Delphinidin

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 Remember that the resolution of a light microscope is 2 micrometers, making it useful for the study of eukaryotic cells. 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 onehalf 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? 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 takes 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.

Full-grown organisms have astronomical numbers of cells. For example, it has been calculated that a single mature leaf of a pear tree contains 50 million cells and that the total number of cells in the roots, stem, branches, leaves, and fruit of a full-grown pear tree exceeds 15 trillion. Can you imagine how many cells there are in a 3,000-year-old redwood tree of California, which may reach heights of 90
meters (300 feet) and measure up to 4.5 meters (15 feet) in diameter near the base?

Some cells are boxlike with six walls, but others assume a wide variety of shapes, depending on their location and function. The most abundant cells in the younger parts of plants and fruits may be more or less spherical, like bubbles, when they are first formed, but as they press against each other, they commonly end up with an average of 14 sides by the time they are mature.