CIE AS/A Level Biology -14.2 Homeostasis in plants- Study Notes- New Syllabus
CIE AS/A Level Biology -14.2 Homeostasis in plants- Study Notes- New Syllabus
Ace A level Biology Exam with CIE AS/A Level Biology -14.2 Homeostasis in plants- Study Notes- New Syllabus
Key Concepts:
- explain that stomata respond to changes in environmental conditions by opening and closing and that regulation of stomatal aperture balances the need for carbon dioxide uptake by diffusion with the need to minimise water loss by transpiration
- explain that stomata have daily rhythms of opening and closing
- describe the structure and function of guard cells and explain the mechanism by which they open and close stomata
- describe the role of abscisic acid in the closure of stomata during times of water stress, including the role of calcium ions as a second messenger
Stomatal Regulation
📌 Overview
- Stomata are pores on the leaf surface that allow gas exchange.
- They respond to environmental changes by opening or closing, balancing CO₂ uptake for photosynthesis and water loss via transpiration.
🌱 Mechanism
- Opening of stomata:
- Triggered by light (especially blue light) and low CO₂ levels inside the leaf.
- Guard cells absorb water, become turgid, and open the pore → allows CO₂ in for photosynthesis.
- Closing of stomata:
- Triggered by darkness, high temperature, or water stress.
- Guard cells lose water, become flaccid, and close the pore → reduces water loss by transpiration.
🌱 Functional Importance
- Ensures sufficient CO₂ enters for photosynthesis.
- Minimizes excess water loss, helping the plant survive in dry conditions.
- Stomatal regulation is an example of homeostasis in plants.
🧠 Key Points:
- Stomata open and close in response to environmental stimuli.
- The aperture is regulated by guard cells through changes in turgor.
- This mechanism balances photosynthetic needs with water conservation.
Daily Rhythms of Stomatal Opening and Closing
📌 Overview
- Stomata exhibit daily (diurnal) rhythms, opening and closing in a predictable pattern over 24 hours.
- This is linked to light availability and the plant’s photosynthetic activity.
🌱 Pattern of Stomatal Activity
| Time of Day | Stomatal Status | Reason |
|---|---|---|
| Morning | Open | Light triggers guard cells → turgid → CO₂ uptake for photosynthesis. |
| Midday | Partially open or closed | High temperature and potential water stress → minimize water loss. |
| Evening/Night | Closed | No light → photosynthesis stops → guard cells flaccid → water conservation. |
🌱 Functional Importance
- Optimizes photosynthesis: Stomata open when light is available to take in CO₂.
- Minimizes water loss: Stomata close during darkness or heat stress.
- Supports the plant’s daily water and energy balance.
🧠 Key Points:
- Stomatal rhythms are endogenous but influenced by light and temperature.
- They demonstrate plant adaptation to environmental cycles.
- Essential for efficient gas exchange and water conservation.
Guard Cells and Stomatal Movement
📌 Overview
- Guard cells are specialized cells surrounding each stoma (pore) on the leaf surface.
- They control the opening and closing of stomata, regulating gas exchange and water loss.
🌱 Structure of Guard Cells
| Feature | Description & Function |
|---|---|
| Shape | Bean-shaped or kidney-shaped; enables pore opening when turgid. |
| Cell wall thickness | Thicker on inner wall (toward stoma), thinner on outer wall → allows curvature. |
| Chloroplasts | Contain chlorophyll → photosynthesis generates ATP for ion transport. |
| Vacuole | Stores water; turgor changes drive opening and closing. |
🌱 Mechanism of Stomatal Opening
- Light triggers ATP production in guard cell chloroplasts.
- Active transport of K⁺ ions into guard cells from surrounding epidermal cells.
- Water enters by osmosis → guard cells become turgid.
- Cell walls curve outward → stoma opens.
🌱 Mechanism of Stomatal Closing
- K⁺ ions are pumped out of guard cells.
- Water leaves by osmosis → guard cells become flaccid.
- Cell walls relax → stoma closes.
🌱 Functional Importance
- Opening: Allows CO₂ in for photosynthesis.
- Closing: Reduces water loss during high temperature or water stress.
- Balances photosynthesis with transpiration, maintaining plant water status.
🧠 Key Points:
- Guard cells regulate stomatal aperture via changes in turgor pressure.
- Ion transport and water movement are central to this mechanism.
- Structural features like cell wall thickness and chloroplasts are essential for function.
Abscisic Acid (ABA) and Stomatal Closure
📌 Overview
- Abscisic acid (ABA) is a plant hormone that helps prevent water loss during drought or water stress.
- It triggers stomatal closure by affecting guard cell turgor.
🌱 Mechanism of Action
- Water stress detected → ABA is synthesized in roots and leaves.
- ABA binds to receptors on guard cells.
- Calcium ions (Ca²⁺) act as second messengers:
- ABA stimulates Ca²⁺ release inside guard cells.
- Ca²⁺ activates ion channels in guard cell membranes:
- K⁺ ions leave the guard cells.
- Cl⁻ and other anions also exit.
- Water leaves by osmosis → guard cells become flaccid.
- Stoma closes, reducing transpiration.
🌱 Functional Importance
- Prevents excessive water loss during drought.
- Helps maintain cell turgor and plant water balance.
- Part of plant stress response to environmental conditions.
🧠 Key Points:
- ABA = stress hormone → triggers stomatal closure.
- Ca²⁺ = second messenger, linking ABA signal to ion movement.
- Guard cell turgor changes cause stoma to close, conserving water.