Every living thing on Earth, from a bacterium in a hot spring to a blue whale in the open ocean, shares a common set of activities that distinguish it from non-living matter. These activities are traditionally grouped into seven life processes: nutrition, respiration, movement, sensitivity, growth, reproduction, and excretion. Often remembered by the mnemonic MRS GREN (or MRS NERG, rearranged), these processes describe what an organism must do, at minimum, to stay alive and perpetuate its kind. The list is a teaching framework rather than a law of nature, but it holds up remarkably well across the full spectrum of life, and the places where it gets fuzzy turn out to be some of the most interesting corners of biology.
Nutrition
Nutrition is how an organism takes in and processes the raw materials it needs for energy and building new structures. For animals, this means eating food, digesting it, and absorbing the useful molecules. For plants, it means pulling carbon dioxide from the air, water from the soil, and using sunlight to stitch those ingredients into sugars through photosynthesis. But the story is broader than the plant-versus-animal split most people learn in school.
Microorganisms have invented ways of feeding that look nothing like either plants or animals. Some deep-sea bacteria, for instance, build their own organic molecules using chemical energy from inorganic compounds like hydrogen sulfide or iron, a lifestyle called chemolithoautotrophy. At hydrothermal vents on the ocean floor, entire ecosystems depend on bacteria that fix carbon dioxide using chemical reactions rather than sunlight, channeling energy through pathways unrelated to photosynthesis.1PubMed Central. Evidence of chemolithoautotrophy in the bacterial community associated with Alvinella pompejana, a hydrothermal vent polychaete Other microbes are mixotrophs, switching between making their own food and scavenging organic molecules depending on what is available.2PubMed Central. Microorganisms from deep-sea hydrothermal vents The sheer variety of nutritional strategies among microbes dwarfs what we see in the multicellular world. The point is that nutrition does not always mean “eating” in the way we picture it. It means acquiring matter and energy from the environment, however an organism manages to do that.3PubMed Central. Natural carbon fixation and advances in synthetic engineering for redesigning and creating new fixation pathways
Respiration
Respiration is not the same thing as breathing. Breathing is how your lungs move air in and out. Respiration is the chemical process inside cells that releases energy from nutrients. Every living cell on Earth performs some version of it. In organisms that use oxygen, glucose and oxygen react in a series of steps to produce a molecule called ATP, which acts as the universal energy currency of cells. ATP powers nearly everything a cell does, from contracting a muscle fiber to copying DNA to pumping molecules across a membrane.4PubMed Central. ATP synthesis and storage
Not all organisms need oxygen for respiration, though. Many bacteria and some fungi carry out anaerobic respiration, using molecules other than oxygen as the final electron acceptor. Yeast, for example, ferments sugars into ethanol and carbon dioxide when oxygen is scarce, which is why we have bread and beer. Some deep-subsurface bacteria use sulfate or even iron ions in place of oxygen. These are not fringe oddities; anaerobic respiration dominated the early Earth for billions of years before photosynthesis filled the atmosphere with oxygen. The common thread is always the same: break chemical bonds, capture the released energy as ATP, and spend that ATP on the business of living.
Movement
When people think of movement as a life process, they usually picture animals walking, swimming, or flying. But movement at the cellular level is just as fundamental and far more universal. Even organisms that never change their position, like plants rooted in soil, have intense motion happening inside every cell. Plant cells drive their internal fluid in organized currents called cytoplasmic streaming, which shuttles organelles and nutrients around the cell interior. In large plant cells and in the giant cells of certain algae, molecular motors push this fluid at speeds up to 100 micrometers per second.5PubMed Central. A physical perspective on cytoplasmic streaming This streaming is driven by the same basic actin-and-myosin machinery that powers muscle contraction in animals, adapted for a completely different purpose.6PubMed. Cytoplasmic streaming in plants
The reason this internal movement matters is practical: in very large cells, simply waiting for molecules to drift from one end to the other by diffusion would take far too long. Streaming solves that problem by actively circulating materials. Beyond the cellular level, plants also move their organs. Sunflowers track the sun, Venus flytraps snap shut, and roots grow downward in response to gravity. These movements are slower than an animal sprinting, but they are genuine, active responses to the environment. Movement as a life process is less about locomotion and more about the ability to physically rearrange matter in purposeful ways.
Sensitivity
Sensitivity, sometimes called irritability or responsiveness, is the ability to detect changes in the environment and respond to them. Your eyes detecting light, your skin sensing heat, your immune cells recognizing a pathogen: these are all examples of sensitivity. At the cellular level, the process works through signaling pathways. A cell detects a signal outside its membrane, relays that information inward through a chain of molecular events, and triggers an appropriate response, which might be anything from dividing to migrating to self-destructing.7PubMed Central. Dynamics and Sensitivity of Signaling Pathways
Single-celled organisms show sensitivity too. Bacteria swim toward food and away from toxins using chemical sensors on their surfaces. Amoebas extend and retract pseudopods in response to their surroundings. Even organisms without nervous systems manage to integrate environmental information in sophisticated ways. Sponges and corals, for example, use their internal fluid-circulation systems to generate local patterns of pressure, flow, and chemical signals that coordinate organism-wide responses to environmental change.8Integrative and Comparative Biology. Model Systems for Environmental Signaling You do not need a brain to be sensitive. You need a way to detect what is happening around you and adjust your behavior accordingly.
