1a Characteristics of Living Organisms¶

Part of 1 The Nature and Variety of Living Organisms.

Every living organism carries out the same eight processes, whatever it looks like. The mnemonic MRS C GREN names them, and together they give one framework for comparing organisms as different as a bacterium, a fungus and a whale.

What You Need to Learn¶

Further detail: Pearson Edexcel International GCSE Biology specification.

The eight life processes and what each one means, and how organisms keep conditions suitable for life. No single process proves that something is alive. The full set does.

Life Processes¶

Each process is defined by what it achieves for the organism. The same process looks different in an animal and in a plant, so the definition matters more than any one example.

Process	What it means	In an animal	In a plant
Movement	An action by the organism, or part of it, that changes position or place	A cat runs	A shoot bends towards light
Respiration	Chemical reactions in cells that release energy from food molecules, with or without oxygen	Muscle cells release energy for contraction	Root cells release energy to take in mineral ions
Sensitivity	Detecting a stimulus, which is a change in the surroundings, and responding to it	The pupil narrows in bright light	A root grows towards water
Control	Keeping internal conditions stable, which is homeostasis	Body temperature is held near 37 °C	Guard cells close stomata to limit water loss
Growth	A permanent increase in size or mass, by more cells, bigger cells or both	A child gets taller	A seedling's shoot lengthens
Reproduction	Producing offspring, sexually from two parents or asexually from one	Eggs are laid or young are born	Seeds form, or runners grow
Excretion	Removing toxic waste products of metabolism, and substances in excess of need	Carbon dioxide leaves the lungs and urea leaves in urine	Carbon dioxide from respiration leaves through the leaves
Nutrition	Taking in the molecules needed for energy, growth and repair	Food is eaten and digested	Carbon dioxide and water are taken in to make glucose

The processes depend on each other. Growth needs materials from nutrition and energy from respiration. Detecting a change (sensitivity) is the first step in correcting it (control). Excretion removes the by-products of respiration and of the other chemical reactions in cells, which are together called metabolism.

Exam technique

Definitions are marked on their key words, so learn them precisely. Growth is a permanent increase, which is why a person who gains weight after a large meal has not grown. Nutrition is taking in molecules, which covers a plant absorbing carbon dioxide as well as an animal eating.

Keeping Conditions Suitable for Life¶

Cells work only within a narrow range of conditions. Enzymes need a suitable temperature and pH, and cells need the right amount of water. Control keeps those conditions steady, and sensitivity supplies the information that control acts on. Human body temperature, held near 37 °C, is the standard example.

Movement does not have to mean walking or swimming. Animals move their whole bodies, but plants show movement too, through growth responses such as shoots bending towards light.

Illustration: why one process is not enough

A candle flame moves, grows and uses oxygen, yet nobody calls it alive. It does not reproduce, respond to stimuli or keep any internal conditions stable. This is why all eight processes are used together as the test for life.

Common Confusions¶

Movement in plants: Plants do not move from place to place, but they are still alive. Growth responses such as tropisms count as movement.
Respiration vs breathing: Respiration is the chemical process that releases energy in cells. Breathing is only the way some organisms bring gases to their cells.
Control vs sensitivity: Sensitivity is detecting and responding to a stimulus. Control is the ongoing keeping of stable internal conditions such as temperature and water content.
Excretion vs egestion: Excretion removes metabolic waste, which is the product of reactions inside cells, such as carbon dioxide and urea. Egestion removes undigested food, which was never part of a cell's chemistry. This is also why the lungs count as organs of excretion.

Check Yourself¶

Name the eight life processes that MRS C GREN stands for.
Define growth, and explain why a person who gains 2 kg during a holiday has not necessarily grown.
Explain why a plant is said to show movement even though it stays in one place.
A student claims that a candle flame is alive because it moves, grows and uses oxygen. Name two life processes a flame does not carry out.
State the difference between excretion and egestion, and give one example of each.
Explain why growth depends on both nutrition and respiration.

Answers

Movement, respiration, sensitivity, control, growth, reproduction, excretion, nutrition.
Growth is a permanent increase in size or mass, through more cells, bigger cells or both. A short-term gain, such as from a meal or retained water, can be lost again, so it is not permanent growth.
Parts of a plant change position by growing, for example a shoot bending towards light. That is movement of part of the organism, even though the whole plant does not change place.
Any two of: reproduction, sensitivity (responding to stimuli), control (keeping internal conditions stable), excretion.
Excretion is removal of metabolic waste made inside cells, such as carbon dioxide from the lungs. Egestion is removal of undigested food, such as faeces.
Growth makes new material, and nutrition supplies the molecules to build it. Respiration releases the energy that building needs, so growth cannot happen with only one of them.

Key Terms¶

Nutrition: taking in molecules needed for energy, growth and repair.
Respiration: the chemical reactions in cells that release energy from food.
Excretion: removal of waste products of metabolism and other unwanted materials.
Sensitivity: the ability to detect and respond to stimuli in the environment.
Stimulus: a change in the internal or external environment that can be detected.
Homeostasis: maintenance of a stable internal environment.
Growth: a permanent increase in size or mass.
Reproduction: the process by which organisms produce offspring.
MRS C GREN: a mnemonic for the eight characteristics of living organisms.