Big Ideas: Key Concepts for This Unit
Before diving in, it helps to know the big ideas (called key concepts in MYP) that frame everything in this unit.
Key Concept: Change
This entire unit is built around the concept of change. Organisms constantly face changes — in temperature, light, food availability, and more. The ability to detect and respond to change is what keeps living things alive.
Related Concept: Interaction
Organisms don't exist in isolation. They interact with their environment and with other organisms. Every stimulus-response system is, at its core, an interaction between an organism and its world.
Global Context: Identities and Relationships
How do living things — from bacteria to humans — sense and relate to the world around them? Exploring stimulus and response helps us understand what it means to be alive and how all organisms are connected through shared biological principles.
As you work through these notes, keep asking yourself: What changes? How does the organism detect it? How does it respond — and why does that response help it survive?
What Is Stimulus and Response?
Living things are constantly detecting changes in their surroundings and reacting to them. This ability is one of the key features that separates living organisms from non-living things.
A stimulus (plural: stimuli) is a change in the environment — either internal or external — that is detected by an organism and triggers a reaction.
A response is the reaction or change in behaviour that an organism produces as a result of detecting a stimulus.
Stimuli can come from outside the organism (external stimuli) such as light, sound, temperature, or touch, or from inside the organism (internal stimuli) such as hunger, thirst, or changes in blood sugar level.
Think of a stimulus and response like a text message and a reply. The message (stimulus) arrives, you detect it, and then you send a reply (response). No message — no reply!
The general pathway for any stimulus-response system follows this sequence:
Stimulus → Receptor → Coordinator → Effector → Response
Understanding this pathway helps explain how every organism — from a simple bacterium to a complex human — perceives and reacts to the world around it.
13 more sections in this topic
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