Capturing light energy
antenna complex
primary acceptor
chlorophyll a and b
In the reaction centre there are two chlorophyll a molecules that are able to eject electrons. Only these two molecules in this location have this ability. Surrounding the reaction centre is an arrangement of accessory (antenna) pigments (100 to 1000) that serve to capture different wavelengths of light and to transfer that energy to the reaction centre. The antenna pigments also serve to protect the reaction centre from excessive light energy (photoprotection).
Make a prediction: The antenna pigment array would be larger in a plant that thrives in full sun or a plant that thrives in partial sun or shade?
Antenna pigments include the chlorophylls (both a and b), xanthophylls, carotenoids, and many others. Proteins and primary acceptor molecules are also associated with the reaction centres. Together, all these molecules make up a photosystem.
Photosystem I and II
The first photosystem evolved in prokaryotic autotrophs. Photosystem I, as it's known, absorbs most efficiently at a wavelength of 700 nm (far red portion of the spectrum). This system is sometimes described as P700 for its specialized chlorophyll a molecule.
Photosystem II evolved later in the cyanobacteria and is present in higher plants. Photosystem II is an add-on to P700 and can never operate on its own. The reaction centre chlorophyll a molecule is associated with a different protein than the P700 and absorbs best at a wavelength of 680 nm. This energy difference in wavelength absorption is enough to allow P680, but not P700, to split water molecules. Since oxygen is a byproduct of the photolysis of water, the evolution of P680 changed the Earth's atmosphere from an oxygen-poor to an oxygen-rich composition.
Photosystems I and II are closely situated in the thylakoid membrane and are part of the chloroplast's photocentre. An electron transport chain connects the two photosystems and ATP synthase molecules complete the participants in the light-dependent reactions.
Light-dependent reactions
When photons hit either photosystem, electrons are ejected from the reaction centre. These energetic electrons are captured by molecules known as primary electron acceptors. But the chlorophyll cannot remain electron deficient (a condition known as "bleached"). To sustain the reactions, an electron must replace the one that was lost.
When an electron leaves P700, it travels through a short electron chain ending with NADP+ reductase. The NADP+ syphons off these electrons and together with protons from the stroma becomes reduced to NADPH. This is the reducing power required by the Calvin cycle.
Electrons are replaced in P700 from electrons ejected from P680 as photons hit that reaction centre. An electron transfer chain shuttles electrons from P680 to P700 and proton pumps within the chain translocate protons from the stroma into the thylakoid space. A proton-motive force is generated in the thylakoid space whose potential energy is harnessed to drive the phosphorylation of ADP through ATP synthase molecules. This is how chloroplasts generate chemical energy (ATP) from light energy.
electron ejection
non-cyclic photophosphorylation
energy profile
