1 Definition of Life
Study Guide
Learn Objectives
- Identify the core characteristics that define “life.”
- Identify the commonalities and differences among various definitions of “life.”
- Discuss the challenges of settling on a single definition of life.
Key Concepts and Terms
- Order
- Sensitivity to stimuli
- Reproduction
- Homeostasis
- Adaptation
- Evolution
- Growth and development
- Energy processing
Biology is the science that studies life. What exactly is life? This may sound like a silly question with an obvious answer, but it is not easy to define life. For example, a branch of biology called virology studies viruses, which exhibit some of the characteristics of living entities but lack others. Although viruses can attack living organisms, cause diseases, and even reproduce, they do not meet the criteria that biologists use to define life.
From its earliest beginnings, biology has wrestled with four questions: What are the shared properties that make something “alive”? How do those various living things function? When faced with the remarkable diversity of life, how do we organize the different kinds of organisms so that we can better understand them? Finally, what biologists ultimately seek to understand is how this diversity arises and how it continues. As new organisms are discovered every day, biologists continue to seek answers to these and other questions.
Properties of Life
All groups of living organisms share several key characteristics or functions: order, sensitivity or response to stimuli, reproduction, adaptation, growth and development, regulation, homeostasis, and energy processing. When viewed together, these characteristics serve to define life.
Order (Cells)
Cells are considered the most basic units of life because they represent discrete units that can independently embody all of the other characteristics listed below. All life living things consist of one or more cells. Even very simple, single-celled organisms are remarkably complex. Inside each cell, atoms make up molecules. These in turn make up cell components or organelles. Multicellular organisms, which may consist of millions of individual cells, have an advantage over single-celled organisms in that their cells can be specialized to perform specific functions and even sacrificed in certain situations for the good of the organism as a whole. How these specialized cells come together to form organs such as the heart, lung, or skin in organisms like the toad shown in Figure 1 will be discussed later.
Sensitivity or Response to Stimuli
Organisms respond to diverse stimuli. For example, plants can grow toward a source of light or respond to touch (Figure 2). Even tiny bacteria can move toward or away from chemicals (a process called chemotaxis) or light (phototaxis). The movement toward a stimulus is considered a positive response, while the movement away from a stimulus is considered a negative response.
Reproduction
Single-celled organisms reproduce by first du