Welcome to Characteristics of Living Organisms!

Hi there! Biology can sometimes feel like learning a whole new language, but don't worry—we're going to break down the first big concept: What exactly makes something alive?

This chapter provides the foundation for classifying and understanding every organism we study, from microscopic bacteria to the largest mammals. In Pearson Edexcel IGCSE Biology, living organisms share eight essential characteristics.

Let's dive in!

The Eight Essential Characteristics of Life

To be classified as a living organism, an organism must demonstrate all eight characteristics:

  • Movement: they move
  • Respiration: they respire
  • Sensitivity: they respond to their surroundings
  • Control: they control their internal conditions
  • Growth: they grow and develop
  • Reproduction: they reproduce
  • Excretion: they excrete their waste
  • Nutrition: they require nutrition

A Memory Trick: Use the Mnemonic MRS C. GREN

The easiest way to remember these eight characteristics is using an expanded mnemonic:

M - Movement
R - Respiration
S - Sensitivity (respond to surroundings)
C - Control internal conditions
G - Growth and development
R - Reproduction
E - Excretion
N - Nutrition

Memory Aid: Say "MRS C. GREN" out loud until you remember it!

1. Movement

What is it?

Movement is an action by an organism causing a change of position or place.

Movement does not always mean locomotion from place to place; it can also be subtle movement of parts of an organism.

  • In Animals: Animals often move their whole body from one place to another (e.g. running, flying, swimming).
  • In Plants: Plants are rooted in place, but they move parts of their body, such as shoots bending towards light (phototropism) or flowers opening.

Key Takeaway: All living things move, even if they remain in one place!

2. Respiration

This is one of the most commonly misunderstood concepts! Pay close attention!

What is it?

Respiration is the chemical process in which living cells break down nutrient molecules (such as glucose) to release energy in the form of ATP for cellular processes.

IMPORTANT MISTAKE TO AVOID:

Many students confuse Respiration with Breathing (ventilation / gas exchange).

  • Respiration is a chemical reaction that happens inside all living cells to release energy.
  • Breathing is the physical movement of air into and out of the lungs.

Every living cell must respire continuously to provide the energy needed to stay alive.

Key Takeaway: Respiration = Chemical release of energy from nutrient molecules.

3. Respond to Surroundings (Sensitivity)

What is it?

Living organisms have the ability to detect changes in their surroundings (stimuli) and make appropriate responses.

The changes detected are called stimuli (singular: stimulus).

Stimulus → Receptor → Co-ordination → Effector → Response

  • Stimuli Examples: Changes in light intensity, temperature, moisture, gravity, touch, or sound.
  • Animal Example: Withdrawing your hand rapidly after touching a hot surface.
  • Plant Example: A stem bending and growing towards a light source to maximise photosynthesis.

Key Takeaway: Organisms sense and respond to changes in their environment to survive.

4. Control Internal Conditions

What is it?

Living organisms must control their internal conditions to maintain a stable and relatively constant internal environment, a process known as homeostasis.

Metabolic reactions within cells function efficiently only within specific ranges of temperature, water content, and chemical balance.

  • In Humans & Mammals: Controlling body temperature (thermoregulation), blood glucose levels, and water content (osmoregulation).
  • In Plants: Regulating water loss from leaves by opening and closing stomata via guard cells.

Key Takeaway: Living organisms maintain a constant internal environment so their metabolic reactions can proceed.

5. Growth and Development

What is it?

Growth and development involves a permanent increase in size and dry mass, usually through an increase in cell number, cell size, or both, accompanied by differentiation and development into specialised cells and tissues.

  • Animals: Growth generally occurs until maturity, followed by cell division for tissue repair and maintenance.
  • Plants: Many plants continue to grow and develop new shoots and roots throughout their entire life cycle.

Key Takeaway: Growth is a permanent, internal increase in size and complexity.

6. Reproduction

What is it?

Reproduction is the biological process by which new individual organisms (offspring) are produced, ensuring the survival of the species.

There are two main types of reproduction:

  • Sexual Reproduction: Involves the fusion of male and female gametes (fertilisation) to produce genetically varied offspring.
  • Asexual Reproduction: Involves a single parent organism and produces genetically identical offspring (clones) without the fusion of gametes.

Key Takeaway: Reproduction produces new individuals to ensure species survival.

7. Excretion

What is it?

Excretion is the removal of waste products of metabolism (chemical reactions inside cells), toxic materials, and substances in excess of requirements from the organism.

  • In Animals: The excretion of carbon dioxide by the lungs, and urea and excess water/ions in urine by the kidneys.
  • In Plants: The excretion of waste oxygen produced during daytime photosynthesis and carbon dioxide from respiration.

CRITICAL DISTINCTION: Excretion vs. Egestion

Excretion is the removal of metabolic waste made inside cells (e.g. urea, carbon dioxide).

Egestion is the passing out of undigested food as faeces. Because undigested food has never entered cells or taken part in metabolic reactions, it is NOT excretion.

Key Takeaway: Excretion is the removal of cellular metabolic waste products.

8. Nutrition

What is it?

All living organisms require nutrition — they need raw materials and energy for growth, tissue repair, and metabolism.

  • Autotrophic Nutrition: Organisms such as green plants make their own organic food substances from simple inorganic raw materials (carbon dioxide and water) using light energy during photosynthesis.
  • Heterotrophic Nutrition: Organisms such as animals and fungi obtain their organic nutrients by consuming and breaking down other living or dead organisms.

Key Takeaway: Nutrition provides the raw materials and fuel needed for all metabolic processes.

Quick Review Box: The 8 Characteristics of Living Organisms

Remember the 8 shared characteristics using MRS C. GREN:

M - Movement: changing position or moving parts of the organism.
R - Respiration: releasing energy from nutrient molecules inside cells.
S - Sensitivity: responding to changes in surroundings.
C - Control: controlling internal conditions (homeostasis).
G - Growth & Development: permanent increase in size, mass, and cell complexity.
R - Reproduction: producing new offspring.
E - Excretion: removing toxic metabolic waste products.
N - Nutrition: taking in or making nutrients for energy and growth.