Light Dependent

LIGHT DEPENDENT REACTION

  • First part/process of photosynthesis (1st: Light-Dependent, 2nd: Light-Independent)

  • Process that uses light energy and turns it into chemical energy.

  • Happens during the day as it cannot happen without the presence of sunlight and light.

  • An essential process to make two molecules needed for the next stage of photosynthesis (light-independent):

  • ATP (Adenosine Triphosphate): The energy storage molecule and;

  • NADPH (Nicotinamide Adenine Dinucleotide Phosphate): The reduced electron carrier

FULL PROCESS



STAGE

STEP

  1. Light Absorption

  • Chlorophyll absorbs sunlight and the light energy from here turns into chemical energy.

NOTE: Light strikes and absorbs multiple times, not just once. It strikes and absorbs in the start, in PSII (via P680), and in PSI (via P700).

  1. Photolysis

  • Photolysis = Process of Splitting Water

  • H2O splits through light absorption (as light breaks the H2O molecule to separate atoms).

  • The electron (e) that bonded the H2O together travels to the Photosystem II (PSII) reaction center.

  • Hydrogen (H) goes to the lumen, Oxygen (O) is a byproduct and exits through the stomata

  1. PSII (P680)

  • Then, light strikes the chlorophyll twice in Photosystem II (PSII) which is then absorbed by P680 (a specialized chlorophyll in the reaction center of PSII).

  • P680 absorbs energy and fuels the electron, making it excited.

  1. Electron Transport Chain (ETC)

  • After this, the electron goes down the Electron Transport Chain (ETC) aka a series of membrane proteins.

  • As the electron goes down the ETC and passes through multiple proteins, it releases energy and pumps hydrogen ions across the thylakoid membrane into the thylakoid.

    • Basically, dumadami the hydrogen ions sa loob ng thylakoid membrane.

  • The electron ends up in Photosystem I (PSI) but without any energy.

  1. ATP Formation

  • This is where Chemiosmosis happens (the process of driving electrons to ATP synthase)

  • Energy from the Electron Transport Chain (ETC) is used to push hydrogen down to the ATP synthase.

    • Because of the abundance of hydrogen ions within the thylakoid membrane, it escapes through the ATP synthase. 

  • Hydrogen ions convert ADP to ATP in the ATP synthase.

NOTE: The electron is still moving sa ETC during this process.

  1. PSI (P700)

  • Since the electron in Photosystem I (PSI) is depleted, chlorophyll in Photosystem I (PSI) gets struck with light twice, P700 absorbs light energy.

  • Electron gets fueled by P700 and goes excited again.

  1. NADPH Formation

  • Electrons get absorbed by NADP+, and when this happens it also accepts hydrogen.

  • NADP+ is reduced to become NADPH.

Total End Product: ATP and NADPH (and oxygen but it is considered as a waste product)



*ATP has positive charge and NADPH has negative charge = Perfect combination to fuel the Light-Independent process or Calvin Cycle