What is a Living Organism?

• Since biology involves the study of living things – also known as organisms – it is important to distinguish between things that are living and things that are non-living.

• This may seem quite easy to do at first. A cat is clearly a living thing and a glass bottle is clearly a non-living thing. But what about the black mould that appears on bathroom tiles? What about a piece of wood? Or a river? Or a fire? Or a robot with artificial intelligence? Which of these are living things?

• Since it is not always clear if something is living or non-living, scientists have defined a group of characteristics that something must have to be considered as being alive.

• The characteristics of living things are as follows:

• Movement

• Respiration

• Sensitivity

• Coordination and Control / Circulation / Cells

• Growth

• Reproduction

• Excretion

• Nutrition

• These characteristics can be remembered using the mnemonic MRSCGREN, where each letter corresponds to the first letter of one of the characteristics.

• We will discuss each characteristic in the following sections.

All living things have a defined set of characteristics.