In animals and plants each cell is surrounded by a very thin ceil surface membrane. This is also sometimes referred to as the plasma membrane.
Many of the cell contents are colourless and transparent so they need to be stained to be seen. Each cell has a nucleus, which is a relatively large structure that stains intensely and is therefore very conspicuous.
The deeply staining material in the nucleus is called chromatin and is a mass of loosely coiled threads. This material collects together to form visible separate chromosomes during nuclear division.
It contains DNA (deoxyribonucleic acid), a molecule which contains the instructions that control the activities of the cell.
Within the nucleus an even more deeply staining area is visible, the nucleolus, which is made of loops of DNA from several chromosomes. The number of nucleoli is variable, one to five being common in mammals.
The material between the nucleus and the cell surface membrane is known as cytoplasm. Cytoplasm is an aqueous (watery) material, varying from a fluid to a jelly-like consistency.
Many small structures can be seen within it. These have been likened to small organs and hence are known as organelles. An organelle can be defined as a functionally and structurally distinct part of a cell.
Organelles themselves are often surrounded by membranes so that their activities can be separated from the surrounding cytoplasm. This is described as compartmentalisation.
Having separate compartments is essential for a structure as complex as an animal or plant cell to work efficiently.
Since each type of organelle has its own function, the cell is said to show division of labour, a sharing of the work between different specialised organelles.
The most numerous organelles seen with the light microscope are usually mitochondria (singular: mitochondrion).
Mitochondria are only just visible, but films of living cells, taken with the aid of a light microscope, have shown that they can move about, change shape and divide. They are specialised to carry out aerobic respiration.
The use of special stains containing silver enabled the Golgi apparatus to be detected for the first time in 1898 by Camillo Golgi.
The Golgi apparatus is part of a complex internal sorting and distribution system within the cell. It is also sometimes called the Golgi body or Golgi complex.
The only structure commonly found in animal cells which is absent from plant cells is the centriole. Plant cells also differ from animal cells in possessing cell walls, large permanent vacuoles and chloroplasts.
Under the light microscope the centriole appears as a small structure close to the nucleus (Figure 1.4).
Cell walls a nd plasmodesmat a With a light microscope, individual plant cells are more easily seen than animal cells, because they are usually larger and, unlike animal cells, surrounded by a cell wall outside the cell surface membrane.
This is relatively rigid because it contains fibres of cellulose, a polysaccharide which strengthens the wall. The cell wall gives the cell a definite shape. It prevents the cell from bursting when water enters by osmosis, allowing large pressures to develop inside the cell.
Cell walls may also be reinforced with extra cellulose or with a hard material called lignin for extra strength. Cell walls are freely permeable, allowing free movement of molecules and ions through to the cell surface membrane.
Plant cells are linked to neighbouring cells by means of fine strands of cytoplasm called piasmodesmata (singular: plasmodesma), which pass through pore-like structures in their walls. Movement through the pores is thought to be controlled by the structure of the pores.
Although animal cells may possess small vacuoles such as phagocytic vacuoles, which are temporary structures, mature plant cells often possess a large, permanent, central vacuole.
The plant vacuole is surrounded by a membrane, the tonoplast, which controls exchange between the vacuole and the cytoplasm. The fluid in the vacuole is a solution of pigments, enzymes, sugars and other organic compounds (including some waste products), mineral salts, oxygen and carbon dioxide.
Vacuoles help to regulate the osmotic properties of cells (the flow of water inwards and outwards) as well as having a wide range of other functions.
For example, the pigments which colour the petals of certain flowers and parts of some vegetables, such as the red pigment of beetroots, may be located in vacuoles.
Chloroplasts are found in the green parts of the plant, mainly in the leaves. They are relatively large organelles and so are easily seen with a light microscope.
It is even possible to see tiny 'grains' or grana (singular: granum) inside the chloroplasts using a light microscope.
These are the parts of the chloroplast that contain chlorophyll, the green pigment which absorbs light during the process of photosynthesis, the main function of chloroplasts.