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Just like with Cellular Respiration, where it is important to understand the structure of Mitochondria, it is equally important to understand the structure of a chloroplast to fully understand photosynthesis.
Chloroplast are organelles that are encapsulated by a double membrane and contain stacks Thylakoids called Granum. Chlorophyll is found in the membrane of the Thylakoids, and the space inside these is called the Lumen. Lamellae are tubes that connect the different grana together and the stroma is the general space inside the chloroplast.
When extracted chlorophyll interacts with a photon (from light), one of its electron gets excited to higher energy level, then quickly returns to its ground state (lowest potential energy). This loss of energy is released as fluorescence (red). You might be wondering why don’t leaves fluoresce red? This is because the excited electron transfers its energy to another molecule, or the electron is transferred to a primary electron acceptor.
Chlorophyll is never alone…it is a part of a photosystem. A Photosystem is composed of a reaction center (composed of 2 chlorophyll a and a nearby electron acceptor) and an antenna complex (or light-harvesting complex composed of additional chlorophyll a and accessory pigments).
The antenna complex (represented as purple protein in the image to the left) acts like a funnel to direct light energy captured from a photon to the reaction center.
In the reaction center, the chlorophyll a absorbs the energy, raising a chlorophyll electrons to a higher energy level.
The electron is transferred to the electron acceptor (chlorophyll is oxidized). Now the positively charged chlorophyll is the strongest biological oxidant, strong enough to split water!
There are two different types of photosystems that specialize in harnessing specific wavelengths:
- Photosystem I: contains chlorophyl a that absorbs 700nm wavelengths (called P700)
- Photosystem II: contains chlorophyl a that absorbs 680nm wavelengths (called P680)
When Photosystem I and II are working together it is called Noncyclic Electron Flow. This process begins when a photons strikes PSII, exciting an electron of P680. P680 loses this e-, but it is even more electronegative than oxygen, so it replaces its missing electron by removing it from H2O, which splits water to form H+ + O2. The H+ ions accumulate in thylakoid lumen (sound familiar?)
The electron from photosystem II passes through an electron transport chain: plastoquinone → cytochrome complex → plastocyanin → Photosystem I. This transports H+ ions into the thylakoid lumen each step of the way.
Photosystem I is also losing electrons when struck by photons, these are replaced by the e- moving through the ETC from Photosytem II. The excited electron from Photosystem I is passed through another ETC to ferredoxin and then NADP reductase. NADP reductase combines the electron and H+ in stroma to reduce NADP+ to NADPH (which will be used later in the Dark Reactions)
H+ ions that accumulate in the thylakoid lumen produce an electrochemical gradient. ATP Synthase proteins are embedded in the thylakoid membrane, as as the flow from the H+ passes through this complex, going from the lumen into the stroma, ADP is converted to ATP. Since light creates the gradient it is called photophosphorylation.
When NADP+ levels are low, Photosystem I can function independently in a process called Cyclic Electron Flow . The process is identical to above, but instead the electron is passed from ferredoxin back to plastiquinone (hence Cyclic!). This cycle only produces ATP, since ferredoxin does not pass on the electron to NADP Reductase. This type of Light Reaction can be favourable as the Dark reactions require much more ATP than NADPH.
Textbook Practice
Textbook Practice
Read p.218-225
Questions p.219#5, p.228#1-6
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