8/18/2019 Photosynthesis is a Process Used by Plants and Other Organisms to Convert Light Energy

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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). Thischemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide

and water   hence the name photosynthesis, from the !ree"  # $ῶ , phōs, %light%, and &*&+$, synthesis, %putting

together%.-/0 1n most cases, oxygen is also released as a waste product. 2ost plants, most algae, andcyanobacteria perform photosynthesis3 such organisms are called photoautotrophs. 4hotosynthesis maintains

atmospheric oxygen levels and supplies all of the organic compounds and most of the energy necessary for life

on 5arth.6

7lthough photosynthesis is performed differently by different species, the process always begins when energyfrom light is absorbed by proteins called reaction centres that contain green chlorophyll pigments. 1n 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. 1n these light8dependent reactions, some energy is used tostrip electrons from suitable substances, such as water, producing oxygen gas. The hydrogen freed by water

splitting is used in the creation of two further compounds9 reduced nicotinamide adenine dinucleotide phosphate

(:7;4alvin cycle, but some bacteria use different mechanisms, such as the reverse ?rebs cycle. 1n the

>alvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, suchas ribulose bisphosphate (@uA4).B Csing the 7T4 and :7;4< produced by the light8dependent reactions, theresulting 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 li"ely

used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons.D 

>yanobacteria appeared later3 the excess oxygen they produced contributed to the oxygen catastrophe,E whichrendered the evolution of complex life  possible. Today, the average rate of energy capture by photosynthesis

globally is approximately -0F terawatts,GH-F which is about three times the current power consumption of

human civilization.-- 4hotosynthetic organisms also convert around -FF--B thousand million metric tonnes of

carbon into biomass per year.

4hotosynthetic organisms are photoautotrophs, which means that they are able to synthesize food directly from

carbon dioxide and water using energy from light. arbon fixation is an endothermic 

redox reaction, so photosynthesis needs to supply both a source of energy to drive this process, and the electrons

needed to convert carbon dioxide into a carbohydrate via a reduction reaction. The addition of an electrons to achemical species is called a reduction reaction. 1n general outline and in effect, photosynthesis is the opposite ofcellular respiration, in which glucose and other compounds are oxidized to produce carbon dioxide and water,

and to release chemical energy (an exothermic reaction) to drive the organism's metabolism. The two processes,

of reduction of carbon dioxide to carbohydrate and then the later oxidation of the carbohydrate, ta"e placethrough a different se=uence of chemical reactions and in different cellular compartments.

The general e=uation for photosynthesis as first proposed by >ornelius van :iel is therefore9-6

>I/ J /

8/18/2019 Photosynthesis is a Process Used by Plants and Other Organisms to Convert Light Energy

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Since water is used as the electron donor in oxygenic photosynthesis, the e=uation for this process is9

>I/ J /I-D

carbon dioxide J arsenite J light energy K arsenate J carbon monoxide (used to build other compoundsin subse=uent reactions)

4hotosynthesis occurs in two stages. 1n the first stage, light-dependent reactions or light reactions capture the

energy of light and use it to ma"e the energy8storage molecules 7T4 and :7;4