By Thread Academy · 12 September 2026 · Biology
Everything you can see in a forest — the tallest tree, the moss underfoot, the fungi threading through the soil — is built from the same basic unit: the cell. A whale, a bacterium, and you are all made of cells. Understanding what a cell is, and what it does, is the foundation of almost everything else in biology.
Cell theory: the big idea
Cell theory is one of the most important ideas in science. It has three parts:
- All living organisms are made of cells.
- The cell is the smallest unit of life — the smallest thing that can carry out all the processes of living.
- All cells come from pre-existing cells by division.
The second point is worth pausing on. A cell is alive. It takes in materials, uses energy, grows, responds to its surroundings, and (in many cases) reproduces. A mitochondrion or a nucleus taken out of the cell is not alive on its own — but the cell as a whole is. That is why biologists call it the basic unit of life.
What a cell contains
Open up a typical animal cell and you will find it is not a bag of soup. It is organised, with compartments and structures that each have a job. These are called subcellular structures, or organelles.
The nucleus
The nucleus is the control centre. It contains the cell's DNA — the genetic material that carries the instructions for building and running the cell. When the cell needs to make a protein, the instructions are copied from the DNA in the nucleus. This is why cells that do a lot of protein-making, such as the cells in glands, often have large or prominent nuclei. The nucleus is usually drawn as a large round structure in the centre of the cell, surrounded by its own membrane with pores that let materials pass in and out.
The cytoplasm
The cytoplasm is the jelly-like substance that fills the cell. It is mostly water with dissolved substances such as salts and sugars, and it is where most of the cell's chemical reactions happen. Keeping it at the right conditions — the right temperature, the right water balance — matters enormously, because enzymes in the cytoplasm only work well in a narrow range of conditions.
The cell membrane
The cell membrane is the thin boundary that separates the cell from its surroundings. It holds the cell together, but it is not just a wall: it controls which substances can enter and leave. Dissolved nutrients, oxygen, and waste pass through it, some by simple diffusion and some through special channels or using energy. This selective control is one of the most important jobs any structure performs — the wrong molecules getting in, or the right molecules leaking out, could kill the cell.
Mitochondria
Mitochondria release energy from glucose during respiration, and that energy powers everything the cell does — from building proteins to dividing. Cells that need a lot of energy have more mitochondria: muscle cells are packed with them, for example, while a quiet cell might have very few. They have an inner folded membrane that gives a large surface area for the reactions that release energy.
Ribosomes
Ribosomes are the sites of protein synthesis — the tiny structures where amino acids are joined together to make proteins, following the instructions carried from the nucleus. They are far smaller than mitochondria and can be free in the cytoplasm or attached to other structures. Because proteins do most of the cell's work — enzymes are proteins, and so are many hormones — ribosomes are essential in every living cell.
Quick check: matching structures to functions
A common exam-style question asks you to match each structure to its job. Here is a mini drill to test yourself before continuing:
- Which structure releases energy from glucose? Mitochondria.
- Which structure contains DNA and controls the cell? The nucleus.
- Where do most chemical reactions happen? In the cytoplasm.
- What controls the movement of substances in and out of the cell? The cell membrane.
- Where are proteins made? On ribosomes.
If you can answer these without looking back, you have the core vocabulary sorted.
From cells to organisms: the hierarchy
One cell on its own can be a whole organism — a bacterium, for instance, is a single cell that carries out every process of life alone. But in multicellular organisms such as humans and oak trees, cells specialise and cooperate. The organisation follows a clear hierarchy:
Cells — the basic units.
Tissues — groups of similar cells working together. Muscle tissue, for example, is made of muscle cells that contract in coordination.
Organs — groups of different tissues working together to perform a function. The stomach is an organ: muscular tissue churns food, gland tissue secretes acid and enzymes, and other tissues carry blood and nerve signals.
Organ systems — groups of organs working together. The digestive system links the mouth, stomach, intestines, and accessory organs into one coordinated process.
Organisms — the whole living thing.
Notice how the same principle repeats at every level: structures with different jobs combining into something more capable than any part alone. The cell is simply where the story starts.
A worked example: reading a magnification question
Exam questions on cells often test whether you can work with scale. Try this one:
A drawing of a cell is 40 mm wide. The real cell is 20 micrometres wide. What is the magnification of the drawing?
Use the formula:
magnification = size of image divided by size of real object
First convert to the same units. 40 mm is 40,000 micrometres. Then:
magnification = 40,000 divided by 20 = 2,000
So the magnification is x2,000. The most common mistake in these questions is forgetting the unit conversion — always check that image and object are in the same units before dividing.
Key takeaways
- The cell is the smallest unit of life, and all living things are made of cells that arise from pre-existing cells.
- The nucleus holds DNA and controls the cell; the cytoplasm is where most reactions occur; the cell membrane controls what enters and leaves.
- Mitochondria release energy from glucose, and ribosomes make proteins — both are found in essentially all cells.
- In multicellular organisms, cells form tissues, tissues form organs, organs form organ systems, and systems form the organism.
- For magnification questions, use magnification = image size divided by real size, and always convert to the same units first.