Cell Biology

References

Light and photosynthesis in aquatic ecosystems, by John T.O. Kirk. 1983. Cambridge Univ. Press.

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Cell Biology

Photosynthesis

Photosynthesis is the process where plants convert sunlight into energy, then store it as carbohydrates, sugars, such as glucose. Photosynthesis may be the most important process in ecosystems, for it both brings in energy needed within the ecosystem, and produce oxygen (O2) needed for cellular respiration, and the production of more ATP.

Photosynthesis has three basic steps:

Energy is captured from the sunlight.
Light energy is converted into chemical energy in the form of ATP and NADPH.
Chemical energy is used to power the synthesis or organic molecules (e.g. carbohydrates) from carbon dioxide (CO2).

This process can be summed with the following chemical equation:

CO2 + H2O + light ---> C6H12O6 + O2

In terrestrial plants, this process takes place in leaves, specifically within the organelle chloroplast So what exactly happens?

First lets look at the structure of chloroplast. Chloroplast contains stacks of flattened organelles called a thylakoid. One stack of thylakoids is called a grana. Grana float within a cytoplasm-like fluid in the chloroplast called stroma.

How does this work? Think light as a packet of energy, like a battery, called photons. When sunlight shines on a plant, the photons hits the plants, plugging into pigments called chlorophyll. Chlorophyl fills the thylakoids . But photons have different colors, similar to having AA and AAA batteries. AA batteries are longer than AAA batteries, and they also have slightly different charges. So each battery needs a different type of plug, and so does the different color photons. So there are two primary types of chlorophyll, chlorophyll a and chlorophyll b. Photons with a wavelength peaking at 680nm plugs into chlorophyll a, while photons with a wavelength peaking at 650nm plugs into chlorophyll b. By having both pigments, plants more than double the amount of photons that is can convert. Other pigments, such as carotenoids, also increases the range of photons which can be captured. But carotenoids are not a good at absorbing photons as chlorophyll.

Once chlorophylhas absorbed the photons, the energy is transferred down a chain until ATP and NADPH is charged. This process occurs in two separate, but connected systems, photosystem I and photosystem II. Photosystem I picks up photons at 700nm, while photosystem II photons at 680nm. The energy absorbed by photosystem II is passed on to photosystem I, which charges NADPH.

When light hits a plant, chlorophyll absorbs the photons. The energy that it absorbs is picked up by photosystem II. Photosystem II takes the energy, along with H2O, and passes that energy to an electron acceptor, Q. O2 is released at this point. The electron acceptor, Q, now has energy. Q takes that energy, and shuttles it off to photosystem I. As that energy is being shuttled from Q to photosystem I, ADP gets charged, becoming ATP. Photosystem I now takes that energy, and charges NADP+. When NADP+ is charged, it loses a hydrogen, and becomes NADPH. Notice that NADPH has now loosed its positive charge by picking up a negative electron. This entire process can be summarized with the following chemical equation.

H2O + light + ADP + P ---> O2 + ATP + e-

After the above steps occur in photosystem II, the electron is finally sent to photosystem I, where the following happens.

e- + NADP+ + H ---> NADPH

Now there are two high energy molecules, fully charged and ready to be used. Plants makes more energy that it needs immediately, so the NADPH and ATP is used to make glucose as follows:

CO2 + ATP + NADPH ---> C6H12O6

This happens through another process called the Calvin cycle.

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Photosynthesis