Characteristics of Life

Objective: Explain why the cell is the basic unit of life and compare unicellular and multicellular organization.

Cell theory holds that organisms consist of one or more cells, that cells are the smallest units able to carry out all processes of life, and that new cells arise from existing cells. A unicellular organism performs all essential processes within one cell. In multicellular organisms, specialized cells cooperate in tissues and larger systems. Cell structures and functions vary across organisms, so a drawing of one cell is a model rather than a universal template.

Objective: Distinguish energy use, matter exchange, and internal regulation.

Organisms obtain and transform energy to support cellular work and exchange matter with their environment. Plants capture light energy and build sugars, while many other organisms obtain chemical energy from food; these processes do not mean matter or energy is created from nothing. Homeostasis describes regulated internal conditions within workable ranges despite external change. A feedback response can counter a disturbance, but homeostasis is dynamic rather than an absolutely fixed value.

Objective: Separate an individual response from reproduction and inherited variation.

Organisms detect and respond to environmental information. Reproduction produces new organisms and transfers hereditary information, but an individual organism can be alive without currently reproducing. DNA carries hereditary information in cellular life. Variation among individuals arises from inherited differences and other influences; over generations, heritable variation can affect populations. Growth, development, and response are coordinated processes, not a checklist that every individual must display at every moment.