Biologists use the seven characteristics of living things to decide whether something is alive. The mnemonic MRS GREN helps students remember these life processes: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, and Nutrition. These processes matter because living organisms can look very different, from a tree to a mushroom to a bacterium, but they all carry out the same basic functions.
A checklist based on MRS GREN helps separate living organisms from nonliving objects and once-living materials.
Understanding Biology: Seven characteristics of living things (MRS GREN)
Each life process depends on cells doing controlled chemical work. Respiration releases usable energy from food. Cells use that energy to build proteins, repair damage, pump substances across membranes, and make muscles contract.
Aerobic respiration needs oxygen, but some organisms can release energy without oxygen for a short time or throughout life. Human muscle cells can respire without oxygen during intense exercise. This produces lactic acid, which contributes to fatigue.
Respiration is not the same as breathing. Breathing moves air into and out of lungs, while respiration happens inside cells.
Nutrition provides the raw materials for respiration, growth, and repair. Animals usually obtain food by eating other organisms or their products. Green plants make glucose using light energy, then use or store it.
They still need mineral ions from the soil, including nitrates for making proteins. A plant can photosynthesise in daylight while respiring all the time. This is why a plant does not gain mass from soil alone.
Much of its new mass comes from carbon dioxide taken from the air. This idea can seem surprising because carbon dioxide is a gas.
Sensitivity means detecting a change and responding in a useful way. The change is called a stimulus. Light, temperature, touch, sound, chemicals, and gravity can all act as stimuli.
In humans, sense organs detect many stimuli and the nervous system produces fast responses. Plants have no nerves, yet they respond through chemical signals and uneven growth. A shoot growing toward light allows leaves to receive more light for photosynthesis.
Roots often grow downward because of gravity, helping the plant reach water and minerals. Movement therefore includes more than animals walking or swimming.
Growth is a permanent increase in size or dry mass. In multicellular organisms, this commonly happens when cells divide and then enlarge. Development is related but different.
Development means changes in form and function, such as a caterpillar becoming a butterfly. Reproduction produces new organisms, either sexually or asexually. An individual organism may not reproduce because it is young, infertile, or unable to find a mate.
The species still has the capacity to reproduce. This is important when applying the characteristics to a single organism.
Excretion is often confused with removing undigested food. Excretion removes waste substances made by metabolic reactions. Carbon dioxide leaves the body through the lungs after respiration.
Urea is made in the liver when excess amino acids are broken down, then removed in urine by the kidneys. Faeces contain undigested material and are removed by egestion, not excretion.
When studying examples, identify the substance, where it was made, and how it leaves the organism. This prevents common mistakes and shows how the seven processes work together rather than as separate facts.
Key Facts
- MRS GREN = Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition.
- Living things are identified by carrying out all 7 life processes, not just one or two.
- Aerobic respiration word equation: glucose + oxygen = carbon dioxide + water + energy.
- Photosynthesis word equation: carbon dioxide + water + light energy = glucose + oxygen.
- Movement can include slow growth responses, such as plant shoots bending toward light.
- Excretion removes waste products made by cells, such as carbon dioxide, urea, or excess oxygen in plants.
Vocabulary
- Movement
- Movement is a change in position or direction by the whole organism or part of the organism.
- Respiration
- Respiration is the chemical process that releases energy from food inside cells.
- Sensitivity
- Sensitivity is the ability to detect and respond to changes in the environment.
- Excretion
- Excretion is the removal of waste substances made by the organism's cells.
- Nutrition
- Nutrition is obtaining or making food materials needed for energy, growth, and repair.
Common Mistakes to Avoid
- Thinking movement only means walking or running is wrong because plants, fungi, and microbes can move through growth, spreading, or cell movement.
- Counting fire or a car as alive because it uses energy is wrong because it does not carry out all seven MRS GREN life processes.
- Confusing excretion with egestion is wrong because excretion removes metabolic waste from cells, while egestion removes undigested food from the digestive system.
- Forgetting that plants respire is wrong because plants use respiration to release energy from glucose, even though they also photosynthesize.
Practice Questions
- 1 A student observes an organism for 14 days and records that it grows from 2 cm to 5 cm tall. How many centimeters did it grow, and which MRS GREN process does this show?
- 2 In a pond sample, 8 out of 20 observed microbes swim toward a light source. What percentage moved toward the light, and which two MRS GREN processes are shown?
- 3 A crystal gets larger over time when more mineral is added to it, but it does not respire, reproduce, excrete, or respond to stimuli. Explain why it is not considered living using MRS GREN.