What Makes Something Living?
Learn how scientists define life using the MRS GREN characteristics
The 7 Life Processes
MRS GREN
All living organisms share seven key characteristics. Scientists use these to determine whether something is alive or not. We remember them using the acronym MRS GREN.
Movement means organisms can change position or move parts of their body. Animals walk, swim, or fly. Plants grow towards sunlight and their roots grow downwards.
Respiration is releasing energy from food. This happens in all cells, in all living things, all the time. It is not the same as breathing.
Sensitivity means responding to changes in the environment. Animals use eyes, ears, and skin to detect light, sound, and temperature. Plants respond to gravity and light direction.
Growth is increasing in size and mass by making new cells. All organisms grow from young to adult forms.
Reproduction is producing offspring. Animals and plants can reproduce sexually (two parents) or asexually (one parent). Bacteria reproduce by splitting in two.
Excretion is removing waste products made by chemical reactions inside cells. Animals excrete carbon dioxide through breathing and urea in urine.
Nutrition is taking in nutrients for energy and growth. Animals eat food. Plants make their own food using photosynthesis. Fungi absorb nutrients from dead material.
Are Viruses Alive?
Viruses are debatable. They can reproduce, but only by invading living cells. They do not respire, grow, or respond to their environment on their own. Most scientists classify viruses as non-living.
Movement
The ability to change position or move body parts
Examples: Animals walk or swim. Plants grow towards light. Bacteria use flagella to swim.
Respiration
Releasing energy from food in cells
Examples: Happens in all cells, all the time. Not the same as breathing.
Sensitivity
Detecting and responding to changes in the environment
Examples: Eyes detect light. Skin detects temperature. Plants grow towards sunlight.
Growth
Increasing in size and mass
Examples: Animals grow from babies to adults. Plants grow from seeds to full size.
Reproduction
Producing offspring
Examples: Animals have babies. Plants make seeds. Bacteria split in two.
Excretion
Removing waste products from the body
Examples: Animals breathe out CO₂. Plants release oxygen. Kidneys remove urea.
Nutrition
Taking in nutrients for energy and growth
Examples: Animals eat food. Plants make food via photosynthesis. Fungi absorb nutrients.
Question:
A crystal grows larger over time when placed in a salt solution. Is it alive?
Answer:
No, the crystal is not alive. While it does show growth, it does not show the other six characteristics. It cannot move, respire, respond to stimuli, reproduce, excrete waste, or take in nutrition. All seven characteristics must be present for something to be classified as living.
A plant grows towards a window with sunlight. Which characteristic is this?