Photosynthesis: How Organisms Convert Light into Chemical Energy

Key Points

• ↳Photosynthesis is a biological process by which organisms convert light energy into chemical energy stored as carbohydrates, and it produces most of the oxygen in Earth's atmosphere.
• ↳Oxygenic photosynthesis, which splits water and releases oxygen, is the most common form and is used by plants, algae, and cyanobacteria.
• ↳Anoxygenic photosynthesis, used by some bacteria and archaea, does not produce oxygen and may represent the earliest form of photosynthesis on Earth.
• ↳The process occurs in two stages: light-dependent reactions that capture energy to produce NADPH and ATP, followed by light-independent reactions (the Calvin cycle) that use those molecules to fix carbon dioxide into sugars.
• ↳Global photosynthesis captures approximately 130 terawatts of energy per year, about eight times total human civilization energy consumption, and converts 100–115 billion tons of carbon into biomass annually.
• ↳In plants and algae, photosynthesis takes place in chloroplasts, where pigments such as chlorophyll, carotenes, and xanthophylls are organized into light-harvesting antenna complexes embedded in thylakoid membranes.

Types of Photosynthesis

Photosynthesis takes two primary forms: oxygenic and anoxygenic. Oxygenic photosynthesis, performed by plants, algae, and cyanobacteria, splits water to release oxygen and is by far the most common type. Anoxygenic photosynthesis, found in certain bacteria and archaea, uses alternative electron donors such as hydrogen sulfide and does not release oxygen. Some archaeal forms using retinal-based pigments may represent the most ancient type of photosynthesis, predating cyanobacteria.

Two-Stage Reaction Process

Photosynthesis proceeds in two sequential stages. In the light-dependent reactions, photosynthetic pigments absorb photons, strip electrons from water, and produce the energy carriers ATP and NADPH. These molecules then drive the light-independent Calvin cycle, in which atmospheric carbon dioxide is incorporated into organic compounds and reduced to form carbohydrates such as glucose. Other bacteria achieve carbon fixation through alternative pathways, such as the reverse Krebs cycle.

Cellular Structures Involved

In plants and algae, photosynthesis occurs within chloroplasts, organelles enclosed by a double phospholipid membrane and filled with an aqueous fluid called the stroma. Within the stroma, stacks of thylakoid membranes called grana house the pigment-protein complexes responsible for capturing light. In bacteria, photosynthetic proteins are embedded directly in cell membranes, which are often folded into thylakoids or intracytoplasmic vesicles to maximize light-absorbing surface area.

Ecological and Evolutionary Significance

Photosynthesis has profoundly shaped life on Earth by producing and sustaining the oxygen-rich atmosphere that enabled complex organisms to evolve. Early photosynthetic organisms relied on hydrogen or hydrogen sulfide as electron donors, while the later emergence of cyanobacteria introduced oxygen as a byproduct, transforming Earth's chemistry. Today, photosynthetic organisms fix hundreds of billions of tons of carbon into biomass each year and capture far more energy than all of human civilization consumes.