Responding to the Environment

Term 1, Week 5, Lesson 1

A plant responds to the stimulus of the landing fly.

Do Now

Look at the image below of a plant on a windowsill.

Plant bending toward window phototropism light stimulus response

In your book, answer the following:

1. What do you notice about the direction the plant is growing?
2. What do you think is causing this?
3. What do you predict would happen if the plant was rotated 180° so the leaves faced away from the window?

You have 3 minutes.

Daily Review

Answer the following 5 multiple choice questions in your book:

1. Which WA plant produces woody follicles that open after fire to release seeds?
   • 1. Jarrah
   • 2. Banksia
   • 3. Tuart
   • 4. Sheoak
2. A lignotuber helps a plant survive bushfire by:
   • 1. Insulating the cambium layer from heat
   • 2. Releasing seeds when the temperature rises
   • 3. Producing new shoots directly from the bark
   • 4. Storing energy and dormant buds underground
3. Nocturnal behaviour in the bilby is classified as:
   • 1. A structural adaptation
   • 2. A physiological adaptation
   • 3. A behavioural adaptation
   • 4. A learned trait
4. Countercurrent heat exchange in penguin flippers is classified as:
   • 1. A structural adaptation
   • 2. A behavioural adaptation
   • 3. A physiological adaptation
   • 4. An ecological adaptation
5. Blubber in seals is classified as:
   • 1. A behavioural adaptation
   • 2. A physiological adaptation
   • 3. A structural adaptation
   • 4. An ecological adaptation

NoteAnswers

1. B 2) D 3) C 4) C 5) C

Learning Intentions

Today we are learning about stimulus–response pathways and homeostasis — the mechanisms that allow all organisms to detect changes in their environment and respond in ways that support their survival.

Success Criteria

You will be successful if you have:

• Defined the terms stimulus, receptor, coordinator, effector and response, and used them to describe a stimulus–response pathway
• Given at least two examples each of how plants and how animals respond to environmental change
• Explained why the ability to detect and respond to stimuli is essential for an organism’s survival

Keywords

Stimulus

What changes — a detectable change in the internal or external environment that triggers a response in an organism. Plural: stimuli. Examples include changes in temperature, light, moisture, chemicals, touch, pressure and sound.

Receptor

What detects the change — a specialised cell or organ that detects a specific type of stimulus and converts it into a signal that can be processed by a coordinator. Different receptors respond to different stimuli (e.g. thermoreceptors detect temperature; photoreceptors detect light).

Coordinator

Decides what to do — a structure such as the brain, spinal cord or an endocrine (hormone-producing) gland that receives signals from receptors, processes the information, and directs an appropriate response.

Effector

Makes the change — a muscle or gland that carries out the response directed by the coordinator. Muscles cause movement; glands release hormones or other substances.

Response

The overall response — the action taken by an organism as a result of receiving and processing a stimulus.

Homeostasis

The maintenance of a stable internal environment within an organism, despite changes in external conditions. From the Greek homoios (similar) and stasis (standing still). Examples include temperature regulation, blood pH and blood glucose control.

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Learning Activities

Activity 1 — I DO: How Organisms Detect and Respond to Change

The Stimulus–Response Model

All responses in living organisms follow the same basic pathway:

Stimulus → Receptor → Coordinator → Effector → Response

Stimulus response pathway diagram

Component	Role	Example
Stimulus	The change that is detected	Rising temperature, drop in light levels
Receptor	Detects the stimulus	Thermoreceptors in skin; photoreceptors in eyes
Coordinator	Processes the signal and coordinates a response	Brain, spinal cord, hypothalamus, hormones
Effector	Carries out the response	Muscles, glands
Response	The action taken	Sweating, moving to shade, stomata closing

Types of Stimuli

Organisms respond to many different types of stimuli:

• Physical: temperature, light, sound, pressure, touch, gravity
• Chemical: CO₂ levels, oxygen, nutrients, toxins, hormones
• Biological: presence of a predator, competitor, or mate

Plant Responses

Plants cannot move away from danger, but they respond using tropisms — directional growth responses to stimuli.

Phototropism geotropism tropism diagram plant responses light gravity

Tropism	Stimulus	Plant Response
Phototropism	Light	Shoots grow toward light (positive); roots grow away (negative)
Geotropism (gravitropism)	Gravity	Roots grow downward; shoots grow upward
Stomatal response	CO₂ levels, light, water stress	Stomata open in the light to allow CO₂ in; close when water stress is high

Stomata response to low water environment

WA Example: During hot summer afternoons in the jarrah forest, banksias and eucalypts close their stomata to reduce water loss by transpiration. This is a stimulus–response to the combined stimuli of high temperature and falling water availability.

