Cells, Tissues & Levels of Organisation

Why cells matter

Every living thing is built from cells. They are the smallest units that can carry out all the processes of life, which is why we call the cell the basic unit of structure and function in an organism. Some organisms, like bacteria, are a single cell. Others, like you, are made of trillions of cells working together. In this chapter we look at what cells are made of, how plant, animal and bacterial cells differ, how cells become specialised for particular jobs, and how cells are organised into larger structures.

Key terms

Organelle — a specialised structure inside a cell that carries out a particular function.

Cell membrane — the thin layer surrounding a cell that controls what enters and leaves.

Cytoplasm — the jelly-like fluid where most chemical reactions take place.

Animal cells

All animal cells share a common set of structures. Learn each part with its function, because Edexcel often asks you to match a structure to the job it does.

Plant cells

Plant cells contain everything an animal cell has, plus three extra features. Remember: animal cells do not have a cell wall, chloroplasts or a permanent vacuole.

Watch out

A common slip is saying "a plant cell has a cell wall instead of a cell membrane". This is wrong. A plant cell has both — the cell wall lies outside the cell membrane. The wall is fully permeable; the membrane is the part that actually controls what enters and leaves.

Bacterial cells

Bacteria are much simpler. They are prokaryotes, which means their DNA is not held inside a nucleus. They also lack the membrane-bound organelles found in animal and plant cells.

A bacterial cell has:

A cell wall (not made of cellulose) and a cell membrane.
Cytoplasm.
A single loop of circular DNA, free in the cytoplasm — there is no nucleus.
Small rings of extra DNA called plasmids.
Some have a flagellum (plural: flagella) for movement.

Bacteria have no nucleus, no mitochondria and no chloroplasts. Make sure you can state these three absences clearly, as it is a favourite comparison question.

Key terms

Plasmid — a small circular piece of DNA, separate from the main circular DNA, found in bacteria.

Flagellum — a whip-like tail that some bacterial cells use to move.

Specialised cells

In a multicellular organism, cells become specialised — their structure is adapted to suit a particular function. Learn how each adaptation links to the job.

Exam tip

When asked "explain how this cell is adapted", always link the feature to the function using a word like so or to. For example: "The red blood cell has no nucleus so there is more space for haemoglobin to carry oxygen." Stating the feature alone scores no marks.

Levels of organisation

Specialised cells do not work alone. In larger organisms they are arranged into a hierarchy, with each level building on the one below.

1. Organelles — structures inside a cell, e.g. mitochondria.
2. Cells — the basic unit, e.g. a muscle cell.
3. Tissues — groups of similar cells working together, e.g. muscle tissue.
4. Organs — different tissues working together for a function, e.g. the heart.
5. Organ systems — groups of organs working together, e.g. the circulatory system.
6. Organism — the whole living thing.

Real world

Your heart is an organ built from muscle tissue, nervous tissue and blood. It works with blood vessels as part of the circulatory system — one of several organ systems that together keep you, the organism, alive.

The light microscope and magnification

Cells are far too small to see with the naked eye, so we use a light microscope. Light passes through a thin specimen and is magnified by two lenses: the eyepiece lens and the objective lens. The image we see is the magnified image; the cell itself is the actual object.

Magnification tells you how many times bigger the image is than the real object:

$$\text{magnification} = \dfrac{\text{image size}}{\text{actual size}}$$

You can rearrange this triangle to find any value:

image size $=$ magnification $\times$ actual size
actual size $=$ image size $\div$ magnification

Cells are usually measured in micrometres (µm). The key conversions are:

Worked example

A plant cell appears 40 mm wide in a photograph taken under a microscope. The real cell is 80 µm wide. What is the magnification?

Step 1 — use the same units. Convert the image size to µm: $40 \text{ mm} \times 1000 = 40\,000 \text{ µm}$.

Step 2 — apply the formula. $\text{magnification} = \dfrac{40\,000}{80} = 500$.

Exam tip

Most magnification mistakes come from mismatched units. Before dividing, always convert image size and actual size into the same unit — usually µm. Remember $1 \text{ mm} = 1000 \text{ µm}$.

Quick recap

Animal cells have a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.
Plant cells have all of those plus a cellulose cell wall, chloroplasts and a permanent vacuole of cell sap.
Bacterial cells have a cell wall, membrane, cytoplasm, circular DNA and plasmids, sometimes a flagellum — but no nucleus, mitochondria or chloroplasts.
Specialised cells are adapted in structure to their function; always link feature to job.
Organisation runs: organelles → cells → tissues → organs → organ systems → organism.
$\text{magnification} = \dfrac{\text{image size}}{\text{actual size}}$, with units matched (1 mm = 1000 µm).