Cells — the building blocks of life
By the end of this topic you'll tell a eukaryotic cell from a prokaryotic one, name every organelle, and say exactly what each one does.
Part 1Two types of cell
All living things are made of cells. There are two big groups. Eukaryotic cells have a nucleus that holds the genetic material — animals, plants and fungi are all made of them. Prokaryotic cells are much smaller and have no nucleus; bacteria are prokaryotes.
In a prokaryote the genetic material is a single loop of DNA floating free in the cytoplasm, plus small extra rings called plasmids. A handy memory hook: prokaryotes come before a proper nucleus.
Organelles and their jobs
- Nucleus
- Holds the DNA; controls the cell's activities.
- Cytoplasm
- Jelly-like fluid where most chemical reactions happen.
- Cell membrane
- Controls what enters and leaves the cell.
- Mitochondria
- Where aerobic respiration happens — release energy.
- Ribosomes
- Where proteins are made (protein synthesis).
Plant cells have everything an animal cell has, plus three extras: a cell wall made of cellulose for strength and support, a permanent vacuole filled with cell sap, and chloroplasts that contain chlorophyll and carry out photosynthesis. Bacterial cells also have a cell wall (but not cellulose) and sometimes a flagellum to swim.
⚠ Watch out — what plants have that animals don't
Don't say a plant cell "has a cell wall instead of a membrane" — it has both. The cell wall sits outside the membrane. And only the green parts of a plant have chloroplasts: a root cell has no chloroplasts because it's underground and never photosynthesises.
A cell has DNA but no nucleus, a cell wall, and a plasmid. What is it?
- AAn animal cell
- BA plant cell
- CA prokaryotic (bacterial) cell
- DA red blood cell
Show answer
Part 2How small is a cell?
Cells are tiny, so we measure them in micrometres (µm). One micrometre is a millionth of a metre — that's 1 µm = 0.001 mm, or 1 × 10⁻⁶ m. A typical animal cell is about 10–30 µm across; a bacterium is roughly 1 µm. Getting comfortable with these units now will make the microscopy maths in Topic 3 far easier.
⚠ Watch out — bacteria are much smaller
Prokaryotic cells are typically 100 to 1000 times smaller than eukaryotic cells. If an exam asks you to compare sizes, that's the headline point — not just "no nucleus".
State two differences between a prokaryotic and a eukaryotic cell.
Prokaryotic cells have no nucleus (DNA is a free loop plus plasmids) and are much smaller. Eukaryotic cells have a nucleus and are larger.Name three structures found in a plant cell but not an animal cell.
A cell wall (cellulose), a permanent vacuole, and chloroplasts.What is the job of the mitochondria?
They are where aerobic respiration happens, releasing energy for the cell.Where in the cell are proteins made?
At the ribosomes.Why does a root hair cell have no chloroplasts?
It is underground and gets no light, so it cannot photosynthesise — chloroplasts would be useless.
Specialised cells & differentiation
How one type of cell becomes many — and how each specialised cell's shape is matched perfectly to its job.
Part 1Differentiation
Cells differentiate — they develop different structures so they can carry out a particular function. A cell that has become specialised is good at one job but can no longer turn into other cell types.
Most animal cells differentiate early in development, and after that an adult animal mostly only divides cells to repair and replace them. Many plant cells, by contrast, keep the ability to differentiate throughout life — that's a key difference between plants and animals.
Specialised cells you must know
- Sperm cell
- Long tail to swim; many mitochondria for energy; carries half the genes (one parent).
- Egg cell
- Large with food stores for the embryo; membrane changes after fertilisation.
- Ciliated cell
- Tiny hairs (cilia) that beat to sweep mucus along the airways.
- Root hair cell
- Long projection — huge surface area for absorbing water and mineral ions.
- Nerve cell
- Long and thin to carry electrical impulses around the body.
- Muscle cell
- Packed with mitochondria and able to contract (shorten).
⚠ Watch out — explain the adaptation, don't just name it
"A sperm has a tail" earns little. Say why: "a tail so it can swim to the egg." Marks come from linking the feature to the function. A root hair cell's long shape gives a large surface area, which speeds up absorption of water and mineral ions.
A cell lining the airways has many tiny beating hairs. What is it, and what do the hairs do?
- ARoot hair cell — it absorbs water
- BCiliated cell — the cilia sweep mucus and trapped dirt out of the airways
- CMuscle cell — the hairs help it contract
- DSperm cell — the hairs help it swim
Show answer
What does "differentiation" mean?
The process by which a cell becomes specialised for a particular job, developing a structure suited to its function.Give two ways a sperm cell is adapted to its function.
