants, algae, fungi and protists are eukaryotes.
- prokaryotic cell
- A small cell with no nucleus; its main DNA loop lies in the cytoplasm. Bacteria are prokaryotes.
- plasmid
- A small ring of DNA in a bacterial cell that may carry additional genes.
- differentiation
- The process by which a cell changes to become specialised for a particular function.
- resolution
- The ability of a microscope to distinguish two close points as separate details.
- magnification
- How many times larger an image is than the real object; it is a ratio with no unit.
- chromosome
- A long DNA molecule carrying many genes; chromosomes are found in the nucleus of eukaryotic cells.
- gene
- A section of DNA that contains the information for a particular product or characteristic.
- mitosis
- The stage of the cell cycle in which copied chromosomes separate and the nucleus divides.
- stem cell
- An undifferentiated cell that can divide to make more cells and can produce certain specialised cell types.
- diffusion
- The net movement of dissolved particles or gas particles from higher concentration to lower concentration.
- osmosis
- The diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane.
- partially permeable membrane
- A membrane that allows some particles, such as water molecules, through but blocks others.
- active transport
- The energy-requiring movement of substances from a more dilute solution to a more concentrated solution.
- surface area to volume ratio
- The surface area of an object divided by its volume, written as a ratio to compare available exchange surface with internal demand.
Calculate with confidence
Equations
Magnification
magnification = image size / real size
Compares the measured size of an image with the object's actual size.
| Symbol | Meaning | Unit |
|---|---|---|
| magnification | number of times larger the image appears | no unit, often written with × |
| image size | measured size of the image | mm, µm or nm |
| real size | actual size of the object | the same unit as image size |
Exam tip: Convert first: 1 mm = 1000 µm and 1 µm = 1000 nm. Magnification itself has no unit.
Percentage change in mass
percentage change = (final mass − initial mass) / initial mass × 100
Expresses a gain or loss in tissue mass relative to its starting mass.
| Symbol | Meaning | Unit |
|---|---|---|
| initial mass | mass before immersion | g |
| final mass | mass after immersion and blotting | g |
| percentage change | relative gain or loss in mass | % |
Exam tip: Divide by the initial mass, not the final mass; keep the sign because a negative value means mass was lost.
Separate Biology: colony or inhibition-zone area
area = πr²
Calculates the cross-sectional area of a circular colony or clear zone from its radius.
| Symbol | Meaning | Unit |
|---|---|---|
| r | radius of the colony or clear zone | mm or cm |
| area | cross-sectional area | mm² or cm² |
Exam tip: If given a diameter, divide it by two before squaring; the area unit must also be squared.
Follow it step by step
Processes to remember
How to choose a membrane-transport mechanism
- Identify the substance: osmosis applies only to water.
- Check whether a partially permeable membrane is involved.
- Compare concentrations on the two sides and state the direction of net movement.
- Choose diffusion for particles moving down the gradient, osmosis for water down its gradient, or active transport for movement against the gradient.
- Mention energy from respiration only for active transport.
Exam tip: Do not decide from the cell type alone; the substance and concentration gradient determine the mechanism.
How to describe the cell cycle in order
- The cell grows and increases the number of sub-cellular structures.
- Its DNA replicates, making two copies of every chromosome.
- During mitosis, one chromosome set moves to each end and the nucleus divides.
- The cytoplasm and cell membrane divide to form two genetically identical daughter cells.
Exam tip: Reserve ‘mitosis’ for nuclear division; growth, DNA replication and cytoplasmic division are other stages of the cell cycle.
Separate Biology: calculate bacterial population
- Put the total growth time and mean division time in the same units.
- Calculate the number of complete divisions: total time / mean division time.
- Double the starting population once for each complete division: final population = starting population × 2ⁿ.
- Higher Tier: express a very large answer in standard form if requested.
Exam tip: The exponent is the number of divisions, not the number of minutes.
See the thinking
Worked example
Worked example: microscope magnification
A cell image is 48 mm wide. The cell's real width is 80 µm. Calculate the magnification.
- Write the equation: magnification = image size / real size.
- Convert the image size: 48 mm = 48,000 µm.
- Substitute values in matching units: magnification = 48,000 µm / 80 µm.
- Calculate and cancel the units: magnification = 600.
Answer: The image has a magnification of ×600.
