Cells: the basic unit of life

What makes something alive

Every living thing carries out the same seven life processes. The mnemonic MRS GREN lists them: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion and Nutrition. A car moves and uses fuel, but it never grows or reproduces, so it is non-living. A seed looks lifeless but can do all seven once it germinates, so it is living.

Some things were once alive — a wooden table, a leather shoe, a fossil. These are called dead or once-living, not non-living. Non-living things such as rocks and water were never alive.

Cell theory

A cell is the smallest unit that can carry out all the life processes. Cell theory sums up what scientists agree about cells:

• All living things are made of one or more cells.
• The cell is the basic unit of structure and function in living things.
• All cells come from pre-existing cells.

Bacteria, amoebas and yeast are unicellular — one cell does everything. Plants, animals and fungi are multicellular: they contain millions of cells that share the work.

The light microscope

Cells are far too small to see with the naked eye, so we use a light microscope. Know the parts:

• Eyepiece (ocular lens) — what you look through, usually $10\times$.
• Objective lenses — on the revolving nosepiece, usually $4\times$, $10\times$ and $40\times$.
• Stage and stage clips — hold the slide.
• Coarse focus knob — large movements, used first on low power only.
• Fine focus knob — small movements to sharpen the image.
• Light source or mirror and diaphragm — control the light passing through the specimen.

Total magnification is the eyepiece multiplied by the objective:

$$\text{total magnification} = \text{eyepiece} \times \text{objective}$$

So a $10\times$ eyepiece with the $40\times$ objective gives $400\times$.

Using the microscope

Always start on the lowest-power objective, centre the specimen, then change to a higher power and use only the fine focus. The image is upside down and back to front, so to move the specimen up on the screen you move the slide down.

To make a wet mount, put a drop of water on the slide, add a thin slice of specimen, then lower a coverslip at an angle with a mounted needle so no air bubbles get trapped. A stain such as iodine or methylene blue makes clear parts of the cell easier to see.