A tail to swim to the egg, and many mitochondria to release the energy needed for swimming. (It also carries half the genetic information.)How is a root hair cell adapted for absorbing water?
Its long, thin projection gives a large surface area, so water and mineral ions are absorbed faster.State one difference in how plant and animal cells differentiate.
Many plant cells keep the ability to differentiate all through life; most animal cells lose it early, dividing later mainly to repair and replace.
Microscopy & the magnification sum
Light versus electron microscopes, one equation to recall, and how to handle the standard-form numbers without panicking.
Part 1Two kinds of microscope
A light microscope uses light and lenses. It's cheap, portable and lets you see living cells, but its resolution is limited — it can't separate the smallest sub-cellular structures. An electron microscope uses a beam of electrons instead of light. It has a much higher magnification and resolution, so it reveals tiny details like the internal structure of mitochondria — which is why our understanding of cells leapt forward when it was invented.
Magnification is how many times bigger the image is. Resolution is the ability to tell two close points apart as separate — higher resolution means a sharper, more detailed image. Electron microscopes win on both.
Equation
- magnification = image size ÷ real size recall
- Rearranged: real size = image size ÷ magnification. Keep both sizes in the same unit before dividing. Magnification has no units (it's a ratio).
⚠ Watch out — same units, and standard form
If the image is in mm but the real size is in µm, convert first (1 mm = 1000 µm). Very small or very large answers are written in standard form: 0.002 mm = 2 × 10⁻³ mm. Practise moving the decimal point — it's where easy marks are won and lost.
Worked example — finding magnification
A cell is really 0.05 mm wide. In a drawing it measures 20 mm wide. Calculate the magnification.
Worked example — finding the real size (standard form)
An image of a cell is 8 mm long at a magnification of ×2000. Find the real length, in mm and in standard form.
An image is 30 mm wide at a magnification of ×1500. What is the cell's real width?
- A45 000 mm
- B0.02 mm (2 × 10⁻² mm)
- C50 mm
- D0.5 mm
Show answer
Part 2Using a light microscope
Observing cells with a light microscope
Aim: use a light microscope to observe, draw and label cells, and work out the magnification used.
- Prepare a slide: place a thin specimen (e.g. onion epidermis) on the slide and add a drop of stain (such as iodine) to make the structures show up.
- Lower a coverslip gently at an angle to avoid trapping air bubbles.
- Clip the slide on the stage. Start with the lowest-power objective lens and use the coarse focus to bring cells into view, then switch to higher power and fine-focus.
- Make a clear, labelled biological drawing with sharp pencil lines (no shading).
- Record the total magnification: eyepiece lens × objective lens (e.g. ×10 eyepiece × ×40 objective = ×400).
Control / improve: add the stain and coverslip carefully to avoid air bubbles, which can be mistaken for cells. Including a scale bar on your drawing makes the real size clear.
A microscope has a ×10 eyepiece and a ×40 objective lens. What is the total magnification?
- A×50
- B×4
- C×400
- D×30
Show answer
State the magnification equation.
magnification = image size ÷ real size. Keep both sizes in the same unit.Give two advantages of an electron microscope over a light microscope.
Much higher magnification and much higher resolution, so it shows far more detail (e.g. inside organelles).A cell is 0.02 mm wide and appears 10 mm wide. Find the magnification.
magnification = 10 ÷ 0.02 = ×500.Why is a stain such as iodine added when preparing a slide?
It makes the cells and their structures show up (increases contrast) so they're easier to see.Write 0.0008 mm in standard form.
8 × 10⁻⁴ mm (which is the same as 0.8 µm).
The cell cycle, mitosis & stem cells
How a cell copies itself to make two identical cells — and why stem cells, which can become anything, are so useful.
Part 1Chromosomes & the cell cycle
The nucleus contains chromosomes — long molecules of DNA carrying the genes. Body cells have chromosomes in pairs; human body cells have 23 pairs (46 chromosomes).
The cell cycle is how a cell grows and divides. In the long first stage, the cell grows, makes more sub-cellular structures (like ribosomes and mitochondria), and copies its DNA so each chromosome becomes two identical strands. Then comes mitosis: the chromosomes line up, are pulled to opposite ends, and the cell divides into two genetically identical daughter cells.
Mitosis is used for growth, repair of damaged tissue, and replacing worn-out cells — and for asexual reproduction in some organisms. Because the cells are identical clones, mitosis never produces variation.