The sizes must use the same unit before division. The micrometre units cancel, so magnification is a ratio rather than a measurement with a unit.
Protect the marks
Common mistakes
Watch out: Saying every cell has a nucleus.
Do this instead: Bacterial cells are prokaryotic and have no nucleus; their main DNA loop lies in the cytoplasm.
Watch out: Writing that mitochondria make energy.
Do this instead: Mitochondria are the main site of aerobic respiration, which transfers energy from glucose.
Watch out: Treating magnification and resolution as synonyms.
Do this instead: Magnification enlarges an image; resolution determines whether close details can be distinguished.
Watch out: Using mitosis as the process that produces gametes.
Do this instead: At GCSE, mitosis produces genetically identical cells for growth, development, repair and replacement.
Watch out: Defining osmosis as any movement from high to low concentration.
Do this instead: Name water, a partially permeable membrane, and movement from dilute to more concentrated solution.
Watch out: Saying active transport is a type of diffusion.
Do this instead: Active transport uses energy from respiration to move substances against a concentration gradient.
Watch out: Claiming that bigger organisms have a larger surface area to volume ratio.
Do this instead: As similarly shaped organisms grow, volume increases faster than surface area, so the ratio becomes smaller.
Watch out: Promising that stem cells will cure a condition.
Do this instead: Use evidence-based language: treatments may help, but benefits, risks and ethical issues must be evaluated.
Plan it like the exam
Required practicals
Use a light microscope to observe plant and animal cells
Combined Science and separate Biology
Aim: Prepare, observe, draw and label plant and animal cells using a light microscope, including a magnification scale.
Method
- Place a thin specimen on a clean slide; add a suitable stain if instructed, then lower a coverslip at an angle to reduce trapped air bubbles.
- Put the slide on the stage and begin with the lowest-power objective lens.
- Use the coarse focus to find the specimen, then sharpen it with fine focus; use fine focus only at high power.
- Centre the specimen before changing to a higher-power objective and adjust the light if needed.
- Draw several representative cells using clear single lines, add labels and include a scale or calculated magnification.
- Repeat with the other specimen type and compare visible structures.
Variables
- Independent
- type of prepared cell specimen or magnification being compared
- Dependent
- structures resolved and measurements made in the microscope image
- Controls
- specimen thickness and preparation method
- stain identity and amount
- lighting and focus procedure
- calibrated scale or the same image-measurement method
Analysis: Label only structures supported by the image. Calculate total magnification from the eyepiece and objective, or use an image scale to estimate real size, stating units and sensible precision.
Safety
- Wear eye protection when using stains and avoid skin contact; follow the school's reagent guidance.
- Handle glass slides and coverslips by their edges and report chips or breakages.
- Carry the microscope with two hands and keep it away from the bench edge.
Improvements
- Use a thin, flat specimen so light passes through and structures do not overlap.
- Calibrate the scale for each magnification used.
- Observe several fields of view rather than choosing one unusually clear cell.
Separate Biology: investigate antibiotics or antiseptics
Separate Biology only
Aim: Use aseptic technique to compare how antibiotics or antiseptics affect bacterial growth on agar by measuring clear zones of inhibition.
Method
- Disinfect the bench, wash hands and label the base of a sterile agar plate before inoculation.
- Sterilise the inoculating loop in a flame as instructed, let it cool, then use it to spread the bacterial culture evenly across the agar.
- Open the lid only slightly and briefly; place sterile treatment discs on the agar with sterile forceps.
- Include a control disc carrying only the solvent, then replace the lid promptly.
- Tape the lid in a few places without sealing all the way around, invert the plate and incubate at no more than 25 °C.
- Keep the incubated plate closed; measure two perpendicular clear-zone diameters and calculate a mean, or calculate area with πr².
- Repeat each treatment on separate plates and compare mean clear-zone sizes.
Variables
- Independent
- identity or concentration of the antibiotic or antiseptic on each disc
- Dependent
- mean diameter or calculated area of the clear zone around the disc
- Controls
- bacterial strain and starting culture
- agar depth and plate size
- disc size and treatment volume
- incubation temperature and time
- solvent-only control disc
Analysis: A larger clear zone suggests stronger inhibition under the tested conditions. Compare repeats and the solvent control; zone size does not by itself show that a treatment is safe or effective in a patient.
