Edexcel iGCSE Biology-3.33 - 3.35 Response to Stimuli: Plants- Study Notes- New Syllabus
Edexcel iGCSE Biology-3.33 – 3.35 Response to Stimuli: Plants- Study Notes- New syllabus
Edexcel iGCSE Biology-3.33 – 3.35 Response to Stimuli: Plants- Study Notes -Edexcel iGCSE Biology – per latest Syllabus.
Key Concepts:
3.33 understand that plants respond to stimuli
3.34 describe the geotropic and phototropic responses of roots and stems
3.35 understand the role of auxin in the phototropic response of stems
Plant Responses to Stimuli
🌱 Introduction
Like animals, plants also need to respond to their environment for survival and growth.
These responses are usually slower because plants don’t have a nervous system.
Controlled mainly by plant hormones (e.g., auxins).
🌞 What is a Stimulus?
A stimulus = any change in the environment.
Plants detect stimuli and respond by growing in a particular direction.
🌿 Examples of Plant Responses
- Light (Phototropism) → Shoots grow towards light to maximise photosynthesis.
- Gravity (Geotropism/Gravitropism) → Roots grow downwards into soil (positive geotropism); shoots grow upwards (negative geotropism).
- Water (Hydrotropism) → Roots grow towards water to ensure absorption for photosynthesis and transport.
- Touch (Thigmotropism) → Seen in climbing plants, where tendrils coil around supports.
🌼 Why Plant Responses are Important
- Ensure better chances of survival.
- Help in absorbing water and minerals.
- Maximise light capture for photosynthesis.
- Anchor plants securely in soil.
📊 Summary Table
| Stimulus | Plant Response | Example |
|---|---|---|
| Light | Shoots bend towards light | Sunflower shoot |
| Gravity | Roots grow down, shoots grow up | Seed germination |
| Water | Roots grow towards water | Root system |
| Touch | Tendrils coil around support | Pea plant |
📌 Quick Recap
Plants respond to light, gravity, water, and touch.
These responses are called tropisms.
Controlled mainly by plant hormones like auxin.
Purpose → survival, growth, and reproduction.
Geotropism & Phototropism in Roots and Stems
🌱 Introduction
Tropism = growth response of a plant to a stimulus.
Two key types:
- Geotropism → response to gravity.
- Phototropism → response to light.
These are mainly controlled by the plant hormone auxin.
☀️ Phototropism (Response to Light)
- Shoots: Grow towards light (positive phototropism).
→ Auxin collects on shaded side → cells elongate more → shoot bends towards light. - Roots: Grow away from light (negative phototropism).
→ Helps roots stay underground to absorb water and minerals.
🌍 Geotropism (Response to Gravity)
- Roots: Grow towards gravity (positive geotropism).
→ Auxin gathers on lower side → slows growth there → upper cells elongate → root bends downward. - Shoots: Grow away from gravity (negative geotropism).
→ Auxin collects on lower side → cells elongate more → shoot bends upward.
📊 Summary Table
| Tropism | Roots Response | Shoots Response |
|---|---|---|
| Phototropism | Negative (away from light) | Positive (towards light) |
| Geotropism | Positive (towards gravity) | Negative (away from gravity) |
📌 Quick Recap
Shoots → positive phototropism & negative geotropism.
Roots → negative phototropism & positive geotropism.
Controlled by auxin distribution.
Purpose → shoots seek light for photosynthesis; roots anchor plant and absorb water/minerals.
Role of Auxin in Phototropic Response of Stems
🌱 What is Auxin?
Auxin = plant growth hormone.
Mainly produced at the tip of the stem.
Controls cell elongation.
🔆 How Auxin Works in Phototropism
- Light falls on one side of the stem.
- Auxin redistributes → moves to the shaded side.
- Higher auxin concentration on shaded side → cells elongate more.
- Stem bends towards light → positive phototropism.
🎯 Why is this Important?
- Helps shoots grow towards light.
- Ensures maximum photosynthesis.
- Advantage → better growth and survival.
📊 Key Points Table
| Step | What Happens | Effect |
|---|---|---|
| 1 | Light on one side | Auxin shifts to shaded side |
| 2 | Auxin builds up | Shaded cells elongate more |
| 3 | Stem bends | Towards light (positive phototropism) |
📌 Quick Recap
Auxin is produced at the tip of stems.
Redistributes to the shaded side when light is uneven.
Causes unequal growth → stem bends towards light.
Ensures efficient photosynthesis.