A thylakoid is a phospholipid bilayer membrane internal to chloroplasts. The membrane is folded repeatedly into a stack of disks called grana, sort of like a stack of pancakes. The stacks connect by channels and form a single functional compartment. Chlorophyll and other integral membrane proteins use light and electron transport chains to power the formation of NADPH and ATP during photophosphorylation.

There are two different electron transport systems in photosynthesis:

  • Noncyclic electron transport produces NADPH + H+ and ATP.
  • Cyclic electron transport produces only ATP.

The noncyclic variety involves the participation of two different photosystems, while the cyclic is dependant on only one. These photosystems are light-driven molecular units, each consisting of many chlorophyll molecules and accessory pigments bound to proteins in separate energy-absorbing antenna complexes. Each system uses light to energize an electron to an unstable orbital, which is then easily passed to a chain of carrier molecules, each of which absorbs some of the electron's energy. Photosystem I contains chlorophyll designate P700 that absorbs 700 nm light maximally, while photosystem II contains P680 chlorophyll that absorbs 680 nm light best (note that these wavelengths correspond to deep red - see the visible spectrum).

  • Photosystem I uses light energy to reduce NADP+ to NADPH + H+, and is active in both noncyclic and cyclic electron transport. In cyclic mode, the energized electron is passed down a chain that ultimately returns it (in its base state) to the chlorophyll that energized it.
  • Photosystem II uses light energy to oxidize water molecules, producing electrons (e-, protons (H+), and diatomic oxygen (O2), and is only active in noncyclic transport. Electrons in this system are not conserved, but are rather continually entering from oxidized H2O (O2 + 2 H+ + 2 e-) and exiting with NADP+ when it is finally reduced to NADPH. It is interesting to note that this oxidation of water conveniently produces the waste product O2 - the very stuff we need to breath.

The molecular mechanism of ATP generation in chloroplasts is similar to that in mitochondria. A major function of the thylakoid membrane and its integral photosystems is the establishment of an ion gradient (typically 10,000:1 protons inside:outside, which corresponds to a pH difference of 4). The carriers in the electron transport chains use little bits of the electron's energy to actively transport protons from the stroma into the thylakoid, As the protons travel back down the gradient through channels in ATP synthase, ADP + Pi is combined into ATP. See electrochemical potential.



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