Characteristics of Life Notes

There are seven characteristics that biologists use to define life. In order for something to be considered alive, it must meet all of the following characteristics. All living things--

  1. are organized into one or more cells
  2. respond to stimuli
  3. reproduce and are able to pass traits to offspring
  4. grow and develop
  5. adapt and evolve
  6. maintain homeostasis
  7. obtain and use energy

1. Organized into one or more cells

The smallest unit of life is the cell. All living things are made of cells. Even though a cell is extremely small, it is still incredibly organized because it must carry out all attributes of life. In order to do this, there are specialized structures inside the cell called organelles. Some cells may not have these specialized structures but accomplish the attributes of life through chemical reactions. Organisms may be single celled like the Paramecium in pond water or multicellular like the goldfish or plant.

2. Respond to stimuliThe ability of an organism to respond to stimuli or changes is critical for survival. Stimuli that may cause a response in organisms include light, temperature, sound, pressure, gravity, odor, water and chemicals. Many organisms respond to stimuli through movement. Plants are able to move their leaves in response to touch or move their stem in response to gravity or light; and single-celled protists are able to move in the water with specialized structures.

3. Reproduce and are able to pass traits to offspring

All living organisms reproduce. Since organisms do not live forever, they must have an ability to pass on traits. Organisms may reproduce sexually or asexually. Reproduction is important for the species to continue; however, it is not necessary for organisms to reproduce sexually to survive. When organisms reproduce sexually, they transfer genetic information from one generation to the next; this is called heredity.Organisms that reproduce asexually must still copy the genetic information in order for the next cell to survive. All cells can reproduce asexually, which is important for survival and growth.

4.Grow

All organisms grow in some way. Even organisms that are made of only one cell grow; however it is not as noticeable as an organisms made of many cells. Cells get slightly larger in size and reach a point where they can no longer function efficiently at that size and the must divide. Multicellular organisms grow by adding more cells to their structure. As cells are added, they differentiate, or change to perform a specialized function. For example, in humans the fertilized egg continues to divide and adds more cells. Some of the cells become nerve cells; some become muscle cells; and some become skin cells, etc. As an organism adds more specialized cells, the organism changes or develops.

5.Adapt and evolve

Some organisms are more suited to respond to environmental conditions better than others within the same species. These traits that enable an organism to better survive are called adaptations. Adaptations are inherited traits that give an advantage to individuals within the population and are passed on to future generations through reproduction.

Over time, the genetic makeup of a population changes due to increasing numbers of individuals within the species inheriting these new traits. This change over time is called evolution.

6.Maintain homeostasis

All organisms must maintain a stable internal environment as they respond to changes that surround them. The maintenance of stable internal conditions within organisms is called homeostasis. Homeostasis has to do with keeping the body of an organism regulated, stable, maintaining balance and at equilibrium so that the organism can carry out all necessary life functions in order to survive.

In humans, homeostasis happens when the body regulates body temperature in an effort to maintain an internal temperature around 98.6 degrees Fahrenheit. For example, we sweat to cool off during the hot summer days, and we shiver to produce heat during the cold winter season.

An example of temperature control. On the left how it is controlled in a home, on the right how it is controlled in the body. The right is an example of maintaining homeostasis.

7.Obtain and use energy

Living things must get nutrients and energy from their environment. They require energy to reproduce, respond to stimuli, grow and develop. The energy that organisms need is the result of chemical reactions that occur inside their cells.