Photosynthesis

Photosynthesis is the process by which light energy is converted into chemical energy, primarily in the form of glucose, using carbon dioxide and water as raw materials. The overall equation is 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. This process is carried out by plants, algae, and cyanobacteria, and it is the primary source of organic matter for most ecosystems. It also produces the oxygen that makes aerobic life possible.

The light-dependent reactions occur in the thylakoid membranes of chloroplasts, where light energy is absorbed by chlorophyll and other pigments, driving the splitting of water (photolysis) and the production of ATP and NADPH. These reactions take place in photosystem II and photosystem I, which work in series to transfer electrons. The oxygen released comes from water, not carbon dioxide, as demonstrated by the Hill reaction and later confirmed with isotopic labeling.¹

The Calvin cycle, also known as the C3 pathway, takes place in the stroma of the chloroplast and uses ATP and NADPH from the light reactions to fix carbon dioxide into three-carbon molecules (3-PGA), which are then converted into G3P, a precursor to glucose. The enzyme RuBisCO catalyzes the initial fixation step, but it also reacts with oxygen in a process called photorespiration, which reduces efficiency. Many plants have evolved C4 and CAM pathways to minimize photorespiration.²

C4 photosynthesis, found in plants like maize and sugarcane, spatially separates initial carbon fixation and the Calvin cycle, using PEP carboxylase to fix CO₂ into a four-carbon compound in mesophyll cells, then releasing CO₂ in bundle sheath cells. CAM (Crassulacean Acid Metabolism) plants, such as cacti and succulents, temporally separate these steps, opening stomata at night to fix CO₂ into organic acids, which are decarboxylated during the day. These adaptations are particularly advantageous in hot, dry environments.³

Photosynthesis evolved over 3 billion years ago in anoxygenic bacteria, which used hydrogen sulfide or other electron donors. Later, cyanobacteria developed oxygenic photosynthesis, leading to the Great Oxidation Event. Chloroplasts in plants and algae are derived from a cyanobacterial endosymbiosis, and subsequent secondary endosymbiotic events gave rise to diverse algal groups. Some organisms, like certain sea slugs, can steal chloroplasts from algae (kleptoplasty) and use them for photosynthesis.⁴

Beyond the textbook pathways, there are many intriguing facets. For instance, some plants exhibit 'C3-C4 intermediate' photosynthesis, which is an evolutionary transitional state. Additionally, certain parasitic plants like Monotropa have lost photosynthetic ability entirely, relying on mycorrhizal fungi for carbon. There are also 'photosynthetic animals' such as the pea aphid, which can synthesize carotenoids, and the spotted salamander, which hosts algae in its eggs. Furthermore, artificial photosynthesis, which mimics natural processes to produce fuels, is a growing field of research.⁵

Photosynthesis is measured using techniques like gas exchange (measuring CO₂ uptake or O₂ evolution), chlorophyll fluorescence, and stable isotope analysis. The Hill reaction, discovered by Robert Hill in 1937, demonstrated that isolated chloroplasts can evolve oxygen in the presence of an artificial electron acceptor. More recently, ultrafast spectroscopy has been used to study energy transfer in light-harvesting complexes, revealing quantum coherence effects.⁶

Chlorophyll

Green pigment that absorbs light energy for photosynthesis.

RuBisCO

Enzyme that catalyzes the first step of carbon fixation in the Calvin cycle.

Photorespiration

Process where RuBisCO reacts with oxygen, reducing photosynthetic efficiency.

Thylakoid

Membrane-bound compartment in chloroplasts where light reactions occur.

Stroma

Fluid-filled space in chloroplasts where the Calvin cycle takes place.