1a Characteristics of Living Organisms¶

Part of 1 The Nature and Variety of Living Organisms.

The idea of life in this course is built from a shared set of processes. These characteristics are useful because they give one framework for comparing organisms that look very different on the surface. All living things share eight core life processes, remembered by the acronym MRS C GREN.

Learning Objectives¶

Life Processes¶

Living organisms take in materials from their surroundings and use them in organised ways. Eight processes are shared by all living things:

• Nutrition: obtaining materials for energy, growth and repair.
• Respiration: releasing usable energy from food molecules, which can happen with or without oxygen.
• Sensitivity: detecting stimuli — changes in the environment such as light, temperature or chemicals — and responding to them.
• Control (homeostasis): maintaining a stable internal environment so that cells and enzymes can work correctly.
• Growth: increasing in mass through making new cells or enlarging existing ones.
• Reproduction: producing offspring, either sexually (combining gametes from two parents) or asexually (from a single parent).
• Excretion: removing toxic waste products and the by-products of metabolism before they build up to harmful levels.
• Nutrition (movement): all living organisms can change position at some scale — whole-body movement in animals, growth responses in plants.

The acronym MRS C GREN groups these processes: Movement, Respiration, Sensitivity, Control, Growth, Reproduction, Excretion, Nutrition.

Keeping Conditions Suitable for Life¶

• Organisms control their internal conditions so that enzymes and cells can keep working. This is the general idea behind homeostasis.
• Movement does not always mean walking or swimming. Animals may move whole bodies, while plants still show movement through growth responses such as shoots bending towards light.
• No single characteristic on its own proves something is alive, but the full pattern gives a reliable picture of living activity.

Using the Characteristics Together¶

• The characteristics overlap. For example, growth requires nutrition and respiration, while reproduction depends on cells maintaining suitable internal conditions.
• When comparing unfamiliar organisms later in the course, this shared list helps you decide how they obtain food, exchange materials and respond to their environment.

Common Confusions¶

• Movement in plants: Plants are still living even though they do not walk from place to place. Growth responses such as tropisms count as movement.
• Respiration vs breathing: Respiration is the chemical process that releases energy in cells. Breathing is only one way some organisms obtain gases for respiration.
• Control vs sensitivity: Sensitivity is detecting and responding to a stimulus. Control (homeostasis) is the ongoing maintenance of stable internal conditions such as temperature and water content.

Key Terms¶

• Nutrition: taking in materials 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: Movement, Respiration, Sensitivity, Control, Growth, Reproduction, Excretion, Nutrition.

Connected Pages¶

• 1b Variety of Living Organisms
• IGCSE Biology Key Terms Glossary