Light-Dependent Reactions (The Z-pathway (The series of reactions involved…
The series of reactions involved in the light-dependent stage are known as collectively, as the Z-pathway because of the movement of electrons across the photosystems and thylakoid membrane.
In the presence of light a water molecule is split using an enzyme found within photosystem II, with a peak absorption of wavelength of 680 nm, into an atom of oxygen as well as two electrons and two protons. The oxygen is a by-product which is not used in this reaction.
The two hydrogen ions (protons) remain inside the thylakoid space, but the electrons are accepted by photosystem II when light is present, as the photons of light excite the electrons so that they are accepted by an electron acceptor and passed along various electron carriers (cytochromes).
The movement of electrons along the thylakoid membrane between different cytochromes releases energy, which pumps protons across the membrane from the stroma into the thylakoid inner space.
This creates a concentration gradient of protons over the membrane as they begin to accumulate, and so protons begin to diffuse back through to the stroma through channel proteins, which are associated with the enzyme ATP synthase. As protons flow through the protein, the enzyme is activated. Protons drive the rotation of the enzyme meaning ADP and one phosphate group forms ATP, which I known as phosphorylation (production of ATP using light energy).
Cyclic phosphorylation only occurs in photosystem I. Electrons are lost from PSI as photons of light excite them, and passed around electron carriers and then back to the chlorophyll of PSI where they first came form in the cycle.
This movement of electrons does generate small amounts of ATP, but does not involve the photolysis of water or reduce and NADP. The ATP produced may be used for other functions where energy is required.
Light-Dependent Stage: General
Light-dependent reactions occur in thylakoid membranes, inside of chloroplasts.
The light dependent stage converts light energy to chemical energy.
The entire light-dependent stage is the process of non-cyclic phosphorylation.
Water is an electron donor.
All of the steps of the Z-pathway are non-cyclic phosphorylation
Photolysis of Water
PSII has an enzyme which, in the presence of light, splits a molecule of water into protons, electrons, and oxygen.
2H₂O -> 4H+ + 4e− + O₂
The oxygen is a waste product.
Photolysis - the splitting of a molecule using photons (light is a type of photon)
The oxygen produced in this reaction is what we breathe in.
Formation of ATP: Chemiosis
Protons diffuse through the thylakoid membrane through channel proteins which are associated with ATP synthase.
The protons that are accepted by the coenzyme NADP (nicotinamide adenine dinucleotide phosphate), alongside electrons, to form reduced NADP, or NADPH (also sometimes written as NADPH₂).
NAPDH is used in the second stage of photosynthesis, the light-independent reactions.