What is Photosynthesis?
Photosynthesis is how trees make their own food using sunlight, water, and carbon dioxide from the air. It's the fundamental process that powers nearly all life on Earth and produces the oxygen we breathe.
The Process
Simple Equation
6 CO₂ + 6 H₂O + Light Energy → C₆H₁₂O₆ + 6 O₂
In words:
- Inputs: Carbon dioxide + Water + Sunlight
- Outputs: Sugar (glucose) + Oxygen
Where It Happens
Chloroplasts: Green organelles in leaf cells
- Thylakoids: Membrane sacks where light reactions occur
- Stroma: Fluid where sugar is assembled
- Chlorophyll: Green pigment that captures light
Two Stages
1. Light-Dependent Reactions
In thylakoid membranes:
- Chlorophyll absorbs light energy
- Water molecules are split (photolysis)
- Oxygen is released as waste
- Energy is stored in ATP and NADPH
Products: ATP, NADPH, O₂
2. Calvin Cycle (Light-Independent)
In stroma:
- CO₂ is captured from air
- ATP and NADPH provide energy
- Carbon is assembled into glucose
- Process repeats continuously
Products: Glucose (sugar)
Why Trees Are Green
Chlorophyll Absorption
Chlorophyll absorbs:
- Red light (600-700 nm)
- Blue light (400-500 nm)
Chlorophyll reflects:
- Green light (500-600 nm) ← We see this!
That's why leaves are green - they reflect the green wavelengths they don't need.
Photosynthesis in Costa Rican Trees
Tropical Efficiency
Advantages:
- Year-round warmth
- Abundant sunlight
- High rainfall (water supply)
- Long growing season
Results:
- Rapid growth rates
- High productivity
- Dense forests
- Carbon sequestration
Adaptations
Emergent trees (Ceiba, Almendro):
- Full sun exposure
- Maximum photosynthesis
- Thick, waxy leaves (reduce water loss)
Understory trees:
- Adapted to low light (1-5% sunlight)
- Larger, thinner leaves
- Slower photosynthesis
- More chlorophyll per leaf
Factors Affecting Rate
Light Intensity
- Low light: Limits photosynthesis
- Optimal light: Maximum rate
- Too much light: Can damage chlorophyll
Temperature
- Too cold (<10°C): Enzymes slow
- Optimal (25-35°C): Peak rate
- Too hot (>40°C): Enzymes denature
Water Availability
- Drought: Stomata close, stops CO₂ entry
- Adequate water: Normal rate
- Flooding: Root damage, reduced uptake
CO₂ Concentration
- Atmospheric CO₂: ~415 ppm (2023)
- Higher CO₂: Increased rate (to a point)
- Low CO₂: Limits sugar production
Products of Photosynthesis
Glucose Uses
Immediate energy: Cellular respiration Storage: Starch in roots, trunk Growth: Cellulose (cell walls) Reproduction: Flowers, fruits, seeds Defense: Resins, tannins, alkaloids
Oxygen Production
One large tree produces:
- ~120 kg oxygen per year
- Enough for ~2 people
- Absorbed by ~0.5 trees' respiration
- Net positive oxygen production
Amazon rainforest:
- Produces ~20% of world's oxygen
- "Lungs of the Earth"
Types of Photosynthesis
C3 Photosynthesis (Most Trees)
- Standard Calvin cycle
- Most common (85% of plants)
- Less efficient in hot, dry conditions
- Examples: Most Costa Rican trees
C4 Photosynthesis
- Modified pathway
- More efficient in heat/drought
- Rare in trees (mostly grasses, crops)
- Separates CO₂ capture and Calvin cycle
CAM Photosynthesis
- Opens stomata at night
- Stores CO₂ as malic acid
- Uses stored CO₂ during day
- Examples: Cacti, some epiphytes
Seasonal Variations
Dry Season (December-April)
Water stress:
- Deciduous trees shed leaves
- Reduced photosynthesis
- Evergreens maintain slower rate
- Some trees flower (energy from storage)
Rainy Season (May-November)
Optimal conditions:
- Full foliage
- Maximum photosynthesis
- Rapid growth
- Fruit production
Measuring Photosynthesis
Field Methods
Gas exchange: Measure CO₂ uptake Chlorophyll fluorescence: Assess plant health Growth rates: Indicate productivity Leaf area index: Total photosynthetic surface
Global Importance
Carbon Cycle
Trees remove CO₂:
- ~2.4 billion tons CO₂ per year (Costa Rica's forests)
- Store carbon in wood (long-term)
- Critical for climate regulation
Climate Change Mitigation
Forests:
- Absorb ~30% of human CO₂ emissions
- Old-growth forests = huge carbon sinks
- Reforestation helps offset emissions
- Protect existing forests = #1 priority
Why It Matters
Understanding photosynthesis helps with:
- Climate action: Trees absorb CO₂
- Forest conservation: Protect oxygen producers
- Reforestation: Plant more photosynthesizers
- Agriculture: Shade-grown crops optimize light
- Tree health: Diagnose stress from poor photosynthesis
Threats to Photosynthesis
Deforestation: Removes photosynthesizers Air pollution: Damages leaves, blocks light Climate change: Drought, heat stress Pests/disease: Damage leaves Habitat loss: Reduces total leaf area
Fun Facts
- One tree can have 200,000+ leaves
- Leaf surface area: Can exceed 10x ground area
- Energy capture: ~1-2% of sunlight converted
- Ancient process: Evolved 3.5 billion years ago
- Oxygen source: All atmospheric O₂ from photosynthesis
Field Recognition
Signs of active photosynthesis:
- Green, healthy leaves
- New growth
- Vigorous canopy
- Rapid growth
- Flowering and fruiting
Signs of reduced photosynthesis:
- Yellow leaves (chlorosis)
- Leaf drop
- Slow growth
- Die-back
- Sparse canopy