1. PHOTOSYNTHESIS The Light Reactions

2. Photosynthesis: An Overview of the Light and ‘Dark’ Reactions Occurs in Photoautotrophs (organisms that can make their own using energy from the sun). Photosynthesis takes place in the chloroplasts. Photosynthesis includes two processes… LIGHT REACTIONS Requires sunlight Occurs in the granna of chloroplasts Produces ATP and NADPH (used to power the Calvin cycle) ‘DARK’ REACTIONS (a misnomer…aka Calvin cycle) Doesn’t require sunlight (happens 24/7). Occurs in the stroma of chloroplasts Produces PGAL (which can later be used to make glucose) http://simple animationsimple animation

3. LIGHT REACTION Step 1 (This process occurs twice): A photon strikes photosystem II and excites an electron of chlorophyll P680. The excited electron is captured by a primary electron acceptor called pheophytin. Through a series of redox reactions, the electron is transferred to an electron carrier called plastoquinone (PQ) and then to an electron transport chain.

4. LIGHT REACTION Step 2: A Z protein, associated with photosystem II and facing the thylakoid lumen, splits water into oxygen, hydrogen ions, and electrons. Two of these electrons are used to replace the missing electrons in chlorophyll P680. Oxygen leaves the chloroplast as a byproduct, and the protons remain in the thylakoid space.

5. LIGHT REACTION Step 3: The electrons that leave photosystem II pass from the stroma into the thylakoid lumen. Four protons are translocated into the thylakoid lumen for each pair of electrons that passes through the transport chain. The electrons eventually replace the two electrons that were lost by photosystem I when it was struck by photons.

6. LIGHT REACTION Step 4: The electrons from photosystem I pass through another electron transport chain containing an iron-containing protein called ferredoxin (Fd). They then move to the enzyme NADP reductase that uses the two electrons and H ions from the stroma to reduce NADP to NADPH.

7. LIGHT REACTION Step 5: Protons that accumulate in the thylakoid lumen contribute to an electrochemical gradient that drives the phosphorylation of ADP to ATP. As protons move through the ATPase complex from the thylakoid lumen into the stroma, ATP is formed. Since light is required for the establishment of the proton gradient, this process is called photophosphorylation.

9. Phosphorylation Phosphorylation: The chemical addition of a phosphate group (phosphorous and oxygen) to a compound. i.e. adding Pi to ADP to get ATP Photophosphorylation is addition of a phosphate using the sun’s energy! There are two types of photophosphorylation; cyclic and non-cyclic.

10. CYCLIC ENERGY FLOW Photosynthesis that incorporated both Photosystems I and II is referred to as Noncyclic Energy Flow. The process is noncyclic because electrons emitted from PS II eventually end up in NADPH.

11. CYCLIC ENERGY FLOW In some cases, excited electrons take a cyclic pathway called cyclic electron flow that uses Photosystem I only. In this pathway, a photon ejects an electron from Photosystem I. The electron is passed to back to chlorophyll via the electron transport chain. Cyclic photophosphorylation is also seen in certain photosynthetic bacteria. Note that the bacteria have no chloroplasts. All structures are embedded in the membrane. The proton gradient is created between the cell membrane and the capsule.

12. CYCLIC ENERGY FLOW This cyclic pathway generates a proton gradient for chemiosmotic ATP synthesis, but does not release electrons to generate NADPH. What is the function of cyclic electron flow?

13. CYCLIC ENERGY FLOW Noncyclic energy flow produces roughly the same quantities of ATP and NADPH yet the Calvin cycle consumes more ATP than NADPH. Cyclic energy flow makes up the difference.

14. Light Reactions and Non-Cyclic Photophosphorylation Hmmmm… Try to interpret this diagram in laymen’s terms. Non-cyclic photophosphorylation

15. Overview of light dependent reactions