Cells are the building blocks of life. Living things begin with one cell, which splits to make two cells. The two cells split and make four cells. This splitting process, known as "differentiation," continues until millions of cells come together and form organs, like eyes and lungs, and, ultimately, a whole, living organism. Cells of an organism have different jobs depending on their location and type, working together to keep the organism alive. There is only one exception to the presence of cells in living things. Viruses are considered living, but they have no cells.
Respiration occurs in all living things. Every organism exchanges gases with its environment in order to breathe. Many animals, including humans, have lungs and breathe oxygen from the air. Fish use their gills for respiration, taking in oxygen from the water in which they live. Regardless of how they do it, all living things breathe.
All living things use energy. It is needed for everything they do. The total sum of chemical energy used is known as "metabolism". And, food is the source of energy. Some organisms, such as plants, make their own food from sunlight. They are called "autotrophs." Bacteria are "chemoautotrophs," making food from various chemical reactions. And "heterotrophs," such as humans, must ingest their food obtained from outside sources. Whichever way an organism eats, nutrition from a food source is necessary for survival.
Temperature, heartbeat, water content and oxygen levels in the blood are some of the many internal conditions that must be kept at certain levels for an organism to survive. These levels can fluctuate due to external conditions, by ingested chemicals or by injuries to the organism. Most of the time when this happens, an automatic, internal mechanism makes the necessary adjustments, stabilizing the organism's internal environment so it can survive. This process is known as "homeostasis."
All living organisms grow and change, following similar developmental paths. First, growth occurs through "differentiation," or successive cell splitting. It quickly becomes increasingly complex, as it ultimately forms a whole, living being with several organs and parts. Growth continues, and the organism becomes larger in size until if reaches adulthood. A plateau is reached at this point, and growth stops for the remainder of the organism's lifespan.
All living organisms reproduce. Some do so asexually, by splitting into two organisms and creating exact replicas. Others reproduce sexually, mating to combine genes and produce offspring with one half made up of characteristics inherited from the father and one half from the mother. By creating more life forms like itself, regardless of which method of reproduction it uses, a living organism helps the entire species to continue living.
All life forms move in one way or another. Some movements are obvious and easy to see, like running or walking, whereas others are more subtle, such as blinking eyes or a smiling face. Other movements are not visible. Such internal processes are, nevertheless, very important, like the transportation of blood from one part of the body to another or the contraction of muscles, as in the case of a beating heart.