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Difference Between Photosystem I and Photosystem II

Photosystem I vs Photosystem II

The process of photosynthesis is essential in plants as it is responsible for deriving energy from light to be used by the plant to grow and reproduce. In photosynthesis, there are two structures that are responsible for the actual conversion of light to chemical energy; they are respectively named photosystem I and II. The main difference between the two is their order in the photosynthesis process.

Although it seems to go against logical reasoning, photosystem II begins the process of photosynthesis by removing an electron from water molecules and breaking it down to oxygen and hydrogen. The excited electron produces ATP as it returns to its rest state. At this point, the electron is once again excited by the photosystem I to a much higher energy level. The excited electron then produces a NADPH molecule which is used by the Calvin cycle.

With the process, as outlined above, the naming of the two photosystems seems contrary to the flow of the process. The reason behind this isn’t really related to their actual function. It just so happened that photosystem I was the first one to be discovered by scientists and thus given the designation. When photosystem II was discovered, the names were not interchanged for the reason of consistency.

Aside from what they do in photosynthesis and the order they come in, there is also a difference in the light frequencies that they are most receptive to. Photosystem I is very receptive to light waves at the 700 nm wavelength. In comparison, photosystem II is very receptive to light wavelengths of around 680 nm.

Both photosystem I and II are necessary in most plants to produce the energy they need from the sun. Although both do the same thing, it is the way they do it that sets apart their differences. Still, the ATP and NADPH they produce are used by the plant as energy sources for its biological processes.


1.Photosystem II appears sooner than photosystem I in the process of photosynthesis.
2.Photosystem II produces ATP while photosystem I produces NADPH.
3.Photosystem I was discovered before photosystem II.
4.Photosystem I is sensitive to light wavelengths of 700 nm while photosystem II is sensitive to light wavelengths of 680 nm.

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