CoachAn organelle is a structure inside a cell, enclosed by its own membrane, that performs a specific task. The principal ones are:
- the nucleus, which contains the chromosomes — the DNA of Chapter 3 — and controls the cell’s activity;
- the mitochondria, some long, where organic molecules are broken down with oxygen to release usable energy (cellular respiration, Chapter 4);
- in plant cells, the chloroplasts, green with chlorophyll, where photosynthesis takes place, and a large water-filled vacuole that keeps the cell taut.
Outside the membrane, a plant cell is boxed in a rigid cell wall of cellulose; animal cells have none. The cytoplasm of every cell also contains millions of ribosomes, particles of about that assemble proteins, visible only with the electron microscope.
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Example 2.7 (Cheek cells)
A cheek cell drawn wide from a image is across, and its nucleus, drawn , is about . A red blood cell is ; the bacteria of the mouth, long, appear as barely resolved dots at the same magnification.
Example 2.10 (Reading a bacterium)
An electron micrograph of the gut bacterium Escherichia coli shows a rod long and wide: a wall, a membrane just inside it, a granular cytoplasm packed with ribosomes, and a paler central region where the single circular DNA molecule lies. No nucleus, no mitochondria. Yet the cell feeds, grows and divides every twenty minutes in a rich broth: everything the cell theory demands, in a thousandth of the volume of a cheek cell.