Growth
Growth means a permanent increase in size and complexity, typically achieved through cell division and cell enlargement. In animals, growth involves cells dividing to produce more cells, those cells specializing into different types, and the organism increasing in overall mass. In plants, the process has a somewhat different architecture: growth is concentrated in special zones called meristems, where cells divide rapidly, then elongate, then mature into their final form as they move away from the dividing zone.9PubMed. Cell division and turgor-driven stem elongation in juvenile plants: a synthesis
Growth is not unlimited or random. Cells communicate with each other mechanically and chemically to control where and when growth happens. In tissues, cells in crowded regions tend to stop growing, while cells at the edges where there is more room keep dividing. This mechanical feedback between a tissue’s physical constraints and individual cell behavior shapes how organisms develop their form.10PubMed Central. Mechanical control of cell proliferation patterns in growing tissues Growth also differs from simple accumulation. A crystal getting larger is not growth in the biological sense because no new cells are being formed and no metabolism is driving the process. Biological growth is an active, energy-consuming process that produces organized, living tissue.
Reproduction
Reproduction is how organisms produce new individuals of their kind. It comes in two broad flavors: sexual and asexual. Sexual reproduction combines genetic material from two parents, shuffling genes in ways that create offspring genetically distinct from either parent. Asexual reproduction produces offspring that are genetically identical (or nearly so) to the single parent, whether through binary fission in bacteria, budding in yeast, or vegetative propagation in plants like strawberries sending out runners.
Many organisms do both, and the balance between the two strategies turns out to be an evolutionary puzzle. In plants that can reproduce both ways, the mix of sexual and asexual propagation may shift depending on environmental conditions. When conditions are stable, cloning yourself asexually is efficient. When conditions fluctuate from year to year, the genetic variety generated by sexual reproduction gives offspring a better shot at surviving whatever comes next.11Journal of Evolutionary Biology. The balance between sexual and asexual reproduction in plants living in variable environments Even in populations that appear almost entirely asexual, a tiny amount of sexual reproduction can maintain surprisingly high genetic diversity. Modeling work has shown that as few as three sexually produced individuals per generation can give a population the same pattern of genetic variation as a fully sexual species.12PubMed. Genetic variation in organisms with sexual and asexual reproduction
Excretion
Excretion is the removal of metabolic waste products from the body. It is not the same as egestion (getting rid of undigested food). Excretion deals specifically with the chemical leftovers of metabolism that would become toxic if they accumulated. The most familiar example in humans is the kidneys filtering urea out of the blood and excreting it in urine, but the scope of excretion across the animal kingdom is much wider.
The three main nitrogen-containing waste products animals produce are ammonia, urea, and uric acid. Which one dominates depends heavily on how much water is available. Aquatic animals generally excrete ammonia directly into the surrounding water because ammonia is highly toxic but dissolves easily and washes away quickly. Land animals cannot afford that luxury, so they convert ammonia into the less toxic urea (mammals, amphibians) or the nearly insoluble uric acid (birds, reptiles, insects), which requires less water to excrete.13PubMed. Nitrogen excretion: three end products, many physiological roles Fish are an interesting case because, despite having functional kidneys, they rely heavily on their gills for excretion. The gill epithelium handles not just gas exchange but also the removal of nitrogenous waste, acid-base regulation, and osmoregulation, roles that in land vertebrates are split across lungs, kidneys, and skin.14PubMed. The multifunctional fish gill: dominant site of gas exchange, osmoregulation, acid-base regulation, and excretion of nitrogenous waste
Plants excrete too, though people rarely think of it that way. They release oxygen as a waste product of photosynthesis. Some plants store waste compounds in leaves that are eventually shed, or deposit them in bark or resin. Excretion in living things is less glamorous than nutrition or reproduction, but it is every bit as necessary. Without it, an organism poisons itself.
How These Processes Connect
It is tempting to treat the seven life processes as a checklist, but in practice they are deeply intertwined. Nutrition provides the raw materials that respiration converts into energy. That energy powers movement, growth, and reproduction. Sensitivity coordinates all of the above by detecting what the organism needs and when. Excretion clears out the metabolic debris generated by everything else. Cells maintain this whole balancing act through feedback loops: when a variable drifts out of range, signaling mechanisms detect the shift and trigger a corrective response. These loops operate at every scale, from individual cells adjusting their internal chemistry to organs coordinating across entire bodies.15PubMed. Biological feedback control-Respect the loops The seven life processes are not seven independent systems running in parallel. They are one integrated system described from seven angles.
