Characteristics of living organisms

To be classified as a living organism, a thing must demonstrate seven key characteristics. These processes are all interconnected and rely on each other. A useful mnemonic to remember them is MRS GREN: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, and Nutrition. While an individual might not show all of these all the time (e.g., not reproducing), they must have the capability to do so. All these processes require energy, which is released from food through respiration.

Nutrition is the process of taking in materials (food) needed for energy, growth, and development. There are two main methods. Plants are 'autotrophs' (meaning 'self-feeders'). They make their own food using simple inorganic substances like carbon dioxide, water, and minerals. They capture light energy for this process, called photosynthesis. Animals, fungi, and some bacteria are 'heterotrophs' ('other-feeders'). They cannot make their own food, so they must obtain it by eating other organisms. Animals ingest complex organic compounds and digest them.

Respiration is not breathing. It is a fundamental chemical process that occurs in every living cell, 24 hours a day. It involves the breakdown of nutrient molecules (like glucose) to release the energy stored within them. This energy is vital for all life processes, including movement, growth, and keeping warm. The most efficient type is aerobic respiration, which requires oxygen. The energy released is used to power all metabolic reactions in the organism.

Sensitivity (also called irritability) is the ability to detect changes, or stimuli, in the internal or external environment and make appropriate responses. A stimulus is any change that an organism can detect (e.g., light, temperature, sound, chemicals). A response is the action taken by the organism. Responses often involve movement, which is an action by an organism or part of an organism causing a change of position or place. For example, a snail retracts into its shell (movement) when touched (stimulus). Plants also show movement, such as flowers opening in the morning or roots growing down towards gravity.

Viruses are fascinating because they challenge our definition of life. They are not made of cells and are much smaller and simpler than bacteria. A virus is essentially a strand of genetic material (DNA or RNA) inside a protein coat. Outside of a living cell, a virus is inert – it does not respire, feed, excrete, move, or grow. It shows no characteristics of life. However, once it infects a living host cell, it uses the cell's machinery and energy to make thousands of copies of itself. This is a form of reproduction. Because they can only reproduce inside a host and show no other life processes independently, most biologists classify them as non-living.