Sprinkle salt on slices of cucumber and leave them for twenty minutes. Puddles of water appear, and the slices go limp. Nobody squeezed them: the water left the cells by itself. By the end of this lesson you'll be able to explain exactly why.
Two kinds of cell
- Prokaryotes (bacteria and archaea) are small and simple: no nucleus, no membrane-bound organelles. Their DNA sits in a region called the nucleoid.
- Eukaryotes (plants, animals, fungi and protists) keep their DNA in a nucleus and divide the work between membrane-bound organelles.
Both have a cell membrane, cytoplasm, ribosomes and DNA.
The cell as a factory
Follow a protein that a cell will export:
- Ribosomes on the rough ER build it.
- The rough ER folds it and sends it on in a vesicle.
- The Golgi modifies, sorts and packages it.
- A vesicle carries it to the membrane and releases it.
Other key organelles: the smooth ER makes lipids and detoxifies; lysosomes digest waste; mitochondria make ATP; chloroplasts capture light energy in plants. Mitochondria and chloroplasts have their own DNA and ribosomes, which is evidence for endosymbiotic theory: they started out as free-living prokaryotes.
Why cells stay small
As a cell gets bigger, its volume grows faster than its surface area. Before long, the membrane can't move materials in and out fast enough. That's why cells are tiny, and why cells that exchange a lot, like those lining your gut, have folds called microvilli.
The membrane: who gets in
The membrane is a phospholipid bilayer with proteins moving through it: the fluid mosaic model. It's selectively permeable:
- Small nonpolar molecules (O₂, CO₂) slip straight through by simple diffusion.
- Ions and polar molecules like glucose need transport proteins (facilitated diffusion).
- Moving something against its gradient takes energy: active transport, like the sodium–potassium pump.
Osmosis is water diffusing across the membrane towards the side with more solute. Back to the cucumber: the salt made the outside hypertonic, so water left the cells.
Common mistakes students make
- Saying water moves towards "more water". It moves towards more solute, which means lower water potential.
- Mixing up hypotonic and hypertonic. The words describe the solution compared with the cell: hyper = more solute outside.
- Thinking prokaryotes have no ribosomes. All cells have ribosomes; prokaryotes just lack membrane-bound organelles.
- Calling glucose "small enough to diffuse". It's polar, so it needs a carrier protein.
Try it
Say each answer before you flip. Then ask yourself: what would you expect to see if you put a raisin in a glass of plain water overnight?
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