Photosynthesis

Photosynthesis is the biological process by which photoautotrophic organisms — including green plants, algae, and cyanobacteria — use light energy to synthesise organic compounds from carbon dioxide and water, releasing oxygen as a byproduct. First investigated systematically in the eighteenth and nineteenth centuries, photosynthesis underpins nearly all life on Earth by supplying both the organic matter and the molecular oxygen on which most ecosystems depend.

Overview

The overall chemical reaction for oxygenic photosynthesis is commonly summarised as:

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

This equation represents the net transformation: carbon dioxide and water, powered by sunlight, are converted into glucose and oxygen. In practice the process consists of dozens of enzymatic steps occurring in two broad stages — the light-dependent reactions and the light-independent reactions — each taking place in distinct compartments of the chloroplast.

Photosynthesis is responsible for producing virtually all the oxygen in Earth's atmosphere. The Great Oxidation Event, estimated to have occurred roughly 2.4 billion years ago, is attributed to the proliferation of cyanobacteria capable of oxygenic photosynthesis, fundamentally altering the planet's atmospheric chemistry and enabling the evolution of aerobic life.

Mechanism

Light-Dependent Reactions

The light-dependent reactions occur in the thylakoid membranes of the chloroplast. Chlorophyll and accessory pigments embedded in protein complexes called Photosystem I and Photosystem II absorb photons, primarily in the red and blue wavelengths of the visible spectrum. The absorbed energy drives the following key events:

• Water splitting (photolysis): Photosystem II catalyses the oxidation of water molecules, releasing oxygen gas, protons, and electrons.
• Electron transport chain: Energised electrons pass through a series of protein carriers, releasing energy that is used to pump protons across the thylakoid membrane, creating a proton gradient.
• ATP synthesis: The proton gradient drives ATP synthase, producing ATP from ADP and inorganic phosphate.
• NADPH formation: Photosystem I uses additional light energy to reduce NADP⁺ to NADPH.

The ATP and NADPH generated in this stage provide the chemical energy and reducing power needed for the next stage.

Light-Independent Reactions (Calvin Cycle)

The light-independent reactions, collectively known as the Calvin cycle, take place in the stroma of the chloroplast. The cycle was elucidated by Melvin Calvin, Andrew Benson, and James Bassham at the University of California, Berkeley, in the early 1950s, work for which Calvin was awarded the Nobel Prize in Chemistry in 1961.

The Calvin cycle proceeds in three phases:

1. Carbon fixation: The enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) incorporates CO₂ into a five-carbon acceptor molecule, ribulose-1,5-bisphosphate (RuBP), producing an unstable six-carbon compound that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).
2. Reduction: ATP and NADPH from the light-dependent reactions are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar that serves as the primary product of the cycle.
3. Regeneration of RuBP: Most G3P molecules are used to regenerate RuBP using ATP, allowing the cycle to continue.

G3P can be used to synthesise glucose, sucrose, starch, and other organic molecules required by the plant.

Pigments and Light Absorption

Chlorophyll is the principal photosynthetic pigment in plants and algae, existing in two main forms: chlorophyll a and chlorophyll b. Chlorophyll absorbs light most efficiently in the red (around 680 nm) and blue (around 450 nm) portions of the spectrum, reflecting green light — which accounts for the characteristic green colour of most plant tissue.

Accessory pigments, including carotenoids (carotenes and xanthophylls) and, in certain algae, phycobilins, broaden the range of wavelengths that can be harvested and also play a role in photoprotection by dissipating excess light energy that could otherwise cause oxidative damage.

Ecological and Biogeochemical Significance

Photosynthesis is the foundation of the carbon cycle on Earth. By fixing atmospheric carbon dioxide into organic matter, photosynthetic organisms form the base of nearly all food webs. The organic compounds produced pass through successive trophic levels as heterotrophs consume autotrophs.

On a geological timescale, photosynthesis has contributed to the formation of fossil fuels: the remains of ancient photosynthetic organisms, subjected to heat and pressure over millions of years, gave rise to coal, oil, and natural gas. The combustion of these fuels releases carbon dioxide that was sequestered over geological time, contributing to the contemporary rise in atmospheric CO₂ concentrations.

Aquatic photosynthesis, carried out primarily by phytoplankton in the world's oceans, accounts for roughly half of global primary production, though precise current estimates vary. These microscopic organisms also produce a large fraction of Earth's atmospheric oxygen.

Variations and Adaptations

Not all photosynthetic organisms use the same biochemical pathways. Three main carbon-fixation strategies are recognised in plants:

• C3 photosynthesis: The ancestral and most common pathway, in which CO₂ is fixed directly by RuBisCO into a three-carbon compound. Most plants, including wheat and rice, use this pathway. It is less efficient under hot, dry conditions because RuBisCO can mistakenly fix oxygen rather than CO₂ (photorespiration).
• C4 photosynthesis: An adaptation found in plants such as maize and sugarcane that pre-concentrates CO₂ around RuBisCO, minimising photorespiration and improving efficiency in warm environments.
• CAM (Crassulacean Acid Metabolism): A water-conserving adaptation used by succulent plants such as cacti, in which stomata open at night to take in CO₂, which is stored and then used during daylight hours when stomata are closed to minimise water loss.

Anoxygenic photosynthesis, practised by certain bacteria such as purple sulfur bacteria and green sulfur bacteria, uses electron donors other than water (such as hydrogen sulfide) and does not produce oxygen as a byproduct.

History of Research

Early investigations into photosynthesis include the experiments of Jan Baptist van Helmont in the seventeenth century, who demonstrated that plants do not derive their mass solely from soil. Joseph Priestley's experiments in the 1770s showed that plants could restore air depleted by combustion. Jan Ingenhousz demonstrated in 1779 that light was essential to the process. The term photosynthesis itself was coined by the American botanist Charles Barnes in 1893, though the word was later popularised by the German plant physiologist Wilhelm Pfeffer.

The molecular details of the photosystems and the electron transport chain were resolved through much of the twentieth century, and the three-dimensional structure of Photosystem II was determined by X-ray crystallography in the early 2000s, providing detailed insight into the mechanism of water splitting.