Plants make use of sunlight for energy by converting solar energy into the chemical energy of food (glucose). People have also used this conversion process. We have many devices, including possibly your calculator, that do precisely the same thing. Solar energy is converted into another form of energy. In the case of your calculator, solar energy is converted into electrical energy.

In this lesson, you explored the following essential questions:

• How does the energy from light flow through living systems?

• Why is oxygen produced during photosynthesis?

In order for photosynthesis to occur, there has to be light energy. There also has to be a way to harness light because light travels at many different wavelengths. To account for this, plants contain many different pigments in their chloroplasts. To illustrate this, you worked through a chromatography investigation.

Plants contain pigments, such as chlorophyll, that absorb various wavelengths of light except for greens, yellows, and some oranges (carotene). Once this light energy is absorbed, it is transferred to electrons that become highly excited and enter an electron-transport chain.

The production of oxygen happens when photosystem II oxidizes water molecules. Oxidized water molecules provide electrons for the electron-transport chain and hydrogen ions for chemiosmosis or ATP synthesis. Oxygen is a by-product that diffuses into the atmosphere. Photosystem I also transports highly-energized electrons and reduces NADP⁺ to NADPH.

In summary, the light-dependent reactions rely on the reactants of water and light energy. These reactions produce the end product of oxygen and the intermediate products of ATP and NADPH, which are used in light-independent reactions to produce glucose. You will discover the final step of photosynthesis in Lesson 3. As you can see, photosynthesis is a series of reactions more complex than the simple statement of

6 CO_{2 (g)} + 6 H₂O_(l) + energy → C₆H₁₂O_{6 (s)} + 6 O_{2 (g)}

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Lesson Glossary

ATP synthase: an enzyme that bonds free phosphates to ADP to form ATP

chemiosmosis: the process by which ATP is generated through the movement of protons down a concentration gradient

The protons move across the inner membranes to the mitochondrion and thylakoid membrane of the chloroplast and combine with ADP and phosphate molecules to form ATP.

NADP⁺: nicotinamide adenine dinucleotide phosphate in its oxidized state (has lost an electron)

NADPH: nicotinamide adenine dinucleotide phosphate in its reduced state; used in light-independent reactions to form glucose

photosystem: a cluster of light-absorbing pigment molecules in the thylakoid membrane of chloroplasts

reference flow: a measure of how far a substance molecule is transported in chromatography

thylakoid membrane: flattened stacks within the stroma that contain chlorophyll