Characteristics, Classification & Features of Organisms

The Seven Characteristics of Living Organisms

Every living organism on Earth must carry out seven key life processes. These define what "alive" means in biology. If something doesn't do all seven, it's not truly alive — this is why viruses don't count!

Movement

Any action by an organism or part of an organism causing a change of position or place. This includes obvious movement like a dog running, but also cells moving inside your body, or roots growing down into soil. Even plants move — they just do it slowly.

Respiration

The chemical reactions that break down nutrient molecules in living cells to release energy for metabolism. Don't confuse this with breathing (that's one method, but respiration happens in every cell). This is how all life gets usable energy.

Sensitivity

The ability to detect or sense stimuli in the internal or external environment and to make appropriate responses. Your eyes see light, your ears hear sound — that's sensitivity. Plants are sensitive too: they grow towards light, bend away from obstacles.

Growth

A permanent increase in size and dry mass by an increase in cell number or cell size or both. Key word: permanent. Eating a meal and getting heavier is temporary; building new muscle or bone is growth.

Reproduction

The processes that make more of the same kind of organism. Can be sexual (with another organism) or asexual (from one organism alone). This ensures species survive.

Excretion

The removal from organisms of toxic materials, the waste products of metabolism, and substances in excess of requirements. Your kidneys remove urea; your lungs remove carbon dioxide. Plants excrete too — they just do it more quietly.

Nutrition

The taking in of materials for energy, growth, and development.

• Plants need: light, carbon dioxide, water, and ions
• Animals need: organic compounds, ions, and usually water

How Organisms Are Classified

What Is a Species?

different species

Why Classify?