Hold up your hand and look at the skin. It seems smooth and continuous, but under a microscope it dissolves into a patchwork of tiny living units, packed together like bricks in a wall. Those units are cells, and they are the reason you are alive. Every leaf, every insect, every human being is built from them, and nothing smaller is considered truly alive. Understanding cells is the starting point for understanding all of biology. This guide explains what a cell is.
What it is
A cell is the smallest unit of life, the basic building block from which every living organism is made. It is the smallest thing that can carry out the processes we associate with being alive, such as taking in energy, growing, responding to its surroundings and, usually, reproducing.
Some living things are made of just a single cell. A bacterium, for instance, is one cell doing everything it needs to survive. Other organisms are vastly more complex: an adult human is built from tens of trillions of cells, organised into tissues and organs. Yet whether an organism has one cell or trillions, the cell remains the fundamental unit. This idea, that all life is made of cells, is one of the cornerstones of modern biology and is known as cell theory.
Cells are also extremely small. A typical human cell is around a hundredth of a millimetre across, far too small to see with the naked eye, which is why they remained hidden until microscopes were invented in the 1600s.
What is inside a cell
A cell is not just a blob of jelly. It is a busy, organised structure with specialised parts, each doing a particular job. The technical name for these internal parts is organelles, which loosely means "little organs". While different cells contain different combinations, several features are common to nearly all of them.
- Cell membrane. A thin, flexible boundary that holds the cell together and controls what goes in and out. It is semi-permeable, letting some substances cross while blocking others, which is what makes processes like osmosis possible.
- Cytoplasm. A jelly-like fluid that fills the cell. Most of the cell's chemical activity happens here, suspended in this watery interior.
- Genetic material. Every cell carries a set of instructions written in a molecule called DNA. This acts as a recipe book, telling the cell how to build the proteins it needs and how to make copies of itself.
Beyond these basics, cells contain tiny machines for releasing energy, building proteins and breaking down waste. The whole thing runs on a constant flow of chemistry, a web of countless small chemical reactions happening every second.
Two great families of cells
Not all cells are built to the same plan. Biologists divide them into two broad types, based on one key difference: whether or not the genetic material is enclosed in a special compartment called a nucleus.
Prokaryotic cells are the simpler, older type. They have no nucleus, so their DNA floats freely in the cytoplasm. They also lack most other membrane-bound organelles. Bacteria are the classic example. These cells are typically very small and are thought to resemble the earliest forms of life on Earth.
Eukaryotic cells are larger and more complex. Their DNA is safely packaged inside a nucleus, and they contain a range of specialised organelles. All animals, plants, fungi and many microbes are made of eukaryotic cells. The word "eukaryote" comes from Greek words meaning "true kernel", a reference to that organised nucleus.
This single distinction, nucleus or no nucleus, is one of the most important dividing lines in all of biology.
The nucleus and the power plants
Within a eukaryotic cell, two organelles deserve special mention because they explain so much about how a cell works.
The nucleus is often called the control centre of the cell. It holds the DNA and acts as the headquarters, issuing the instructions that direct the cell's activities. When a cell divides to make new cells, it is the nucleus and its DNA that must be carefully copied first, so that each new cell gets a complete set of instructions.
The mitochondria (a single one is a mitochondrion) are often described as the powerhouses of the cell. Their job is to release energy from food in a usable form, a bit like a tiny power station keeping the lights on. Cells that need lots of energy, such as muscle cells, are packed with mitochondria. This energy ultimately traces back to the wider food chain, through which energy passes from the Sun to plants and on to the animals that eat them.
Plant cells versus animal cells
Plant and animal cells are both eukaryotic and share many features, but plants have some important extras suited to their way of life. The differences are easiest to see side by side.
| Feature | Animal cell | Plant cell |
|---|---|---|
| Cell membrane | Yes | Yes |
| Nucleus | Yes | Yes |
| Mitochondria | Yes | Yes |
| Cell wall | No | Yes |
| Chloroplasts | No | Yes |
| Large central vacuole | No | Yes |
The cell wall is a tough outer layer outside the membrane that gives plant cells their rigid, boxy shape and provides support, which is part of why plants can stand upright. Chloroplasts are the green structures that capture sunlight, allowing plants to make their own food through photosynthesis. The large vacuole is a fluid-filled sac that stores water and helps keep the cell firm. Animal cells lack all three, which is why they tend to be rounder and more flexible.
Why cells matter
Cells are not just an academic curiosity; they are the foundation of health, growth and disease. Your body constantly makes new cells to grow, heal wounds and replace worn-out tissue. When this process goes wrong and cells start dividing uncontrollably, the result can be cancer. Many medicines work by acting on specific cells or the chemistry inside them, and understanding cells underpins almost every advance in medicine, from vaccines to organ transplants.
Cells are also where life connects to chemistry. The energy you draw from food, the way water balances inside your tissues, and the signals that travel through your nerves all come down to processes happening at the level of individual cells.
The bottom line
A cell is the smallest unit of life, the building block from which every living organism is made, whether that organism is a single bacterium or a human made of trillions of cells. Each cell is wrapped in a membrane, filled with jelly-like cytoplasm and carries DNA instructions for staying alive. Simple cells without a nucleus are called prokaryotes, while complex cells with a nucleus and specialised organelles are eukaryotes, the type that makes up plants and animals. Plant and animal cells share much in common but differ in features such as the cell wall and chloroplasts. Tiny, hidden and almost unimaginably numerous, cells are the quiet machinery on which all life depends.