An overview of the cell

Two cells under the microscope

Peel a thin layer from an onion and look at it under a microscope. Then scrape the inside of your cheek gently, and look at those cells too.

• The onion cell is a typical plant cell. Its outer boundary is a firm cell wall. Just inside lies the plasma membrane.
• The cheek cell is an animal cell. It has no wall, so the plasma membrane is its outer layer.

Inside each cell you see a dense round body, the nucleus. It holds the chromosomes, which carry the genetic material, DNA.

Prokaryotic and eukaryotic

The nucleus divides all cells into two kinds.

• Eukaryotic cells have a membrane-bound nucleus. Protists, fungi, plants and animals have them.
• Prokaryotic cells have no membrane-bound nucleus. Bacteria and blue-green algae have them.

In both kinds the DNA carries the instructions of the cell. In a prokaryote, though, it lies free in the cytoplasm.

What every cell has

All cells share a few parts.

• The plasma membrane wraps the cell and controls what goes in and out.
• The cytoplasm is a semi-fluid matrix that fills the cell. Most of the cell's chemistry happens there.
• Organelles are small structures with special jobs.

Eukaryotic cells have organelles bound by membranes. These include the endoplasmic reticulum, the Golgi complex, lysosomes, mitochondria, microbodies and vacuoles. Prokaryotic cells have none of these.

Some parts have no membrane at all. Ribosomes are found in every cell. They lie free in the cytoplasm, and also inside chloroplasts and mitochondria, and on the rough endoplasmic reticulum. Animal cells also have a centrosome, which helps in cell division.

You will meet each organelle in the next two courses.

Sizes and shapes

Cells come in a huge range of sizes.

• Mycoplasma, the smallest cells known, are only about $0.3\ \mu\text{m}$ long.
• Most bacteria are $3$ to $5\ \mu\text{m}$.
• A human red blood cell is about $7.0\ \mu\text{m}$ across.
• Nerve cells are among the longest cells.
• The egg of an ostrich is the largest isolated single cell.

Shapes vary too. Cells may be disc-like, polygonal, columnar, cuboid, thread-like or irregular. The shape suits the job. A red blood cell is a flat disc, which squeezes through narrow vessels. A nerve cell has long threads that carry messages far.

Why cells stay small

A cell takes in food and oxygen through its surface. But every part of its volume needs them. Make a cell bigger, and its volume grows faster than its surface. Think of a cube of side $s$:

• surface area $= 6s^2$,
• volume $= s^3$,
• surface area to volume $= \dfrac{6}{s}$.

Double the side, and the ratio halves. A large cell would not get enough through its surface. That is one reason organisms grow by adding cells, not by making each cell huge.

You should find that a $1\ \mu\text{m}$ cube has a ratio of $6$ per µm. At $20\ \mu\text{m}$ it falls to $0.3$ per µm.

Try it in Python

The program works out the area, the volume and the ratio for five cube-shaped cells.

Practise with new numbers

Common mistakes

• Thinking prokaryotes have no DNA. They have DNA, but no membrane round it.
• Saying ribosomes have a membrane. They have none, and they are found in every cell.
• Calling the ostrich egg the longest cell. It is the largest single cell; nerve cells are the longest.
• Thinking big cells take in more for their size. A bigger cell has less surface for each unit of volume.

Remember these

• Eukaryotic: a membrane-bound nucleus and membrane-bound organelles. Prokaryotic: neither.
• Every cell: plasma membrane, cytoplasm, ribosomes.
• Sizes: Mycoplasma about $0.3\ \mu\text{m}$; bacteria $3$ to $5\ \mu\text{m}$; red blood cell about $7.0\ \mu\text{m}$.
• Surface area to volume of a cube is $\dfrac{6}{s}$; it falls as the cell grows.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is the outer boundary of an onion cell?

2. What separates a eukaryotic cell from a prokaryotic one?

3. Name an organelle without a membrane.

4. Which organisms have the smallest cells known?

5. What happens to the surface area to volume ratio when a cube-shaped cell doubles its side?