Life Made Simple

What does it mean to be alive? It seems like it would be a pretty obvious answer. If something can move, breathe and eat, then it must be alive. What about plants and sea anemones? They do not necessarily move. Some animals, like sponges, filter feed instead of chewing up food. Then there are plants, which do not eat other animals. How can we decide what is living and what is not?

Life can be defined by several characteristics that living organisms display, including cellular organization, reproduction, metabolism, homeostasis, responsiveness, growth, and adaptation.

All living things are composed of at least one or more cells. Unicellular organisms are made up of one single cell, the smallest units capable of life, such as a bacterium. Multi-cellular organisms consist of many cells, which have specialized jobs within the organism. For example, skin cells have a different job than muscle cells.

All organisms reproduce in order to continue the species. In sexual reproduction, the genetic information is combined from the mother and father. A new organism will have some characteristics from both parents. In asexual reproduction, a new organism is an exact copy of the first, because the parent organism split an existing cell into two offspring cells.

All organisms use energy. Sum of the chemical energy used is called metabolism. Organisms need energy to grow, develop, repair damage, and reproduce. Autotrophs, like plants, use energy from the sun to make their own “food.” Heterotrophs, like animals, must ingest food to obtain energy. All organisms have internal conditions that must be maintained in order for the organism to remain alive. This is called homeostasis. Conditions like body temperature, blood volume, pH balance, and water balance must be kept stable. Homeostasis is related to energy use in that a certain level of energy must also be maintained.

All living things respond to their environment. A response to stimuli is usually a response of an organism to some external condition or event. Stimuli include light, temperature, odor, sound, heat, and water. An example is a plant’s leaves and stems growing toward the light. All organisms grow and develop. Cells divide to form new, identical cells. Cells mutate into other forms of cells, making a more complex organism. Organisms growing, changing and becoming more complex is called development. Unicellular organisms grow as well, but they only become slightly larger.

All organisms adapt to their environment through evolution. Adaptation is the process that enables organisms to become better suited to their environment. Life requires a complex definition for it to make sense. The characteristics described above detail all the current conditions it takes for something to be considered alive.