PHOTOSYNTHESIS LIGHT DEPENDENT REACTIONS OVERVIEW OF LIGHT DEPENDENT

PHOTOSYNTHESIS: LIGHT DEPENDENT REACTIONS

OVERVIEW OF LIGHT DEPENDENT REACTION

LIGHT DEPENDENT REACTION: 5 STEP PROCESS 1. Light energy forces electrons to enter a higher energy level in the two chlorophyll a molecules of photosystem II. This energizes electrons or “excites” them and cause them to LEAVE the chlorophyll a molecule. This is called Oxidation. � Reduction is when the chlorophyll a molecule GAINS an electron. It gains an electron by taking one from H 2 O inside thylakoid (broken by an enzyme) � (By being reduced it allows chlorophyll a to repeat the process. ) �

REVIEW STEP ONE: (NOTICEH O DECOMPOSITION ENZYME) 2

2. The lost electron from chlorophyll a is taken by a molecule in the thylakoid membrane called the primary electron acceptor.

3. The primary electron acceptor donates the electron to a series of molecules in the thylakoid membrane. The series of molecules are called the electron transport chain. � As electrons pass through the chain they lose energy by passing it on to protons (H+). � The excited protons (H+) move into the thylakoid lumen. �

ENZYME 2 H O 4(H+) + 4(E-) + O 2 2 This decomposition enzyme replenishes electrons in PII, places protons into the thylakoid, and releases oxygen into the atmosphere.

4. Light is absorbed by Photosystem I (P 1) As in Photosystem II, electrons (e-) are excited and break away from chlorophyll a in P 1 (oxidation). � The chlorophyll a molecules lost e- is replaced by an electron which entered from photosystem II. (reduction). � As in PII, e- excited in P 1 are passed through molecules in an electron transport chain. � The chain brings e- to the thylakoid membrane where it connects with NADP+. � 5. When e- connects with NADP+ it becomes NADPH.

CHEMIOSMOSIS AND THE PROTON PUMP Adenosine Triphosphate (ATP) – is the main energy currency for cells. Chemiosmosis: 1. 2. 3. 4. Chemiosmosis relies on build-up of protons (H+) which is a concentration gradient of protons. These protons came from the water splitting enzyme and those brought into the thylakoid in PII. There is a higher concentration of protons in thylakoid which want to move out of the thylakoid. (potential energy) ATP synthase enzyme acts as a “proton pump” which harnesses energy from the flow of protons (H+) going through it.

ATP Synthase uses energy to catalyze a reaction of adensosine diphosphate (ADP) to create ATP. 5. ATP Synthases adds a phosphate atom to ADP making it ATP

Summary: � The creation of NADPH and ATP is the purpose of light dependent reactions. � These two powerful chemicals are used in the next part of photosynthesis called the Calvin Cycle.