Photosynthesis in plants• Light energy is used to transform carbon

dioxide and water to energy rich food molecules composed of glucose monomers

• There are 2 stages in this process

Photosynthesis: The DetailsPhotosynthesis: The DetailsPhotosynthesis is divided into 2 sequential processes: the light reactions (stages 1 & 2) and carbon fixation (stage 3)

The Light Reactions: Noncyclic Electron Flow

-Convert solar energy to chemical energy

-The process is divided into 3 parts:

1.Photoexcitation

2.Electron Transport

3.Chemiosmosis

1. Photoexcitation• Electrons in chlorophyll molecules are initially at ground

state• When a molecule absorbs a photon, one of the electrons is

elevated to an orbital where it has more potential energy

Photoexcitation• In the photosynthetic membrane, a nearby molecule referred to

as a Primary Electron Acceptor traps a high energy electron that has absorbed a photon

• This a redox reaction• In chloroplasts – independent pigments do not absorb light,

instead clusters of chlorophyll molecules and accessory pigments associated with proteins called photosystems absorb light

The Light Reactions

• Photosystems are embedded in the thylakoid membrane.

• They contain chlorophyll and accessory pigments that are associated with proteins.

• A photosystem consists of an antenna complex and a reaction centre.

Photosystems•Photosystems I and II

•Of the many chlorophyll a molecules only one can trigger the light reactions by donating its excited electron to a primary electron acceptor

•The other chlorophyll a, chlorophyll b and carotenoid molecules function collaboratively as a light-gathering antenna that absorbs photons and passes the energy from pigment to pigment until it reaches the one chlorophyll a molecule in an area called the reaction centre

Photosystem I and II

• Photosystem I contains a specialized chlorophyll a molecule known as P700 since it best absorbs light with an average wavelength of 700 nm

• Photosystem II contains a specialized chlorophyll a molecule known as P680 since it best absorbs light with an average wavelength of 680 nm

• P700 and P680 chlorophyll a molecules are identical – they simply absorb at slightly different wavelengths because of the effects of the proteins they are associated with in the reaction centre

The Light Reactions

Photosystem II (P680)• Two photons strike photosystem II

and excite 2 electrons from chlorophyll P680.

• The excited electrons are captured by a primary electron acceptor and are then transferred to plastoquinone (PQ) and the ETC.

The Light Reactions

Photosystem II (P680)• In the ETC, the 2 electrons pass

through a proton pump (Q cycle).• The Q cycle transports 4 protons

from the stroma into the thylakoid lumen to create a proton gradient.

The Light Reactions

Photosystem II (P680)• The electrochemical gradient

drives the photophosphorylation of ADP to ATP.

• 1 ATP forms for every 4 protons that pass through ATPase from the thylakoid lumen into the stroma.

The Light Reactions

Photosystem II (P680)• A Z protein splits water into 2 protons, 2

electrons and 1 oxygen atom.– The electrons replace those lost from

chlorophyll P680.– The protons remain in the thylakoid space to

add to the proton gradient.– Oxygen leaves as a byproduct.

Noncyclic Electron Transport and Chemiosmosis

• Photon excites 2 electrons of chlorophyll P680

• Through series of redox reactions, electron transferred to PQ and then to ETC

• Z protein splits water and replaces missing electrons in P680

• Electrons flow down an ETC to P700 providing energy to make ATP since light is required for the establishment of proton gradient, this process is called photophosphorylation

• Excited electrons are stored as high energy-electrons in NADPH

http://vcell.ndsu.edu/animations/photosynthesis/movie.htm

http://vcell.ndsu.edu/animations/photosystemII/movie.htm

The Light Reactions

Photosystem I (P700)• Two photons strike photosystem I and

excite 2 electrons from chlorophyll P700 (replaced by electrons from P680).

• These electrons pass through another ETC.

• The enzyme NADP reductase uses the 2 electrons and a proton from the stroma to reduce 1 NADP+ to 1 NADPH.

http://highered.mcgraw-hill.com/sites/0070960526/student_view0/chapter5/animation_quiz_1.html

Cyclic flow

• Photosystem I only• Electron excited and trapped by primary electron acceptor• Electron passed to Fd• Passes through Q cycle, b6-f complex and back to chlorophyll

P700• Generates proton gradient for ATP synthesis, does NOT

release electrons to generate NADPH• Without NADPH, carbon fixation cannot occur

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio12.swf::Cyclic%20and%20Noncyclic%20Photophosphorylation