Characteristics of Life I

What is life?

What separates a mouse from a rock?

What is the difference between humans and robots?

Biology is the study of life and living matter. In order to understand biology we first need to classify what life is. How do we know if what we are looking at is alive?

There are 5 different characteristics that something must have to be considered alive. These are:

  • Organization
  • Nutrition
  • Excretion
  • Response
  • Reproduction


Organization is the different levels of complexity in an organism. An organism is defined as an individual animal, plant, or single-celled life form. Obviously, you cannot organize living things by shoving them into folders and putting them on shelves. Instead they are organized based on levels, also known as the hierarchy of life.


Nutrition is how living things obtain and use food.

All energy used by organisms originally comes from the sun. Autotrophic organisms use energy from the sun to produce energy they can use in the form of food. Plants, algae, and some bacteria are examples of autortophic organisms who use energy from the sun in photosynthesis to create carbohydrates.

Animals and other kinds of bacteria then eat the plants, algae, bacteria or even other animals in order to obtain energy. These are known as heterotrophic organisms. They rely on other organisms for food.


Metabolism is the sum of all chemical reactions that occur in an organism. The waste products from these reactions must be kept at a reasonable level, otherwise they may become toxic to the organism. Excretion is getting rid of these waste products from metabolism. The skin, lungs and kidneys are involved in excretion in animals and the stromata and lenticels remove waste in plants. less complex organisms, like bacteria, remove waste through diffusion, which is the movement of molecules from areas of high concentration to areas of low concentration.


Response is the way an organism reacts to a stimulus in their environment. An example of a stimulus is the sun and the response would be the direction in which plants grow (phototropism). Response can be hormonal, caused by chemicals, or nervous, caused by electrical impulses. It is incredibly important in survival as responses include catching prey, escaping predators, looking for water etc.


The final characteristic of life is reproduction. Reproduction is the way in which new offspring are produced from parent(s). There are two methods of reproduction: asexual (one parent) and sexual (two parents). Bacteria and other members of the Protista family use asexual reproduction, whereas animals and plants prefer sexual reproduction.

This covers all of the basic information of what defines life. this is all that is required for Leaving Cert. Of course, if this was everything there would be no need for college or university. There is more detail in Characteristics of Life II.

Quizlet for this section:

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