Characteristics of Living Things
Biology defines life by function rather than appearance, since living things range from a single bacterial cell to a blue whale. Something is generally classified as a living organism when it displays all of the following processes, usually remembered by biology students under the acronym MRS GREN.
Last updated: 1 September 2026
Key characteristics
Movement
Living things move all or part of themselves, whether that is an animal walking, a plant's leaves turning toward light, or cytoplasm shifting inside a single cell.
Respiration
Organisms release energy from food through chemical reactions inside their cells, most commonly by combining glucose with oxygen, though some organisms respire without oxygen at all.
Sensitivity
Living things detect and respond to changes in their surroundings, such as light, temperature, touch, or chemical signals, which is why a plant bends toward a window and an animal flinches from heat.
Growth
Organisms increase in size or complexity over their lifetime through cell division and development, following a pattern that is largely programmed by their genetics.
Reproduction
Living things produce offspring, either sexually by combining genetic material from two parents or asexually by copying themselves, ensuring the continuation of the species.
Excretion
Organisms remove waste products created by their own internal chemical processes, such as carbon dioxide from respiration or nitrogenous waste from breaking down protein.
Nutrition
Living things take in and use materials for energy and growth, either by making their own food through photosynthesis, as plants do, or by consuming other organisms, as animals do.
Frequently asked questions
Why is a virus not classified as a fully living thing?
Viruses cannot reproduce, grow, or carry out metabolism on their own. They can only replicate by hijacking the machinery inside a host cell, which is why most biologists classify them as being on the boundary between living and non-living rather than fully alive.
Do all living things need to move to be classified as alive?
Not in the sense of walking or swimming. Movement in biology also includes internal movement, such as a plant's stomata opening and closing or blood circulating inside a body, so plants still satisfy this characteristic despite being rooted in place.
Is fire alive because it grows, moves, and consumes fuel?
No. Fire is a chemical reaction, not a biological one. It lacks cells, cannot reproduce a genetically similar copy of itself, and does not regulate its own internal processes the way a living organism does.