Welcome to IGCSE Combined Science Biology!

In this first chapter of Biology (B1), we tackle a fundamental question: What makes something alive?

If you see a cat running (living) and a car moving (non-living), how do you tell the difference? Scientists have agreed upon seven key processes that must be carried out by any organism to be classified as alive.

These notes will break down each characteristic clearly. Don't worry if this seems like a lot to remember—we have a brilliant trick for you!

B1.1 The Characteristics of Living Organisms

To ensure you remember all seven characteristics, use the classic acronym:

M. R. S. G. R. E. N.

Memorise these letters, and you've memorised the building blocks of life!

M is for Movement

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

  • Example (Animals): Running, swimming, flying.
  • Example (Plants): The slow movement of a root growing deeper into the soil, or leaves turning towards the light (a process called phototropism).

Did you know? Even organisms stuck in one place, like fungi, show movement internally, growing and extending their hyphae into new areas for feeding.

R is for Respiration

Definition: Respiration is the chemical reactions in cells that break down nutrient molecules and release energy for metabolism.

  • This energy is vital for *all* other life processes (movement, growth, reproduction, etc.).
  • We often breathe, but breathing is just one part of the process (gas exchange). Respiration is the key chemical reaction happening inside every cell.
  • The most common form is aerobic respiration, which uses oxygen to break down glucose.

Quick Tip: Avoid the Common Mistake!

Respiration is not breathing. Breathing is taking air in and out; respiration is the cellular process of releasing energy.

S is for Sensitivity

Definition: Sensitivity is the ability to detect and respond to changes in the internal or external environment.

Living organisms need to react to survive. These changes are called stimuli.

  • External Stimuli: Light, temperature, sound, touch, or the presence of predators. (A plant bending towards the window is a response to light).
  • Internal Stimuli: Changes within the body, such as hunger or temperature changes. (Shivering when cold is a response to an internal drop in temperature).

G is for Growth

Definition: Growth is a permanent increase in size and dry mass.

  • Growth involves synthesizing new materials (like proteins and cellulose) to become larger and more complex.
  • The term dry mass is important: if an organism just takes in water, it increases in size but doesn't genuinely grow. Permanent growth means building new, stable living matter.

Analogy: Imagine blowing up a balloon (temporary size increase from air/water) versus building a house (permanent increase in size and structure, or dry mass).

R is for Reproduction

Definition: Reproduction refers to the processes that make more of the same kind of organism.

  • This characteristic is essential for the survival of the species, ensuring that the living line continues.
  • It can be sexual (involving two parents) or asexual (involving one parent).

E is for Excretion

Definition: Excretion is the removal of waste products of metabolism and substances in excess of requirements.

Metabolism (all the chemical reactions in the body) produces waste products that are often toxic, such as urea and carbon dioxide. These must be removed.

  • Examples:
  • Carbon dioxide removed via lungs (in animals).
  • Urea removed via kidneys (in animals).
  • Excess salts or water removed via sweat glands or kidneys.

Quick Tip: Excretion vs. Egestion

Do not confuse excretion with egestion. Egestion is the removal of undigested food (faeces). Excretion is the removal of chemical waste products generated by cells.

N is for Nutrition

Definition: Nutrition is the taking in of materials for energy, growth and development.

  • Plants (producers) carry out photosynthesis, taking in raw materials (CO2 and water) to make their own food (glucose).
  • Animals (consumers) must ingest (eat) complex organic substances (carbohydrates, proteins, fats) from other organisms.

Key Takeaway Review

We use the mnemonic MRS GREN to easily recall the seven characteristics of living organisms:

M: Movement

Change of position or place.

R: Respiration

Chemical reactions releasing energy from nutrients inside cells.

S: Sensitivity

Ability to detect and respond to stimuli.

G: Growth

Permanent increase in size and dry mass.

R: Reproduction

Making more organisms of the same kind.

E: Excretion

Removal of metabolic waste and excess substances.

N: Nutrition

Taking in materials for energy, growth, and development.