Life Processes

It is usually easy to recognize life, but it is often much harder to define it. All living things are made of cells. Some organisms are unicellular and consist of only a single cell. Other organisms are multicellular and are composed of many cells. To determine whether an object is living or nonliving, biologists have agreed on several characteristics that define living things. They are referred to as life processes or activities. These life processes include such activities as growth, metabolism, movement, and reproduction. Living things also react, or respond, to their environment. The ability to respond to an environment stimulus is called irritability (no, this word does not mean cranky in this case).

Living things grow. Growth is an increase in size. Most organisms also go through a series of changes called development. The beginning form of an organism may not resemble its adult form. For example, a tadpole does not look the same as an adult frog. Growth in multicellular organisms is due to an increase in the number of cells. Humans begin life as a single cell. However, when they are adults, they consist of even more cells.

Metabolism refers to the chemical activities that are needed for life. Ingestion, digestion, respiration, and excretion are the processes of metabolism. Ingestion is taking in food. The process of breaking down food into simpler substances is called digestion. The breaking down of food particles to release energy is called respiration. For biologists, respiration has two meanings. Respiration occurs at the cellular level when food is broken down to release energy. Respiration is also the process of taking in oxygen and giving off carbon dioxide as a waste gas. In humans, breathing and respiration often refer to the same process. Excretion is getting rid of wastes. Excretion usually refers to the removal of solid and liquid metabolic wastes that are produced during respiration.

Catabolism and anabolism are two processes in living things that are involved in metabolism. Catabolism is the break down of complex substances into simpler substances. Anabolism is the formation of complex substances from simpler substances.

Reproduction is the process of producing new organisms of the same kind. Reproduction of living things can occur asexually, requiring only one parent, or sexually, requiring two parents. Organisms that consist of a single cell reproduce asexually by dividing. Organisms that reproduce sexually contain genetic material contributed from each parent. If a group of living things does not reproduce, extinction of that group occurs.

Living things react to changes in their environment. A response is a reaction to a change. Responding to a change in the environment may increase an organism’s ability to survive.

Organisms must be able to get and use energy in order to survive. Energy is needed to carry out all cellular processes. For example, organisms use energy from food to grow, develop, and reproduce.

Energy flows through individual animals, communities, and the environment. It is passed from one organism to another organism, usually in the form of food. For example, a goat eats the energy from the sun that is contained in the cells of a plant. Man then eats the goat and gets its energy. In this way, energy moves through living systems from one organism to another. The sun is the ultimate source of energy, for most of the organisms that live on Earth, although there are exceptions. Some bacteria for example, are able to use the energy trapped in chemical compounds rather than the energy from the sun as plants do.

Living things are highly ordered. A tree grows into a form typical of its species. All humans have the same general form, although there are differences in size. The chemical reactions that occur in living things do not occur randomly. The chemicals that make up living organisms are, in general, more complex than the chemicals found in nonliving things, such as rocks. All living things are complex. All are composed of small units of life called cells. Cells are able to carry out all the life processes that insure their survival.