5.2 Light Dependent Reactions Unit 5: Cell Energy 5.2 Light Dependent Reactions

Light Dependent Reactions Two sets of chemical reactions that occur in the thylakoid membrane Light-dependent reactions use energy from sunlight to: Convert ADP into ATP Convert NADP+ into the high energy carrier NADPH Produce oxygen as waste

Photosystems Photosystems: Clusters of proteins surrounded by accessory pigments in the membrane of the thylakoid (Similar structure to the cell membrane!) Absorb sunlight and generate high-energy electrons that are passed to a series of proteins embedded in the thylakoid membrane

Chemical Reaction #1: Photosystem II Light energy is absorbed High energy electrons are produced and passed to the electron transport chain Water molecules are split to replace those electrons H+ ions and oxygen are released ATP is synthesized

Electron Transport Chain A series of proteins that shuttle electrons Electrons move down the chain to photosystem I Energy from electrons is used to pump H+ ions across the thylakoid membrane and into the thylakoid space to create a concentration gradient of H+ ions

Electron Transport Chain Analogy

Electron Transport Chain-ATP Synthesis H+ then diffuse back into the cell across the enzyme ATP synthase. This causes ATP synthase to rotate, creating energy. ATP synthase captures the energy and uses it to create a bond between ADP and an additional phosphate. This creates ATP!

Chemical Reaction #2: Photosystem I The electrons have lost energy in the electron transport chain and need to be reenergized in photosystem I by absorbing more sunlight A second electron transport chain transfers these electrons to produce NADPH

Animation https://www.youtube.com/watch?v=BK_cjd6Evcw