Safety
- Use only the school-approved microorganism and follow trained supervision for flame sterilisation.
- Incubate at no more than 25 °C, tape without fully sealing to avoid anaerobic conditions, store plates upside down and never reopen them.
- Wear eye protection, disinfect spills as instructed and dispose of cultures through the school's sterilisation procedure.
Improvements
- Spread the same volume of one well-mixed bacterial culture evenly on every plate.
- Use discs with equal diameter and equal treatment volume.
- Repeat treatments and measure zones in two directions to reduce random error.
Investigate osmosis in plant tissue
Combined Science and separate Biology
Aim: Measure how a range of salt or sugar concentrations changes the mass of equal plant-tissue pieces.
Method
- Prepare labelled tubes containing equal volumes of a range of salt or sugar concentrations, including distilled water.
- Cut plant-tissue cylinders to equal length and diameter, remove any skin, blot them consistently and record each initial mass.
- Place one cylinder in each solution for the same time at the same temperature.
- Remove each cylinder, blot it in the same way and record its final mass.
- Calculate percentage change in mass for every concentration.
- Repeat each concentration, calculate a mean and plot mean percentage change against concentration.
Variables
- Independent
- concentration of the salt or sugar solution
- Dependent
- percentage change in mass of the plant tissue
- Controls
- plant species and source tissue
- initial cylinder length and diameter
- solution volume
- immersion time and temperature
- blotting method before each weighing
Analysis: Positive percentage change means net water entry; negative change means net water loss. The graph's zero-change concentration estimates where there was no net movement of water.
Safety
- Use a tile and cut away from fingers when using a cork borer or scalpel; follow teacher supervision.
- Wear eye protection if the chosen solutions require it and wipe up spills promptly.
Improvements
- Use a cork borer and ruler to make cylinders with equal dimensions.
- Use several concentrations around the zero-change point.
- Repeat each concentration and calculate a mean after checking anomalies.
Try it before you move on
Quick check
Say your answer first, then open the card to check it.
Where is the main genetic material in a bacterial cell?
Answer: As a loop of DNA free in the cytoplasm, rather than enclosed in a nucleus.
Bacteria are prokaryotes; they may also contain smaller DNA rings called plasmids.
An image is 30 mm wide and the real cell is 50 µm wide. What is the magnification?
Answer: ×600
30 mm = 30,000 µm, then magnification = 30,000 / 50 = 600.
What two preparations happen before mitosis?
Answer: The cell grows and increases sub-cellular structures, and its DNA replicates.
Mitosis then separates the copied chromosomes as the nucleus divides.
A potato cylinder gains mass in a dilute solution. What caused the gain?
Answer: Net movement of water into its cells by osmosis through partially permeable membranes.
Water moved from the more dilute solution towards the more concentrated cell contents.
Why can root hair cells absorb mineral ions when their concentration is lower in the soil?
Answer: Active transport moves the ions against their concentration gradient using energy from respiration.
Diffusion cannot produce net movement from lower concentration to higher concentration.
Separate Biology: why are school microbial cultures incubated at no more than 25 °C?
Answer: To reduce the likelihood of growing pathogens that are harmful to humans.
Aseptic technique protects the culture from contamination; the temperature limit also reduces biological risk.
Good questions, clear answers
Frequently asked questions
What is the difference between a cell wall and a cell membrane?
A cell membrane controls movement into and out of a cell. A wall lies outside it and provides support; plant and algal cell walls are made of cellulose.
Why are electron microscopes more useful for tiny cell structures?
They have much greater resolving power as well as magnification, so nearby structures can be distinguished and smaller details can be revealed.
Are mitosis and the cell cycle the same thing?
No. The cell cycle includes growth, increasing sub-cellular structures, DNA replication, mitosis and division of the cytoplasm and cell membrane.
Why are stem cells controversial?
They may enable useful treatments, but embryonic sources raise ethical or religious objections and treatments can carry risks such as transfer of viral infection.
How do diffusion, osmosis and active transport differ?
Diffusion moves particles down a concentration gradient. Osmosis is water diffusion through a partially permeable membrane. Active transport uses energy to move against a gradient.
Is culturing microorganisms part of Combined Science?
Not in this AQA scope. Culturing microorganisms and its required practical are separate Biology only; the guide labels that material explicitly.
See the whole topic
Illustrated notes
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