8.2 Gas Exchange in Plants: Easy-to-Learn Notes

These notes are structured for quick revision—bold key terms, bullet points for flow, and simple explanations with visuals in mind (e.g., refer to Figs. 8.4 & 8.5). No point missed from the text. Study tip: Read aloud, draw diagrams, and quiz yourself on mechanisms!

1. Overview of Gas Exchange

Gas exchange in plants involves intake of CO₂ (for photosynthesis) and O₂ (for respiration), plus release of byproducts.
Plants lack lungs; exchange happens via diffusion through tiny pores.
Key site: Stomata (singular: stoma)—tiny openings/pores in plant tissues for gaseous exchange.
- Location: Mostly on leaves (underside for protection), but also on some stems.
Stomata control transpiration rate (water loss as vapor) alongside gas flow.

2. Structure of Stomata

Surrounded by guard cells: Specialized, bean-shaped cells with chloroplasts (for photosynthesis/energy).
Guard cells act as "multisensory hydraulic valves" (Fig. 8.4 shows SEM of open/closed stomata on lavender leaf—open = turgid/swollen; closed = flaccid/shrunk).
Function: Open/close pores based on environmental conditions (light, CO₂, water, temperature).

Daily Rhythm

Daytime (light present): Stomata open → CO₂ enters for photosynthesis; O₂ exits.
Nighttime (no light): Photosynthesis stops → Stomata close to conserve water (minimize transpiration).
- But respiration continues: O₂ in, CO₂ out (via diffusion, even when closed—gases still pass slowly).

3. Mechanisms of Opening & Closing Stomata

Two main hypotheses explain turgor changes (water pressure) in guard cells. Both involve osmosis (water movement from high to low water potential).

A. Starch-Sugar Hypothesis (Proposed by H. Van Mohl, 1856)

Day (Opening):
- Guard cells photosynthesize → Produce soluble sugars (e.g., glucose).
- High sugar concentration → Low water potential in guard cells.
- Water enters via osmosis → Guard cells become turgid (swollen) → Stomata open.
Night (Closing):
- No photosynthesis → Sugars convert to insoluble starch or used in respiration.
- Low sugar → High water potential → Water exits guard cells.
- Guard cells become flaccid (limp) → Stomata close.
Limitation: Doesn't explain rapid turgor changes during quick stomatal movements.

B. Influx of K⁺ Ion Hypothesis (Potassium Pump Theory)

Day (Opening):
- Active transport pumps K⁺ ions (potassium) into guard cells → Lowers osmotic potential (more solutes inside).
- Water enters via osmosis → Guard cells turgid → Stomata open.
- Blue light boosts this: Acidifies surroundings → Enhances K⁺ uptake → More water absorption.
Night (Closing):
- K⁺ passively diffuses out (no energy for pumping).
- Water follows out → Guard cells flaccid → Stomata close.
Advantage: Explains fast responses to light/CO₂ changes.

Quick Tip: Remember "Sugar Swells, Potassium Pumps"—both lower water potential to pull in water!

4. Role of Hormones in Stomatal Movement

Abscisic Acid (ABA): Stress hormone released by mesophyll cells during high temperature or wilting (water shortage).
- Effect: Stops active transport of K⁺ into guard cells (overrides light/CO₂ signals).
- Result: K⁺ pumping halts → Stomata close quickly to prevent further water loss.
Why? Protects plant from dehydration—emergency override!

5. Leaf Tissues Involved in Gas Exchange

Palisade Mesophyll:
- Location: Just beneath upper epidermis of leaf.
- Structure: Elongated, tightly packed cells rich in chloroplasts.
- Function: Maximizes light absorption → Efficient conversion of light to chemical energy (photosynthesis hub).
Spongy Mesophyll:
- Location: Below palisade layer, near lower epidermis.
- Structure: Loosely packed cells with large air spaces.
- Function: Allows gas diffusion; increases surface area for exchange.
Other Layers (Fig. 8.5: Full leaf cross-section):
- Cuticle: Waxy upper layer (reduces water loss).
- Epidermis: Outer skin (upper/lower); stomata here.
- Guard Cells & Stoma: At lower epidermis for gas entry/exit.

6. Pathway of Gas Diffusion in Leaves

CO₂ Entry (from atmosphere):
1. Diffuses through stomata (open pores).
2. Travels via air spaces in spongy mesophyll.
3. Enters palisade mesophyll cells → Used in photosynthesis (Calvin cycle).
O₂ Exit (produced in photosynthesis):
1. Diffuses out of palisade cells.
2. Through spongy mesophyll air spaces.
3. Exits leaf via stomata.
Key Principle: All via passive diffusion (high to low concentration gradient).
Nighttime Note: Reverse for respiration—O₂ in, CO₂ out (limited by closed stomata).

Quick Revision Table: Open vs. Closed Stomata

Aspect	Open Stomata (Day)	Closed Stomata (Night/Stress)
Trigger	Light, low CO₂, K⁺ influx/sugars	Darkness, high CO₂, ABA, K⁺ efflux
Guard Cells	Turgid (water in via osmosis)	Flaccid (water out)
Gas Flow	CO₂ in, O₂ out (photosynthesis)	Minimal (respiration only)
Water Loss	High (transpiration)	Low (conservation)