Dictionary » P » Photosystem



noun, plural: photosystems

A multisubunit complex found mainly in the thylakoid membranes of plants and algae, and in the cytoplasmic membranes of photosynthetic bacteria. It is primarily involved in capturing light to cause a series of redox reactions.


The photosystem is where the light reactions take place. Two photosystems have been found in plants and algae: photosystem I and photosystem II. Each of them consists of chlorophyll molecules, accessory pigments, proteins and other molecules. Both of them are involved in the release and transfer of excited electrons to the electron transport chain. They differ in their reaction system chlorophylls (P700 and P680) and in distinct biochemical reactions which they are involved.

Word origin: ancient Greek: phos = light and systema = assembly

See also: light reactions

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I just taught that course (gr 12 photosynthesis): It appears that it is probably extremely unclear. I think it uses 4 photons per photosystem for each H2O broken. But since you need a mix of cyclic and non cyclic light reactions to produce anough ATP and NADPH, and that the cyclic reactions ...

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by canalon
Sat Apr 14, 2012 6:23 pm
Forum: Physiology
Topic: Photosynthesis
Replies: 2
Views: 3948


Is this statement true? Photosystem II is at highe energy level than Photosystem I.

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by chiayen
Sun Jan 15, 2012 4:43 am
Forum: General Discussion
Topic: photosystems
Replies: 1
Views: 1088

reverse photosynthesis and get respiration

... respiration (as mith said) think how mitochondrias work ( the thylakloid membranes and ETC's and then the chemeoosmosis) and compare that with photosystem... an abvious answer it is a big NO... and the reasons you can get even by thinking a little good luck thinking ;) thhough if you still ...

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by jwalin
Sun Apr 11, 2010 11:55 am
Forum: Cell Biology
Topic: reverse photosynthesis and get respiration
Replies: 8
Views: 5044

Re: Why do intact chloroplasts fluoresce?

... acceptor. So at what point is energy released as light? Thanks for your reply. Don't they drop back down to near the ground state when going from Photosystem II to Photosystem I? Would that cause the fluorescence in intact photosystems?

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by mamoru
Tue Mar 09, 2010 12:32 am
Forum: Botany Discussion
Topic: Why do intact chloroplasts fluoresce?
Replies: 10
Views: 8675

Biology IQ test: How smart are you = look

... electron sink in respiration. 15) RUBISCO is optimally evolved to work in an oxygen atmosphere. 16) The chlorophyll A molecules of the two photosystems are chemically different which causes their difference in absorbance spectra. 17) The large +G of life (all life collectively) is ultimately ...

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by DemiMike
Sat Nov 28, 2009 6:21 am
Forum: General Discussion
Topic: Biology IQ test: How smart are you = look
Replies: 3
Views: 13935
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