Plant cells typically have a cell wall surrounding the protoplasm, which consists of all the living components of a cell. These living components are bounded by a membrane called the plasma membrane. All cellular components between the plasma membrane and a relatively large body called the nucleus are known as cytoplasm. Within the cytoplasm is a souplike fluid called cytosol, in which various bodies called organelles are dispersed. Organelles are persistent structures of various shapes and sizes with specialized functions in the cell; most, but not all, are bounded by membranes (Figs. 3.4 and 3.5).
Most plant and animal cells are so tiny they are invisible to the unaided eye. Cells of higher plants generally vary in length between 10 and 100 micrometers.1 Since there are roughly 25,000 micrometers to the inch, it would take about 500 average-sized cells to extend across 2.54 centimeters (1 inch) of space; 30 of them could easily be placed across the head of a pin. Some prokaryotic (bacterial) cells are less than one-half micrometer wide, while cells of the green alga, mermaid's wineglass (Acetabularia), are mostly between 2 and 5 centimeters long, and fiber cells of some nettles are about 20 centimeters long.
Why are cells so small, and why aren't they larger? Consider that as a cell increases in size, its volume grows much more than its surface area. The increase in surface area of a spherical cell, for example, is equal to the square of its increase in diameter, but its increase in volume is equal to the cube of its increase in diameter. This means that a cell whose diameter increases 10 times would increase in surface area 100 times (10 squared) but in volume 1,000 times (10 cubed). Since all substances enter or leave cells through their surfaces, which are the only contact areas with their surroundings, larger cells are at a disadvantage. Furthermore, the nucleus regulates all aspects of a cell's activities, and the greater the volume of the cell, the longer it can take for instructions from the nucleus to reach the surface. On the other hand, smaller cells have a clear advantage because they have relatively larger surface area to volume ratios, thereby enabling faster and more efficient communication between the nucleus and other parts of the cell.
1. See Appendix 6 for metric conversion tables.
ribosome ribosome vacuolar membrane (tonoplast)
nuclear pore nucleolus nucleus nuclear envelope cell wall cytoplasmic fluid dictyosome primary cell wall secondary cell wall mitochondrion intracellular space middle lamella cell wall of adjacent cell nuclear pore nucleolus nucleus nuclear envelope rough ER
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