Gen Bio 1 L7: Light-dependent Reactions of Photosynthesis

• Light-dependent Reaction: takes place in the thylakoid membrane of the chloroplasts in plant cells.
• Light-dependent Reaction: involves the absorption of light energy by pigments, such as chlorophyll, and the conversion of this energy into chemical energy in the form of ATP and NADPH.
• Energy-rich Molecules: used in the light-independent reaction to produce glucose.
• Within the chloroplasts, light energy is absorbed by pigments located in two types of photosystems:

  1. Photosystem I (PSI)

  2. Photosystem II (PSII)
• Photosystem I (PSI): responsible for capturing light energy and transferring electrons to produce NADPH.
• Several Components of PSI: P700 Chlorophyll, Antenna Complex, Reaction Center, Electron Transport Chain, and NADP+ Reductase
• P700 Chlorophyll: primary pigment molecules in PSI that absorbs light with a wavelength of around 700nm.
• P700 Chlorophyll: it is called P700 because it absorbs light best at this specific wavelength (700nm).
• Antenna Complex: surrounding the P700 chlorophyll are several accessory pigments, such as chlorophyll b and carotenoids mounted on two proteins.
• Antenna Complex: These pigments help to capture light energy from a wider range of wavelengths and transfer it to the P700 chlorophyll.
• Reaction Center: The P700 chlorophyll transfers the captured light energy to a special pair of chlorophyll molecules in the iron-sulphur (FeS) type reaction center.
• Reaction Center: Here, the energy is used to excite electrons, which are then transferred to an electron acceptor molecule (iron-sulphur proteins).
• Electron Transport Chain: The excited electrons are passed through a series of electron carriers in the thylakoid membrane, creating a flow of electrons.
• Electron Transport Chain: This flow of electrons generates a proton gradient across the membrane, which is used to produce ATP through a process called chemiosmosis.
• NADP+ Reductase: At the end of the electron transport chain, the electrons are transferred to NADP+ (nicotinamide adenine dinucleotide phosphate), along with protons from the surrounding medium.
• NADP+ Reductase: This reaction converts NADP+ into NADPH, which is an energy-rich molecule used in the light-independent reaction.
• Photosystem II (PSII): functions to capture light energy and transfer electrons to generate ATP.
• Components of Photosystem II (PSII): P680 Chlorophyll, Antenna Complex, Reaction Center, and Electron Transport Chain.
• PSII P680 Chlorophyll: primary pigment molecule in PSII that absorbs light with a wavelength of around 680 nanometers.
• PSII P680 Chlorophyll: It is called P680 because it absorbs light best at this specific wavelength (680nm).
• PSII Antenna Complex: composed of 6 types of light-havesting complexes(Lhcb 1-6).
• PSII Antenna Complex: Lhcb 1-3 produce LHCII, form heterotrimers.
• PSII Antenna Complex: These accessory pigments capture light energy from a wider range of wavelengths and transfer it to the P680 chlorophyll.
• PSII Reaction Center: The P680 chlorophyll transfers the captured light energy to a special pair of chlorophyll molecules in the quinone or Q-type reaction center.
• Reaction Center: Here, the energy is used to excite electrons, which are then transferred to an electron acceptor molecule (pheophytin).
• Electron Transport Chain: The excited electrons from PSII are passed through a different electron transport chain in the thylakoid membrane.
• This electron transport chain also generates a proton gradient, which is used to produce ATP through chemiosmosis.