Thylakoid
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Thylakoids are a membrane-bound compartment inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. The word "thylakoid" is derived from the Greek thylakos, meaning "sac". Thylakoids consists of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as "grana" (singular: granum). "Grana" is Latin for "stacks of coins". Grana are connected by intergrana or stroma thylakoids, which join granum stacks together as a single functional compartment. Thylakoids are embedded into the chloroplast stroma.
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[edit] Thylakoid formation
Chloroplasts develop from proplastids when seedlings emerge from the ground. Thylakoid formation requires light. In the plant embryo and in the absense of light, proplastids develop into etioplasts that contain semicrystalline membrane structures called prolamellar bodies. When exposed to light, these prolamellar bodies develop into thylakoids. This does not happen in seedlings grown in the dark, which undergo etiolation. An underexposure to light can cause the thylakoids to fail. This causes the chloroplasts to fail resulting in the death of the plant.
[edit] Thylakoid membrane proteins
Thylakoid membranes contain integral membrane proteins which play an important role in light harvesting and the light-dependent reactions of photosynthesis. The main thylakoid membrane proteins include:
Photosystem II is located mostly in the grana thylakoids, whereas photosystem I and ATP synthase are mostly located in the stroma thylakoids and the outer layers of grana. The cytochrome b6f complex is distributed evenly throughout thylakoid membranes.
Together with mobile electron carriers such as plastoquinone and plastocyanin, these proteins make use of light energy to drive electron transport chains that generate a chemiosmotic potential across the thylakoid membrane and NADPH, a product of the terminal redox reaction. The ATP synthase uses the chemiosmotic potential to make ATP during photophosphorylation.
[edit] Photosystems
These photosystems are light-driven redox centers, each consisting of an antenna complex that uses chlorophylls and accessory photosynthetic pigments such as carotenoids and phycobiliproteins to harvest light at a variety of wavelengths. 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 chlorophyll 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.
[edit] Electron transport chains
Due to the separate location of the two photosystems in the thylakoid membrane system, mobile electron carriers are required to shuttle electrons between them. These carriers are plastoquinone and plastocyanin.
Two different variations of electron transport are used during photosynthesis:
- Noncyclic electron transport or Non-cyclic photophosphorylation produces NADPH + H+ and ATP.
- Cyclic electron transport or Cyclic photophosphorylation produces only ATP.
The noncyclic variety involves the participation of both photosystems, while the cyclic electron flow is dependent on only photosystem I.
- 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 molecular 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.
[edit] 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 of chemiosmotic potential. The carriers in the electron transport chain use some of the electron's energy to actively transport protons from the stroma to the lumen. During photosynthesis the pH difference across the membrane is 4 pH units representing a 10,000:1 proton gradient from inside:outside. As the protons travel back down the gradient through channels in ATP synthase, ADP + Pi is combined into ATP.
[edit] Thylakoid lumen proteins
While plastoquinones are lipid-soluble and therefore move within the thylakoid membrane, plastocyanin moves through the thylakoid lumen. Plastocyanin carries electrons from the cytochrome b6f complex to photosystem I.
The lumen of the thylakoids is also the site of water oxidation by the oxygen evolving complex associated with the lumenal side of photosystem II.
[edit] See also
[edit] References
- Heller, H. Craig; Orians, Gordan H.; Purves, William K.; & Sadava, David (2004). LIFE: The Science of Biology (seventh edition). Sinauer Associates, Inc.. ISBN 0-7167-9856-5.
- Raven, Peter H.; Ray F. Evert, Susan E. Eichhorn (2005). Biology of Plants, 7th Edition. New York: W.H. Freeman and Company Publishers, 115-127. ISBN 0-7167-1007-2.