Animal Responses

Animals have two main communication systems for responding to stimuli:

System	Speed	Duration	Examples
Nervous system	Very fast (milliseconds)	Short-term	Reflex actions, withdrawal from pain, eye-blink reflex
Endocrine (hormonal) system	Slower (seconds to hours)	Long-term	Adrenaline release during stress, reproductive hormones

Reflex arc: An automatic, rapid response that bypasses the brain for speed. When you touch a hot object, signals travel from receptor → spinal cord → effector muscle, and your hand withdraws before your brain consciously registers pain.

Homeostasis

All the stimulus–response systems in an organism work together to maintain homeostasis — a stable internal environment.

Homeostasis negative feedback loop

• Variables that are regulated include: body temperature, blood pH, blood glucose, water balance.
• The key mechanism is negative feedback: when a variable moves away from its set point, the body responds in a way that opposes and reverses the change — returning the variable to normal.

WA Examples of Homeostatic Responses:

Animal	Stimulus	Response	Type
Banksia	High temperature, low humidity	Stomata close → reduces transpiration	Physiological
Quokka (Rottnest Island)	Rising midday temperature	Retreats into dense shade, reduces activity	Behavioural
Thorny devil	Cool morning temperatures	Orients body perpendicular to sun rays to maximise solar radiation absorption	Behavioural
Red kangaroo	Rising core body temperature	Licks forearms; increased panting	Physiological + Behavioural

Check for Understanding

Fill in the blank boxes to complete the stimulus–response chain:

Scenario: A bilby emerges from its burrow and detects the scent of a predator on the night breeze.

Stimulus	Receptor	Coordinator	Effector	Response
Chemical scent of predator

Answer: Receptor — olfactory (smell) receptors in the nose; Coordinator — brain (processes the threat signal); Effector — leg muscles; Response — retreats rapidly back into burrow

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Activity 2 — WE DO: Stimulus–Response Table for WA Organisms

Examples of homeostatic responses.

As a class, we will complete a stimulus–response table for three WA organisms — one plant, one ectotherm and one endotherm.

Organism	Stimulus	Receptor	Coordinator	Effector	Response	Type (S/B/P)
Banksia	Hot, dry afternoon — water stress				Stomata close
Thorny devil	Low body temperature at dawn				Orients body toward sun
Red kangaroo	Core body temperature rising above 38°C				Licks forearms; seeks shade

Discussion Questions

• What do all three responses have in common? What does each organism gain from responding to its stimulus?
• The thorny devil’s morning orientation is voluntary. Does that make it less effective as a response? Why or why not?
• Can you think of a scenario in which a stimulus–response might fail — where the response doesn’t return the organism to its set point? What might happen?

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Activity 3 — YOU DO: Stimulus–Response Pathways

Organisms responding to stimuli

Complete the worksheet: 151-responding-to-environment-you-do.docx

You will complete stimulus–response chains for given scenarios and explain why each response improves the organism’s survival chances.

Work independently. You have 10 minutes.

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Notes

Use this space to write any important points from today’s lesson.

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Reflection

1. A specialised cell that detects a stimulus is called:
   • 1. An effector
   • 2. A receptor
   • 3. A coordinator
   • 4. A response
2. Which of the following is the correct order in a stimulus–response pathway?
   • 1. Response → receptor → effector → stimulus
   • 2. Stimulus → effector → receptor → response
   • 3. Stimulus → receptor → coordinator → effector → response
   • 4. Receptor → stimulus → response → effector
3. Homeostasis refers to:
   • 1. An organism’s ability to move toward a food source
   • 2. The maintenance of a stable internal environment
   • 3. The process of growing new leaves after fire
   • 4. The study of organisms in ecosystems
4. A banksia closing its stomata on a hot afternoon is responding to which stimulus?
   • 1. Predator detection
   • 2. Change in light direction
   • 3. Water stress and rising leaf temperature
   • 4. Soil nutrient levels
5. Short answer: Using the stimulus–response model, describe what happens when you touch a hot object and quickly pull your hand away. Identify the stimulus, receptor, coordinator, effector and response in your answer.

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Home-study

Think of a time you responded to a change in your environment today (e.g. turning on a light, putting on a jumper, eating when hungry). Write a short paragraph (3–4 sentences) describing your stimulus–response using the correct vocabulary: stimulus, receptor, coordinator, effector and response.