3. Characteristics of Living Things (Organisms)

All living organisms, from microscopic bacteria to complex animals, share a specific set of seven characteristics. These functions define what it means to be "alive."

1. The MRS GREN Acronym

The easiest way to remember the seven characteristics that all living things share is by using the acronym MRS GREN:

  • Movement

  • Respiration

  • Sensitivity

  • Growth

  • Reproduction

  • Excretion

  • Nutrition

2. Breakdown of the Seven Characteristics

Movement

This refers to an organism's ability to move its whole body or parts of it. While animals might walk or swim to find food, plants also move by turning their leaves toward the sun or growing roots toward water.

Respiration

Respiration is a series of chemical reactions that occur inside cells. It breaks down nutrient molecules (like glucose) to release the energy needed for metabolism—the sum of all chemical processes that keep an organism functioning.

Sensitivity (and Control)

Living things can detect and respond to changes in their internal or external environment. This allows them to stay in control of their internal conditions, such as temperature and water levels. For example, humans shiver when cold, and plants grow toward light.

Growth

Growth is a permanent increase in size and dry mass (the mass of an organism once all water is removed). In a single-celled organism like bacteria, this means the individual cell getting larger.

Reproduction

This is the process by which organisms make more of the same kind of organism. This ensures the survival of the species over time.

Excretion

Excretion is the removal of:

  1. Waste products of metabolism (such as carbon dioxide or urea).

  2. Substances in excess of requirements (such as extra water or mineral ions that the body does not currently need).

Nutrition

All organisms must take in materials for energy, growth, and development.

  • Animals eat and drink to get nutrients.

  • Plants create their own food through photosynthesis.

  • Fungi and Bacteria often absorb nutrients directly from their surroundings.

3. Key Distinctions

  • Living vs. Non-Living: While a car can move and a crystal can grow, only living organisms perform all seven of these functions simultaneously.

  • The Case of Viruses: Viruses are generally not considered living organisms. They do not fulfill all the MRS GREN requirements; for example, they do not grow on their own or excrete waste.

  • Cells: All living things are made of at least one cell.