Organisation of the organism

• If you can say what a structure is, what its shape or position causes, and what the cell therefore can do, you can answer almost every question in Topic 2.
• Three returns beat one long session. Redraw, rebuild, then test yourself cold.

• I can describe and compare a plant cell with an animal cell, using the eight structures the syllabus lists: cell wall, cell membrane, nucleus, cytoplasm, chloroplasts, ribosomes, mitochondria and vacuoles.
• I can describe a bacterial cell using the six structures the syllabus lists: cell wall, cell membrane, cytoplasm, ribosomes, circular DNA and plasmids.
• I can identify every one of those structures in a diagram or an image of a plant, animal or bacterial cell.
• I can state a correct function for every one of those structures in a single accurate sentence.
• I can explain why a plant cell needs both a cell membrane and a cell wall, and say which one controls entry.
• I can state that new cells are produced by the division of existing cells.
• I can state the specific function of each named specialised cell: ciliated cells, root hair cells, palisade mesophyll cells, neurones, red blood cells, and sperm and egg cells.
• I can link a feature of a specialised cell to the function it makes possible, rather than listing features on their own.
• I can describe the meaning of all five terms — cell, tissue, organ, organ system and organism — precisely enough to separate them from each other.
• I can place those terms in the correct order and give a complete worked example from one organism.
• I can state the formula magnification = image size ÷ actual size, and use it in all three of its rearrangements.
• I can calculate a magnification, an image size and an actual size for a biological specimen working in millimetres.
• I can explain why a magnification carries no unit, and I never write one.
• I can convert a measurement between millimetres and micrometres in both directions.
• I can carry out a magnification calculation in which the image size and the actual size are given in different units, converting before I divide.
• I can prepare a temporary stained slide of plant cells and, under safe school practice, describe the preparation of cheek cells.
• I can make a biological drawing that follows the marking standard: large, single clear lines, no shading, ruled horizontal labels, a title, and magnification or scale when asked.
• I can recognise the common faults in a slide from what they look like down the microscope.

Why this chapter carries so much weight. Almost every later topic rests on it. Diffusion and osmosis happen across the cell membrane. Photosynthesis happens in chloroplasts. Respiration happens in mitochondria. Enzymes work in the cytoplasm. Transport, gas exchange and excretion are all organ-system stories that begin with a specialised cell. If Topic 2 is secure, the rest of the course has somewhere to attach itself.

• “The magnification is \(400\ \mathrm{mm}\).” Fix A magnification is a ratio of two lengths in the same unit, so it has no unit. Write \(\times 400\).
• “I multiplied by the magnification to get the actual size.” Fix The image is the enlarged one, so the actual size must come out smaller. Divide the image size by the magnification, and check that your answer is smaller than what you started with.
• “\(48 \div 120 = 0.4\), so the magnification is 0.4.” Fix The two lengths were in different units. Convert first: \(48\ \mathrm{mm} = 48\,000\ \mu\mathrm{m}\), then \(48\,000 \div 120 = 400\). Being out by exactly a factor of one thousand is the signature of this mistake, so if your answer is 0.4 or 400 000, check the conversion line before you check anything else.
• “The cell wall controls what enters the cell.” Repair The cell membrane controls exchange; the wall supports. The wall is fully permeable, so it cannot select anything.
• “The image is \(60\ \mathrm{mm}\) and the cell is \(0.15\ \mathrm{mm}\), so the magnification is \(60 \times 0.15 = 9\).” Repair Magnification is image size divided by actual size, never multiplied: \(60 \div 0.15 = \times 400\). If a magnification comes out smaller than the numbers you started with, you multiplied where you should have divided.
• Drawing every organelle you know into a photomicrograph. Repair Draw only what can actually be observed. A light microscope shows no ribosomes at all, and mitochondria only as specks.
• “A tissue is many cells.” Repair A tissue is a group of similar cells working together to perform a function. Both halves are needed.
• “Bacteria have no DNA because they have no nucleus.” Repair They have DNA. It is a circular molecule lying free in the cytoplasm, and there may also be plasmids. What is absent is the nucleus.
• “All plant cells have chloroplasts.” Repair Only photosynthetic cells do. A root hair cell has none, because no light reaches it. An onion epidermal cell has none either.
• “Mitochondria make energy for the cell.” Repair Mitochondria are the site of aerobic respiration, which releases energy from nutrient molecules. Energy is never made.
• “The magnification is \(400\ \mathrm{mm}\).” Repair Magnification is a ratio of two lengths in the same unit, so the units cancel and it has none. Write \(\times 400\).
• Listing features when the question said explain. Repair Use the FLE chain: feature, then what it physically causes, then what the cell can therefore do. “Many mitochondria” is a feature; “so energy is released for active transport” is the explanation.
• Writing two separate lists when the question said compare. Repair Put both organisms in the same sentence: “A plant cell has a nucleus, whereas a bacterial cell does not.”
• “\(48 \div 120 = 0.4\)” — dividing before converting millimetres and micrometres. Repair Put both lengths in the same unit first: \(48\ \mathrm{mm} = 48\,000\ \mu\mathrm{m}\), then \(48\,000 \div 120 = \times 400\). Being wrong by exactly a factor of one thousand is the signature of this error.
• “\(1\ \mathrm{mm} = 1000\ \mu\mathrm{m}\), so to change micrometres into millimetres I multiply by 1000.” Repair The conversion runs the other way: micrometres are the smaller unit, so a length in micrometres is a bigger number than the same length in millimetres. Going from micrometres to millimetres you divide by 1000. Check the direction against something you know — a cell is about \(60\ \mu\mathrm{m}\), which is \(0.06\ \mathrm{mm}\), not \(60\,000\ \mathrm{mm}\).

