What is a Living Organism?
Look around you. A tree growing outside, a cat on a windowsill, a mushroom pushing through soil, all of these are living organisms. A stone, a glass of water, a computer are not living organisms. So how do we know the difference? What exactly makes something alive?
A living organism is any individual form of life capable of growing, reproducing, and carrying out the fundamental processes that sustain existence. This includes animals, plants, fungi, and even bacteria. All organisms (regardless of how simple or complex) share a set of defining characteristics that separate them from non-living matter.
Scientists have agreed on seven characteristics that every living organism must demonstrate. These seven characteristics are known collectively as the life processes. Miss even one, and the organism cannot be considered truly alive.
Why does this matter?
Understanding life processes is foundational to all of biology. It explains how doctors detect illness (a failure of a life process), why certain environments cannot support life, and how scientists search for life beyond Earth — they look for evidence of these same seven processes.
What is MRS GREN?
MRS GREN is a mnemonic (a memory aid) used in biology to help you recall the seven life processes. Each letter stands for one of the essential characteristics that every living organism must possess. It is one of the most widely used acronyms in school science, and once you know it, you will never forget it.
Now that you have the overview, let's go deep into each one — understanding not just the what but the why behind every life process.
The 7 Life Processes
1. Movement
The M in MRS GREN
Every living organism has the ability to move some part of itself and this is true even for organisms that appear completely stationary.
In animals, movement is obvious: birds fly, fish swim, humans walk and blink. But movement in biology is defined more broadly than just locomotion. It refers to any self-powered change in position or orientation, whether that's an entire body moving or just a specialised structure doing so.
Plants are a perfect example of this. A sunflower turns its face to track the sun across the sky throughout the day (a process called phototropism). A Venus flytrap snaps shut in a fraction of a second when an insect touches its trigger hairs. Tree roots grow toward water sources deep underground. None of these plants are running or jumping, but all of them are unambiguously moving.
Why does movement matter for survival? Movement improves the chances of catching food, escaping predators, finding mates, and positioning the body to absorb sunlight or water. It is how organisms interact with and navigate their world.
2. Respiration
The first R in MRS GREN
Respiration is one of the most commonly misunderstood life processes. It is not the same as breathing. Breathing (also called ventilation) is simply the mechanical act of moving air in and out of the lungs. Respiration is the biological process that happens inside every single cell.
Cellular respiration is a series of chemical reactions that converts glucose (a sugar) into ATP (Adenosine Triphosphate) — the molecule that powers virtually every activity in a cell, from muscle contraction to cell division to brain function. Without respiration, cells cannot function, and the organism dies.
There are two types. Aerobic respiration uses oxygen and is highly efficient, releasing large amounts of ATP along with carbon dioxide and water as by-products. This is the type most animals and plants rely on. The equation is: Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP).
Anaerobic respiration occurs without oxygen and produces less ATP. In human muscles, it produces lactic acid — the cause of the burning sensation during intense exercise. Some microorganisms use anaerobic respiration permanently, surviving in oxygen-poor environments like deep soil or the ocean floor.
3. Sensitivity
The S in MRS GREN
Sensitivity in biology does not mean being emotionally sensitive. It refers to an organism's ability to detect changes in its environment and respond to them appropriately. All living things (from bacteria to blue whales) can sense their surroundings.
These environmental changes are called stimuli (singular: stimulus). They can be physical (light, sound, temperature, touch) or chemical (pH, oxygen levels, the presence of nutrients or toxins). Organisms detect stimuli through specialised structures called sensory receptors.
In complex animals like humans, the nervous system coordinates responses to stimuli with extraordinary speed. A reflex arc (the automatic response to touching something hot) takes only milliseconds. In simpler organisms, sensitivity works through chemical signals rather than nerves. Even a single-celled bacterium can detect chemical gradients in its environment and move toward nutrients or away from toxins.
Why is sensitivity so critical? An organism that cannot detect danger cannot avoid it. An organism that cannot detect food cannot find it. Sensitivity is the biological interface between the organism and its world.
4. Growth
The G in MRS GREN
Growth in biology has a precise definition: it is an irreversible increase in the dry mass of an organism. The word "irreversible" is important here. When you inflate a balloon, it gets bigger — but that isn't growth. Biological growth represents an increase in the amount of living material in an organism, driven by the production of new cells and tissues.
Growth requires energy, which is why it depends directly on respiration. The ATP produced through cellular respiration powers the production of new proteins, the replication of DNA, and ultimately the division of cells — the fundamental mechanism behind all biological growth.
Different organisms grow in remarkably different ways. Humans and most animals grow throughout childhood before slowing and eventually stopping. Many plants continue growing throughout their entire lives. Some trees (like the giant sequoia) grow for thousands of years. Bacteria can double in population every 20 minutes under ideal conditions, a form of growth that is almost incomprehensibly rapid.
