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Step 1
The light dependent reactions begin in photo system II. This photo system is called photosystem II simply because it was discovered after photosystem I. Chlorophyll molecules in photosystem II absorb light. This absorption of light raises electrons in chlorophyll to a higher energy level.
Step 2
These high-energy electrons are passed from chlorophyll to the electron transport chain. An electron transport chain is a series of electron proteins that shuttle high-energy electrons during ATP-generating reactions.
Step 3
As light continues to shine, more and more high-energy electrons are passed to the electron transport chain. Does this mean that chlorophyll eventually runs out of electrons? No. the thylakoid membrane contains a system that provides new electrons to chlorophyll to replace the ones it has lost. These new electrons come from water molecules (H2O).
Step 4
Enzymes on the inner surface of the thylakoid pull apart each water molecule into two electrons, two hydrogen ions H+, and one oxygen atom (O). Because the negatively-charged electrons move to the outside of the membrane while the positively-charged H+ ions are released inside, a charge separation is built up across the membrane.
Step 5
These two electrons from water replace the high-energy electrons that chlorophyll lost to the electron transport chain. As electrons are taken from water, oxygen gas (O2) is left behind and is released into the air. (This is the source of nearly all of the oxygen in the Earth's atmosphere, and another way God designed photosynthesis to make our live possible.
Step 6
As the electrons move along the electron transport chain, energy from the electrons is used by the proteins in the chain to pump still more H+ ions from the stroma inside the thylakoid sac. At the end of the electron transport chain, the electrons themselves pass to a second photosystem called photosystem I.
Step 7
In the first step of photosystem I, because some energy has been used to pump H+ ions across the thylakoid membrane, electrons do not contain as much energy as they used to when they reach photosystem I. Pigments in photosystem I use energy from light to reenergize these electrons and pass them to other carriers and eventually to the electron carrier NADP+. At the end of a short second electrons transport chain, NADP+ in the stroma picks up the high-energy electrons, along with H+ ions, at the outer surface of the thylakoid membrane, to become NADPH.
Step 8
The thylakoid membrane contains a protein complex called ATP synthase that spans the membrane and allows H+ to pass through it. Powered by the H+ concentration difference, H+ pass through ATP synthase and force it to rotate, almost like a turbine being spun by water in the Chickamauga hydroelectric power plant. As it rotates, ATP synthase binds ADP and a phosphate group together to produce ATP. This process enable light-dependent electrons transport to synthesize not only NADPH but ATP, as well.
Step 9
Carbon dioxide molecules (CO2) enter the Calvin Cycle (light-independent cycle) from the atmosphere. An enzyme in the stroma of the chloroplast combines these carbon dioxide molecules with 5-carbon compounds already present in the organelle, producing 3-carbon compounds that continue into the cycle. For every six carbon dioxide molecules that enter the cycle, a total of twelve 3-carbon compounds are produced. Other enzymes in the chloroplast then convert these compounds into higher energy forms in the rest of the cycle. The energy for these conversions comes from the ATP and high-energy electrons from NADPH produces in the light dependent state.
Step 10
At midcycle, in the Calvin Cycle, two of the twelve 3- carbon molecules are removed from the cycle. This is a very special step because these two molecules are used by the plant cell to synthesize sugars, lipids, amino acids, and other compounds.
Step 11
The remaining ten 3-carbon molecules are converted back into six 5-carbon molecules. These molecules combine with six new carbon dioxide molecules to begin the next cycle. As the cycle continues, more and more carbon dioxide is removed from the air and converted into the compounds the plant needs for growth and development.