Photosynthesis Explained

4 min read

Photosynthesis is the ancient biological process by which plants and certain bacteria use light, water, and CO₂ to create organic molecules (sugars, lipids, amino acids) and release oxygen [1]. It unfolds in two phases: a photochemical phase that captures light energy and produces NADPH and ATP, and a dark phase (Calvin cycle) that uses these resources to reduce CO₂ into triose phosphates and sugars, regenerating RuBP to allow the cycle to continue [1]. It is the primary source of carbon-based organisms and oxygen on the planet [1].

The plant captures carbon dioxide present in the air and water drawn up by the roots, uses light energy to transform them into carbohydrates (sugars), and releases dioxygen as a byproduct [2]. In other words, the plant does not "eat" the soil. The soil provides it with water and essential mineral salts, but the carbon that makes up its wood, leaves, and fruits indeed comes from the atmosphere [2]. Photosynthesis takes place in microscopic structures called chloroplasts, found mainly in leaf cells [2]. These chloroplasts contain chlorophyll, a pigment that effectively absorbs red and blue light and reflects green—hence the color of plants [2][3].

Biologists traditionally distinguish between two complementary phases [2]. Light-dependent reactions capture light energy and produce NADPH and ATP [3]. A frequently misunderstood point: the oxygen released does not come from carbon dioxide, but from the water molecule, which is split during the first phase [2]. The second phase can continue for a while without direct light as long as it has the energy produced previously [2].

Chlorophyll is the green pigment responsible for capturing light energy. It primarily absorbs red and blue wavelengths and reflects green, which gives plants their color [2][3]. Located in the thylakoids of the chloroplasts, it converts photon energy into chemical energy in the form of ATP and NADPH [3].

Light reactions take place in the thylakoids of the chloroplasts. Solar energy is absorbed by the chlorophyll of the photosystems, exciting electrons that initiate a transport chain. This causes the photolysis of water (releasing O₂), the creation of a proton gradient to synthesize ATP, and the reduction of NADP⁺ to NADPH [3][1]. In summary, they transform light into ATP and NADPH while producing oxygen [3].

The Calvin cycle (or carbon fixation reactions) takes place in the stroma of the chloroplasts and does not directly require light. It uses the ATP and NADPH produced by the light reactions to fix atmospheric CO₂ via the enzyme Rubisco, forming 3-phosphoglycerate [1][4]. This cycle consists of three phases: carbon fixation, reduction (production of G3P), and regeneration of RuBP. The G3P formed is used to synthesize glucose and other sugars [1].

Photosynthesis is essential because it produces the oxygen necessary for the respiration of most living organisms [3]. It forms the base of the food chain by converting solar energy into organic matter usable by consumers [1][4]. Furthermore, it regulates CO₂ and O₂ levels in the atmosphere, contributing to climatic stability and the balance of ecosystems [1]. Photosynthesis is the primary energy source for life on Earth and the only biological pathway that directly converts solar energy into usable chemical energy [4].

Three main factors determine the speed of the process, and it is always the most deficient one that imposes the limit: light intensity (the more it increases, the more photosynthesis accelerates until saturation), CO₂ concentration (a limiting factor at low levels), and temperature (optimum around 25-35°C for enzymes such as Rubisco) [2]. The availability of water and nutrients affects photolysis and chlorophyll synthesis; in the event of drought, the plant closes its stomata to avoid dehydration, which also blocks the entry of carbon dioxide [2][1].

The general scheme remains the same everywhere, but evolution has produced variants adapted to very different climates [2]. These three strategies (C3, C4, and CAM) clearly illustrate a general principle of biology: the same fundamental need can be satisfied by very different mechanisms depending on environmental constraints [2].

Photosynthesis links sunlight, the air we breathe, and the food we eat in a single reaction. Understanding it means understanding why a forest matters, why a houseplant languishes in a dark corner, and how agriculture works [2].