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Cells

Cells. All livings things are made of cells. Cells are very small, so large organisms contain mil­lions of cells. Some organisms are unicellular. These are the bacteria, protozoa and some algae . Microscopes are used to study cells. To see cells clearly, you need to use a microscope

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Cells

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  1. Cells All livings things are made of cells.

  2. Cells are very small, so large organisms contain mil­lions of cells. • Some organisms are unicellular. • These are the bacteria, protozoa and some algae

  3. Microscopes are used to study cells • To see cells clearly, you need to use a microscope • The kind of microscope used in a school labo­ratory is called a light microscope because it shines light through the piece of animal or plant you are looking at

  4. Light microscope • . It uses glass lenses to magnify and focus the image. • A very good light microscope can magnify about 1500 times, and then show all the structures shown next

  5. Electron microscope • To see even smaller things inside a cell, an electron microscope is used. • This uses a beam of electrons instead of light, and can magnify up to 500 000 times. This means that a lot more detail can be seen inside a cell, as shown in Next

  6. All cells have a cell membrane • Whatever sort of animal or plant they come from, all cells have a cell membrane around the outside • Inside the cell membrane is a jelly-like substance called cytoplasm,

  7. Cell structure • In the cytoplasm is found many small structures called organelles. • The most obvious of these organelles is usually the nucleus. • The whole content of the cell is called protoplasm.

  8. Plant cells • All plant cells are surrounded by a cell wall mademainly of cellulose • Paper, which is made from cell walls, is also made of cellulose • Animal cells never have cell walls made of cellulose

  9. Cell Wall • Cellulose forms fibres which criss-cross over one another to form a very strong covering to the cell • This helps to protect and support the cell. • Because of the spaces between fibres, even very large molecules are able to go through the cellulose cell wall. • It is therefore said to be fully permeable

  10. Cell Membranes • All cells have a membrane surrounding the cell. • It is called the cell surface membrane or plasma membrane. • In a plant cell, it is very difficult to see, because it is right against the cell wall.

  11. Partially Permeable • The cell surface membrane is a very thin layer of protein and fat. • It is very important to the cell because it controls what goes in and out of it. • It is said to be partially permeable, • it will let some substances through but not others.

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