Free energy is released during the redox reactions of the electron transport chains. This energy is used in two ways.
In the electron transport chain that follows photosytem II, the movement of electrons through the chain releases energy that is used to move protons across the thylakoid membrane just as in the electron transport chain of cellular respiration in the mitochondria. The protons are moved into the thylakoid space where they accumulate and produces a voltage across the membrane. Also associated with this electron transport chain is the enzyme ATP synthase. The ATP synthase uses the voltage formed by the build-up of protons in the thylakoid space to produce ATP. Functionally, the electron transport chain that follows photosystem II is similar to the electron transport chain of respiration; the accumulation of protons on one side of a membrane provides the free energy that ATP synthase needs to drive the production of ATP.
In the electron transport chain that follows photosystem I, the electrons that move through the chain are used to reduce NADP+ to NADPH .
The two products of the light-dependent reactions of photosystem are ATP and NADPH. The movement of high energy electrons releases the free energy that is needed to produce these molecules. The ATP and NADPH are used in the light-independent reactions to make sugar.
Where do the electrons come from? Water molecules are split at the beginning of the first electron transport chain. The splitting of water provides electrons that become energized in the photosystems, and protons that accumulate across the membrane.
As water is split and electrons and protons are removed, oxygen is released. Thus, oxygen is a by-product of photosynthesis. Recall from the tutorial Prokaryotes III that the oxygen that is released as a by-product of photosynthesis had a major impact on the early earth's atmosphere and is the source of the oxygen that we breathe.
Energy IV Photosynthesis Part 2 VoiceThread Transcript
Light energy raises electrons associated with pigment molecules to a higher energy state and these electrons are used to reduce NADP+ to NADPH, and to supply energy for the accumulation of protons into the thylakoid space.
The charge separation across the thylakoid membrane represents stored energy in the form of a voltage (similar to what happens within mitochondria during cellular respiration). The figure above compares ATP synthesis in the two organelles. As shown, chloroplasts also make use of the voltage via ATP synthase proteins in the membrane. These are similar to the ATP synthase molecules associated with cellular respiration. In a process called photophosphorylation , they harness the energy from protons moving across the membrane to make ATP from ADP and phosphate.
Both the ATP produced by the electron transport chain following photosystem II and the NADPH produced by the electron transport chain following photosystem I will be used by the light-independent reactions to build a sugar. How a plant uses this ATP and NADPH to build a sugar will be covered in the next tutorial.