Photosystem II The light-dependent reactions, shown in Figure 8–10, begin when pigments in photosystem II absorb light. (This first photosystem is called photosystem II simply because it was discovered after photosystem I.) Light energy is absorbed by electrons in the pigments found within photosystem II, increasing the electrons' energy level. These high-energy electrons (e^–) are passed to the electron transport chain. An electron transport chain is a series of electron carrier proteins that shuttle high-energy electrons during ATP-generating reactions.

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). Enzymes on the inner surface of the thylakoid break up each water molecule into 2 electrons, 2 H⁺ ions, and 1 oxygen atom. The 2 electrons replace the high-energy electrons that have been lost to the electron transport chain. As plants remove electrons from water, oxygen is left behind and is released into the air. This reaction is the source of nearly all of the oxygen in Earth's atmosphere, and it is another way in which photosynthesis makes our lives possible. The hydrogen ions left behind when water is broken apart are released inside the thylakoid.

FIGURE 8–9 Why Green? The green color of most plants is caused by the reflection of green light by the pigment chlorophyll. Pigments capture light energy during the light-dependent reactions of photosynthesis.

In Your Notebook Explain in your own words why photosynthetic organisms need water and sunlight.

Electron Transport Chain What happens to the electrons as they move down the electron transport chain? Energy from the electrons is used by the proteins in the chain to pump H⁺ ions from the stroma into the thylakoid space. At the end of the electron transport chain, the electrons themselves pass to a second photosystem called photosystem I.

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 the electrons. At the end of a short second electron transport chain, NADP⁺ molecules in the stroma pick up the high-energy electrons, along with H⁺ ions, at the outer surface of the thylakoid membrane, to become NADPH. This NADPH becomes very important, as you will see, in the light-independent reactions of photosynthesis.

Hydrogen Ion Movement and ATP Formation Recall that in photosystem II, hydrogen ions began to accumulate within the thylakoid space. Some were left behind from the splitting of water at the end of the electron transport chain. Other hydrogen ions were “pumped” in from the stroma. The buildup of hydrogen ions makes the stroma negatively charged relative to the space within the thylakoids. This gradient, the difference in both charge and H⁺ ion concentration across the membrane, provides the energy to make ATP.

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