Sunday, January 9, 2011

Plants! Photosynthesis!

Photosynthesis, a process that converts sunlight into sugars which can then be used for energy. This process happens in autotrophs, organisms like plants, algae and many bacteria. For explanation, I will be using a plant cell. In a plant cell, there are chloroplasts, which is where photosynthesis takes place. In the chloroplast (diagram on left), there are stacks of thylakoids where the first step of photosynthesis, the Light Reaction, takes place.

Process of Photosynthesis: 6CO2 + 6H2O + light --> C6H12O6 + 6O2

The Light Reaction:
The light reaction begins with taking photons from sunlight, sending these photons into the Photo Synthesis II and PS I. When in the PS II and I, the light bounces off pigments, eventually landing at the bottom, at the reaction center. Here, an electron gets "excited" from the energy collected from the photon, and the electron is shot from the PS II to the PS I. When this happens, energy is able to be used and push an H+ ion from the Stroma to the Lumen of the chloroplast (See upper diagram). Then, another electron in the PS I is "excited" and pushed to the stroma, where NADP+ picks it up and transfers to NADPH. The electron from PS II then replaces the electron from PS I. This brings up the problem: Where does the electron from PS II get replaced? To answer this, water is broken down into H+, e- and O2, where the e- replaces the other electron, and the H+ waits to be pushed into the Lumen.
After this process, there are H+ in the Lumen, but because of diffusion, the H+ wants to go back down into the Stroma. They do this by going through the ATP synthase, where when they go down, ATP is reduced, producing ATP from the Light Reaction.

Inputs: Photons, water
Outputs: NADPH and O2

*NADPH to be used in the next step, the Calvin Cycle

The Calvin Cycle:
In the Stroma of a chloroplast, the calvin cycle takes place, inputting CO2, ATP and using the NADPH produced in the light reaction. To start the cycle,  3 RuBP, a product already there in the cycle (a five carbon sugar) is joined with 3 C02 to produce 3 six carbon molecules. Because this six carbon molecule is so unstable, it is quickly broken down into 6 3-carbon molecules. Then, 6ATP and 6NADPH are oxidized and the energy from them is used to rearrange the 3 carbon molecules. After this, one 3 Carbon molecule (PGAL) is released, and the other 5 3 carbon molecules are rearranged into the starting RuBP, using the help of 3ATP. The PGALs taken out of this cycle are then used to form glucose, a sugar that can then be used in cellular respiration to make energy.

Inputs: CO2, NADPH and ATP
Outputs: Glucose

These two processes make up photosynthesis, producing sugars in plants for future use as energy.