2.2 What Is a Cell?

Every living being — a towering tree, a buzzing bee, you — is built from the same kind of tiny building block: the cell. A cell is to a living body what a brick is to a wall. To meet one, we only need a microscope and something alive to look at.

Activity 2.2 — Look at an onion cell Peel the thin, transparent film from the inner surface of an onion scale. Lay it flat on a glass slide, add a drop of safranin (a red stain) so the parts show up, rinse, add a drop of glycerin to stop it drying, and lower a coverslip gently so no air bubbles are trapped. Under the microscope you will see neat, almost rectangular boxes packed tightly together, with no gaps — exactly like bricks in a wall. Those boxes are onion cells.

Activity 2.3 — Look at your own cheek cell Gently scrape the inside of your cheek with the blunt end of a clean toothpick, smear it on a slide, add a drop of methylene blue (a blue stain) for contrast, then glycerin and a coverslip. You will see flat, polygon-shaped cheek cells — the cells that line the inside of your mouth. Compare them with the onion cells: both are cells, yet they look quite different.

Inside a cell

A cell is not a simple bag of liquid. It is a busy, organised structure with several parts, each doing a special job:

  • the cell membrane is the thin outer boundary that holds the cell together and controls what goes in and out;
  • the cytoplasm is the jelly-like material filling the cell, where many activities happen;
  • the nucleus is the control centre — it directs all the cell’s activities and its growth;
  • mitochondria release energy the cell needs to live and work.

Plant cells carry a few extra parts that animal cells lack:

  • a tough cell wall outside the membrane, which gives the plant rigidity and strength (this is why plant cells look firm and box-like);
  • chloroplasts, green structures holding chlorophyll, which let the plant make its own food by photosynthesis;
  • a large vacuole, a big fluid-filled space that stores substances, removes waste, and keeps the cell plump and firm. (Animal cells have only small vacuoles, or none.)

The Key Idea Every cell has a cell membrane, cytoplasm, and (in most) a nucleus. Plant cells additionally have a cell wall, chloroplasts, and a large vacuole. That short list of extras is the whole difference between a typical plant cell and a typical animal cell.

Use the sorter below to lock this in. For each cell part, decide whether it is found in both plant and animal cells, or only in plant cells.

Plant cell, animal cell, or both?
Click each part to toggle between "Both" and "Plant only". Then press Check.

Ever Heard Of… The single largest cell in the living world is the yolk of an ostrich egg — about 130–170 mm across. The whole egg also carries non-living material: a shell for protection and the “white” to nourish the cell as it develops.

Exam Tip A classic exam trap: “Name the parts found only in a plant cell.” The answer is cell wall, chloroplast, and (large) vacuole — not the nucleus or membrane, which both cell types share. Learn the three plant extras as a set and you will never lose these marks.