Photosynthesis: Light Dependent Reactions

Overview of Light Dependent Reaction

Light Dependent Reaction: 5 Step Process 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 H2O inside the thylakoid (broken by an enzyme) (By being reduced it allows chlorophyll a to repeat the process.)

Review step one: (notice H2O Decomposition enzyme)

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

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 H2O  4(H+) + 4(e-) + o2 This decomposition enzyme replenishes electrons in PII, places protons into the thylakoid, and releases oxygen into the atmosphere.

Light is absorbed by Photosystem I (P 1) As in Photosystem II, electrons (e-) are excited and break away from chlorophyll a in P1 (oxidation). The chlorophyll a molecules lost e- is replaced by an electron which entered from photosystem II. (reduction). As in PII, e- excited in P1 are passed through molecules in an electron transport chain. The chain brings e- to the thylakoid membrane where it connects with NADP+. When e- connects with NADP+ it becomes NADPH.

Chemiosmosis and the Proton Pump Adenosine Triphosphate (ATP) – is the main energy currency for cells. Chemiosmosis: 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. 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.