Photosynthesis Intro and Light-Dependent Reactions

Photosynthesis

Photosynthesis uses light energy to make organic compounds, especially sugars. It is essential to life because plants’ organic compounds form the basis of much of the food that people and other organisms depend on. Overall, plants take in carbon dioxide and water and produce glucose and oxygen.

Photosynthesis has two stages:

1. Light-dependent reactions, which require light and produce ATP and NADPH.
2. The Calvin cycle, which uses ATP and NADPH to help make sugars.

Light-dependent reactions

These reactions take place in the thylakoid membranes inside chloroplasts. Thylakoids contain light-absorbing pigments such as chlorophyll, grouped into photosystems.

• Photosystem II absorbs light. Light energizes electrons in chlorophyll, and the electrons move into an electron transport chain.
• Water replaces the electrons lost by chlorophyll. Splitting water produces electrons, hydrogen ions, and oxygen. The oxygen released during photosynthesis comes from this step.
• The first electron transport chain builds a hydrogen-ion gradient. As electrons travel through the chain, their energy is used to pump hydrogen ions into the thylakoid.
• ATP synthase makes ATP. Hydrogen ions flow back across the membrane through ATP synthase, which uses that flow to produce ATP.
• Photosystem I re-energizes the electrons. Light excites the electrons again, and they pass through a second, shorter electron transport chain.
• NADP⁺ accepts electrons and hydrogen to form NADPH. NADPH carries them to the Calvin cycle.

ATP and NADPH are key products of the light-dependent reactions. The Calvin cycle uses them to help make sugar.

Speakers and sources

• Narrator: Unidentified narrator for the BioMan Biology video.
• Source featured: BioMan Biology, including a mention of its Photosynthesis Interactive.