What is a Cell?
All living things are made up of cells. Each of us has about 50 million million cells - an enormous number which is difficult to imagine. Each cell is a sort of bag made from a sort of skin called a membrane. The inside of a cell is watery and jelly-like. Cells are very small - you can't see them just using your eyes. You need to use a microscope, which makes them look many times bigger that they actually are.
If a cell is cut in half, it will not survive. So a cell can be considered as the smallest part of an organism that can survive on its own. Some organisms have only one cell, while more complicated organisms are made out of lots of cells. All cells have a membrane, which separates them from the outside world. The membrane protects the cell, and allows the cell to be selective about what is allowed in. The membrane is alive too, and can detect and respond to changes in the outside environment.
Are all cells the same?
No, they're not. Plant cells are different than animal cells. Plant cells in a root are different to those in the stem or in the leaf. Animal cells, including the cells in our bodies are all sorts of different shapes and sizes.
Cells are the units which all organisms are made from. Different cells do different things. Some organisms consist of only one cell, like amoeba, or bacteria. Other, more complex organisms, are made up of lots of different types of cells. For example, muscle cells are long and fat and work together in bundles to let us move about whereas skin cells are flat and fit together like a jigsaw, covering our bodies. In contrast, nerve cells are long and thin and can carry messages from one place to another.
Despite all the differences between types of cells, there are also similarities. For instance, all cells have membranes which allow some substances to pass through, but not others. All cells contain DNA (deoxyribonucleic acid) for storing information and RNA (ribonucleic acid) for building proteins. Proteins are used in all cells for forming the structure of the cell itself, and regulating the cells reactions.
Where do new cells come from?
New cells come from old cells. When the time is right, an animal cell or a plant cell divides into two, forming two new cells. These are called daughter cells. The two new cells are exactly the same as the original cell. This process is called cell division.
You may be thinking, "But that's not right - they should have half of what was in the original cell!" Although a cell may be to small to see, its not stupid - before it divides it makes an extra copy of everything in its nucleus. This means that the two daughter cells get a complete nucleus. This is important because the nucleus contains the 'recipe' which is used to tell the cell what to do, including telling it how to divide to make new cells. They do share the cytoplasm but they can make more and end up the same size as their parent cell.
What is a single-celled animal?
A single-celled animal is an animal cell that lives on its own, without other cells. It can move around. It can get its own food. It gets rid of waste. It divides to make more cells. It can tell something about what is happening around it. the single cells of a multicellular animal (one made up of many cells such as a tiger, an insect or a person) could not do this. They need to live surrounded by lots of other cells.
What is a single-celled plant?
A single celled plant, such as an algae, is a plant cell that lives on its own. It does not need to be part of larger plant structure. Algae always live in water.