It is also worth noting that growth is not just about getting bigger in overall size. It includes the growth of specific structures: the healing of a wound, the regrowth of a lizard's tail, the repair of bone tissue after a fracture. All of these are expressions of the life process of growth.
5. Reproduction
The second R in MRS GREN
Reproduction is the process by which living organisms produce new organisms of the same species. It is the mechanism through which life continues from one generation to the next. Without reproduction, every species would eventually go extinct.
There are two fundamentally different strategies for reproduction in nature: sexual reproduction
and asexual reproduction.
Sexual reproduction involves two individuals combining their genetic material. In humans and most animals, a sperm cell fertilises an egg cell, creating an entirely unique individual with a mix of genetic information from both parents. This genetic variation is enormously important for the long-term survival of species — it is the engine of evolution, producing individuals with different traits that may help them adapt to changing environments.
Asexual reproduction requires only one parent and produces offspring that are genetically identical to that parent — essentially clones. It is quicker and requires no partner, making it highly efficient. Bacteria reproduce asexually through binary fission, splitting in two. Some plants reproduce asexually through runners or bulbs. Certain animals, like aphids, can reproduce asexually under specific conditions.
An important note: reproduction is unique among the life processes in that an individual organism can survive without reproducing. However, without reproduction, the species itself cannot survive. It is essential at the population level rather than the individual level.
6. Excretion
The E in MRS GREN
Excretion is the process by which organisms remove the toxic metabolic waste products produced by normal chemical reactions in their cells and tissues. This is another term that is frequently confused with something else — in this case, with egestion (the removal of undigested food, i.e. faeces). Excretion is specifically about getting rid of the waste your cells produce, not the food you couldn't digest.
The main waste products of metabolism include carbon dioxide (a by-product of respiration), urea (produced when excess amino acids are broken down in the liver), and water (produced in large quantities by cellular respiration and other reactions).
If these waste products were allowed to build up inside an organism, they would quickly become toxic. High concentrations of carbon dioxide lower the pH of the blood, disrupting vital chemical reactions. Excess urea is poisonous to cells. This is why organisms with damaged kidneys (which filter urea from the blood) become seriously ill very quickly.
Different organisms have evolved remarkably different excretion systems. In humans, the kidneys filter urea and excess water into urine; the lungs excrete carbon dioxide with every breath; and even the skin excretes small amounts of urea through sweat. In plants, oxygen (a by-product of photosynthesis) is released through tiny pores called stomata and this is a form of excretion too.
7. Nutrition
The N in MRS GREN
Nutrition is the process by which living organisms obtain and use food to provide the energy and raw materials needed for all life processes. Without a source of nutrition, an organism cannot respire, grow, repair itself, or reproduce.
There are two fundamentally different strategies for nutrition in the living world: autotrophic (self-feeding) and heterotrophic (other-feeding).
Autotrophs make their own food. Plants and algae are the most familiar examples. Through photosynthesis, they use energy from sunlight to convert carbon dioxide from the air and water from the soil into glucose — a sugar they use as their primary energy source and as a raw material for building more complex molecules.
The equation is: Carbon dioxide + Water + Light energy → Glucose + Oxygen.
Some bacteria are also autotrophs, using chemical energy instead of light.
Heterotrophs cannot make their own food and must eat other organisms. Animals are heterotrophs. So are fungi. Humans are heterotrophs. Everything we eat, whether a carrot or a piece of chicken, ultimately traces back to energy that was first captured from the sun by a plant or autotrophs.
Beyond energy, nutrition also provides organisms with the specific nutrients (proteins, fats, vitamins, minerals) that are required for particular biological functions: calcium for strong bones, iron for haemoglobin, vitamin C for immune function. A deficiency in any critical nutrient will impair one or more life processes.
Revision Summary (Key Points to Remember)
- All living organisms must carry out seven life processes, remembered using the acronym MRS GREN.
- Movement : All organisms can move some part of themselves, including plants (e.g. growing toward light).
- Respiration: Cellular respiration converts glucose into ATP energy inside every cell; it is not the same as breathing.
- Sensitivity : Organisms detect and respond to stimuli (changes in their environment) through sensory receptors.
- Growth: An irreversible increase in dry mass, powered by energy from respiration and driven by cell division.
- Reproduction: Organisms produce offspring of the same species, either sexually (two parents, genetic variation) or asexually (one parent, identical clones).
- Excretion: Removal of toxic metabolic waste products (CO₂, urea, water) from cells; not the same as egestion.
- Nutrition: Obtaining food for energy and nutrients; plants are autotrophs (make their own), animals are heterotrophs (eat others).
