Sub-cellular structures and what each one does

Which grows faster: an ivy with plain green leaves, or a variegated ivy splashed white and green, side by side on one windowsill? Same light, same water, so you would expect them to keep pace. Yet the plain green ivy usually grows faster. Cells from the white patches look much like the green ones, but they are short of one part, and that part explains the slower growth. We will return to the white ivy patches once every cell part is in view.

A real cell is not a flat circle. It is a three-dimensional blob, more like a tiny water balloon than a drawing. Everything in this video sits one level below the whole cell. A sub-cellular structure is a part inside a cell; the cell itself is the whole unit. Every cell diagram you meet in the exam is a slice through that blob, like one slice cut from a loaf. Anything sitting above or below the slice is left out of the drawing. Now try this: a student names nucleus as a type of cell. Which level is that answer on? One level too low. A nucleus is a sub-cellular structure inside a cell, never a cell itself.

Start with an animal cell, and with what each part lets the cell do. The nucleus holds the genetic material, so it controls what the cell does. The cytoplasm is the jelly that fills the cell. It is everything that is not a named structure, and many of the cell's chemical reactions happen in it. The cell membrane wraps the cell. It contains receptor molecules and provides a selective barrier to molecules, which means it controls what gets in and out. Ribosomes are tiny structures scattered through the cytoplasm, far too small to see with an ordinary light microscope. A ribosome is where protein synthesis happens, which means it is where the cell makes protein. How it does that is a later chapter's job. A cell lets glucose in but keeps a harmful molecule out. Which structure is doing that? The cell membrane. It is the selective barrier, so it decides which molecules cross. Mitochondria contain enzymes for cellular respiration. Respiration is the reaction that releases energy from glucose, and enzymes are proteins that speed reactions up. So respiration releases the energy, and a mitochondrion is where it happens. Released is the word to use, every time. Here is a sentence a student wrote: the mitochondria create energy for respiration. It is marked wrong. What is wrong with create energy for respiration? There are two faults to find. Energy is never created; respiration releases it. And it is released by respiration, not for it, because respiration is the reaction doing the releasing. A sentence that would score reads: respiration in the mitochondria releases energy for the cell. One examiner's report on a Foundation paper puts it like this. The common error of referring to energy production or creation in the mitochondria was still seen. In addition, students often referred to energy being released for respiration, which is not creditworthy. The fix is two small words. Write releases, never makes or creates, and write released by respiration, never released for it.

A plant cell has every structure an animal cell has, plus up to three more. Outside the cell membrane is a cell wall, made of cellulose, a tough fibre that plants make. The wall strengthens the cell and holds its shape. That gives a plant cell two layers at its edge, and their order is the rule. The cell wall is the stiff outer layer; the cell membrane is the thin layer just inside it. An animal cell has only the membrane. The permanent vacuole is a large space filled with cell sap, a watery solution of sugars and salts. It helps keep the cell firm. A plant cell's vacuole loses most of its sap. What happens to the cell? It loses its firmness and goes floppy, since the full vacuole was what held it out. Chloroplasts contain chlorophyll, the green pigment that absorbs light. They are where photosynthesis happens, the reaction in which the plant makes its own glucose. One report on a Higher paper puts a number on the plant cell extras. Ninety-five per cent of students correctly identified vacuole or cell wall as features found in plant and fungal cells but not in animal cells. If you knew that already, you are in good company. Now a plant cell labelled by a student. The outer layer is labelled cell membrane, the thin layer just inside it is labelled cell wall, and the jelly filling the cell is labelled chloroplast. Three labels, and the drawing itself is enough to check every one. Which labels are wrong, and what in the drawing tells you? All three. The wall is the outermost line and the membrane sits just inside it, so those two are swapped. And the jelly is cytoplasm. A chloroplast is a separate green oval, a body you could draw round; cytoplasm is everything around the named bodies. A third report, on a Foundation labelling question, records what went right and what went wrong. Virtually all students labelled the nucleus correctly. The most common mistake seen was confusing the cell membrane and cell wall. The habit that fixes it: find the outermost line first. For a plant cell, that line is the wall, and the membrane is pressed just inside it.

Now the bacterial cell, with the rule from the video on eukaryotic and prokaryotic cells: no part wrapped in its own membrane. A bacterium has a cell membrane, cytoplasm and a cell wall. Its genetic material is a single loop of DNA, also called chromosomal DNA, and it often carries plasmids, small extra rings of DNA. Which of these can a bacterium have: ribosomes, mitochondria or a nucleus? Ribosomes. A ribosome is not wrapped in anything, so the rule does not exclude it, and every living thing needs to make protein. Mitochondria and a nucleus are both wrapped in membranes, so a bacterium has neither. Chloroplasts are out for the same reason.

Now the argument runs backwards: four jobs, and you name the structure. Job one is a cell that makes a great deal of one protein and sends it out. Job two is a cell that contracts all day and needs a lot of energy released. Job three is a cell that makes its own glucose in the light. Job four is a cell that has to hold its shape with no skeleton around it. Name the structure each job needs plenty of, and say why. Job one needs ribosomes, because a ribosome is where protein is made. Job two needs mitochondria, where respiration releases the energy. Job three needs chloroplasts, where photosynthesis makes glucose. Job four needs a cell wall, because the wall holds the cell's shape. That last answer narrows down the cell, too. Animal cells have no wall, so job four is not an animal cell, though the job never said so. Now back to the windowsill. Why does the variegated ivy grow more slowly than the plain green one? The variegated ivy's white patches have few or no chloroplasts, so less of each leaf can photosynthesise. Less glucose is made, so growth is slower.

Three short questions to close, two of them on cells this video never mentioned. A cell pours out an enzyme, which is a protein. Which structure will it be packed with? Ribosomes, because protein synthesis happens at the ribosomes. Here's another: would cells from an onion bulb, growing underground, contain chloroplasts? No. Chloroplasts absorb light, and the bulb grows in the dark, so its cells have none. Try the last one: what does respiration do to energy, in one word? Releases. It never makes or creates energy, and the release is by respiration, not for it. And the variegated ivy: its white cells are short of chloroplasts, which is why it grows more slowly than the plain green ivy beside it.

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