protein but they do not have a membrane around them. Eukaryotic cells contain 80S ribosomes, which are larger than those found in prokaryotes. As in prokaryotic cells, the plasma membrane controls the movement of materials into and out of the cell, and the gel-like cytoplasm, which fills much of the volume of the cell, provides a medium for many metabolic reactions. Plant cells have three additional structures. All plant cells have an outer cellulose cell wall and most have a large central vacuole. Some plant cells, such as palisade mesophyll cells (Figures 1.14 and 1.15), contain chloroplasts. The chloroplasts are found in cells exposed to the light, as they are the sites of photosynthesis. Chloroplasts have a double membrane and are about the same size as bacteria. Both chloroplasts and mitochondria have their own DNA and ribosomes and are able to reproduce independently of the cell. The large central vacuole contains water and salts. The membrane that surrounds it is under pressure from within and exerts a force on the cytoplasm, which in turn exerts a force on the cell wall, making the cell turgid and firm. The outer cell wall is composed of cellulose and other carbohydrates such as lignin and pectin, giving plant cells further support and a more rigid structure than animal cells. The cell walls and turgidity of plant cells give strength and support to tissues like leaves, holding them in the optimum position to catch the energy from sunlight for photosynthesis. Although they are both eukaryotic cells, there are several key differences between animal and plant cells. These are summarised in Table 1.3. Animal cells
Plant cells
cell wall absent
cell wall present
small vacuoles sometimes present
large central vacuole present in mature cells
no chloroplasts
chloroplasts often present
cholesterol in plasma membrane
no cholesterol in plasma membrane
DFOUSJPMFT QSFTFOU TFF QBHF
centrioles absent
stores glycogen
stores starch
Table 1.3 Differences between animal and plant cells.
Differences between prokaryotic and eukaryotic cells Comparisons of images of prokaryotic and eukaryotic cells show numerous differences between them. These are summarised in Table 1.4. Note, for example, the difference in size of ribosomes between prokaryotic and eukaryotic cells.
1 CELL BIOLOGY
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