⚠ Watch out — mitosis makes identical cells
The two daughter cells are genetically identical to the parent and to each other. Don't muddle this with gametes (sperm and egg), which are not made by mitosis and are not identical. And remember the DNA is copied before the cell divides, not during.
Part 2Stem cells
A stem cell is an undifferentiated cell that can keep dividing and can differentiate into many different cell types.
Embryonic stem cells (from a very early embryo) can turn into almost any type of cell. Adult stem cells, found in places like bone marrow, can form a more limited range — for example, blood cells. In plants, stem cells are found in the meristems at the tips of roots and shoots, and these can produce any kind of plant cell throughout the plant's life.
Uses & issues
- Medical uses
- Replace faulty cells — e.g. treating diabetes or paralysis; bone-marrow transplants already use adult stem cells.
- Therapeutic cloning
- Embryo made with the patient's genes, so stem cells aren't rejected.
- Plant meristems
- Clone whole plants quickly and cheaply — useful for crops and rare species.
- Issues
- Ethical objections to using embryos; risk of transferring viral infection; some find it religiously or morally wrong.
⚠ Watch out — give a balanced argument
"Evaluate" questions need both sides. Benefits: stem cells could cure conditions like paralysis or type 1 diabetes. Concerns: using embryonic stem cells raises ethical issues, and there's a small risk of viral infection being passed on. End with a justified conclusion.
Where are stem cells found in a plant, and what can they do?
- AIn the leaves — they photosynthesise
- BIn the meristems (root and shoot tips) — they can differentiate into any plant cell
- CIn the bone marrow — they make blood cells
- DIn the vacuole — they store sap
Show answer
Name two things that happen before a cell divides by mitosis.
The cell grows and makes more sub-cellular structures, and the DNA is copied so each chromosome becomes two identical strands.Give two uses of mitosis in the body.
Any two of: growth, repair of damaged tissue, and replacing worn-out cells.How are the daughter cells from mitosis related to the parent cell?
They are genetically identical (clones) to the parent and to each other.What is a stem cell?
An undifferentiated cell that can keep dividing and can differentiate into many different cell types.State one benefit and one concern of using embryonic stem cells.
Benefit: could treat conditions such as diabetes or paralysis. Concern: ethical objections to using embryos (or the risk of passing on a viral infection).
Diffusion — spreading out
The passive spreading of particles from high to low concentration — and why small cells exchange substances faster than big ones.
Part 1Down the gradient
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration — that is, down a concentration gradient. It happens because particles move randomly, and it needs no energy from the cell: it's a passive process.
In living things, oxygen and carbon dioxide diffuse in and out of cells, and urea diffuses out into the blood plasma. "Net" movement matters: particles move both ways, but more move from the high side, so overall they spread out until evenly mixed.
Three things speed up diffusion: a steeper concentration gradient (bigger difference), a higher temperature (particles move faster), and a larger surface area of the membrane.
⚠ Watch out — diffusion is passive
Diffusion needs no energy from respiration — that's what separates it from active transport (Topic 7). And the word is "net" movement: particles still move both ways, just more from high to low. Don't say they "all" move one way.
Part 2Surface area : volume ratio
For a substance to be exchanged fast enough, a cell needs a big surface area compared with its volume. As an object gets bigger, its volume grows faster than its surface area, so the surface-area-to-volume ratio falls. That's why single-celled organisms manage by diffusion alone, but large animals need specialised exchange surfaces (like lungs) and a transport system.
Worked example — surface area : volume ratio
Compare the surface-area-to-volume ratio of a cube of side 1 cm and a cube of side 2 cm.
Which change would make oxygen diffuse into a cell faster?
- AA smaller surface area
- BA lower temperature
- CA steeper concentration gradient
- DA smaller concentration difference
Show answer
Define diffusion.
The net movement of particles from a higher to a lower concentration (down a concentration gradient). It is passive — no energy needed.Name three factors that increase the rate of diffusion.
A steeper concentration gradient, a higher temperature, and a larger surface area.Name two substances that diffuse across cell membranes in the body.
Any two of: oxygen, carbon dioxide, and urea.Why can't a large multicellular animal rely on diffusion alone?
Its surface-area-to-volume ratio is too small, so diffusion across the outer surface is too slow — it needs special exchange surfaces and a transport system.
Osmosis — the movement of water
Diffusion's water-only cousin — across a partially permeable membrane — plus the potato required practical that everyone gets asked about.
Part 1Water across a membrane
Osmosis is the movement of water molecules across a partially permeable membrane, from a dilute solution (high water concentration) to a more concentrated solution (low water concentration). It's really just diffusion of water — passive, no energy needed.
