11.2: Light Reactions

light

form of electromagnetic radiation and while propagated in waves can also have particle-like behaviours

photons

particles of light that carry energy = energy source for photosynthesis

relationship between energy and wavelengths

shorter wavelengths = greater the energy

what happens when photons meet receptive molecules in plants

either bounce off molecule and is scattered/reflected, pass through molecule or absorbed and used for energy - depends on type of wavelength

ground state

The lowest energy state of an atom or molecule, where electrons are in their lowest energy configuration.

excited state

when electron absorbs energy and elevated to a higher level = more unstable and reactive

chlorophyll

A green pigment found in plants that absorbs light energy during photosynthesis, primarily in the blue and red wavelengths.

photosystem

protein and pigment complex in chloroplasts - role in capturing and converting light energy into chemical energy during light reactions

2 stages

light reactions where light drives synthesis of ATP and NADPH and Calvin cycle where ATP and NADPH powers conversion of CO2 into sugar

light-harvesting complexes

arrangement of pigments in photosystems that absorb light and convert into chemical energy

overview of how energy is passed to reaction centre

photons absorbed by chlorophyll →excited state→ releases electrons (energy) as returns to ground state → electrons absorbed by adjacent molecules → passed until reaches chlorophyll a in reaction centre

reaction centre

part of photosystem where excited electrons are transferred to primary electron acceptors, initiating the electron transport chain. Contains chlorophyll a (Chl)

overview of light reactions

Light enters as photons → electrons from water flow through photosystem ll into photosystem l = electrons transferred to electron carrier NADPH = production of ATP

photosystem 2

Chlorophyll P680 in reaction centre enters excited state = electrons transferred to primary electron acceptor via redox reaction = electrons passed to photosystem 1 thru ETC = creates proton gradient providing energy. P680 now missing negative charge = water molecule split into 2 electrons, 2 protons and ½ oxygen = the 2 electrons replace the missing ones in P680, the protons released into thylakoid space and oxygen combines with 2nd oxygen to make O2 diffused out of chloroplasts

photosystem 1

P700 in reaction centre becomes excited transferring electrons to primary electron acceptor = making P700+ which accepts electrons from electron transport chain from PS2. electrons passed down a 2nd ETC via ferredoxin (Fd) but no proton gradient = no ATP. Enzyme NADP+ reductase transfers electrons from Fd to NADP+.

production and maintenance of pH gradient

pumping of H+ into thylakoid lumen during the electron transport chain causes gradient. High H+ from hydrolysis and low H+ from by reaction of H+ with NADP+ forming NADPH