Bhaskara
Mr. Vinay Resch
AP Biology
13 December 2011
Photosynthesis Pathway Description
Photosynthesis begins when a photon (1) hits the cluster of chlorophyll pigments (13).
The photons excite one of electrons in the P680 chlorophyll a molecules in the reaction center of
Photosystem II (PS II). This excited electron is then captured by the primary electron acceptor of
PS II. Simultaneously, a water molecule (11) binds to an enzyme (12) and then splits into two
electrons, two Hydrogen ions, and an oxygen atom (which quickly combines with another
oxygen atom to form O2). The electrons from the splitting of water are used to replenish to restock the electrons lost by the P680 molecules. Each photoexcited electron passes from the
primary acceptor of PS II to PS I via an electron transport chain (2) that is made up of
plastoquinone, a cytochrome complex, and plastocyanin. The exergonic flow of electrons down
the electron transport chains are used to pump protons (H+ ions) (5) across the thyakloid
membrane (14). Once the excited electron reaches Photosystem I (PS I), it is used to replenish a
P700 chlorophyll molecule that was previously excited by light from a light harvesting complex
and lost an electron to PS Is primary electron acceptor. From PS Is acceptor, the electron is
passed down through another electron transport chain containing ferredoxin (Fd). The enzyme
NADP+ reductase transfers two electrons from Fd to NADP+ (3), thereby reducing it to NADPH.
Meanwhile, the protons (6) which were pumped across the thyakloid membrane by the
first transport electron chain now diffuse back across the membrane via the enzyme ATP
synthase (9). The ATP synthase catalyzes ADP into ATP inside the stroma via phosporylation.
Both ATP and NADPH are used in the next step of photosynthesis; the Calvin cycle. The
primary input into the Calvin cycle is carbon dioxide (15), 3 molecules to be exact. These CO2
molecules are attached to a 5 carbon sugar known as ribulose biphosphate (RuBP), a reaction
catalyzed by RuBP carboxylase, or rubisco. The 6 carbon intermediate that is formed is very
unstable, and immediately breaks down into 2 molecules of 3-phosphoglycerate. ATP (17)
donates a phosphate group to 3-phosphoglycerate, becoming ADP, and the 3-phosphoglycerate
becomes 1, 3-biphosphoglycerate. The 1, 3-biphosphoglycerate is then reduced by NADPH (19)
to glyceraldehyde 3 phosphate (G3P); specifically, the electrons from NADPH reduce the
carboxyl group of 1, 3-biphosphoglycerate to the aldehyde group of G3P (21). Of the 6 G3P (21)
molecules produced for every 3 CO2 molecules inputted, only one (22) is actually sent to the rest
of the cell to form sugars; the other 5 (23) are used to replenish the cycles stock of RuBP. 3
molecules worth of ATP (24) are converted into ADP (25). The resultant energy release is used to
drive a complex set of reactions that rearrange the 5 G3P molecules into 3 molecules of RuBP
(26). All told, the input of three CO2 molecules yields just one actually usable sugar, and the net
change in carbon molecules is zero (3 5-carbon RuBPs converted into 3 molecules of RuBP).