Photosynthesis

Lore & Background

The general equation for photosynthesis was first proposed by Cornelis van Niel. In oxygenic photosynthesis, water is used as the electron donor, releasing oxygen as a byproduct. The process begins when light energy is absorbed by reaction centers containing photosynthetic pigments such as chlorophylls in plants and algae, or bacteriochlorophyll in some bacteria. Some organisms perform anoxygenic photosynthesis, which does not produce oxygen. Purple bacteria use bacteriochlorophyll to split hydrogen sulfide, releasing sulfur instead of oxygen. Archaea such as Halobacterium use retinal and microbial rhodopsin to absorb green light and produce a proton gradient, directly synthesizing ATP. However, this light-driven proton pumping is not classified as photosynthesis because it lacks carbon fixation and typical electron transport chains. The earliest known photosynthetic organisms on Earth are cyanobacteria-like, not archaeal rhodopsin-based systems. In plants, algae, and cyanobacteria, sugars are synthesized via the Calvin cycle, using ATP and NADPH from light-dependent reactions to fix carbon dioxide. Other bacteria use mechanisms like the reverse Krebs cycle. The first photosynthetic organisms probably used reducing agents such as hydrogen or hydrogen sulfide as electron sources.

Reader's Guide

Photosynthesis is fundamental to life on Earth, as it produces the oxygen in the atmosphere and supplies the biological energy that supports complex life. The process is the opposite of cellular respiration: photosynthesis reduces carbon dioxide to carbohydrates, while respiration oxidizes carbohydrates to carbon dioxide. Oxygenic photosynthesis, which releases oxygen, is the most common type, but anoxygenic variants exist, including those using hydrogen sulfide or arsenite as electron donors. Some archaea use a simpler, non-carbon-fixing form of photosynthesis involving bacteriorhodopsin. Understanding photosynthesis is crucial for fields such as agriculture, bioenergy, and climate science, as it underpins the global carbon cycle and food production.

Did You Know?

• All oxygenic photosynthetic plants, including shade-loving sciophytes, release at least some oxygen into the atmosphere.
• Archaea such as Halobacterium use retinal and microbial rhodopsin to absorb green light and produce ATP directly, but this process is not considered photosynthesis because it does not fix carbon dioxide or involve typica

The Two-Stage Engine of Light and Carbon

Photosynthesis operates as a two-stage biochemical engine. The first stage, the light-dependent reactions, kicks off when photons strike reaction centers—protein complexes studded with photosynthetic pigments. In plants, those pigments are chlorophylls, a porphyrin derivative that harvests red and blue wavelengths while reflecting green, and they sit inside chloroplasts packed throughout leaf cells. In cyanobacteria, the same role is played by pigments embedded directly in the plasma membrane. During this stage, energy is spent ripping electrons away from water molecules, liberating oxygen gas as a waste product. The freed hydrogen is then channeled into building two vital energy carriers: NADPH and ATP. The second stage, the Calvin cycle, is entirely light-independent. Atmospheric carbon dioxide is folded into existing organic scaffolds such as ribulose bisphosphate. The ATP and NADPH from the first stage then drive the reduction of those intermediates into usable sugars like glucose. Some bacteria bypass the Calvin cycle entirely, relying instead on the reverse Krebs cycle to fix carbon.

Beyond Oxygen: The Anoxygenic and Archaeal Variants

Not every photosynthetic organism follows the familiar oxygen-releasing blueprint. Purple bacteria, for instance, employ bacteriochlorophyll and split hydrogen sulfide rather than water, venting elemental sulfur as their byproduct. This strategy dominated the euxinic Canfield oceans during the so-called Boring Billion, a vast stretch of Earth's past when oxygen was scarce. Even more radically, certain archaea such as Halobacterium perform a form of photosynthesis that does not fix carbon at all. They rely on retinal and its microbial rhodopsin derivatives to capture green light, generating a proton gradient across the cell membrane. The resulting ion flow drives transmembrane proton pumps that synthesize ATP directly, bypassing the need for sugar production. Researchers speculate this archaeal mechanism may represent the earliest photosynthesis to evolve on Earth, potentially stretching back to the Paleoarchean eon and predating cyanobacteria—a notion captured in the Purple Earth hypothesis. Meanwhile, some shade-adapted plants called sciophytes produce so little oxygen during their photosynthesis that they consume it all internally, releasing nothing to the atmosphere.

From Sulfur to Oxygen: The Evolutionary Arc

The earliest photosynthetic organisms likely did not split water. Instead, they drew electrons from reducing agents such as molecular hydrogen or hydrogen sulfide, a strategy that left no oxygen behind. This anaerobic mode of energy capture probably emerged very early in life's history, long before the atmosphere contained meaningful amounts of free oxygen. Cyanobacteria arrived later and changed the story dramatically. Their oxygenic photosynthesis generated surplus molecular oxygen that accumulated in the atmosphere, a process known as the Great Oxygenation. That oxygenation event was a prerequisite for the evolution of complex, multicellular life as we know it. The general equation for photosynthesis, first articulated by Cornelis van Niel, captures the essence of this transformation: carbon dioxide and an electron donor are combined using light energy to yield a carbohydrate and an oxidized byproduct. In oxygenic systems, water serves as the electron donor and oxygen is the oxidized product. In the older anoxygenic systems, the electron donor might be hydrogen sulfide, and the oxidized byproduct is elemental sulfur.

A Planetary Engine and Its Discovery

The sheer scale of photosynthesis on Earth is staggering. Beyond energy and carbon, photosynthesis is the principal mechanism that produces and sustains the oxygen content of Earth's atmosphere, and it supplies the vast majority of the biological energy that underpins complex life. It is also worth noting that photosynthesis and cellular respiration, though often described as mirror images, are mechanistically distinct: they proceed through different sequences of chemical reactions and occur in different cellular compartments, with respiration taking place in mitochondria. Photosynthesis reduces carbon dioxide into carbohydrates; respiration oxidizes those carbohydrates back to carbon dioxide and water.