Differences between animal and plant cells
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.
Centrioles
Under the light microscope the centriole appears as a small structure close to the nucleus.
Cell walls and plasmodesmata
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 plasmodesmata (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.
Vacuoles
Although animal cells may possess small vacuoles such as phagocytic vacuoles (page 87), 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
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.
Points to note
- You can think of a plant cell as being very similar to an animal cell, but with extra structures.
- Plant cells are often larger than animal cells, although cell size varies enormously.
- Do not confuse the cell wall with the cell surface membrane. Cell walls are relatively thick and physically strong, whereas cell surface membranes are very thin. Cell walls are freely permeable, whereas cell surface membranes are partially permeable. All cells have a cell surface membrane.
- Vacuoles are not confined to plant cells; animal cells may have small vacuoles, such as phagocytic vacuoles, although these are not usually permanent structures.