Thylakoid
From Wikipedia, the free encyclopedia
Thylakoids (commonly referred to as Thylakoid membranes) are a phospholipid bilayer membrane-bound compartment internal to chloroplasts, and represent the majority of its internal structure. The word "thylakoid" is derived from the Greek thylakos, meaning "sac". Thylakoid membranes fold on top of each other into stacks of disks referred to singularly as a granum and in the plural as "grana". "Grana" is Latin for "stacks of coins". The term "thylakoid" also refers to the intergrana or stroma thylakoid, which joins granum stacks together as a single functional compartment known as the lumen.
-
- See Photosynthetic Pigments for more information.
Thylakoids contain many integral membrane proteins which play an important role in the light-dependent stage of photosynthesis due to their abundance in chlorophyll. Thylakoids also play a key role in light harvesting due to photosystems which harvest light at different wavelengths, utilising accessory pigments such as carotenoids and phycobiliproteins to harvest light at a greater spectrum of wavelengths.
These make use of 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 chemi-osmotic potential to make ATP during photophosphorylation.
Contents |
[edit] Photosystems
These photosystems are light-driven molecular units, each consisting of many chlorophyll molecules and Photosynthetic Pigments or 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 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. An underexposure to light can cause they thylakoids to fail, this causes the chloroplasts to fail resulting in the death of the plant.
[edit] Electron transport chains
Two different variation 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 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.
[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 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 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] 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.
