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On to Part Two

Photosynthesis is a two part process. On this page is a diagram and explanation of the first part, photochemical reactions (light dependent). The second part, thermochemical reactions (light independent) are on another page.

The Balanced Equation for Photosynthesis:

or another way to look at it


There are two stages of photochemical reactions.

Stage 1
[1.] Light energy strikes photosystem 2 and chlorophyll molecules absorb energy.

[2.] Excited electrons leave the chlorophyll and move to electron carriers. (loss of and electron leaves the chlorophyll molecule unstable)

[3.] Electrons move along a chain of carrier molecules to the lower energy level of photosystem 1.

[4.] The electron is finally added to a compound called NADP (Nicotine Adenine Dinucleotide Phosphate) as well as one hydrogen and forms NADPH.

[5.] A water molecule is split- an electron is added to the unstable chlorophyll molecule and a hydrogen is added to NADP in step 4 above (the oxygen from the water is realeased)

Electrons lost by photosystem 1 are replaced by electrons from photosystem 2, which in turn are replaced by electrons released from the breakdown of HO

The bonds in NADPH hold energy for later use

Stage 2
[1.] As electrons move along the electron transport chain, a gradient of ions is formed across the thylakoid membrane.

[2.] Ions move from a higher concentration to a lower one.

[3.] Electrochemical energy of the moving ions causes a phosphate to be added to ADP making it ATP (phosphorylation)

[4.] The ATP is used immediately in carbon fixation.