Light Dependent Reactions - Non Cyclic

What is this known as

Known as light reactions since they require light to process

What does it produce

• Produces two high energy compounds NADPH and ATP by trapping light

Where does it SPECIFICALLY occur

Occurs on the thylakoid membrane of the chloroplasts

Summary of light dependent reaction

• Photon lights excites electrons to photosystems 

• Electrons are shuttled in between protein complexes from an electron transport chain 

• Products are ATP and NADPH

  • ATP produces energy for dark reactions

  • NADPH+ provides electrons for dark reactions 

  • Oxygen is a by-product

Photosystems I and II

• Photosystem I (P700) is best at absorbing light at a wavelength of 700nm 

• Photosystem II (P680) is best at absorbing light at a wavelength of 680nm 

• These 2 photosystems are key players in light reactions 

Step 1 in the PSII

1. A photon of light strikes PSII, exciting an electron in a chlorophyll molecule 

   • Light energy passes from pigment until it reaches the reaction center P680

   • The electron leaves P680 and is picked up by it's primary electron acceptor 

   • PSII is now oxidized (loses electrons and becomes P680+) and must be reduced and gain an electron for the reaction to continue 

   • In order to gain this electron, H2O is split and gains an electron 

     • Process of this is called Photolysis of Water 

   • Oxygen and hydrogen ions are produced, which is the source of oxygen that is released from plants

   • Hydrogen ions remain in the thylakoid space  (lumen)

   • Occurs up to 200 times a second 

Step 1 outside PSII

• The electron that left PSII is transported by a series of redox reactions (electron transport chain)

• Electron is passed from the first electron acceptor to a carrier 

  • Electron carrier is called plastoquinone 

• Plastoquinone transports electron to the b6f complex which is then passed to another carrier called plastocyanin 

• Plastocyanin takes the electron to PSI 

Step 2

1. As each electron passes through the proteins, it releases energy and H+ ions into the thylakoid space via the b6f complex 

   • Creates an ion gradient that is used in chemiosmosis (phosphorylation) 

Step 3

1. At the same time PSI is also capturing light energy

   • When energy reaches PSI reaction center P700, an electron is excited and is picked up by an electron carrier ferredoxin and leaves PSI

   • The lost electrons from PSI are replaced by the moving electrons from PSII 

Step 4

1. NADP reductase must be oxidized so that the chain can continue working

   • NADP+ acts as the final electron acceptor, pulling the electrons out of the electron transport chain, producing NADPH

Chemiosmosis and ATP production

• The electron transport chain are arranged with the photosystems in the thylakoid membranes and pump H+ within it

  • The flow of H+ is harnessed by the ATP synthase to make ATP

  • Also known as phosphorylation since the original energy source is light 

  • In the stroma, the H+ ions combine with NADP+ to form NADPH 

Cyclic Reactions

• In the cyclic flow, only P700 is used (PSI)

  • Electrons cycle back to the P700

  • Electrons are passed through the same pathway as the non-cyclical path, generating the proton gradient due to b6-f pumping H+ from the stroma into the thylakoid space 

  • ATP is made from the ATP synthase but NADPH  is not produced 

Why Cyclic?

• In non cyclic, the same amount of ATP and NADPH is produced 

• In cyclic, more ATP is consumed (3) than NADPH (2)

• Cyclic flow makes up the difference