Thylakoid

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Thylakoids are a phospholipid bilayer membrane-bound compartment internal to chloroplasts. The membrane is folded repeatedly into a stack of disks called grana, similar to a stack of pancakes. The stacks are interconnected by stroma thylakoids and form a single functional compartment; the inside is termed the lumen.

Thylakoids contain many integral membrane proteins that are important for photosynthesis. This includes two different photosystems with chlorophyll and accessory pigments to harvest light. Electron transport chains that function to generate a chemiosmotic potential across the membrane and NADPH, a product of the terminal redox reaction. In addition, the ATP synthase enzymes use the chemiosmotic potential to make ATP during photophosphorylation.

1 Photosystems
2 Electron transport chains
3 ATP synthase
4 See also
5 References

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Photosystems

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 antenna complex has between 250 and 400 pigment molecules and the energy they absorb is shuttled by resonance energy transfer to a specialized chlorophyll a at the reaction center of each photosystem . When either of the two chorophyll a molecules at the reaction center absorb energy an electron is excited and transferred to an electron-acceptor molecule. Photosystem I contains a pair of chlorophyll a molecules, designated P700, at its reaction center that maximally absorbs 700 nm light. Photosystem II contains P680 chlorophyll that absorbs 680 nm light best (note that these wavelengths correspond to deep red - see the visible spectrum). The P is short for pigment and the number is the specific absorption peak in nanometers for the chlorophyll molecules in each reaction center.

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Electron transport chains

Two different variation of electron transport are used during 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 dependent on only one.

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 that is vital for cellular respiration.

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ATP synthase

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 a chemiosmotic potential. The carriers in the electron transport chain use some of the electron's energy to actively transport protons inside the thylakoids from the stroma to the lumen. During photosynthesis the pH difference across the membrane is 4 pH units representing a 10,000:1 protons gradient from inside:outside. As the protons travel back down the gradient through channels in ATP synthase, ADP + P_i is combined into ATP.

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