IGCSE Co-ordinated Sciences (0654) - Biology Study Notes

Chapter B1: Characteristics of Living Organisms

Welcome to Biology! Before we study cells, digestion, or respiration, we need to answer a fundamental question: What does it mean to be alive?


In this chapter, you will learn the seven characteristics shared by all living organisms, from the smallest bacteria to the largest blue whales. These definitions are essential foundation knowledge for the entire Biology section, so let’s make sure we nail them!


The Seven Characteristics of Life: MR. S GREN

To help you remember all seven characteristics required by the syllabus, scientists often use the simple mnemonic: MR. S GREN. If an object shows all seven of these features, it is generally considered a living organism.


1. M – Movement

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

  • Example (Animals): A lion running to catch prey (change of place).
  • Example (Plants): A sunflower turning its head to follow the Sun (movement of a part).

Did you know? Even organisms that seem static, like trees, are constantly moving parts internally (e.g., growing roots or leaves, moving water and nutrients).

Key Takeaway: It means changing position, whether the whole organism moves or just a part of it moves relative to its original position.


2. R – Respiration

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

  • This is a crucial definition! It is NOT just breathing (which is gas exchange).
  • This energy release powers all life processes, collectively known as metabolism.
  • Word Equation (Aerobic Respiration): Glucose + Oxygen → Carbon Dioxide + Water (+ Energy)

Quick Tip: Respiration vs. Breathing

Don't confuse the two! Breathing is the physical act of inhaling and exhaling (Gas Exchange). Respiration is the chemical process happening inside every cell to produce energy.

Key Takeaway: All life needs energy, and respiration is the universal process (chemical reactions) that releases this energy inside the cells.


3. S – Sensitivity (or Response)

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

  • External Stimulus: Bright light, loud noise, touch. (E.g., shutting your eyes in bright light.)
  • Internal Stimulus: Changes in body temperature, blood sugar levels. (E.g., sweating when your internal temperature rises.)

Key Takeaway: Sensing the world (or your internal state) and reacting appropriately is essential for survival.


4. G – Growth

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

  • Permanent increase: The change is irreversible.
  • Dry mass: This is important! It means the increase is due to making new cells and increasing the amount of cellular material (like proteins and cellulose), not just absorbing water (which is temporary swelling).

Analogy: If you blow up a balloon, it increases in size but not in *dry mass* (it's not adding new material). A human baby growing into an adult adds new tissue and cells—that is true biological growth.

Key Takeaway: Growth means adding new, permanent living material (increase in dry mass).


5. R – Reproduction

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

  • Reproduction ensures that the species survives beyond the lifetime of a single individual.
  • There are two main types: Asexual (one parent, identical offspring) and Sexual (two parents, varied offspring). You will cover these later in B15.

Key Takeaway: The process of creating offspring to continue the species.


6. E – Excretion

Definition: Excretion is the removal of waste products of metabolism (chemical reactions in cells) and substances that are in excess of requirements.

  • Waste products: Carbon dioxide from respiration, urea from protein breakdown.
  • Removal methods: Urine, sweat, exhaled air (carbon dioxide), and sometimes through plant leaves.

⚠️ Common Confusion Alert! Excretion vs. Egestion

These two words look similar but are completely different processes:

Excretion: Removal of toxic or excess *chemical waste* produced by the body's cells (metabolism). Think CO₂ and urine.
Egestion: Removal of *undigested food* (faeces) from the gut. This food was never actually taken into the body's cells, so it's not a metabolic waste product.

Key Takeaway: Excretion is specifically about removing cellular chemical waste.


7. N – Nutrition

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

  • Plants (Autotrophs): Take in simple raw materials like carbon dioxide, water, and mineral ions to make their own complex food (glucose) via photosynthesis.
  • Animals (Heterotrophs): Take in complex organic food substances (eating other organisms).

Key Takeaway: Obtaining the necessary substances to fuel life and build new structures.


📝 Quick Review Box: MR. S GREN

Memorize these definitions, as you will often be asked to state or define them in exams!

  • Movement: Change in position or place.
  • Respiration: Chemical release of energy from food.
  • Sensitivity: Detecting and responding to stimuli.
  • Growth: Permanent increase in size and dry mass.
  • Reproduction: Making more organisms of the same kind.
  • Excretion: Removal of metabolic waste and excess substances.
  • Nutrition: Taking in materials for energy, growth, and development.

You’ve successfully identified the criteria for being a living thing! Now you're ready to explore how these living things are structured, starting with cells (Chapter B2).