Unicellular and Multicellular Organisms

Some organisms, called unicellular, are single cells. In these organisms, this unique cell performs all functions required for organism’s life processes. Among those, we may find bacteria and other monerans. All monerans are prokaryotes, that is, their cells lack a true nucleus and membrane-bound organelles such as chloroplasts and mitochondria. The genetic material of monerans is a single, continuous loop of nucleic acid in direct contact with the cytoplasm. The ribosomes of monerans are smaller than those in eukaryotes.

Like plant cells, moneran cells have cell wall. However, almost all moneran cell walls contain acids and sugars instead of the cellulose found in plant cell walls.

Other living things, called multicellular, are made up of more than one cell. Some multicellular organisms have only a few cells. Others, such as mammals, consist of trillions of cells. Most cells in more complex organisms are highly specialized, that is, each of many types of cells has its own special functions to perform.

Cells Vary in Shape, Size and Arrangement

Our own body contains over 100 different kinds of cells. Some of these cells are round. Others, such as the nerve cells, are shaped like long, tangled strings. Cells tend to be spherical. They take other shapes because of cell walls found in most plant cells and in many one-celled organisms or because of attachments to and pressure from other, neighboring cells or surfaces.

Cells vary greatly in size too (Fig. 1.1). Most of the cells that make up a plant or animal body are within a size range of between 10 and 30 micrometers in diameter. The yolk of a single ostrich egg is one of the largest cells known. It has a circumference of nearly 8 centimeters. In contrast, the bacteria that naturally live deep inside your body are microscopic. And the most metabolically active cells are usually small too. The ostrich egg cell is 800,000 times bigger than the bacterial cells.

The living arrangements of cells also vary.

Fig. 1.1. The cells of a plant (left), a rat (center),
and a human being (right) appear to be quite different