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[Biologie] fosforylatie


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#1


  • Gast

Geplaatst op 02 augustus 2005 - 15:01

ik heb twee vrage, namelijk
1) cyclische en niet- cyclische foto-fosforylatie met elkaar vergelijken
2) de verschillende soorten fosforylering op sommen en kaderen

ik zoek die voor mijn herexamen zou iemand mij kunne helpe
bedankt op voorhand.

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#2


  • Gast

Geplaatst op 03 augustus 2005 - 00:58

Okť, ben een beetje lui, maar hier is een link. Op het laatste stuk word de niet cyclische en cyclische fosforilatie uitgelegd. De tekst komt letterlijk uit Campbell's 'Biology'.

Hopelijk heb je hier wat aan, en succes met je hertentamen!

http://occawlonline....hap10SEC2-5.htm

#3

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Geplaatst op 03 augustus 2005 - 01:05

Oke, misschien iets te lui.... :shock:

Noncyclic Electron Flow

Light drives the synthesis of NADPH and ATP by energizing the two photosystems embedded in the thylakoid membranes of chloroplasts. The key to this energy transformation is a flow of electrons through the photosystems and other molecular components built into the thylakoid membrane. During the light reactions of photosynthesis, there are two possible routes for electron flow: cyclic and noncyclic. Noncyclic electron flow, the predominant route, is shown in FIGURE 10.12 on p. 186 The numbers in the text description correspond to the numbered steps in the figure.

Figuur 10.12:
Geplaatste afbeelding

The energy changes of electrons as they flow through the light reactions are analogous to the cartoon in FIGURE 10.13. As complicated as the scheme is, do not lose track of its functions: The light reactions use solar power to generate ATP and NADPH, which provide chemical energy and reducing power, respectively, to the sugar-making reactions of the Calvin cycle.

Figuur 10.13:
Geplaatste afbeelding

Cyclic Electron Flow


Under certain conditions, photoexcited electrons take an alternative path called cyclic electron flow, which uses photosystem I but not photosystem II. You can see in FIGURE 10.14 that cyclic flow is a short circuit: The electrons cycle back from ferredoxin (Fd) to the cytochrome complex and from there continue on to the P700 chlorophyll. There is no production of NADPH and no release of oxygen. Cyclic flow does, however, generate ATP. This is called cyclic photophosphorylation, to distinguish it from noncyclic photophosphorylation.

Figuur 10.14:

Geplaatste afbeelding

What is the function of cyclic electron flow? Noncyclic electron flow produces ATP and NADPH in roughly equal quantities, but the Calvin cycle consumes more ATP than NADPH. Cyclic electron flow makes up the difference. The concentration of NADPH in the chloroplast may help regulate which pathway, cyclic versus noncyclic, electrons take through the light reactions. If the chloroplast runs low on ATP for the Calvin cycle, NADPH will begin to accumulate as the Calvin cycle slows down. The rise in NADPH may stimulate a temporary shift from noncyclic to cyclic electron flow until ATP supply catches up with demand.

Whether photophosphorylation is driven by noncyclic or cyclic electron flow, the actual mechanism for ATP synthesis is the same. This is a good time to review chemiosmosis, the process that uses membranes to couple redox reactions to ATP production.

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