Light Dependent Reactions

Define Photosynthesis

• the process by which cells synthesize organic molecules (e.g. glucose) from inorganic molecules (CO₂ and H₂O) in the presence of sunlight

• requires a photosynthetic pigment (chlorophyll)

• only occur in certain organisms (plants, some bacteria)

• occurs within the chloroplast

Light dependent versus light independent reactions

Light dependent reactions:

• convert light energy into chemical energy (ATP) and split water to release hydrogen atoms

• need NADP to carry hydrogen atoms

Light independent reactions:

• use chemical energy to fuse hydrogen atoms to carbon source => producing organic compounds

• Calvin cycle requires NADPH and ATP to occur

Photosystem II info

• made up of energy chemicals

• P680: strongest oxidizing agent (cholorphyll)

• light activates P680 bc it is photo excitable => causes electrons to become energized => loss of electrons (oxidation)

• Water photolysis => products of ½ O2 + 2H+

• ½ O2 is released into the atmosphere

• regenerates the e- P680 lost

H+ info

• H+ pump creates a concentration gradient (H move from the stroma into the thylakoid lumen)

• makes chemiosmosis possible => ATP production

Photosystem I

• P700: light excitable, oxidation agent

• excited by light => energize 2e- => transfer 2e- to Fd

ATP synthase info

• electrochemical gradient causes the flow of H+ from high to low => exiting the lumen into the stroma

• H+ goes through ATP synthase => rotor spins => converting ADP+ Pi to ATP