HookThe machine that let us see a ribosome
On 9 March 1931, in a Berlin laboratory, the engineer Ernst Ruska switched on a machine that focused a beam of electrons instead of light. Within two years his electron microscope resolved detail no optical instrument ever could, and biology's hidden machinery — ribosomes, the folds inside a mitochondrion, the double line of the cell membrane — swam into view for the first time. Ruska waited until 1986 to collect his Nobel Prize, by which point every textbook diagram of a cell was really a portrait drawn by electrons.
The reason a school microscope can never match it comes down to one word the exam loves: resolution. Visible light waves are simply too fat — roughly 400 to 700 nm across — to separate two objects closer together than about \(2 \times 10^{-7}\) m, so a light microscope tops out near a ribosome's front door and stops. Everything in B1 sits on the other side of that door: what the sub-cellular structures are, how we measure things too small to see, how one fertilised egg becomes 200 specialised cell types, and how substances cross the very membrane Ruska was first to photograph. Master the difference between magnifying an image and resolving it, and half of this topic is already yours.
ModelTwo blueprints — prokaryotic and eukaryotic cells
Every living thing is built from one of two cell designs. Eukaryotic cells — the cells of animals, plants, fungi and protists — keep their DNA sealed inside a nucleus and carry membrane-bound organelles. Prokaryotic cells — bacteria — are far simpler and far smaller: no nucleus, no membrane-bound organelles, just a single loop of DNA floating free in the cytoplasm, usually with one or more small extra rings of DNA called plasmids.
Size is the fact examiners test hardest, because it is quantitative. A typical bacterium is about 1 µm long; an animal cell is ten to a hundred times bigger, roughly 10 to 100 µm. Written in metres that is \(1 \times 10^{-6}\) m against about \(2 \times 10^{-5}\) m — a gap of one to two orders of magnitude, which is just a compact way of saying 'powers of ten'. Being fluent at moving between mm, µm and nm earns more marks in this topic than any single fact.
Why does the design matter? Because compartments let eukaryotes specialise. Sealing DNA in a nucleus, packaging respiration into mitochondria and photosynthesis into chloroplasts lets a eukaryotic cell run many chemical processes at once without them interfering — the biological equivalent of separate rooms. A prokaryote does everything in one open-plan space, which is why bacteria stay small and simple but can still divide astonishingly fast.
ModelInside an animal and a plant cell
Some structures appear in nearly every cell. The nucleus controls the cell and holds the genetic material as chromosomes. The cytoplasm is the jelly where most chemical reactions happen. The cell membrane controls what enters and leaves. Mitochondria are where most aerobic respiration happens, transferring the energy the cell runs on. Ribosomes are where proteins are made.
Plant cells carry three extras. A cell wall made of cellulose strengthens the cell and stops it bursting. A permanent vacuole filled with cell sap keeps the cell firm (turgid). Chloroplasts, packed with green chlorophyll, absorb light for photosynthesis — though root cells, which never see light, have none.
A bacterial cell has cytoplasm, a cell membrane and a cell wall, a single loop of DNA and often plasmids — but no nucleus, mitochondria or chloroplasts. The exam's favourite trap lives here: a cell wall is not unique to plants. Bacteria and fungi have walls too, just made of different materials — and a plant cell still has a membrane inside its wall.
MechanismMicroscopy — seeing bigger versus seeing clearer
Two words get confused constantly. Magnification is how many times larger the image is than the real object. Resolution is the smallest distance between two points that can still be seen as separate. A light microscope magnifies up to about ×1500 and resolves down to roughly 200 nm; an electron microscope magnifies over ×500,000 and resolves to about 0.2 nm, because electrons behave as waves far shorter than light. That leap in resolution is exactly why electron microscopes revealed the inside of organelles that light had blurred into nothing.
The formula is the workhorse of this section: \(\text{magnification} = \dfrac{\text{image size}}{\text{actual size}}\). Rearrange it for whichever quantity is missing. The universal error is mixing units — image and actual must be in the same unit before you divide. Convert first, every time: 1 mm = 1000 µm and 1 µm = 1000 nm.
Required practical 1 puts this to work. Prepare a slide of onion epidermis or human cheek cells: add a drop of water, lower a coverslip with a mounted needle to avoid trapping air bubbles, and stain with iodine (or methylene blue) so the structures show up. View on low power first to find the cells, then switch to high power and refocus. Draw with a sharp pencil in clean, unshaded lines, label the parts, and add a magnification or a scale bar.
