The Open Door Web Site
Part XI : Cells Index
A paramecium as seen under the microscope
Cells from the leaflet of a moss seen under a microscope
All forms of life, from the biggest trees to the microscopic organisms that swim in the smallest drop of water, are made up of cells.
Most organisms are made up of many types of different cells. Some organisms, called single-celled organisms, have all they need for life in one single cell. They can move, breathe, feed, grow, reproduce, excrete and are sensitive even though each individual exists as a single cell. An example of a single-celled organism is a paramecium.
Organisms which have more than one cell are called multi-cellular organisms. Each type of cell in the organism has a special function. In plants, for example, some cells are in charge of making food using photosynthesis, other cells have the job of absorbing water in the roots and still others are responsible for making the plant rigid so it will not fall over.
Animals, too, have many different types of cells. In humans, for example, there are red blood cells which carry oxygen, skin cells which protect the body, bone cells to make the skeleton and nerve cells to send messages around the body.
In multi-cellular organisms, each of the vital functions of life have different specialized cells to perform them. Nerve cells are not capable of carrying oxygen like red blood cells can. Skin cells cannot carry messages the way nerve cells do. All the different types of cells work together in tissue and organ systems to make up complex living bodies.
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