From the moment a human zygote is formed, the cells are busy dividing and specializing into the many different types of cells they will become. These specialized cells will perform numerous functions in the human body, from digestion and excretion to message transmission and oxygen distribution. The structure of each type of human cell depends on what function it will perform in the body. A direct relationship exists between the size and shape of every cell and the tasks it needs to accomplish.
Red Blood Cells
Red blood cells carry the protein hemoglobin, which attaches to oxygen and delivers it to all of the body's tissues. Red blood cells are flat and round, and very small, allowing them to easily turn corners with the flow of the blood and fit through the capillaries, the tiniest blood vessels, where oxygen is transferred to body cells.
Nerve cells, or neurons, carry electrical messages to and from the brain and spinal cord, helping the body respond to various stimuli, regulate mechanisms, and absorb and store information. To most efficiently transmit these electrical messages, neurons have a long, thin structure, allowing for very quick and accurate communication and responses. Length is beneficial to the structure of a neuron because electrical messages within a neuron travel more quickly than the chemical messages between neurons. Thus, fewer, longer neurons means faster transmission of signals.
Skeletal muscle cells are arranged in bundles of linear fibers. A single muscle cell is elongated in shape, containing within it many myofibrils, or thin strands made up of the proteins actin and myosin, that perform muscle contraction. The elongated shape allows the contraction proteins to line up in an overlapping pattern that makes muscle flexing possible. Nuclei and other organelles that are normally within a cell lay at the perimeter of muscle cells, making space for the ordered patterns of the proteins.
The sperm cells in males are the only human cell with flagella, or whiplike cell extensions. This is because of their need to "swim" long distances to reach an egg for fertilization. Also due to their need to travel, the body of a sperm cell is very light, carrying not much more than the chromosomes containing the DNA for a potential zygote. The other organelles found in most other body cells are nonexistent in sperm cells, and are given to a zygote by its mother's egg.