Life processes are the series of actions essential to determine if an organism is alive – every living thing known to humans performs these actions. The seven processes are both signs of life and vital for sustaining it.
While different living organisms may seem significantly different or potentially seem not to be alive at all, as long as they perform all seven of the following processes, we can identify them as living organisms.
- M – Movement – All living things move, even plants
- R – Respiration – Getting energy from food
- S – Sensitivity – Detecting changes in the surroundings
- G – Growth – All living things grow
- R – Reproduction – Making more living things of the same type
- E – Excretion – Getting rid of waste
- N – Nutrition – Taking in and using food
As you can see, it’s possible to organize the letters for each life process into a handy acronym: MRS GREN. It makes them easier to remember!
However, let’s look at each point in a little more detail to understand what each of these things looks like in the natural world!
Movement
The movement’s one of the easier ways to spot a living thing, although it’s easier to see in some types of organisms than in others. All living things can move in some way without outside help, but there are many different ways to move, and sometimes those movements can be very small. It’s vital that a living organism can move in some way, or it’ll have no way to find food or reproduce!
It’s easiest to observe movement in animals, which seek food and shelter and try to escape danger. There’s a range of different ways that animals have evolved to allow movement: vertebrates (like us humans!) have an endoskeleton that provides us with a stable structure that supports our muscles and grows as we grow, but invertebrates who don’t have that internal support have other solutions, such as exoskeletons, or hydrostatic systems.
Although it’s much harder to spot in plants, movement is also present. For example, a plant will turn and grow towards the light above ground and towards water below, although they do so much, much slower than animals do.
Respiration
Respiration is the process by which nutrients an organism takes are converted to energy. Respiration is a chemical reaction that occurs in every cell in living things. Without respiration, an organism won’t have the energy to keep all the systems within their bodies working to sustain them.
Almost all organisms rely on aerobic respiration, which converts oxygen and glucose into water and carbon dioxide, releasing the energy the body requires in the process. For tiny, single-celled organisms like bacteria, the gas exchange between the vital oxygen and the waste products occurs across the organism’s cellular membrane. Still, multicellular life is a bit more complicated and usually requires a specially adapted organ.
Larger multicellular land animals, such as birds and mammals, have to breathe in oxygen from the air around us, which travels to the lungs and is transferred into our blood. The vascular system carries this fresh oxygen to all the tissues and cells of the body, where it’s used for respiration, and the carbon dioxide produced is transferred into the blood and carried back to the lungs, where it’s exhaled before it builds up to dangerous levels in the body.
Plants respire too, but they do it through openings called stomata, which are found on the underside of their leaves. These openings allow gases to travel in and out, so the plant doesn’t need to breathe actively.
However, some organisms can respire even without oxygen. For example, certain types of bacteria use a variety of respiration aerobic respiration, in which other reactants carry out the role of oxygen. Anaerobic respiration may use carbon dioxide or nitrate, nitrite, or sulfate ions, allowing the organism to live in an environment without oxygen. However, it’s not as efficient as aerobic respiration since it produces more waste products. Still, it’s an excellent solution for environments with insufficient oxygen for respiration, such as at the bottom of the sea.
Sensitivity
It might be easy to overlook, but this one’s vital! All living things can detect and respond to the conditions around them, which is essential for staying alive: if you can’t find food or notice a predator, you’re not going to survive for very long.
Animals rely on a wide range of sensory organs, although there are five vital senses that most of us use:
- Sight (eyes)
- Hearing (ears)
- Smell (nose)
- Touch (skin, feelers)
- Taste (tongue)
Sometimes there’s some overlap between them – some reptiles ‘hear’ by placing their jaw on the ground to feel vibrations because they don’t have an ear fully evolved to receive sounds from the air. Hence, they use touch to strengthen it, and there’s a lot of evidence that taste and smell are connected to many different species, but most animals will have a way of using all 5 of these senses to investigate the world around them and respond to it.
