Cell wall, and prokaryotic and eukaryotic cells

Plants mostly stay in one place, so they cannot run from wind, rain or a grazing animal. They need a rigid frame. Each plant cell has one: a cell wall, an extra covering outside the cell membrane. Walls of millions of cells together keep stems upright and let leaves and flowers hold their shape.

Rigid does not mean sealed. Water and some dissolved minerals cross the wall without trouble, because the cell wall is permeable. The wall lets things through and the membrane behind it chooses, and together they let roots soak up water and nutrients from the soil.

Soak an onion peel or a Rhoeo leaf in strong sugar solution and osmosis draws water out of its cells. The cells do not get smaller, because the rigid wall holds its shape; instead the membrane and everything inside it pull away from the wall. You may also meet the name for this: plasmolysis.

Animal cells, such as your cheek cells, have no wall, so in the same solution they simply shrink. The upside of having no wall is flexibility: animal cells can change shape, which suits tissues that move.

Chemically, a plant cell wall is mostly cellulose, a carbohydrate: many glucose units linked into long chains. We cannot digest it, but in our food it works as roughage and helps digestion. Fungi and bacteria also have cell walls for protection and support.

Look at almost any cell and you will find three basic parts: a selectively permeable plasma membrane around it, a semi-fluid, jelly-like cytoplasm filling it, and a prominent nucleus.

Floating in the cytoplasm are smaller structures with special jobs, called organelles, along with other substances. Most organelles are too small for a light microscope and show up only under an electron microscope.

Switch between a typical bacterial cell, plant cell and animal cell below. Before you look, predict: which of the six structures in Table 2.1 does each one have? Tap the boxes to tick or cross, then check.

Look first at the bacterial cell. Its DNA has no well-defined nucleus around it, and none of its parts sit inside a membrane of their own (it has no membrane-bound organelles). Such cells are prokaryotic, from pro, primitive, and karyon, nucleus. Most of their work happens directly in the cytoplasm.

Now the plant and animal cells. Each keeps its DNA inside a well-defined nucleus and has many organelles wrapped in membranes. They are eukaryotic cells: eu means true, so "a true nucleus".

Table 2.2 of the book puts the two kinds of cell side by side. Here are the same points as a quick comparison.