What is photosynthesis...?
2 answers:
In plants and algae, chloroplasts are the organelles for photosynthesis takes place. This is what they look like:
The stroma is a semi-fluid region found inside the inner membrane. Thylakoids, which are held inside the stroma and attached to the membranes, are formed of the same phospholipid bilayers that make up the cell membrane. Granum is the name for a collection of thylakoids.
Light-dependent reactions
Below is a photo of the thylakoid membrane (Attachment #2). Even while it appears sophisticated, there are really just a few components that are significant unless you are really researching the chemical mechanisms underlying photosynthesis.
Cyclic photophosphorylation and non-cyclic photophosphorylation are the two possible mechanisms that could occur here. The non-cyclic variety is the most fascinating of them and is the one that is frequently taught as a requirement in classes. Ask me about the cyclic kind somewhere else if you're interested in learning more.
Consider the world from left to right. A protein complex called Photosystem II is located on the far left (abbreviated PSII). Chlorophyll and other pigments are found in the reaction centers of PSII and its partner, Photosystem I (PSI), which both function to absorb solar light.
So what occurs is that a photon from the sun strikes PSII, ejecting some very excited electrons from the reaction center. A water-splitting complex in PSII can also split water into two hydrogen ions, two highly energetic electrons, and one oxygen molecule, the last of which is discarded, thanks to the energy of the photon. The electrons knocked off of PSII are replaced by ones from water.
The energized electrons are then transported along a protein chain (plastoquinone, cytochrome b6f, and plastocyanin above). The cytochrome utilizes the energy they lose at each stage to force the hydrogen ions that are outside the thylakoid into the cell. Since the concentration increases outdoors, this creates a gradient of hydrogen ions.
The ATP synthase protein (the huge orange one) is powered as the hydrogen ions from water splitting travel through it with the gradient. Adenosine triphosphate (ATP), the fundamental energy "currency" of life, is created when the enzyme ATP synthase adds a phosphate to the molecule known as adenoside diphosphate (ADP). These hydrogen ions can also be drawn back inside by the transport chain's electrons since they end up on the exterior.
Returning to our electrons now. These electrons arrive to PSI exhausted and ravenous since the majority of their energy has been used up. What good fortune! Another photon strikes PSI, increasing the energy of the electrons. They subsequently travel through a few additional protein intermediates before being absorbed by the molecule nicotinamide adenine dinucleotide phosphate, or NADP+, coupled with a hydrogen ion. It's a mouthful, therefore it's a good thing there's an abbreviation! When NADP+ is transformed into NADPH, the energy-rich hydrogen and electrons are transported to the light-independent processes.
The net reaction of the light-dependent reactions is:
2H2O + 2NADP+ + 3ADP + 3Pi → O2 + 2NADPH + 3ATP
Light-independent reactions
So now we've got a whole bunch of NADPH in the stroma; how do we take the energy in it and convert it to sugars for long-term storage?
The solution is the Calvin cycle: (Attachment #3)
Because they don't require light to take place, the Calvin cycle and associated pathways are known as the light-independent reactions. The enzyme RuBisCo, on which the Calvin cycle is built, is so grossly inefficient that it may serve as the strongest argument against intelligent design. In specifically, RuBisCo is intended to accept carbon dioxide and connect it to a molecule known as ribulose 1,5-biphosphate. However, occasionally, it will take an oxygen molecule instead of carbon dioxide. Oops.
Anyway, let's start from the beginning. We have five of a 3-carbon molecule called 3-phosphoglycerate (PGA). Through a series of complicated reactions which use up the ATP and NADPH produced in the light-dependent reactions, these five 3-carbon molecules are converted into three 5-carbon molecules, the aforementioned ribulose 1,5-biphosphate. Then, RuBisCo fixes three carbon dioxide molecules onto the three ribulose 1,5-biphosphate molecules. This forms three 6-carbon molecules, which immediately split to form six PGAs. One of these PGAs leaves the reaction as a product. This means there are five PGAs left, and we're back where we began. (That's why it's called a cycle!)
(Now, look at the photosynthesis equation again: Attachment #4)
The PGA that left the cycle is usually converted into glyceraldehyde 3-phosphate (PGAL), which can be rearranged and combined to form molecules like glucose, which is the end product of photosynthesis. Ultimately, it takes two PGALs to form one glucose molecule.
Thank you,
Eddie
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