Turning Light Into Life
Photosynthesis is the biological process by which plants, algae, and cyanobacteria convert light energy into chemical energy stored in glucose. It is arguably the most important chemical reaction on Earth: it produces virtually all the food in the biosphere and generates the oxygen we breathe.
The overall equation for oxygenic photosynthesis is:
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
This deceptively simple equation conceals a remarkable series of electron transfers, proton pumping, and enzymatic reactions occurring in two distinct but interconnected stages.
Where Photosynthesis Occurs
In plants, photosynthesis takes place in chloroplasts — organelles with three membrane systems: - Outer and inner membranes: the chloroplast envelope - Thylakoid membranes: stacked into coin-like discs called grana, containing the photosynthetic machinery - Stroma: the fluid-filled space outside the thylakoids, where carbon fixation occurs
Photosynthetic Pigments
Light is captured by pigment molecules embedded in the thylakoid membrane. The primary pigment is chlorophyll, a porphyrin ring with a central Mg²⁺ ion and a long hydrophobic phytol tail.
- Chlorophyll a: absorbs light most strongly at ~430 nm (blue-violet) and ~680 nm (red); the primary reaction-center pigment
- Chlorophyll b: absorbs at ~453 nm and ~642 nm; an accessory pigment that widens the absorption spectrum
- Carotenoids (carotenes and xanthophylls): absorb blue-green light (400–500 nm); protect against photooxidative damage and transfer energy to chlorophyll a
The green color of plants results from chlorophyll reflecting green light (~530 nm), which neither chlorophyll a nor b absorbs efficiently.
Pigments are organized into photosystems — large protein-pigment complexes that funnel absorbed light energy to reaction centers where photochemistry occurs.
Stage 1: The Light Reactions
The light reactions (also called the light-dependent reactions) occur in the thylakoid membranes and accomplish three tasks: splitting water, producing ATP, and producing NADPH.
Photosystem II (PSII)
The process begins at Photosystem II (confusingly named because it was discovered after Photosystem I). When PSII's reaction center pigment, P680 (absorbs at 680 nm), absorbs light, it is excited to a higher energy state and donates an electron to an acceptor molecule.
To replace the lost electron, PSII splits water molecules in a process called the oxygen-evolving complex (OEC):
2H₂O → 4H⁺ + 4e⁻ + O₂
This photolysis of water is the source of all atmospheric oxygen. The protons (H⁺) released here contribute to the proton gradient across the thylakoid membrane.
The Electron Transport Chain
Electrons from PSII pass through an electron transport chain similar to that in mitochondria: - Plastoquinone (PQ): shuttles electrons through the membrane, pumping additional H⁺ into the thylakoid lumen - Cytochrome b₆f complex: a proton pump analogous to mitochondrial Complex III - Plastocyanin (PC): a copper-containing protein that carries electrons to PSI
Photosystem I (PSI)
Photosystem I (reaction center: P700, absorbs at 700 nm) re-energizes electrons received from the cytochrome b₆f complex. These excited electrons are used to reduce NADP⁺ to NADPH via ferredoxin and the enzyme NADP⁺ reductase:
NADP⁺ + 2e⁻ + H⁺ → NADPH
ATP Synthesis: Photophosphorylation
Protons accumulate in the thylakoid lumen from both water splitting and plastoquinone pumping. This proton gradient drives ATP synthase (CF₁F₀-ATP synthase) in the thylakoid membrane, producing ATP from ADP + Pᵢ in a process called photophosphorylation.
Net Products of the Light Reactions (per 2 NADPH produced)
- 3 ATP
- 2 NADPH
- O₂ released
These products — often summarized as ATP and NADPH — are the "energy currency" transferred to the Calvin cycle.
Stage 2: The Calvin Cycle (Light-Independent Reactions)
The Calvin cycle (also called the dark reactions, though it occurs in daylight too) takes place in the stroma and uses the ATP and NADPH from the light reactions to fix CO₂ into organic molecules. Melvin Calvin elucidated this cycle using radioactive ¹⁴C tracers, earning the 1961 Nobel Prize in Chemistry.
Phase 1: Carbon Fixation
CO₂ is attached to a five-carbon molecule called ribulose-1,5-bisphosphate (RuBP) by the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase):
CO₂ + RuBP → 2 × 3-phosphoglycerate (3-PGA)
RuBisCO is the most abundant protein on Earth and the primary entry point for inorganic carbon into the biosphere. However, it is a notoriously slow enzyme (~3 reactions/second) and can also react with O₂ instead of CO₂ (photorespiration), wasting energy.
Phase 2: Reduction
Each 3-phosphoglycerate (3-PGA) is reduced to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH:
3-PGA + ATP + NADPH → G3P + ADP + NADP⁺ + Pᵢ
G3P is the first stable organic product of photosynthesis and can be used to synthesize glucose, sucrose, starch, fatty acids, and amino acids.
Phase 3: Regeneration of RuBP
Most G3P molecules (5 out of every 6 produced per CO₂) are used to regenerate RuBP, consuming additional ATP. This regeneration keeps the cycle running continuously.
Net Calvin Cycle for One Glucose (requires 6 CO₂)
- 18 ATP consumed
- 12 NADPH consumed
- 1 glucose (C₆H₁₂O₆) produced
- 12 water molecules produced
C4 and CAM Photosynthesis: Adaptations to Hot Environments
Plants in hot, dry environments face a problem: closing stomata to conserve water also limits CO₂ entry, causing RuBisCO to react with O₂ and waste energy (photorespiration).
C4 plants (corn, sugarcane, sorghum) solve this by pre-concentrating CO₂ in bundle-sheath cells using the enzyme PEP carboxylase (which cannot react with O₂) before passing it to RuBisCO. This spatial separation of CO₂ collection from the Calvin cycle nearly eliminates photorespiration.
CAM plants (cacti, agave, pineapple) open their stomata only at night, fixing CO₂ into organic acids. During the day with stomata closed, they release this CO₂ to run the Calvin cycle — a temporal separation strategy.
The Importance of Photosynthesis
Photosynthesis has shaped Earth's atmosphere, chemistry, and biology over 3 billion years. Cyanobacteria performing oxygenic photosynthesis produced the Great Oxidation Event ~2.4 billion years ago, transforming Earth's atmosphere and enabling the evolution of aerobic life. Today, photosynthesis absorbs roughly 120 billion tonnes of carbon per year, forming the base of almost all food chains and moderating atmospheric CO₂ levels. Improving photosynthetic efficiency is a key research target in the effort to feed a growing population on a warming planet.