A cheek cell measures 30 mm across in your drawing, made at ×3000 magnification. How wide is the real cell? Rearrange the formula: \[\text{actual size} = \dfrac{\text{image size}}{\text{magnification}} = \dfrac{30\ \text{mm}}{3000} = 0.01\ \text{mm}.\] Convert to sensible units: \(0.01\ \text{mm} = 10\) µm — a textbook animal-cell width. Now the reverse: an electron micrograph shows a mitochondrion 40 mm long that is really 2 µm across. Put both into the same unit — 40 mm is 40000 µm — so the magnification is \(40000 \div 2 = \times 20000\). Same formula, both directions; the only place the mark is lost is forgetting to convert the units first.
ModelSpecialisation and differentiation
A specialised cell has a structure that fits its job. A sperm cell has a tail to swim, many mitochondria for energy and an enzyme-filled tip to break into the egg. A nerve cell is long and branched to carry impulses across the body. A muscle cell is full of protein fibres that contract. A root hair cell has a huge surface area to absorb water and minerals and, being underground, has no chloroplasts. Xylem cells are hollow and dead for water transport; phloem cells have sieve plates for carrying sugars.
Differentiation is the process by which a cell gains those specialised features, by switching particular genes on and others off. In most animals, differentiation happens early — during embryo development — and once a cell is specialised it largely loses the ability to change; adult cell division is mostly for growth, repair and replacement. In plants, by contrast, many cells keep the ability to differentiate for the whole of the organism's life.
That single difference explains a lot: why a cut plant stem can grow entirely new roots but you cannot regrow a lost limb, and why cloning a plant from a cutting is trivial while cloning an animal is not. It also opens the door to the next idea — stem cells, the undifferentiated cells that all specialisation begins from.
MechanismChromosomes, the cell cycle, mitosis and stem cells
Chromosomes are long molecules of DNA in the nucleus, carrying the genes, and normally found in pairs — human body cells have 23 pairs. Cells make more of themselves through the cell cycle, which has three stages. First, a long growth stage: the cell grows, makes new sub-cellular structures such as ribosomes and mitochondria, and copies its DNA so each chromosome becomes two identical strands. Second, mitosis: the chromosomes line up in the middle and the copies are pulled to opposite ends, then the nucleus divides. Third, the cytoplasm and membrane divide, giving two genetically identical daughter cells.
The cell cycle does three jobs — growth and development of an organism, replacing worn-out or damaged cells, and asexual reproduction. Because every daughter cell is an exact genetic copy, mitosis never introduces variation; that is meiosis's job, later in B6.
A stem cell is an undifferentiated cell that can keep dividing and can become other cell types. Embryonic stem cells can form any type; adult stem cells, such as those in bone marrow, form only a limited range; and plant meristem tissue at root and shoot tips produces stem cells for the plant's whole life. Medically, stem cells may treat diabetes (making insulin-producing cells) or paralysis (making nerve cells), and therapeutic cloning can grow cells carrying the patient's own genes so they are not rejected. Against that sit real objections — the ethics of using embryos and the risk of transferring viral infection. In farming, cloning from meristems produces large numbers of identical, disease-resistant plants and can rescue rare species cheaply.
MechanismDiffusion, osmosis and active transport
Diffusion is the net movement of particles from a region of higher concentration to one of lower concentration — down a concentration gradient — and it is passive, needing no energy from the cell. Oxygen and carbon dioxide move across the lungs this way, and dissolved food and urea move across cell membranes. Diffusion speeds up with a steeper concentration gradient, a higher temperature (faster-moving particles) and a larger surface area.
Osmosis is the diffusion of water only, across a partially permeable membrane, from a dilute solution (lots of water) to a more concentrated one (less water). It too is passive. Osmosis is why a wilted plant stiffens again in water and why a red blood cell placed in pure water swells and bursts.
Active transport moves substances the 'wrong' way — from a lower to a higher concentration, against the gradient — so it must use energy transferred by respiration. Root hair cells absorb mineral ions from very dilute soil water this way, and the small intestine absorbs glucose into the blood even when blood glucose is already higher. Efficient exchange also depends on surface area to volume ratio: thin, folded surfaces such as villi, alveoli and gills maximise area and shorten the diffusion distance.
