HookThe cells that never stopped dividing
In 1951, surgeons at Johns Hopkins Hospital took a sample of cervical-cancer cells from a 31-year-old patient named Henrietta Lacks. Every previous human sample the laboratory had tried to grow had died within days. Hers did not. They divided roughly once every 24 hours, and they never stopped. The 'HeLa' line has since been grown on every continent, used in more than 70,000 studies from the first polio vaccine to early COVID-19 research, and — cell for cell — has outlived the woman it came from by more than seventy years.
HeLa is the whole of B1 in one story. The cells divide by mitosis, the same mechanism that built you from a single fertilised egg. You can only see them because of the microscope, and only count them with a magnification sum. They are eukaryotic, wrapped in a membrane that lets some substances through and blocks others. Strip the section back and it is four questions: what is a cell built from, how do we see something too small to see, how does one cell become many, and how does anything cross the membrane that holds it together?
ModelTwo kinds of cell, and the thousand-fold size gap
Every living thing is built from one of two cell types, and the exam expects you to tell them apart. Prokaryotic cells — bacteria — are small, typically about \(1\,\mu\text{m}\) across. They have cytoplasm, a cell membrane and a cell wall, but no true nucleus: their genetic material is a single loop of DNA floating free in the cytoplasm, often with small extra rings called plasmids. Eukaryotic cells — those of animals, plants, fungi and protists — are far larger, roughly \(10\) to \(100\,\mu\text{m}\), and keep their DNA sealed inside a membrane-bound nucleus.
An animal cell carries a nucleus, cytoplasm, a cell membrane, mitochondria (where aerobic respiration transfers energy) and ribosomes (where proteins are made). A plant cell has all of those plus three extras: a cell wall of cellulose for strength, a permanent vacuole filled with cell sap that keeps the cell firm, and chloroplasts holding the chlorophyll that absorbs light for photosynthesis.
The size gap is the point examiners hide. A bacterium is about a thousand times smaller in each direction than a plant cell, so you are expected to compare sizes in standard form and to know that it is the nucleus, not the cell wall, that separates the two groups — bacteria and plants both have walls, but only eukaryotes have a nucleus.
DataMicroscopy: seeing bigger versus seeing clearer
Two numbers describe any microscope, and students confuse them 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 told apart — the true limit on detail. A light microscope reaches a magnification of about ×1500 and a resolution near 200 nm; an electron microscope reaches over ×100,000 with a resolution better than 1 nm, which is why organelles such as ribosomes were invisible until electron microscopes arrived. Higher resolution, not just higher magnification, is what reveals fine structure.
The formula runs the whole topic: \(\text{magnification} = \dfrac{\text{image size}}{\text{actual size}}\). Rearrange it for whichever quantity is missing, and always convert both sizes into the same unit before dividing, remembering \(1\,\text{mm} = 1000\,\mu\text{m}\) and \(1\,\mu\text{m} = 1000\,\text{nm}\).
Required practical 1 puts this to work. To view plant cells, peel a thin layer of onion epidermis, lay it flat on a slide, add a drop of iodine stain for contrast, and lower a cover slip with a mounting needle to avoid trapping air bubbles. Cheek cells are stained with methylene blue instead. Focus on low power first to find the cells, then switch to high power. Your drawing must use clean, unshaded lines, take up at least half the space, show only what is really there, and carry a title, a magnification and a scale — the marks are for accuracy and convention, not artistry.
An onion cell measures 45 mm across on a drawing made at ×1000 magnification. Rearrange to find its real width: \[\text{actual size} = \dfrac{\text{image size}}{\text{magnification}} = \dfrac{45\,\text{mm}}{1000} = 0.045\,\text{mm}.\] Convert to micrometres: \(0.045 \times 1000 = 45\,\mu\text{m}\) — a believable plant-cell width. Now the reverse. A chloroplast about \(5\,\mu\text{m}\) wide appears \(30\,\text{mm}\) across in an electron micrograph; put both into the same unit first — \(30\,\text{mm} = 30000\,\mu\text{m}\) — so the magnification is \(30000 \div 5 = ×6000\). One formula, both directions; the only mark ever lost here is forgetting to convert the units before you divide.
MechanismSpecialisation and differentiation
A fertilised egg divides into cells that are at first identical, then become specialised — each shaped for one job. A sperm cell has a tail to swim and many mitochondria for energy. A nerve cell is long and branched to carry electrical impulses. A muscle cell is packed with protein fibres that contract. In plants, a root hair cell has a long extension that increases surface area for absorbing water and mineral ions; xylem cells are hollow, dead and strengthened for water transport; phloem cells form living tubes that carry dissolved sugars.
Differentiation is the process of gaining those features, by switching particular genes on and others off so the cell builds the sub-cellular structures it needs. The timing is the examinable contrast. Most animal cells differentiate at an early stage and, once specialised, largely lose the ability to change — later cell division is mainly for repair and replacement. Most plant cells keep the ability to differentiate throughout the plant's life, which is why a single cutting can grow entirely new roots and leaves.
