Photosynthesis & Plant Physiology

Plants are the foundation of almost every food chain on Earth. Through photosynthesis, they convert sunlight into chemical energy, producing the oxygen we breathe and the food we eat. This process is arguably the most important chemical reaction on the planet.

The Overall Equation

6CO₂ + 6H₂O + Light Energy --> C₆H₁₂O₆ + 6O₂

Photosynthesis occurs in the chloroplasts, specifically using the green pigment chlorophyll which absorbs light energy.

Light Reactions (Light-Dependent Reactions)

Occur in the thylakoid membranes of chloroplasts.

What happens:

1. Chlorophyll absorbs light energy
2. Water molecules are split: 2H₂O --> 4H⁺ + 4e⁻ + O₂ (this is where the oxygen comes from!)
3. Light energy is converted to chemical energy: ATP and NADPH are produced
4. These energy carriers move to the stroma for the dark reactions

Dark Reactions (Calvin Cycle / Light-Independent Reactions)

Occur in the stroma of chloroplasts. Called "dark" not because they need darkness, but because they do not directly require light.

What happens:

1. CO₂ is fixed (combined with a 5-carbon compound called RuBP)
2. Using ATP and NADPH from light reactions, CO₂ is reduced to form glucose
3. RuBP is regenerated to continue the cycle

The Calvin cycle "fixes" 3 CO₂ molecules to produce one molecule of G3P (a 3-carbon sugar).

Factors Affecting Photosynthesis

Several factors can limit the rate of photosynthesis:

• Light intensity: Increases rate up to a saturation point
• CO₂ concentration: Increases rate up to a saturation point
• Temperature: Optimal around 25-35 C; too high denatures enzymes
• Water availability: Essential as a raw material

The factor in shortest supply is the limiting factor (Blackman's Law of Limiting Factors).

Transpiration

Transpiration is the loss of water vapour from plant leaves through stomata. It may seem wasteful, but it serves vital functions:

• Creates a pull (transpiration pull) that draws water up from roots through xylem
• Cools the plant (like sweating in humans)
• Helps in mineral transport from soil to all plant parts

Factors affecting transpiration: Temperature, humidity, wind speed, light (stomata open in light)

Transpiration Pull Theory

Water moves from roots to leaves through xylem due to:

1. Root pressure: Osmotic pressure pushes water up from roots
2. Transpiration pull (cohesion-tension theory): As water evaporates from leaves, it creates a negative pressure that pulls water up. Water molecules stick together (cohesion) forming an unbroken column.

Nepal Connection

Nepal's tea plantations in Ilam and Jhapa depend on optimal photosynthesis conditions -- adequate rainfall, moderate temperature, and good sunlight. Farmers manage shade trees to control light intensity for tea plants.

Key Takeaways

• Photosynthesis: 6CO₂ + 6H₂O + light --> C₆H₁₂O₆ + 6O₂
• Light reactions occur in thylakoids (produce ATP, NADPH, O₂)
• Dark reactions (Calvin cycle) occur in stroma (fix CO₂ into glucose)
• Transpiration drives water transport and cools the plant