DataRequired practical 3 — measuring osmosis in potato
The method is deliberately simple so the calculation carries the marks. Cut cylinders of potato with a cork borer, trim them to equal length, blot off surface moisture and weigh each. Place them in a range of sucrose (or salt) concentrations — for example 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm³ — for a fixed time. Remove each cylinder, blot it the same way, and reweigh.
The independent variable is the solution concentration; the dependent variable is mass; and you must control the potato source, temperature, time and cylinder size. Two habits protect the result: blotting identically each time (surface water fakes a mass gain), and reporting percentage change rather than raw grams, so cylinders that started at slightly different masses can be compared fairly.
Interpreting it is where understanding shows. In dilute solutions, water enters the cells by osmosis and mass rises; in concentrated solutions, water leaves and mass falls. Where the line of best fit crosses zero change, the solution's concentration matches the concentration inside the cells (isotonic) — a headline result you read straight off the graph.
A cylinder starts at 4.20 g and, after 30 minutes in 0.2 mol/dm³ sucrose, weighs 4.62 g. Percentage change in mass is \[\dfrac{m_f - m_i}{m_i} \times 100 = \dfrac{4.62 - 4.20}{4.20} \times 100 = +10\%.\] A second cylinder in 1.0 mol/dm³ sucrose falls from 4.00 g to 3.40 g: \(\dfrac{3.40 - 4.00}{4.00} \times 100 = -15\%\). Plot percentage change (not mass) on the y-axis so cylinders of slightly different starting size compare fairly, draw a line of best fit, and read off where it crosses 0% — that concentration equals the potato cells' own internal concentration, the point of no net osmosis.
MechanismCulturing microorganisms and Required practical 2
Microorganisms are grown on nutrient agar plates or in nutrient broth. To keep a culture pure you use aseptic technique: sterilise the inoculating loop by passing it through a flame until red-hot, use sterilised Petri dishes and pre-sterilised media, lift the lid as little as possible, and secure the lid with two short strips of tape rather than sealing it right round — some oxygen must still reach the plate to discourage harmful anaerobic bacteria.
In school laboratories, cultures are incubated at a maximum of 25°C. That is a safety rule, not a biology one: warmer temperatures near human body temperature would encourage pathogens that could infect people. Industrial laboratories incubate hotter under tightly controlled conditions.
Required practical 2 measures how antiseptics or antibiotics affect bacterial growth. Spread bacteria evenly across the agar, add paper discs soaked in different antibiotics or antiseptics — plus a control disc soaked in sterile water — and incubate. A clear ring, a zone of inhibition, forms where bacteria could not grow, and the larger the clear area, the more effective the substance. Compare substances fairly by calculating the area of each zone, not its radius.
A disc soaked in antiseptic A leaves a clear zone of radius 8 mm; a disc of antiseptic B leaves a radius of 5 mm. Compare them by area, because area is what the bacteria actually lost: \[\text{area} = \pi r^2.\] Antiseptic A: \(\pi \times 8^2 \approx 201\ \text{mm}^2\). Antiseptic B: \(\pi \times 5^2 \approx 79\ \text{mm}^2\). So A is not merely '3 mm better' — it clears about 2.5 times the area, because area grows with the square of the radius. The water-disc control matters just as much: if it grows a zone too, the effect came from the paper, not the chemical, and your comparison is void.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Magnification calculations are guaranteed marks if you are disciplined. Convert image and actual size into the same unit first, quote the formula, state whether you are finding image, actual or magnification, rearrange, then substitute — and give a sensible unit (µm for whole cells, nm for organelles). Sloppy unit conversion is the single biggest source of lost marks in this topic.
'Explain how this cell is adapted' wants a structure-then-function pair every time: a root hair cell has a large surface area, which speeds up absorption of water and minerals. For the three transport processes, decide two things — which way the substance moves relative to the concentration gradient, and whether energy is needed. Those two questions alone separate diffusion, osmosis and active transport, and examiners reward the comparison, not just a definition.
Required-practical marks are for method detail and control, so learn them as facts: iodine or methylene blue as the stain, incubation at a maximum of 25°C, blotting before every weighing, using percentage change of mass, and the sterile water-disc control. Where a practical produces numbers, show the calculation — percentage change for osmosis, zone area for antimicrobials — because the interpretation sentence is the mark most candidates drop.