Cells are the basis of all living things. Some organisms are single-celled whereas others, like humans and fungus, are multi-celled. Single-celled organisms are self-sufficient, meaning that all necessary processes are carried out by the single cell. In multicellular organisms, cells specialize, meaning that certain cells perform specific functions, such as liver or brain cells.
All living organisms take in energy from their environment in the form of proteins, carbohydrates and fats. They metabolize these raw materials and use the resulting energy for movement and growth. The most common form of energy is ATP, or adenosine triphosphate. In humans, ATP is used for synthesizing RNA and DNA, nerve impulses and muscle contractions. All foods produce ATP, and it can be used by all cells, meaning there is no need for any kind of conversion to different types of energy within the body. As a result of many chemical reactions in the body, toxins are produced. These toxins must be excreted so as to not poison the cells.
Organisms grow by using energy to increase the size of the cells, which then divide to create new cells. As the cells divide, an organism grows larger. There are two kinds of cell division, binary fusion in simpler, prokaryotic cells, and mitosis, in the more complex eukaryotic cells. Human cells undergo mitosis. Although reproduction is not necessary for a single organism to survive, it is necessary for the survival of the species. Living things reproduce asexually, meaning without parents, where an organism clones itself, or by sexual reproduction, as in humans, with two parents.
Homeostasis is the body's ability to maintain an internal balance despite what is happening outside the body. Homeostasis describes the parameters, physical and chemical, that must be met to keep the body running optimally. Without properly functioning mechanisms of homeostasis, an organism would die. An example of homeostasis is seen by the body's effort to try to keep a constant temperature of 98.6 degrees, despite the outside temperature. If this temperature increases or decreases, there is a problem with the body's homeostasis. The body combats these changes by such actions as sweating or shivering.
All living things respond to stimuli in their environment. This can been seen in plants as leaves turn toward the sun to maximize the light to photosynthesize better. Humans respond to stimuli with the help of the nervous system as seen when touching a hot pot. The response is movement away from the pot to minimize damage. Living organisms also adapt to their environment through natural selection, which is the mechanism of evolution; as the environment changes, living things change in order to continue surviving in the environment.