It’s much harder to see in plants, but they can also sense their environment. As we’ve already mentioned, plants can detect and grow upwards toward sunlight, sometimes even contorting themselves into bizarre shapes to optimize the light they take in. Likewise, root structures can see water and grow towards it. Some plants will also respond to touch – take the Venus Fly Trap, which has evolved to close its flower on unsuspecting insects when they feel the hairs on it!
Growth
All living things get bigger, taller, and heavier over time, which is the most accessible demonstration of growth. However, unlike non-living objects, which can also get bigger by accumulating more mass, living things grow by making new parts and materials and changing old ones – proportions can change, and new organs can develop.
Vertebrate animals grow at a fairly steady pace until they reach adulthood. Every day, as they get older, their bodies change. Their skeleton grows with them, each bone getting more prominent over time until they reach physical maturity. After that, their bodies will still change, but the rate slows, and they don’t generally get much more significant.
Invertebrates, however, tend to keep growing their whole lives. Arthropods, animals with exoskeletons, have it rough: to grow, they have to shed their old exoskeleton completely and grow a new one, leaving them very vulnerable while their new shell hardens!
Plants also don’t stop growing – if they’ve got enough food, they usually keep growing their whole lives because most plants develop from seeds or tubers and must develop rapidly to survive.
A special kind of growth heals injuries. Shrubs and trees mend wounds by covering them with bark and adding new layers of wood. Crabs grow fresh legs when old ones are lost. Likewise, human beings can heal cut skin and mend broken bones.
Reproduction
All living things can reproduce and make more of their species, although they do this in many different ways. Reproduction methods fall into two main categories: asexual reproduction, which an individual organism can do on its own, and sexual reproduction, which requires a partner.
Standard methods of reproduction for animals include:
- giving birth
- laying eggs
- asexual reproduction (e.g., Hydra jellyfish sprout babies like buds growing from their parent’s body).
Standard methods of reproduction for plants include:
- seeds
- spores
- asexual reproduction (which provides for tubers, rhizomes, and runners)
Excretion
All living organisms create waste products via the processes of living. Much waste comes from food; the rest is produced by movement, growth, respiration, and other living functions. If this waste remained in living things, it would soon cause illness and death. Thus living things must have a way to break down and dispose of waste matter. The process that removes waste products from the body is called excretion.
In animals, excretion removes carbon dioxide, water, and harmful substances (the by-products of respiration) from your body. For example, we exhale carbon dioxide from our lungs, and our kidneys help filter out other waste products from our blood, which are then removed from the body via the bladder.
Plants break down waste products much slower than animals, but they also have to excrete them. So plants use the stomata on their leaves to eliminate waste gases from respiration and photosynthesis, allowing them to diffuse through the delicate tissue in the roots.
Nutrition
All living things require energy to survive. The energy is derived from nutrients, or food, broken down to release energy and chemicals that the body uses as building blocks to keep everything working smoothly. It allows living things to grow and heal and maintain their current physical state.
Green plants, algae, and some bacteria can make food from water and carbon dioxide via a chemical reaction called photosynthesis. Photosynthesis relies on a chemical called chlorophyll, which helps to capture the light energy needed to convert water and carbon dioxide into glucose, an essential sugar that the organism can then use for respiration.
However, animals need to get their nutrients from external sources by eating plants or other animals. Different types of animals have evolved to have very different diets, but there are three main groups:
- Carnivores – exclusively eat meat
- Herbivores – exclusively eat plants
- Omnivores – eat both meat and plants
Homeostasis
There’s some debate over whether Homeostasis should be included as a Life Process, but it’s more commonly considered an extension of Sensitivity. Homeostasis is the ability of a living thing to recognize and maintain its internal conditions, such as temperature. It’s vital to help keep an organism alive, but it can be seen as an extension of some of the other life processes, so it doesn’t usually get added to the vital Mnemonic of MRS GREN.
To be considered a living thing, an object or organism must be able to do ALL of these things since, otherwise, the item in question can’t self-sustain without intervention from others. So, for example, although a train can move and converts fuel into energy to support that movement, it can’t get any bigger, can’t sense anything, and can’t reproduce, so a train couldn’t exist without a living organism there to steer it, keep it fuelled, and repair it.
