The Cell: Building Block of Life

As more and more bricks are added, a wall grows bigger. In the same way, as the number of cells in a body grows, the size of that living thing grows. This is why cells are called the building blocks of life, and why they are considered the basic unit of every living thing.

For most of human history nobody had any idea that living things were made of anything smaller. People studied plants and animals carefully for thousands of years and saw nothing, because a cell is far too small for an eye to catch. Then microscopes were invented, and an English scientist called Robert Hooke put a thin slice of cork under one in 1665. He saw rows of tiny empty boxes, packed side by side like a honeycomb. They reminded him of the small bare rooms that monks lived in, which were called cells, so that is the word he wrote down. The name has stayed ever since.

There is a lesson in that date worth holding on to. Careful looking was not enough on its own. What changed everything was a tool that let people observe something their eyes could not reach. This is exactly the point made in Unit 1 about scientific study: observation is the first step, but observation is limited by what we can actually see, and a great deal of science has been the story of building tools that push that limit further out.

Here is a question the textbook does not ask, and it is the best question in the whole topic. If a body is built from cells, why is it built from billions of tiny ones instead of a few large ones? An elephant could have been made of a hundred huge cells. It is not. Every elephant, every rice plant, and every person is made of cells that are all roughly the same tiny size.

The answer comes from the cell membrane. Everything a cell needs, food, water, oxygen, has to come in through its outer surface, and everything it throws out has to leave the same way. So the surface is the doorway, and the inside is the part being served. Now watch what happens when a cell gets bigger. The inside grows much faster than the surface does.

Take a cube with sides of 1. Its surface is 6 and its inside is 1, so there are 6 units of doorway for every unit of inside. Double the side to 2 and the surface becomes 24, but the inside becomes 8, so now there are only 3. Make the side 3 and it drops to 2. The bigger the cube gets, the less doorway each part of the inside has, and this never stops. A cell that grew large enough would have a middle that food could never reach in time, and it would starve while surrounded by plenty.

So a living thing cannot grow by making its cells bigger. It has to grow by making more of them, each one staying small enough to feed itself. That single fact is why an elephant and a mouse have cells of almost exactly the same size, and why the elephant simply has far more of them.

Living things whose body is made of only one cell are called unicellular organisms, examples include Amoeba and bacteria. Living things whose body is made of many cells are called multicellular organisms, examples include plants, chickens, and humans. In a multicellular body, groups of cells that do the same job join together to form a tissue, and groups of tissues join together to form an organ. A plant's body is built from plant cells, and an animal's body is built from animal cells.

An amoeba is worth thinking about for a moment, because one cell has to do absolutely everything for it. That single cell must take in food, get rid of waste, sense its surroundings, move, and make a new amoeba. In your body those jobs are shared out. A muscle cell only has to pull, a red blood cell only has to carry oxygen, a root hair cell only has to soak up water. Each one can be very good at one job precisely because it does not have to do the others, and that division of work is the real advantage of being multicellular.

A cell has an outer boundary called the cell membrane, found in both plant and animal cells. It is thin, stretchy, and made mostly of fat and protein. It controls which substances can enter or leave the cell, letting in what the cell needs and pushing out what it does not need, so it is often called semi-permeable. Only plant cells have an extra layer outside the cell membrane called the cell wall, which is thick, rigid, and made of a material called cellulose. The cell wall protects the cell and gives it a fixed shape.

The word semi-permeable is worth unpacking, because it is the whole point of the membrane. A wall keeps everything out. An open doorway lets everything through. The membrane is neither. It is more like a school gate with someone standing at it: students go in, strangers do not, and the rubbish goes out. If the membrane were a solid wall the cell would starve, and if it let everything through the cell would not be a separate thing at all. Being choosy is what makes a cell a cell.

Inside the cell membrane, a jelly-like substance fills most of the cell, this is called cytoplasm. It is made of water mixed with salts, minerals, vitamins, and proteins. Many tiny working parts, called organelles, float inside the cytoplasm. At the centre of most cells is a rounded structure called the nucleus, which controls all the activities of the cell and holds the cell's genetic information.

An animal can walk away from trouble, so it needs a soft, flexible body. A plant cannot walk anywhere, so it has to be able to stand up and stay standing. That is what the cell wall is for, and it does that job together with the large vacuole, working as a pair.

When a plant has plenty of water, the vacuole in each cell fills right up and pushes outwards against the cell wall, which pushes back. Millions of cells all pressing outwards at once make a stem stiff, in the same way that a football is only firm because the air inside is pushing on the leather. That is why a soft green plant with no wood in it can still stand upright.

Now think about a plant that has not been watered for two days. Its leaves droop and the whole plant goes floppy. That is not the plant getting tired. The vacuoles have lost water, so they have stopped pushing, and without that push the walls have nothing holding them firm. Water it and within a couple of hours it stands up again. You have almost certainly seen this happen. What you were watching was the vacuole in millions of cells refilling, and now you can say exactly why it worked.