protoplasm(the contents of a cell) is not uniform in structure. Although cells show great variation, they have certain structures in common.
All cells have a clearly defined shape or boundary. This is maintained by a flexible cell membrane which encloses the internal contents of the cell. Sometimes this membrane is called the plasma membrane.
The protoplasm inside most cells reveals definite structures, called
organelles(‘little organs’). Organelles have a particular job to do for the cell. They are either enclosed in or made up of membranes. Organelles and the cell membrane are continually changing, breaking down and reforming in a dynamic pattern of activity.
One organelle that can be seen with the light microscope is the
nucleus. Inside the nucleus is a darker staining area called the nucleolus; the small threads which show up clearly in some nuclei, by staining, are
Because they are so small, cells are measured in micrometres and nanometres.
● One micrometre (µm) is one millionth of a metre, or 10–6 m. ● One nanometre (nm) is one
thousand millionth of a metre, or 10–9 m.
Plant and animal cells range in size from 5 to over 100 µm wide (see Figure 2.8).
50 Heinemann Biology
the chromosomes. The contents of the cell between the nucleus and cell membrane is called the cytoplasm.
Cytoplasm looks like a clear fluid with particles in it. Some of these particles or granules are easily visible with the light microscope, but others cannot be distinguished clearly. Some of the granules visible in the cytoplasm may be food reserves or food vacuoles. The food they contain, such as starch or oils, may be made visible by selective staining techniques.
With the best light microscopes, small structures called mito-
chondria(singular = mitochondrion) can be seen, but not in detail. The smaller details of internal cell structures are distinguishable only with an electron microscope.
When cells were first examined under the light microscope, it became apparent that there are two basic and commonly seen cell types. One is surrounded by a cell membrane only, and the other has a much thicker cell wall around it. Cell walls are non-living structures which give shape and rigidity to the cell they surround. The cell membrane still surrounds the internal cell contents but is often not visible because it is pressed up against the cell wall.
The cell wall, found in plant cells, consists of a network of cellulose microfibrils (long strands of cellulose molecules) in a cement of pectin and other substances. Lignin (wood) may be one of these. The wall protects and supports the cell (see Figure 2.14, p. 53).
Many cells with cell walls may also contain large round or oval membrane-bound organelles called plastids which are easily seen in the cytoplasm. They are found only in cells taken from plants. Plastids often contain pigment. Green-coloured plastids or chloroplasts contain the pigment chlorophyll, giving plants their green colour. Chromoplasts produce the colours of flowers and fruits, and leucoplasts store nutrients such as starch.
Vacuoles are sacs surrounded by a single membrane. Vacuoles may be temporary structures, such as food vacuoles, which contain sub- stances brought into the cell from outside. Plant cells usually also con- tain a large central vacuole. It is filled with a fluid called the cell sap, consisting mainly of water and dissolved substances such as sugars, salts and sometimes coloured pigments. In some plant cells the vacuole may occupy 80–90% of the total cell volume.
cytoplasm chloroplast position of cell membrane nucleus (contains chromosomes) large vacuole
cell wall nuclear membrane cell membrane nucleus (contains chromosomes) nuclear membrane cytoplasm
Note: no cell wall no large vacuoles no chloroplasts
Plant cell
Animal cell
FIGURE 2.10
Generalised cells seen through a light microscope (1000×).
All cells have a clearly defined shape or boundary. This is maintained by a flexible cell membrane which encloses the internal contents of the cell. The cell wall, found in plant cells, consists of a network of cellulose microfibrils (long strands of cellulose molecules) in a cement of pectin and other substances.
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Naammee FFuunnccttiioonn
Nucleus contains the chromosomes; the information in the chromosomes is used to control the development and functioning of the whole cell; without a nucleus most cells will die
Cell membrane forms the boundary between the cytoplasm and the outside environment; controls the entry and exit of substances to and from the cell
Cytoplasm contains many organelles; is where most cell activities are carried out
Cell wall gives protection, support and shape; all plant cells have a cell wall
Chloroplasts contain the green pigment chlorophyll, and are the site of food manufacture (photosynthesis) in plants
Vacuoles store water and other substances; large and important in plant cells
The role of the nucleus was clearly demonstrated in a set of experiments carried out in the 1930s using Acetabularia, a large single-celled marine alga consisting of a branching foot, stalk and cap. The nucleus is located in the foot. Two closely related species with different shaped caps are A. mediterranea and A. crenulata. When a cap is removed from either species, a cap of the same shape grows back (Figure 2.11a).
In the next part of the experiment (Figure 2.11b), caps were removed from individuals of each species, then the stems were cut and transplanted onto roots of the other species. The first caps that regrew were intermediate in form between the two species. When these were removed, all following regenerated caps had the form of the root species. The nucleus was obviously controlling the functions of the cell. A. mediterranea cap stalk branching foot