The ancient biological engine that fuels nearly all life on Earth
In short: Plants don't just eat sunlight; they perform a complex series of chemical reactions to manufacture food from thin air and water. This process, photosynthesis, provides the foundational energy for nearly every food web on the planet and literally built the oxygen-rich atmosphere we breathe today.
Photosynthesis is categorized into two distinct stages. The first is the light-dependent reaction, where light energy is absorbed by pigments like chlorophyll. This energy is used to strip electrons from substances like water, releasing oxygen as a byproduct and creating two essential energy molecules: ATP and NADPH. In plants, these pigments are housed within chloroplasts found in leaf cells. In cyanobacteria, they are located in the plasma membrane. This process allows autotrophs to convert light into the chemical energy needed for metabolism.
The second stage, known as the light-independent reactions or the Calvin cycle, uses those molecules to perform carbon fixation. In this phase, atmospheric carbon dioxide is incorporated into organic compounds like ribulose bisphosphate (RuBP). Through a sequence of reduction and regeneration, these are transformed into carbohydrates such as glucose, fructose, and sucrose. Other bacteria utilize the reverse Krebs cycle to achieve similar results. These reactions store converted energy within the bonds of intracellular organic compounds. These compounds include starches, phytoglycogen, and cellulose.
While oxygenic photosynthesis is the most common form today, it is not the only way to harvest light. Some bacteria perform anoxygenic photosynthesis, which does not produce oxygen. For example, certain purple bacteria split hydrogen sulfide to release sulfur, while some Archaea like Halobacterium use retinal to create a proton gradient to synthesize ATP directly. The anoxygenic variety was a dominant form in the euxinic Canfield oceans during the Boring Billion. This specific archaeal method might have been the earliest form of photosynthesis to evolve on Earth. It preceded the appearance of cyanobacteria.
The historical impact of this process is profound. About 2.4 billion years ago, cyanobacteria drove the Great Oxidation Event, raising atmospheric oxygen to nearly 1 percent of current levels over 600 million years. This shift paved the way for multicellular life. Today, we are rapidly consuming fossil fuels—essentially the stored photosynthetic energy of organisms from millions of years ago—returning carbon dioxide to the atmosphere at an unprecedented rate. If this process stopped, most organisms would disappear from the planet. The atmosphere would eventually become nearly devoid of oxygen. Only chemosynthetic bacteria, which use inorganic compounds, would survive such conditions.
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Sources
- Source: CrashCourse — Photosynthesis: Crash Course Biology #8 · YouTube (embedded player only)
- Also: Photosynthesis
- Also: Photosynthesis | Definition, Formula, Process, Diagram, Reactants, Products, & Facts | Britannica
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