A partially permeable membrane lets water molecules through but not the larger solute particles (like sugar). So water moves to even out the concentration, but the sugar stays put.
This matters for cells. In a dilute solution, a plant cell takes in water and becomes firm (turgid) — that's what holds a plant up. In a concentrated solution, water leaves and the plant wilts. (Animal cells, with no cell wall, can burst or shrivel.)
⚠ Watch out — osmosis is about WATER
Osmosis moves water only, and only across a partially permeable membrane. Be careful which way it goes: water moves towards the more concentrated (lower-water) solution. Putting a cell in pure water makes it gain water; strong sugar solution makes it lose water.
Part 2The potato practical
Effect of solution concentration on potato tissue
Aim: investigate the effect of sugar (or salt) solution concentration on the mass of plant (potato) tissue.
- Use a cork borer and ruler to cut potato cylinders of the same size; blot them dry and record the starting mass of each.
- Make up sugar solutions of different concentrations (and one of pure water, 0 mol/dm³).
- Put one potato piece in each concentration for the same length of time.
- Remove each piece, gently blot dry, and record the final mass.
- Calculate the percentage change in mass for each: % change = (change in mass ÷ start mass) × 100
Control / improve: keep size, time and temperature the same; using percentage change (not raw grams) lets you compare pieces fairly even if they started slightly different. Pieces gain mass in dilute solutions and lose mass in concentrated ones; the concentration where mass doesn't change equals the cells' internal concentration.
Worked example — percentage change in mass
A potato cylinder starts at 4.0 g. After soaking it is 4.6 g. Find the percentage change in mass.
A potato piece is left in concentrated sugar solution. What happens to its mass, and why?
- AIt increases — water moves in by osmosis
- BIt decreases — water leaves the cells by osmosis
- CIt stays the same — sugar can't cross the membrane
- DIt decreases — sugar diffuses out of the potato
Show answer
Define osmosis.
The movement of water across a partially permeable membrane, from a dilute solution to a more concentrated one.In the potato practical, why is percentage change in mass used rather than the change in grams?
It lets you fairly compare pieces that may have started at slightly different masses.A potato piece starts at 5.0 g and ends at 4.5 g. Calculate the percentage change.
(−0.5 ÷ 5.0) × 100 = −10% (it lost water).Why does a plant cell become turgid in pure water?
Water enters the cell by osmosis; the cell wall stops it bursting, so the cell becomes firm (turgid), which supports the plant.
Active transport — uphill
Moving substances the "wrong" way — against the gradient — using energy from respiration. The one that's not passive.
Part 1Against the gradient
Active transport moves substances from a more dilute solution to a more concentrated one — against the concentration gradient. Because this is "uphill", it needs energy released by respiration. That's the big contrast with diffusion and osmosis, which are passive and need no energy.
Two examples you must know. In plants, root hair cells use active transport to absorb mineral ions from the soil, even though the soil is more dilute than the cell. In humans, the gut absorbs sugar (glucose) from low concentrations in the food into the blood by active transport, so none is wasted — important for respiration.
⚠ Watch out — the energy point is the marks
The single most important thing to write is that active transport requires energy from respiration — that's what makes it different from diffusion and osmosis. A cell doing lots of active transport (like a root hair cell) therefore has many mitochondria.
Which statement correctly describes active transport?
- AMovement down a gradient, no energy needed
- BMovement of water across a partially permeable membrane
- CMovement against the gradient, using energy from respiration
- DRandom movement of particles until evenly spread
Show answer
Part 2Telling the three apart
Diffusion, osmosis and active transport are easy to mix up. Lock down the differences: what moves, which way, and whether energy is needed.
Three ways things move in and out of cells
- Diffusion
- Any particle, high → low concentration, no energy.
- Osmosis
- Water only, across a partially permeable membrane, dilute → concentrated, no energy.
- Active transport
- Substances moved low → high (against the gradient), energy from respiration needed.
A root hair cell absorbs mineral ions from very dilute soil into its more concentrated cytoplasm. Which process is this?
- ADiffusion
- BOsmosis
- CActive transport
- DEvaporation
Show answer
Define active transport.
The movement of substances against the concentration gradient (from dilute to concentrated), using energy released by respiration.Give two examples of active transport in living things.
Root hair cells absorbing mineral ions from the soil, and the gut absorbing glucose (sugar) into the blood.What is the key difference between active transport and diffusion?
Active transport goes against the gradient and needs energy from respiration; diffusion goes down the gradient and needs no energy.Why does a root hair cell contain many mitochondria?
To release the energy by respiration needed for the active transport of mineral ions.