IGCSE NOTES : Biology - Cell structure and organisation

Cytoplasm

Under the ordinary microscope (light microscope), cytoplasm looks like a thick liquid with particles in it. In plant cells it may be seen to be fl owing about. The particles may be food reserves such as oil droplets or granules of starch. Other particles are structures known as organelles, which have particular functions in the cytoplasm. In the cytoplasm, a great many chemical reactions are taking place which keep the cell alive by providing energy and making substances that the cell needs. The liquid part of cytoplasm is about 90% water
with molecules of salts and sugars dissolved in it. Suspended in this solution there are larger molecules of fats (lipids) and proteins. Lipids and proteins may be used to build up the cell structures, such as the membranes. Some of the proteins are enzymes. Enzymes control the rate and type of chemical reactions which take place in the cells. Some enzymes are attached to the membrane systems of the cell, whereas others fl oat freely in the liquid part of the cytoplasm.

Cell membrane

This is a thin layer of cytoplasm around the outside of the cell. It stops the cell contents from escaping and also controls the substances which are allowed to enter and leave the cell. In general, oxygen, food and water are allowed to enter; waste products are allowed to leave and harmful substances are kept out. In this way the cell membrane maintains the structure and chemical reactions of the cytoplasm. Nucleus (plural: nuclei) Most cells contain one nucleus, which is usually seen as a rounded structure enclosed in a membrane and embedded in the cytoplasm. In drawings of cells, the nucleus may be shown darker than the
cytoplasm because, in prepared sections, it takes up certain stains more strongly than the cytoplasm. The function of the nucleus is to control the type and quantity of enzymes produced by the cytoplasm. In this way it regulates the chemical changes which take place in the cell. As a result, the nucleus determines what the cell will be, for example, a blood cell, a liver cell, a muscle cell or a nerve cell. The nucleus also controls cell division. A cell without a nucleus cannot reproduce. Inside the nucleus are thread-like structures called chromosomes, which can be seen most easily at the time when the cell is dividing.

Plant cells

A few generalised animal cells are represented by a drawing of two palisade cells from a plant leaf. Outside the cell membrane they all have a cell wall which contains cellulose and other compounds. It is non-living and allows water and dissolved substances to pass through. The cell wall is not selective like the cell membrane. (Note that plant cells do have a cell membrane but it is not easy to see or draw because it is pressed against the inside of the cell wall. Under the microscope, plant cells are quite distinct and easy to see because of their cell walls. Each plant cell has its own cell wall but the boundary between two cells side by side does not usually show up clearly. Cells next to each other therefore appear to be sharing the same cell wall.

2 Most mature plant cells have a large, fl uid-fi lled space called a vacuole. The vacuole contains cell sap, a watery solution of sugars, salts and sometimes pigments. This large, central vacuole pushes the cytoplasm aside so that it forms just a thin lining inside the cell wall. It is the outward pressure of the vacuole on the cytoplasm and cell wall which makes plant cells and their tissues firm. Animal cells may sometimes have small vacuoles in their cytoplasm but they are usually produced to do a particular job and are not permanent.
3 In the cytoplasm of plant cells are many organelles called plastids. These are not present in animal cells. If they contain the green substance chlorophyll, the organelles are called chloroplasts. Colourless plastids usually contain starch, which is used as a food store.