Photosynthesis

Photosynthesis is the biological process by which plants, algae, and certain bacteria convert light energy, usually from the sun, into chemical energy stored in glucose. This process is fundamental to life on Earth, providing the oxygen we breathe and forming the base of nearly all food chains.

Overview

The word "photosynthesis" derives from the Greek words phos (light) and synthesis (putting together). The process can be summarized by the following equation:

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

In words: six molecules of carbon dioxide plus six molecules of water, using light energy, produce one molecule of glucose and six molecules of oxygen.

History of Discovery

Early Observations

The understanding of photosynthesis developed over several centuries:

• 1648: Jan Baptist van Helmont conducted his famous willow tree experiment, concluding that plants gain mass from water
• 1727: Stephen Hales proposed that plants derive nourishment from the air
• 1771: Joseph Priestley discovered that plants produce oxygen
• 1779: Jan Ingenhousz demonstrated that light is necessary for oxygen production
• 1845: Julius Robert von Mayer proposed that plants convert light energy to chemical energy

Modern Understanding

• 1905: Frederick Frost Blackman identified light-dependent and light-independent reactions
• 1931: Cornelis van Niel proposed that oxygen comes from water, not carbon dioxide
• 1937: Robert Hill demonstrated the light reaction in isolated chloroplasts
• 1961: Melvin Calvin mapped the carbon fixation pathway (Calvin Cycle)

The Photosynthetic Apparatus

Chloroplasts

Photosynthesis in plants occurs within specialized organelles called chloroplasts. These oval-shaped structures contain:

• Outer membrane: Permeable to small molecules
• Inner membrane: Contains transport proteins
• Stroma: The fluid-filled space where the Calvin Cycle occurs
• Thylakoids: Membrane-bound compartments arranged in stacks called grana
• Thylakoid membrane: Contains the photosynthetic pigments and electron transport chain

Photosynthetic Pigments

Several pigments absorb light energy:

Chlorophyll a is the primary pigment directly involved in the light reactions. Other pigments serve as accessory pigments, expanding the range of light that can be captured.

The Two Stages of Photosynthesis

Stage 1: Light-Dependent Reactions

These reactions occur in the thylakoid membranes and require direct light:

Photosystem II (PSII)

1. Light energy excites electrons in chlorophyll P680
2. Water molecules are split (photolysis): 2H₂O → 4H⁺ + 4e⁻ + O₂
3. Excited electrons pass to the electron transport chain
4. Oxygen is released as a byproduct

Electron Transport Chain

1. Electrons flow through plastoquinone, cytochrome b6f complex, and plastocyanin
2. This flow pumps H⁺ ions into the thylakoid lumen
3. The proton gradient drives ATP synthesis via ATP synthase (chemiosmosis)

Photosystem I (PSI)

1. Light energy excites electrons in chlorophyll P700
2. Electrons pass through ferredoxin
3. NADP⁺ reductase produces NADPH

Products of Light Reactions: ATP, NADPH, and O₂

Stage 2: Light-Independent Reactions (Calvin Cycle)

Also called the "dark reactions" or carbon fixation, these occur in the stroma:

Phase 1: Carbon Fixation

• CO₂ combines with ribulose-1,5-bisphosphate (RuBP)
• The enzyme RuBisCO catalyzes this reaction
• Produces two molecules of 3-phosphoglycerate (3-PGA)

Phase 2: Reduction

• ATP and NADPH from light reactions are used
• 3-PGA is converted to glyceraldehyde-3-phosphate (G3P)
• For every 3 CO₂ molecules, 6 G3P molecules are produced

Phase 3: Regeneration

• 5 of every 6 G3P molecules regenerate RuBP
• 1 G3P exits the cycle to form glucose
• ATP is consumed in this process

Net Result: 3 CO₂ + 9 ATP + 6 NADPH → 1 G3P + 9 ADP + 8 Pi + 6 NADP⁺

Types of Photosynthesis

C3 Photosynthesis

• Most common pathway (85% of plant species)
• First stable product is 3-carbon molecule (3-PGA)
• Examples: rice, wheat, soybeans
• Less efficient in hot, dry conditions due to photorespiration

C4 Photosynthesis

• Carbon is first fixed into 4-carbon compounds
• Spatial separation of initial fixation and Calvin Cycle
• More efficient in hot, sunny environments
• Examples: corn, sugarcane, sorghum

CAM Photosynthesis (Crassulacean Acid Metabolism)

• Temporal separation of CO₂ uptake and fixation
• Stomata open at night to reduce water loss
• CO₂ stored as malic acid, released during day
• Examples: cacti, pineapple, succulents

Factors Affecting Photosynthesis

Light Intensity

• Rate increases with light intensity up to a saturation point
• Beyond saturation, rate plateaus
• Very high intensities can cause photoinhibition

Carbon Dioxide Concentration

• Rate increases with CO₂ up to a saturation point
• Current atmospheric CO₂ (~420 ppm) is below saturation for C3 plants
• C4 plants are already near saturation at current levels

Temperature

• Optimal range: 25-35°C for most plants
• Too cold: enzyme activity decreases
• Too hot: enzymes denature, photorespiration increases

Water Availability

• Water stress causes stomata to close
• Reduces CO₂ entry into leaves
• Directly affects the light reactions (water as electron source)

Ecological Importance

Oxygen Production

• Photosynthesis produces virtually all atmospheric oxygen
• Estimated 50-80% comes from marine phytoplankton
• Terrestrial plants contribute the remainder

Carbon Cycle

• Removes approximately 120 billion tons of CO₂ from atmosphere annually
• Forests and oceans serve as major carbon sinks
• Critical for regulating global climate

Food Webs

• Primary producers form the base of all food chains
• All heterotrophs depend directly or indirectly on photosynthesis
• Approximately 1-2% of light energy is converted to biomass

Photosynthesis and Human Society

Agriculture

• Crop yields depend on photosynthetic efficiency
• Breeding programs aim to enhance photosynthesis
• Understanding C4 pathway helps develop more efficient crops

Climate Change Mitigation

• Forests and oceans absorb significant CO₂
• Afforestation and reforestation are key strategies
• Enhanced photosynthesis research could help carbon capture

Artificial Photosynthesis

• Scientists are developing systems to mimic natural photosynthesis
• Goals include producing hydrogen fuel and other chemicals
• Could provide clean, renewable energy solutions

See Also

• Climate Change
• DNA
• The Solar System

References

1. Blankenship, R.E. (2014). Molecular Mechanisms of Photosynthesis. Wiley-Blackwell.
2. Taiz, L., et al. (2015). Plant Physiology and Development. Sinauer Associates.
3. Nobel, P.S. (2009). Physicochemical and Environmental Plant Physiology. Academic Press.