Chapter 6 Section 1

1. Chapter 6 Section 1 The Light Reactions of Photosynthesis

2. The Light Reactions The purpose of the light reactions is to convert solar energy into chemical energy that will be used in the light-independent reactions. The reactions that make up the biochemical pathway for the first part of photosynthesis involve only the reactants water and sunlight.

3. The Chloroplast The chloroplast is divided up into stacks of membranes called thylakoids that are surrounded by a stroma solution. The majority of the light reactions take place within the thylakoid membrane. Structures such as photosystems, an ETC and an ATP synthase are all found within the thylakoid membrane.

4. Solar Energy and Photosystems There are two photosystems in the thylakoid membrane that contain hundreds of chlorophyll and caretenoid molecules. Step 1: Solar energy hits these molecules in Photosystem II and excites the electrons in atoms that make up the pigment molecules. The electrons move up to higher energy levels and eventually leave the chlorophyll molecules.

5. Electron Movement Step 2: The electrons move to a primary electron acceptor molecule that is the first in a series called the electron transport chain (ETC). As the electrons move they supply energy for a proton pump that pumps H+ back into the thylakoid space using active transport. This energy is necessary because the [H+] is high inside of the thylakoid but low outside.

6. Photosystems and Electrons Step 3: Solar energy also hits pigment molecules in Photosystem Iand excites the electrons which move to higher energy levels. The electrons move to another primary electron acceptor molecule that is part of a second electron transport chain. The electrons moving out of the first ETC replace those lost from Photosystem I so the energy flow continues.

7. Water splitting Step 4: Two Water molecules move into the thylakoid using osmosis. A water-splitting enzyme is needed to break the bonds between the hydrogen and oxygen atoms in the stable water molecules. The products of the reaction are hydrogen ions, electrons and oxygen atoms.

8. Water splitting Step 5: When the two water molecules split two unstable oxygen atoms are produced. They immediately pair up to share their six valence electrons and achieve stability. Oxygen is a waste product and move out of the chloroplast using simple diffusion.

9. Electrons and Redox Reactions Step 6: An oxidation reaction happens when water loses electrons. The electrons move into photosystem II in a reduction reaction. The chlorophyll molecules in photosystem II accept these electrons because they lose their original ones when they are hit by solar energy. In reality this step is going on simultaneously with the movement of electrons through the photosystems and ETC.

10. Hydrogen Ion Movement Step 7: There is a high [H+] inside of the thylakoid for two reasons. Some H+ is generated from water splitting and some is pumped in through the proton pump that is part of the ETC. The H+ diffuses passively through a special enzyme called the ATP synthase out into the stroma solution.

11. The Formation of ATP Step 8: The ATP synthase uses the power of H+ movement from the thylakoid to the stroma to form ATP. It attaches a phosphate group onto an ADP molecule to from the energy storage molecule ATP which is a form of chemical energy. The coupling of the movement of H+ with the formation of ATP is called chemiosmosis (The hydrogen originally came from water).

12. Formation of NADPH Step 9: The NADP+ molecule does not gain stability by just accepting electrons. It must also accept the H+ that is in the stroma solution. The formation of NADPH accomplishes two goals. One of them is to keep the [H+] in the stromalow so the H+ concentration gradient is maintained between the thylakoid space and the stroma.

13. The Movement of Hydrogen Step 9: Not all of the H+ that diffuses through the ATP synthase stays in the stroma solution. Some of the H+ needs to be pumped back into the thylakoid to go through the ATP synthase a second time. This is because more ATP molecules need to be produced than there are H+ ions available to make the correct amount.

14. Electron Movement Step 9: When the molecules in the second ETC give up their electron they become oxidized. The electrons move out of the thylakoid into the stromawhen they are accepted by NADP+ molecules that are then reduced. The NADP+ form of the molecule is a low energy form.

15. Formation of NADPH Step 9: The second reason that the formation of NADPH is important is to store chemical energy. The molecule provides a way to store hydrogen atoms for use in the light-independent reactions.

16. Overview of the Light Reactions Below is a diagram showing all of the steps involving H+ and e- movement during the light reactions. Make time to review the steps so you don’t end up like this kid!