The cell theory
The cell theory is one of biology's foundations: all living things are made of cells, the cell is the basic unit of structure and function, and all cells come from pre-existing cells. Cells range from bacteria a few micrometres across to a nerve cell over a metre long, but all carry out the same essential jobs of life.
Cells come in two grand types. Prokaryotic cells (bacteria) have no nucleus and no membrane-bound organelles; eukaryotic cells (plants, animals, fungi, protists) keep their DNA in a nucleus and have many organelles.
Organelles and their jobs
A eukaryotic cell is a division of labour among organelles. The nucleus holds the DNA and controls the cell; mitochondria release energy in respiration; ribosomes build proteins; the endoplasmic reticulum and Golgi apparatus process and package them; and the cell membrane controls what enters and leaves.
| Organelle | Function |
|---|---|
| Nucleus | stores DNA; controls the cell |
| Mitochondrion | aerobic respiration — releases energy |
| Ribosome | protein synthesis |
| Chloroplast (plants) | photosynthesis |
| Vacuole (large, in plants) | stores cell sap; keeps the cell turgid |
| Cell wall (plants) | cellulose; support and shape |
Plant versus animal cells
Both plant and animal cells have a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. But a plant cell has three extras: a cellulose cell wall for support, chloroplasts for photosynthesis, and a large permanent vacuole. Animal cells have none of these and are usually more rounded and flexible.
Plant-cell extras
Remember the three things a plant cell has that an animal cell lacks: a cell wall, chloroplasts and a large permanent vacuole. Everything else they share.
Moving substances across the membrane
The cell membrane is partially permeable, and three processes move substances across it. Diffusion is the net movement of particles from high to low concentration, down a gradient — how oxygen enters a cell. Osmosis is the diffusion of water across a partially permeable membrane, from a dilute to a more concentrated solution. Both are passive — they need no energy.
Active transport is different: it moves substances against their concentration gradient, which requires energy from respiration — for example, roots absorbing mineral ions from dilute soil water.
Why cells stay small
As a cell grows, its volume increases faster than its surface area, so a large cell has too little membrane to supply its interior. A high surface-area-to-volume ratio keeps exchange fast enough — which is why cells stay small, and why exchange surfaces like the lungs and small intestine are folded to maximise area.
A worked example
A piece of potato is placed in pure water and gains mass; another placed in concentrated salt solution loses mass. Explain both results.
- In pure water, the water is more dilute outside than inside the potato cells.Cell contents are a more concentrated solution than pure water.
- Water enters the cells by osmosis, so the potato gains mass.Osmosis moves water from dilute to concentrated across the membrane.
- In concentrated salt solution, the outside is more concentrated than the cells.The salt solution has less free water than the cell contents.
- Water leaves the cells by osmosis, so the potato loses mass.Water moves out toward the more concentrated solution.
Water moves by osmosis toward the more concentrated solution — in, in pure water (gain); out, in salt solution (loss).
Comparing the two microscopes' view of a cell
The organelles you can study depend on the microscope. Under a light microscope you can make out the cell wall, the large vacuole, chloroplasts and the nucleus. Only the far more powerful electron microscope reveals the small organelles — mitochondria, ribosomes and the endoplasmic reticulum — and the true structure of the membrane. This is why our modern picture of the cell only emerged once electron microscopes were invented.
Specialised exchange surfaces
The rule that cells stay small because of their surface-area-to-volume ratio shapes whole organs. Where a lot of exchange must happen fast, the surface is folded or branched to add area without adding much volume: the villi of the small intestine, the alveoli of the lungs, and the root hair cells of a plant all use this trick. Thin walls and a good blood (or transport) supply complete the adaptation.
Surface area rules the design
Diffusion is fast enough only when the surface area is large relative to the volume it serves. Exchange surfaces everywhere in biology solve this the same way — by folding to add area.