Size & Complexity of Living Organisms

Living organisms greatly differ in size and complexity of their body. They range from minute unicellular bacteria to very big multicellular organisms such as blue whales and redwood trees. Primitive cell forms such as bacteria and blue-green algae are very simple in organization and function. The cells and organisms are very small and cannot be seen with the naked eye. A microscope is needed for observing these microorganisms. The multicellular organisms and their cells are very complex in organization and function. The body of higher plants and animals consists of billions of various types of specialized, structurally and functionally complex cells. Therefore, their body and its function is highly complicated.

Variations in the body size affect various other body measurements differently. This is because the volume of cells and so the volume of the entire organism increases much faster than the surface area. Entire single-celled organisms and most primitive multicellular organisms use their cell surfaces to acquire nutrients and dispose of wastes. But the amount of nutrients needed, and the quantity of wastes produced, is related to cell volume. Since the surface area to volume ratio of a cell decreases as its size increases, cells have an upper limit on how much volume they can sustain with a given surface area. Large organisms have less surface area relative to mass than do small organisms. This relationship affects the efficient exchange of material between the body and the environment. Allometric relationships describe the effect of body size on biological features. These relationships can reveal general patterns of how organisms function; for example, how much they sleep, their food requirements, and their brain size.


Allometric relationships also have practical applications, as in the proper determination of drug dosages for animals of differing body sizes. For multicellular organisms, increases in overall body size are mostly due to increases in cell number, not cell size. This is also because of surface area to volume ratio limitations on cells.

The evolution of complexity in multicellular organisms is driven by the specialization of cells. Multicellular complexity requires coordination among body cells. Internal communication mechanisms such as hormones and the nervous system help make this possible. Complexity also requires many body cells to give up reproduction in support of a relatively few cells that do reproduce.

Tags: Bio Technology, Bio Genetics , Life Forms, organisms

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