Page 270 - Biology_F5

Nutrition

oxygen does not come from carbondioxide, molecule which accumulates in the stroma
but from the spliting water molecules. to be used in the dark reaction (Figure 5.2).
NADP is therefore important in maintaining
The chlorophyll of photosystem I is also the flow of electrons in the thylakoid
activated and it absorbs light energy of membrane, as it is the final electron acceptor.

the wavelength 700 nm. The absorbed Therefore, the normal flow of electrons
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energy excites the electrons, causing them can be inhibited if this electron acceptor
to move from photosystem I and transfer is limited.
through a series of electron acceptor and
carrier molecules, including ferredoxin Thus, the synthesis of ATP in this
way is described as a non-cyclic
and ferredoxin-NADP reductase. As these photophosphorylation, because the electrons
electrons move through the carrier systems, from photosystem II move in one direction.
they lose their energy but they are not They do not move back to their original
returned to their original position. Instead molecule, instead they move through
they are handed to NADP , where together different electron carriers, and they are
+
with hydrogen proton (H ) from photolysis taken up by photosystem I to fill the gap
+
of water, reduce NADP to NADPH , which of the lost electrons used to reduce NADP.
+
2
is also written as NADPH or NADPH + The formed ATP and NADPH are used in
+
H , as indicated in the following equation: the Calvin cycle or dark reaction to produce
+
NADP + 2H + 2e − NADPH + H + carbohydrates while the oxygen gas from
+
photolysis of water is released into the air
The light energy is now stored in the NADPH as a by-product.
Primary Primary
Electron Electron
Acceptor Acceptor
2e - 2e - 2e - 2e -
Pheophytin
Light rays 2e - Light rays Ferredoxin

Plastoquinone 2e -
ADP+ Pi 2e - NADP + 2H +

+
Cytochrome
ADP
2e - NADPH
Photosystem II 2e -
Photosystem I
H O 1 O + 2H +
2
2 2

by - product

Figure 5.2: Non-cyclic photophosphorylation

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