Welcome to Chapter 1: Features of Organisms

Hello Biologist! This chapter is your starting point for the entire IGCSE Biology course. We are going to answer a fundamental question: What makes something alive?

Understanding the basic features of living things allows us to distinguish between organisms (like plants, animals, and bacteria) and non-living objects (like rocks and water). Mastering these seven characteristics is essential, as they form the foundation for many topics later on!

The Seven Characteristics of Life

Scientists agree that to be considered truly "alive," an object must show all seven characteristics listed below.

Memory Trick: MR SGREN (or Mrs. Gren)

Use this simple mnemonic to remember all seven features:

  • Movement
  • Respiration
  • Sensitivity
  • Growth
  • Reproduction
  • Excretion
  • Nutrition

1. Movement

Definition (Syllabus 1.1.1a)

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

  • In Animals: Movement often means moving the whole body (locomotion), like running, flying, or swimming, allowing them to find food or escape danger.
  • In Plants: Movement is usually slower and limited to parts of the plant, such as flowers turning towards the sun or roots growing downwards.

Did you know? Even stationary plants move! They change position as they grow or when they track light, which is still classified as movement.

Key Takeaway: Movement is about changing position, whether of the whole body or just a part.

2. Respiration

Definition (Syllabus 1.1.1b)

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

  • IMPORTANT: Respiration is not the same as breathing! Breathing (gas exchange) is just moving air in and out. Respiration is the process that happens *inside* the cells.
  • This released energy (often from breaking down glucose) powers all life processes (known collectively as metabolism). Without respiration, organisms cannot move, grow, or excrete.

Analogy: Think of a car engine. Breathing is simply opening the bonnet (hood) to let air in. Respiration is the controlled combustion of fuel (glucose) inside the engine to release the power.

Key Takeaway: Respiration is the controlled chemical process *within* cells that releases energy from food.

3. Sensitivity

Definition (Syllabus 1.1.1c)

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

  • The change being detected is called the stimulus (plural: stimuli).
  • The action taken in response is the response.
  • Examples:
    • When you touch something hot (stimulus), you quickly pull your hand away (response).
    • A plant root grows towards water (stimulus: chemical change in the soil).

Don't worry if this seems tricky at first; we cover sensitivity (nerves, hormones) in much more detail later! For now, remember it's all about responding to the world around you.

Key Takeaway: Sensitivity allows an organism to react appropriately to stimuli for survival.

4. Growth

Definition (Syllabus 1.1.1d)

Growth is a permanent increase in size and dry mass.

  • This involves making more cells (cell division) and increasing the size of existing cells.
  • Why "dry mass"? If you drink a lot of water, you increase in mass, but this isn't true growth. True growth involves the permanent production of new, solid material (like protein, fat, or cellulose).

Analogy: When you inflate a balloon, it gets bigger (not growth). When you build a house by adding permanent bricks and mortar, it increases its permanent size and mass (true growth).

Key Takeaway: Growth is a permanent, irreversible increase in size and mass.

5. Reproduction

Definition (Syllabus 1.1.1e)

Reproduction is the processes that make more of the same kind of organism.

  • Reproduction is essential for the survival of the species (a group of organisms that can reproduce to produce fertile offspring), even if a single organism doesn't need to reproduce to survive itself.
  • It allows genetic material to be passed to the next generation.

Key Takeaway: Reproduction creates new individuals to ensure the continuity of the species.

6. Excretion

Definition (Syllabus 1.1.1f)

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

  • Waste Products of Metabolism: These are toxic chemicals produced by cell activities (like respiration). Examples include carbon dioxide, urea, and excess salts.
  • Common Confusion: Excretion vs. Egestion
    • Excretion: Removes chemical waste (e.g., urea from the kidneys, CO₂ from the lungs).
    • Egestion: Removes undigested food (faeces) from the digestive tract, which was never part of the body's metabolism.

If you burn wood (metabolism), the smoke and ash must be removed (excretion). The solid log that didn't burn (undigested food) is egestion.

Key Takeaway: Excretion removes harmful metabolic waste products to maintain a healthy internal environment.

7. Nutrition

Definition (Syllabus 1.1.1g)

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

  • In Plants (Producers): They make their own food (organic nutrients) using simple raw materials (carbon dioxide and water) and light energy in a process called photosynthesis.
  • In Animals (Consumers): They obtain organic nutrients by eating other organisms (plants or animals).

Key Takeaway: Nutrition is how organisms obtain the necessary materials to fuel their life processes and build new body tissues.

Quick Review: The Characteristics of Life

  • Movement: Change in position or place.
  • Respiration: Release of energy from food inside cells.
  • Sensitivity: Responding to stimuli.
  • Growth: Permanent increase in size and dry mass.
  • Reproduction: Making more organisms of the same kind.
  • Excretion: Removing toxic metabolic waste.
  • Nutrition: Taking in materials for energy and growth.

If something displays all these features, congratulations—it’s a living organism!