What is "photosynthesis"

Photosynthesis \Pho`to*syn"the*sis\, n. (Plant Physiol.) The process of constructive metabolism by which carbohydrates are formed from water vapor and the carbon dioxide of the air in the chlorophyll-containing tissues of plants exposed to the action of light. It was formerly called assimilation, but this is now commonly used as in animal physiology. The details of the process are not yet clearly known. Baeyer's theory is that the carbon dioxide is reduced to carbon monoxide, which, uniting with the hydrogen of the water in the cell, produces formaldehyde, the latter forming various sugars through polymerization. Vines suggests that the carbohydrates are secretion products of the chloroplasts, derived from decomposition of previously formed proteids. The food substances are usually quickly translocated, those that accumulate being changed to starch, which appears in the cells almost simultaneously with the sugars. The chloroplasts perform photosynthesis only in light and within a certain range of temperature, varying according to climate. This is the only way in which a plant is able to organize carbohydrates. All plants without a chlorophyll apparatus, as the fungi, must be parasitic or saprophytic. -- Pho`to*syn*thet"ic, a. -- Pho`to*syn*thet"ic*al*ly, adv.

1898, loan-translation of German Photosynthese, from photo- "light" (see photo-) + synthese "synthesis" (see synthesis). Another early word for it was photosyntax.\n\n[T]he body of the work has been rendered into English with fidelity, the only change of moment being the substitution of the word "photosynthesis" for that of "assimilation." This change follows from a suggestion by Dr. Barnes, made a year ago before the American Association at Madison, who clearly pointed out the need of a distinctive term for the synthetical process in plants, brought about by protoplasm in the presence of chlorophyll and light. He proposed the word "photosyntax," which met with favor. In the discussion Professor MacMillan suggested the word "photosynthesis," as etymologically more satisfactory and accurate, a claim which Dr. Barnes showed could not be maintained. The suggestion of Dr. Barnes not only received tacit acceptance by the botanists of the association, but was practically approved by the Madison Congress in the course of a discussion upon this point.

["The Botanical Gazette," vol. XIX, 1894]

n. (context biology English) The process by which plants and other photoautotrophs generate carbohydrates and oxygen from carbon dioxide, water, and light energy.

n. synthesis of compounds with the aid of radiant energy (especially in plants)

Photosynthesis is a process used by plants and other organisms to convert light energy, normally from the Sun, into chemical energy that can be later released to fuel the organisms' activities ( energy transformation). This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek φῶς, phōs, "light", and σύνθεσις, synthesis, "putting together". In most cases, oxygen is also released as a waste product. Most plants, most algae, and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis maintains atmospheric oxygen levels and supplies all of the organic compounds and most of the energy necessary for life on Earth.

Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centres that contain green chlorophyll pigments. In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas. The hydrogen freed by water splitting is used in the creation of two further compounds: reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), the "energy currency" of cells.

In plants, algae and cyanobacteria, sugars are produced by a subsequent sequence of light-independent reactions called the Calvin cycle, but some bacteria use different mechanisms, such as the reverse Krebs cycle. In the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate (RuBP). Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as glucose.

The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons. Cyanobacteria appeared later; the excess oxygen they produced contributed to the oxygen catastrophe, which rendered the evolution of complex life possible. Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts, which is about three times the current power consumption of human civilization. Photosynthetic organisms also convert around 100–115 thousand million metric tonnes of carbon into biomass per year.