Where the List Gets Fuzzy
The seven life processes are a useful framework, but nature does not always cooperate with tidy categories. Viruses are the most famous edge case. A virus particle sitting on a doorknob performs none of the seven processes. It does not respire, grow, excrete, move under its own power, or feed. But once it enters a host cell, it hijacks the cell’s machinery to reproduce, sometimes spectacularly. Whether viruses count as “alive” has been debated for over a century, and the honest answer is that the question hinges entirely on how you define life. Within what some biologists call the replicator framework, viruses sit at the extreme selfish end of a spectrum of biological entities, from fully cooperative cells to purely parasitic genetic elements.16PubMed Central. Are viruses alive? The replicator paradigm sheds decisive light on an old but misguided question They are biological, undeniably. Whether they are “alive” is more a question about our vocabulary than about viruses.
Tardigrades present a different kind of puzzle. These microscopic animals can perform all seven life processes perfectly well under normal conditions. But when their environment dries out or freezes, they enter a state called cryptobiosis, in which metabolism effectively stops. A desiccated tardigrade is not dead: it can revive when water returns, sometimes after years or even decades. But it is not performing respiration, growth, excretion, or any other metabolic process in the dried state. Researchers describe this as a reversible halt of metabolism, a condition sometimes called a third state between life and death.17PubMed. Cryptobiosis: a new theoretical perspective During this state, genes involved in DNA replication, protein production, and protein degradation are dialed down, consistent with a genuine metabolic shutdown rather than just a slowing.18PubMed Central. Towards decrypting cryptobiosis–analyzing anhydrobiosis in the tardigrade Milnesium tardigradum using transcriptome sequencing The tardigrade challenges the idea that life processes must be continuous. For these animals, being alive seems to include the option of pausing.
The Bare Minimum for Life
If you strip a living cell down to the absolute essentials, what do you end up with? Researchers have tried to answer this by building minimal cells in the lab. In 2016, a team synthesized a bacterial genome containing only the genes deemed essential or nearly essential for survival. The resulting organism could replicate its DNA, produce RNA and proteins, and divide to make new cells, and that was about it.19PubMed Central. Minimal Cells-Real and Imagined It performed the core life processes in their most stripped-down form: nutrition (importing molecules), respiration (generating ATP), growth (increasing in size before division), reproduction (dividing), and excretion (expelling waste). Movement and sensitivity were minimal but not entirely absent, since even the simplest cells respond to chemical gradients and transport materials internally.
What these minimal-cell experiments reveal is that the seven life processes are not an arbitrary checklist invented by textbook authors. They reflect something genuinely fundamental about what it takes to be a self-sustaining, reproducing unit. Researchers cataloguing essential genes across different species keep finding that the functions encoded by those genes map onto the same basic categories: energy metabolism, information copying, membrane maintenance, and waste management.20Trends in Cell Biology. Essential genes as antimicrobial targets and cornerstones of synthetic biology The specific genes vary from species to species, but the functions they support do not.
Life Without Sunlight
The seven life processes are often taught using familiar examples: a plant photosynthesizing, a dog running, a human exhaling carbon dioxide. But some of the most dramatic illustrations come from organisms living in places where those textbook examples break down entirely. At deep-sea hydrothermal vents, where superheated water laced with hydrogen sulfide pours out of the ocean floor, communities of microbes carry out all seven life processes without a single photon of sunlight. These organisms fix carbon dioxide into organic molecules using chemical energy from inorganic compounds, a strategy fundamentally different from anything happening on the sunlit surface.1PubMed Central. Evidence of chemolithoautotrophy in the bacterial community associated with Alvinella pompejana, a hydrothermal vent polychaete
The diversity of metabolic approaches at these vents is staggering. Some microbes oxidize hydrogen, others use sulfur compounds, and still others run their metabolism on iron or methane. Many switch between strategies depending on what is available, a metabolic flexibility that land organisms rarely need.2PubMed Central. Microorganisms from deep-sea hydrothermal vents These communities are not curiosities on the margins of biology. They support entire food webs, including tube worms and shrimp that rely on symbiotic bacteria for nutrition. The seven life processes look the same in principle at a vent as they do in a meadow. The details just get stranger.
Why MRS GREN Endures
Biology classrooms have been teaching the seven life processes for decades, and periodic attempts to replace or refine the list have not gained much traction. One reason is simplicity: seven items are manageable for a student to remember and apply. Another is that the list is genuinely hard to shrink. Drop reproduction, and you lose the ability to distinguish a living population from a collection of sophisticated machines. Drop excretion, and you cannot explain why organisms need kidneys, stomata, or gills. Drop sensitivity, and you cannot account for how a bacterium finds food or how a plant bends toward light.
The more interesting question is whether the list should be expanded. Some biologists have argued for adding homeostasis, the maintenance of stable internal conditions, as an eighth process. Others suggest that adaptation or evolution should be on the list because populations of living things change over generations in ways that non-living matter does not. These are reasonable arguments, and different textbooks draw the boundary differently. Homeostasis, for instance, could be seen as an emergent property of the other seven working together, or it could be seen as fundamental enough to warrant its own slot. The debate is less about what life does and more about how finely we want to slice the description. The seven-process version remains popular because it captures the essentials without becoming unwieldy, and because every known organism, from a minimal synthetic bacterium to a baleen whale, performs recognizable versions of all seven.