Contrast unicellular and multicellular organisms.

Unicellular organisms are made up of a single cell that performs all the functions necessary for the survival of the organism. These functions include metabolism, reproduction, and response to stimuli. Examples of unicellular organisms include bacteria, yeast, and protozoa.

On the other hand, multicellular organisms are made up of many cells that work together to perform the functions necessary for the organism's survival. In multicellular organisms, cells are often specialized to perform specific functions. For example, in humans, there are nerve cells for transmitting signals, muscle cells for movement, and red blood cells for carrying oxygen. This specialization allows multicellular organisms to perform more complex functions than unicellular organisms.

In terms of reproduction, unicellular organisms usually reproduce asexually, meaning they divide into two identical cells. This is a quick and efficient way to reproduce, but it does not allow for much genetic variation.

Multicellular organisms, on the other hand, usually reproduce sexually, meaning they produce offspring through the combination of genetic material from two parent organisms. This allows for a great deal of genetic variation, which can be beneficial for the survival of the species.

In terms of size, unicellular organisms are generally much smaller than multicellular organisms. This is because a single cell can only grow to a certain size before it becomes inefficient. Multicellular organisms, with their many cells, can grow much larger.

Finally, in terms of lifespan, unicellular organisms generally have shorter lifespans than multicellular organisms. This is because unicellular organisms often reproduce by dividing into two, which essentially creates a new organism. Multicellular organisms, on the other hand, can live for many years, as their cells can be replaced when they die.