Cell specialisation

Why are the two cells that meet at fertilisation built so differently? A human egg cell is about a tenth of a millimetre across, one of the biggest cells in the body. A sperm cell's head is about twenty times smaller. You might guess the egg carries more instructions. It does not: each one brings the same number of chromosomes, twenty-three. Hold on to that size difference, because the jobs of the two cells explain it.

A specialised cell is a cell whose structure fits it to one job in the body. Each feature that fits a cell to its job is called an adaptation. Every adaptation is explained the same way. Name the job, name the feature, then say how the feature helps the job. A feature with no reason beside it explains nothing. The words so that join the two halves. Take a sperm cell. Its job is to reach an egg and fertilise it. A sperm has to swim to the egg. What feature would you expect it to have? A tail, so that it can swim to the egg. Its middle section is packed with mitochondria, so that respiration can release the energy it needs for swimming. At the tip of its head is an acrosome, a small pouch of enzymes, so that it can digest a way into the egg. And its nucleus is haploid. It holds half the usual number of chromosomes, so that fertilisation brings the number back to a full set.

The same three steps work on any animal cell, including ones you have never met. A nerve cell's job is to carry electrical impulses around the body, sometimes from your spine all the way to your toe. Which feature would help a nerve cell carry impulses that far? It is long, so that it can carry impulses over a long distance. Its branched endings let it connect with many other cells. A muscle cell's job is to contract. It is packed with protein fibres that can contract, so that the whole cell shortens. It has many mitochondria, so that respiration releases the energy contracting needs. A ciliated epithelial cell lines your airways, and epithelial means it is part of a lining. Tiny hair-like cilia on its surface beat together, again and again. Do the cilia move the cell along, like a sperm's tail, or move something past it? They move something past it. The cilia sweep mucus, with trapped dirt and microbes, up and out of the airways, while the cell stays put. A sperm's tail moves the cell; cilia move what is next to the cell. Different courses name different cells to learn, and your course's list is in the table here. The three steps are the same for every cell, even one a question describes for the first time.

Plant cells are read the same way. A root hair cell's job is to take up water and mineral ions out of the soil. Its long, hair-like extension gives it a large surface area, so that it can absorb more. It has no chloroplasts, because underground there is no light for them to use. A student labels a root hair cell drawing, calling the large space in the middle, P, the nucleus. What is wrong with calling P the nucleus in this root hair cell? P is the permanent vacuole, filled with cell sap. In a plant cell the biggest structure is usually the vacuole. The real nucleus is the small round body, Q. One examiner's report on a Foundation paper says that "A large proportion of candidates seemed to be unfamiliar with the structure of a typical plant cell or plant root hair cell and incorrectly identified X as the nucleus." In that question, X was the vacuole. The fix: find the biggest structure first, which in most plant cells is the vacuole. Now a job with no cell named. Some cells must form a pipe carrying water from the roots to leaves thirty metres up a tree, without the pipe collapsing or anything blocking the flow. What three features do these pipe cells need, and why does each one help? No end walls, so that the cells join end to end into one continuous tube. No contents, so that the tube is hollow and nothing blocks the water. And walls strengthened with lignin, so that the tube does not collapse and helps hold the tree up. These are xylem cells. A report on a Higher paper, about xylem cells, puts the rule like this: "If a correct adaptation was given, the reason had to match the structure. Quite a lot of students talked about long tubes, which was insufficient. A reference to them being hollow or empty was needed." The fix: name the exact feature your reason needs, such as hollow or no end walls, never just long tubes. Phloem cells form long tubes too, but they carry dissolved sugars instead of water. Why would the end walls of phloem cells have pores in them? So that what they carry can flow straight through, from one cell to the next.

Back to the egg and the sperm. The egg's job is different: once fertilised, it has to feed the new cell through its first divisions, and let in only one sperm. Six features are up: four you met on the sperm, and two new ones. Sort the six features: sperm, egg, or both? Sperm: the tail, the acrosome and the middle section packed with mitochondria. Egg: the nutrients, and the membrane that changes. Both: the haploid nucleus. Now a reason: why does only the egg need a store of nutrients? The nutrients feed the fertilised egg through its first cell divisions. The sperm's job ends once it is inside. And the egg's membrane changes after fertilisation, so that no other sperm can get in. That answers the opening question. Every sperm feature is about travelling and getting in. Every egg feature is about what happens after. And the one feature they share, the haploid nucleus, is the one that makes fertilisation add up: half a set plus half a set makes a full set.

To close, the three steps again, on a cell from your gut and one from a leaf. A gut lining cell has a surface folded into tiny projections. What do the folds let it do? They give it a larger surface area, so that it can absorb digested food faster. Now a harder one: a leaf cell near the top surface is packed with chloroplasts. Why? It gets the most light, so packing in chloroplasts lets it photosynthesise as much as possible. Here's the last one: a student writes that mitochondria make energy. What should the verb be? Release. Mitochondria release energy by respiration, and they never make it. And the egg and the sperm: built so differently because their jobs are different. One has to travel and get in; the other has to feed what comes after.

If this one's yours, press the thumbs up and treat it as finished. If it is not there yet, keep it on a playlist and write out one job, feature and reason for each cell; written down once, the pairs tend to stay.

Next in the chapter: Levels of organisation: cells, tissues, organs and organ systems. Hopefully that's next in your feed, but if not, search for ScholaFly B I oh two, oh four.

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