Photosynthesis


Photosynthesis is the conversion of light energy into chemical energy by living organisms. The raw materials are carbon dioxide and water; the energy source is sunlight; and the end-products are oxygen and (energy rich) carbohydrates, for example sucrose, glucose and starch. This process is arguably the most important biochemical pathway,[1] since nearly all life either directly or indirectly depends on it. It is a complex process occurring in higher plants, phytoplankton, algae, as well as bacteria such as cyanobacteria. Photosynthetic organisms are also referred to as photoautotrophs.

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Photosynthesis uses light energy and carbon dioxide to make triose phospates (G3P). G3P is generally considered the prime end-product of photosynthesis. It can be used as an immediate food nutrient, or combined and rearranged to form disaccharide sugars, such as sucrose and fructose, which can be transported to other cells, or packaged for storage as insoluble polysaccharides such as starch.

A commonly used but slightly simplified equation for photosynthesis is:

6 CO2(g) + 12 H2O(l) + photons → C6H12O6(aq) + 6 O2(g) + 6 H2O(l)
carbon dioxide + water + light energy → glucose + oxygen + water

When written as a word equation the light energy appears above the arrow as it is required for photosynthesis but it is not actually a reactant. Here the monosaccharide glucose is shown as a product, although the actual processes in plants produce disaccharides.

The equation is often presented in introductory chemistry texts in an even more simplified form as:[2]

6 CO2(g) + 6 H2O(l) + photons → C6H12O6(aq) + 6 O2(g)

Photosynthesis occurs in two stages. In the first phase, light-dependent reactions or photosynthetic reactions (also called the Light reactions) capture the energy of light and use it to make high-energy molecules. During the second phase, the light-independent reactions (also called the Calvin-Benson Cycle, and formerly known as the Dark Reactions) use the high-energy molecules to capture carbon dioxide (CO2) and make the precursors of carbohydrates.

In the light reactions, one molecule of the pigment chlorophyll absorbs one photon and loses one electron. This electron is passed to a modified form of chlorophyll called pheophytin, which passes the electron to a quinone molecule, allowing the start of a flow of electrons down an electron transport chain that leads to the ultimate reduction of NADP into NADPH. In addition, it serves to create a proton gradient across the chloroplast membrane; its dissipation is used by ATP Synthase for the concomitant synthesis of ATP. The chlorophyll molecule regains the lost electron by taking one from a water molecule through a process called photolysis, that releases oxygen gas.

In the Light-independent or dark reactions the enzyme RuBisCO captures CO2 from the atmosphere and in a process that requires the newly-formed NADPH, called the Calvin-Benson cycle releases three-carbon sugars, which are later combined to form sucrose and starch.

Photosynthesis may simply be defined as the conversion of light energy into chemical energy by living organisms. It is affected by its surroundings and the rate of photosynthesis is affected by the concentration of carbon dioxide, the intensity of light, and the temperature.

The light energy is converted to chemical energy using the light-dependent reactions. This chemical energy production is more than 90% efficient with only 5-8% of the energy transferred thermally. The products of the light-dependent reactions are ATP from photophosphorylation and NADPH from photoreduction. Both are then utilized as an energy source for the light-independent reactions.

Not all wavelengths of light can support photosynthesis. The photosynthetic action spectrum depends on the type of accessory pigments present. For example, in green plants, the action spectrum resembles the absorption spectrum for chlorophylls and carotenoids with peaks for violet-blue and red light. In red algae, the action spectrum overlaps with the absorption spectrum of phycobilins for blue-green light, which allows these algae to grow in deeper waters that filter out the longer wavelengths used by green plants. The non-absorbed part of the light spectrum is what gives photosynthetic organisms their color (e.g., green plants, red algae, purple bacteria) and is the least effective for photosynthesis in the respective organisms.

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