Eukaryotic and prokaryotic cells

Antibiotics go everywhere your blood goes. How do they kill the bacteria in a chest infection and leave your own cells alone? It is tempting to say the drug knows which cells are the enemy, but a tablet knows nothing. It works because a bacterial cell is built differently from yours. How the drug uses that comes later in the course; the difference itself is this video.

Start with what every cell has in common. Every cell has a cell membrane, a thin layer wrapped around the whole cell. Inside it is cytoplasm, the jelly that fills the cell. Somewhere in that jelly is the genetic material, the instructions the cell runs on. All cells split into two groups on one point: where that genetic material is kept. A eukaryotic cell keeps its genetic material inside a nucleus. A nucleus is a compartment with its own membrane wrapped around it. Your own cells are eukaryotic, and so are plant cells. A prokaryotic cell has no nucleus at all. Its genetic material lies loose in the cytoplasm, with nothing wrapped around it. So the two words name one difference. A eukaryotic cell is one with its genetic material inside a nucleus; a prokaryotic cell is one with its genetic material free in the cytoplasm. At GCSE, every prokaryotic cell you are shown is a bacterium, so the two words point at the same picture. Having no nucleus is not a fault, either. Prokaryotes are the commonest cells on the planet, so it is a design that works. A yeast cell keeps its genetic material in a nucleus. Eukaryotic or prokaryotic? Eukaryotic. Its genetic material is inside a nucleus, and that one fact settles it, whatever living thing the cell comes from.

A bacterial cell still needs its genetic material, so here is where it keeps it. Most of it is one single loop of DNA, lying in the cytoplasm. DNA is the chemical that genetic material is made of. Some exam papers call that loop chromosomal DNA. Both names mean the same single loop of DNA. Many bacteria also carry plasmids. A plasmid is a small, separate ring of DNA, carried as well as the main loop. Around its membrane, a bacterial cell has a cell wall, a stiff outer layer. Which other kind of cell also has a cell wall: animal or plant? Plant cells. Animal cells have none, so a cell wall on its own never proves that a cell is a bacterium. That fills in the bacterial side of the comparison. It has everything every cell has, plus a cell wall, a single loop of DNA and plasmids, and still no nucleus. One examiner's report on a Foundation paper records the slip in one line. Two fifths of students incorrectly thought that both types of cell contained a nucleus. The fix is a habit. Every time you look at a bacterial cell, say no nucleus before you name anything else in it. One rule covers everything a bacterium lacks. A prokaryotic cell has no membrane-bound structures, meaning no part wrapped in its own membrane, of any kind. The nucleus is one of those wrapped parts. Eukaryotic cells have several others, and the video on sub-cellular structures names them all. A mitochondrion is a part with its own membrane. Can a bacterium have one? No. It is membrane-bound, and a prokaryotic cell has none, so the rule answers it before you even know what a mitochondrion does. A bacterial cell is also far smaller. Most bacteria are about one to two micrometres across, and a micrometre is a thousandth of a millimetre. An animal or plant cell is about ten to a hundred micrometres across. Put a one-micrometre bacterium beside a plant cell a hundred micrometres wide. It looks like a sesame seed beside a dinner plate, up to about a hundred times smaller across.

Now three cells, described in words with no picture to lean on. The first comes from the lining of a human cheek, and its genetic material sits inside a nucleus. The second is a Salmonella cell, with a cell wall, one loop of DNA in its cytoplasm and two plasmids. The third comes from an onion skin; it has a nucleus, and it has a cell wall too. Each of the three gets one of the two words, and each word needs its reason. Which word fits each cell, and which one feature decides it? The cheek cell is eukaryotic, because its genetic material is inside a nucleus. Salmonella is prokaryotic: no nucleus, just a single loop of DNA and plasmids. The onion cell has a cell wall, like Salmonella, and it is still eukaryotic, because it has a nucleus. A cell wall settles nothing, since plant cells and bacteria both have one. The nucleus is what decides it, every time. One report on a Foundation paper backs a useful habit here. It is good to see the diagrams being annotated. This was a common strategy used by many candidates. Its example has prokaryote and eukaryote clearly written against the bacterial cell and the plant cell. Where points are that clear, it says, marks can be awarded. So in the exam, write prokaryotic or eukaryotic straight onto the drawing, against its own cell. The same report adds that candidates are advised to make clear as to which cell they refer.

Now a student's answer to mark. The question asked where a mutation, a change in the DNA, takes place inside a bacterium. The student wrote: in the nucleus. That answer is marked wrong, and the job now is to say why. What is wrong with answering in the nucleus? A bacterial cell has no nucleus, so nothing can happen in one. The answer places the change inside a structure the cell does not have. An answer that scores names what the bacterium does have. The mutation happens in its DNA: in the single loop, or in a plasmid. One examiner's report on a Higher paper lists the answers students gave. In plasmids or in DNA were the most common responses. DNA was often described as a strand, loop or ball of DNA. Then the slip itself. The most common incorrect response was in the nucleus. The habit that fixes it: before you name a place inside a bacterial cell, check that the place is there at all.

To finish, three quick ones, each on something this video has not described. A moss cell has a cell wall and keeps its DNA in a nucleus. Which kind of cell is it? Eukaryotic, because the nucleus is there. The wall only hints that it is a plant cell, and moss is a plant. Now a different cell: Helicobacter has a single DNA loop and plasmids. Which kind is it? Prokaryotic. A single loop of DNA free in the cytoplasm means no nucleus, so Helicobacter is a bacterium. One more: could Helicobacter have a chloroplast, which has its own membranes? No. It is membrane-bound, and a prokaryotic cell has no membrane-bound structures at all. So the antibiotics at the start never had to know which cells were the enemy. A bacterial cell is built differently from yours, with no nucleus, a cell wall and a single loop of DNA, and you can now name that difference.

If you could teach this one to a friend, give it a thumbs up, so you know you never need it again. If it has not settled, save it, because the rest of the chapter keeps using these two words, so they come back round.

Next in the chapter: Sub-cellular structures and what each one does. Hopefully that's next in your feed, but if not, search for ScholaFly B I oh one, oh two.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.