• Reveal buttons are a study tool, not decoration. Every hidden answer in this chapter is a chance to retrieve before you read. Say or write your answer first, then press the button. Reading the answer straight away feels faster and teaches you far less.
• 2.1 has no Supplement statements. The official table prints the Supplement heading for 2.1 with nothing underneath it. Everything in 2.1 is Core, so an Extended candidate learns exactly the same cell structure as a Core candidate — no more and no less.
• Core never depends on a Supplement block. You can read this chapter skipping every bordered Supplement block and still have a complete, self-sufficient Core course — including every Core calculation, which is worked in millimetres throughout. Nothing you need for Core is hidden inside a Supplement box.
• The single most transferable habit in this topic: when a question gives you a structure, answer with a job; when it gives you a job, answer with a structure. Most of Topic 2 is a one-to-one mapping, and the mapping runs in both directions.
• Read the “Plant” column carefully. Two of its entries are conditional, not absolute. Chloroplasts occur in photosynthetic cells, and the entry for the animal column is no large permanent vacuole rather than a flat “no vacuole”. These two cells of the table are where a loose answer becomes an inaccurate one.
• Do not answer this with mitosis or meiosis. Naming a type of division here is not what the statement asks for, and the processes themselves belong to a later topic on inheritance, where they are Supplement content. At this point in the course the whole of the required answer is that new cells come from the division of cells that already exist. Writing more adds nothing, and a half-remembered version of mitosis can turn a correct answer into a muddled one.
• Two cells, one idea. Whenever a cell has to absorb or exchange something, look for a shape that raises surface area: the long thin projection of a root hair cell, and the biconcave dish of a red blood cell. If you can spot which problem a shape is solving, you can usually work out the function of a specialised cell you have never seen before.
• Organelle is the one word on the ladder that the syllabus does not ask you to define. It is in the diagram because it makes the sequence make sense — a mitochondrion is part of a cell, not a cell — but the five terms in the table above are the ones a question can ask you for.
• Work in millimetres and stay there. For Core, every length you are given and every length you are asked for is in millimetres, so there is no conversion to do and no chance of the factor-of-a-thousand error. Measure the image with a ruler in millimetres, read the actual size in millimetres, divide, and write the answer as a number of times.
• A sanity check that costs five seconds. An image on a page is tens of millimetres across and a cell is a small fraction of a millimetre, so a magnification should come out in the hundreds or thousands. If an “actual size” comes out larger than the image, you multiplied where you should have divided.
• Convert first, or convert last — but write the line either way. The conversion line is the step that makes the method visible. An answer that jumps from mixed units straight to a number leaves no working to fall back on if the arithmetic slips.
• Never share swabs. A swab that has been in one person's mouth must not go anywhere near another's. Follow your school's stated procedure for using and disposing of swabs, and treat used swabs and slides as your teacher directs. A method that describes sharing equipment between students is unsafe, and unsafe practice is not a valid method.
• Nothing else in Topic 2 is Extended-only. Every other statement in this topic — all seven of 2.1 and both Core statements of 2.2 — is Core, and is examined on the Extended papers too. So if these four items are secure, the whole of your Topic 2 Supplement is secure.

• Organisation of the organism is examined in more than one style, and the style changes what a good answer looks like. The descriptions below are about question form, not about how often a topic appears: no one can promise you what will come up.
• Recognition under time pressure. Paper 1 is the Core route, Paper 2 the Extended route. You are shown a labelled diagram, an image or a short comparison and asked which structure, which function or which cell type. Precision of the boundary cases decides these — wall versus membrane, DNA versus nucleus.
• Written answers: state a function, describe a bacterial cell, explain why a root hair cell contains many mitochondria, complete a comparison table, or calculate a magnification and show the working. Paper 3 is the Core route and Paper 4 the Extended route; a Paper 4 calculation may require the millimetre-to-micrometre conversion, a Paper 3 one may not.
• Both routes sit one of these. Slide preparation, use of the microscope, measurement of a specimen, and biological drawing with a title and a magnification or scale. A drawing is judged against a set of stated conventions rather than against artistic skill, so the requirements are ones you can practise deliberately.
• The single most transferable habit in this topic: when a question gives you a structure, answer with a job; when it gives you a job, answer with a structure. Most of Topic 2 is a one-to-one mapping, and the mapping runs in both directions.