In unicellular organisms, a single cell has to carry out all the functions of life. The living things that you can see without microscopes, such as plants and animals, are made up of many cells of different types. In these organisms, each type of cell is specialized to carry out a particular job. This cell is a nerve cell that helps carry messages to and from the brain.
Multicellular organisms
Living things such as humans, fish and the tree and koala in Figure 2.3.1 are made of millions and millions of cells.
They are known as multicellular organisms-multi means many.
In multicellular organisms, all the cells work together but they do not do the same thing. Cells are specialized. This means that they have a special job to do in the body, and they have a structure that makes them better able to do their job.
Animals are complex multicellular organisms with a large number of different cell types. The human body contains about 50 trillion cells and more than 200 different types of cells. A few of these 200 types are described below.
Immune cells
Immune cells are incredibly varied and they are involved in protecting your body. There are many different types of immune cells, each with their own specific function.
Muscle cells
The three types of muscles in your body are voluntary muscles (also called skeletal muscles), involuntary muscles (also called smooth muscles) and cardiac muscles.
Surrounding the bones of your skeleton are muscles that you use to move around. You can choose to make these muscles move (or not) and so they are referred to as voluntary muscles. For example, your biceps and triceps are voluntary muscles that help control your arm. You can see a magnified view of voluntary muscle in Figure 2.3.2.
You have other muscles in your body that work without you having to think about it. They are the muscles involved in breathing and those that keep food moving though your gut. These muscles are known as involuntary muscles. Your diaphragm controls your breathing and is an involuntary muscle, as are the muscles in the wall of your digestive tract or gut. An image of involuntary muscle is shown in Figure 2.3.3.
All muscles (voluntary or involuntary) contain a large number of mitochondria. This is because muscles require a lot of energy to keep working and it is the mitochondria that provide the energy.
Cardiac muscle (Figure 2.3.4) is the type of muscle in the heart. Cardiac muscle is involuntary muscle but it has a striped appearance like voluntary muscles. Unlike other muscles that are striped, cardiac muscle does not get tired. It has very large numbers of mitochondria to provide a continuous supply of energy. These characteristics allow the heart to beat continuously.
Nerve cells
Nerve cells make up your brain. They also carry information from your brain to other parts of your body such as your muscles, and from your muscles back to your brain. As Figure 2.3.5 shows, some nerve cells have very long fibres extending from the cell. These fibers are called axons-they allow the cell to carry messages over long distances. The longest axon of a human nerve cell reaches from the base of the spine to the toes, and can be over a meter long.
Blood cells
Figure 2.3.6 shows some blood cells. Red blood cells carry oxygen from your lungs around your body to the cells, where it is used to release the energy you need. They also carry some of the waste carbon dioxide from the cells back to your lungs so that you can get rid of it from your body. The white blood cells have a very different job. They are part of the immune system and help the body to fight infection.
Fat cells
You have two different types of fat cells in your body. Brown fat cells are used to produce heat for the body, especially when it gets cold and you are shivering. The white fat cells in Figure 2.3.7 are used as a store of energy. They also form an insulating layer under the skin that helps to keep your body at a constant temperature.
