Your body is made of roughly 30 trillion cells, and a single oak leaf contains millions more. Each cell is far too small to see with your eyes, yet each one takes in materials, releases energy, gets rid of wastes and, in many cases, divides to make new cells. This unit follows cells upward: how they divide, how they specialize, and how they team up into tissues, organs and whole organ systems. It starts with the parts inside a cell and the instrument that lets us see them: the compound light microscope.
The cell theory
- All living things are made of one or more cells.
- The cell is the basic unit of structure and function in living things.
- New cells come only from cells that already exist, by cell division.
Plants and animals are both made of eukaryotic cells: cells whose DNA is enclosed in a nucleus and that contain small membrane-bound compartments called organelles. Each organelle does a particular job, much like the departments of a company. Plant and animal cells share most organelles, but a few structures are found only in plant cells.
| Organelle | What it does | Found in |
|---|---|---|
| Cell membrane | Thin, flexible boundary that controls what enters and leaves the cell (it is selectively permeable) | Plant and animal |
| Nucleus | Contains DNA, the instructions for making proteins, and controls the cell's activities | Plant and animal (mature mammalian red blood cells lose theirs) |
| Cytoplasm | Jelly-like fluid that fills the cell and holds the organelles; many chemical reactions happen here | Plant and animal |
| Mitochondria | Carry out cellular respiration, releasing energy from glucose for the cell to use | Plant and animal |
| Ribosomes | Tiny structures that build proteins | Plant and animal |
| Endoplasmic reticulum (ER) | Folded membranes that make and transport proteins (rough ER, with ribosomes) and lipids (smooth ER) | Plant and animal |
| Golgi apparatus | Sorts, packages and ships proteins and other molecules, often to the cell membrane | Plant and animal |
| Vacuole | Stores water, nutrients and wastes; in plants one large central vacuole pushes against the wall and keeps the cell firm | Plant (one large) and animal (several small) |
| Lysosomes | Sacs of enzymes that break down worn-out parts, wastes and invading bacteria | Mainly animal |
| Cell wall | Rigid layer made mostly of cellulose outside the membrane; gives support and shape | Plant only |
| Chloroplasts | Carry out photosynthesis, using light energy to make sugar | Plant only (in green parts) |
Using the compound light microscope
A compound microscope uses two lenses in a row. The objective lens near the slide makes an enlarged image, and the ocular lens (eyepiece) enlarges that image again. So the total magnification is the product of the two. The field of view is the circle of the specimen you can see. When you switch to a stronger objective you see a smaller piece of the slide in more detail, so the field of view shrinks in proportion: ten times the magnification gives one tenth the diameter.
- Carry the microscope with one hand on the arm and the other under the base.
- Always start with the lowest-power (4×) objective in place.
- Use the coarse adjustment knob only with the low-power objective; at higher power, use only the fine adjustment knob so the lens never hits the slide.
- Centre the part you want before switching to a higher objective, because the field of view gets smaller.
- Remember that the image is upside down and backwards: a letter e on the slide looks like an upside-down, reversed e, and moving the slide left moves the image right.