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Photosynthesis

Photosynthesis is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabolism. Photosynthesis usually refers to oxygenic photosynthesis, a process that releases oxygen.

Photosynthetic organisms store the chemical energy in sugars such as glucose and fructose, which are later broken down through cellular respiration. Because it removes carbon dioxide from the air and releases oxygen, photosynthesis is central to the oxygen content of the Earth's atmosphere and supplies most of the biological energy needed for complex life.

Overview

Most photosynthesis in plants happens inside chloroplasts, organelles that contain the green pigment chlorophyll. The process is generally split into the light-dependent reactions, which capture energy from light, and the light-independent reactions (the Calvin cycle), which use that energy to build sugars from carbon dioxide.

The overall efficiency with which plants convert incoming sunlight into stored chemical energy is relatively low — typically only a few percent — but on a global scale photosynthesis captures an enormous amount of energy each year and forms the base of nearly every food chain. It is also responsible for the buildup of fossil fuels, which are the remains of organisms that grew using sunlight millions of years ago.

Photosynthetic membranes and organelles

In plants and algae, photosynthesis takes place in chloroplasts, which are surrounded by a double membrane and contain stacked, disc-shaped sacs called thylakoids. The thylakoid membranes hold the light-absorbing pigments and the protein complexes responsible for the light reactions, while the surrounding fluid, the stroma, is where carbon fixation occurs.

In cyanobacteria, which lack chloroplasts, the same machinery is embedded directly in internal membranes of the cell. Chloroplasts are widely thought to have originated from free-living cyanobacteria through endosymbiosis, a relationship in which one organism comes to live inside another and the two evolve together.

Light-dependent reactions

During the light-dependent reactions, pigments such as chlorophyll and carotenoids absorb photons and pass the energy to reaction centres in two large complexes known as photosystem I and photosystem II. This energy is used to split water molecules — a step called photolysis — releasing oxygen, protons, and electrons.

The excited electrons travel along an electron transport chain, driving the production of the energy carriers ATP and NADPH. Oxygen is released as a byproduct, and it is this reaction that gradually filled Earth's atmosphere with breathable oxygen over billions of years.

Light-independent reactions

The light-independent reactions, or Calvin cycle, take place in the stroma and do not require light directly. Using the ATP and NADPH produced earlier, the enzyme RuBisCO — often described as the most abundant protein on Earth — fixes carbon dioxide onto an existing five-carbon sugar.

Through a series of steps the cycle produces glucose and regenerates its starting compounds so the process can continue. The sugars formed are used for energy, for growth, and to build structural molecules such as cellulose and storage molecules such as starch.

Efficiency and limiting factors

The rate of photosynthesis depends on light intensity, carbon dioxide concentration, temperature, and water availability. Up to a point, increasing light or carbon dioxide raises the rate until another factor becomes limiting — a principle known as the law of limiting factors. Many plants have evolved variations such as C4 carbon fixation and CAM photosynthesis that improve efficiency in hot, bright, or dry environments where water loss is a concern.

Evolution

Photosynthesis appeared very early in the history of life. The earliest forms were anoxygenic, meaning they did not produce oxygen and instead used molecules such as hydrogen sulfide as an electron source. The later evolution of oxygen-producing photosynthesis in ancient cyanobacteria triggered the Great Oxidation Event roughly 2.4 billion years ago, dramatically changing the atmosphere and paving the way for complex, oxygen-breathing life.

Today photosynthesis is studied not only for its role in ecology and agriculture but also as inspiration for artificial photosynthesis, a field that aims to capture and store solar energy in fuels much as plants do.