MechanismChromosomes, the cell cycle, mitosis and stem cells
Inside every eukaryotic nucleus the DNA is coiled into chromosomes, each carrying many genes; human body cells hold 23 pairs, 46 in total. Cells make more of themselves through the cell cycle. In the long first stage the cell grows, makes more ribosomes and mitochondria, and replicates its DNA so each chromosome becomes two identical copies. In mitosis, one copy of each chromosome is pulled to each end of the cell and the nucleus divides. Finally the cytoplasm and cell membranes divide, giving two genetically identical daughter cells. Mitosis is used for growth, repair, replacing worn-out cells and asexual reproduction — and because the copies are identical, it never creates variation.
Stem cells are undifferentiated cells that can keep dividing and can become other types. Embryonic stem cells can form almost any cell type; adult stem cells, such as those in bone marrow, form only a limited range like blood cells; and plant stem cells sit in meristems at root and shoot tips, active for the plant's whole life. Medically, stem cells could replace cells lost to diabetes or paralysis, and therapeutic cloning can grow cells carrying a patient's own genes so they are not rejected. Against that sit genuine objections — the ethics of using embryos and the risk of transferring a viral infection — which is exactly the balance an 'evaluate' question is testing.
MechanismDiffusion, osmosis and active transport
Substances cross membranes in three ways, and one question decides which: does the substance move down its concentration gradient, and does it need energy? Diffusion is the net movement of particles from a higher to a lower concentration, down the gradient. It is passive — no energy from the cell — and it moves oxygen and carbon dioxide in gas exchange and urea from cells into the blood. Its rate rises with a steeper concentration gradient, a higher temperature and a larger surface area, which is why lungs and small intestines are so heavily folded.
Osmosis is a special case of diffusion: the movement of water only, across a partially permeable membrane, from a dilute solution (high water concentration) to a more concentrated one (low water concentration). It too is passive, and only water crosses — the dissolved solute stays put.
Active transport is the odd one out. It moves substances against the gradient, from a lower to a higher concentration, so it must be powered by energy transferred through respiration. Root hair cells use it to absorb mineral ions from very dilute soil water; the small intestine uses it to absorb the last of the glucose when the concentration in the gut has already fallen below that in the blood. If a question says 'against the gradient' or 'requires energy', the answer is active transport — diffusion and osmosis are both passive.
DataRequired practical 2 — measuring osmosis in potato
Required practical 2 measures osmosis using plant tissue, and is designed so that the calculation carries the marks. Cut cylinders of potato with a cork borer, trim them to equal length, blot off surface water and weigh each one. Leave them for a fixed time in a range of sugar (or salt) concentrations — for example \(0.0\), \(0.2\), \(0.4\), \(0.6\) and \(1.0\,\text{mol/dm}^3\) — then blot and reweigh.
The variables are where the marks sit. The independent variable is the solution concentration; the dependent variable is the percentage change in mass; the control variables are temperature, time in solution, the volume of solution, and the size and type of potato. You report percentage change rather than raw grams so that cylinders which started at slightly different masses can be compared fairly, and you blot each cylinder in exactly the same way, because leftover surface water fakes a mass gain — the practical's main source of error.
A cylinder starts at \(5.0\,\text{g}\) and, after 30 minutes in distilled water, weighs \(5.6\,\text{g}\). The percentage change in mass is \[\dfrac{\text{final} - \text{initial}}{\text{initial}} \times 100 = \dfrac{5.6 - 5.0}{5.0} \times 100 = +12\%.\] The positive value means water moved into the cells by osmosis, so the solution outside was more dilute than the cell contents. A second cylinder left in \(1.0\,\text{mol/dm}^3\) sugar falls from \(5.0\,\text{g}\) to \(4.1\,\text{g}\): \(\dfrac{4.1 - 5.0}{5.0} \times 100 = -18\%\), water leaving. Plot percentage change against concentration and read off where the line crosses \(0\%\): at that concentration the solution matches the cell contents and there is no net movement — the classic graph-reading mark.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Combined Science marks split roughly three ways: about 40 per cent for recall (AO1), 40 per cent for applying ideas to unfamiliar contexts (AO2) and 20 per cent for analysis and evaluation (AO3). B1 is where the maths marks bite — around a tenth of every paper is mathematical, and magnification is the classic. Always write the formula, substitute with units, convert millimetres and micrometres before dividing (\(1\,\text{mm} = 1000\,\mu\text{m}\)), and be ready to give an answer in standard form. A bare number with the wrong unit scores zero even when the arithmetic is right.
Around 15 per cent of marks assess practical skills, so expect the required practicals as written questions. For osmosis, name the independent, dependent and control variables, explain why percentage change is fairer than raw change, and describe an anomaly as a point far from the line of best fit — never 'the odd one'. On the 6-mark extended questions, examiners use levels of response: a top-level answer is a single logically ordered chain, not a list of facts. Compare-and-contrast questions — light versus electron microscope, diffusion versus active transport — want the two sides explicitly linked, not written as two separate paragraphs.