Tag: emerson's experiments & hill's reaction

The ATP and NADPH from the light-dependent reactions are used to make sugars in the next stage of photosynthesis. The net result of this reaction is the production of 2 ATP and 9 NADPH and the photolysis of water. Light is absorbed and the energy is used to drive electrons from water to generate NADPH and to drive protons across a membrane. These protons return through ATP synthase to make ATP.

When photosystem II absorbs light, electrons in the reaction-centre chlorophyll are excited to a higher energy level and are trapped by the primary electron acceptors. Photoexcited electrons travel through the cytochrome b6f complex to photosystem I via an electron transport chain set in the thylakoid membrane. The light-dependent reactions use light energy to make two molecules needed for the next stage of photosynthesis: the energy storage molecule ATP and the reduced electron carrier NADPH. In plants, the light reactions take place in the thylakoid membranes of organelles called chloroplasts.

The thylakoid membranes have pigment system, electron transport proteins as well as the ATP synthases. When the transfer of electron down the transport proteins occurs, many H$^{+}$ are transported into the thylakoid lumen from the stroma. This high concentration of H$^{+}$ is then used by the ATP synthases to make ATP by transferring these H$^{+}$ out of the thylakoid lumen in the stroma. The punctured thylakoid will not have an intact thylakoid lumen, so the concentration of the H$^{+}$ ins will not be developed. As a result, ATP synthase will not function and so ATP will not be synthesized. 

The overall function of light-dependent reactions is to capture the energy from the light for the generation of ATP and NADPH molecules. The dark reactions of the C$ _{3}$ cycle utilize the energy from short-lived electronically excited carriers to convert CO$ _{2}$ & H$ _{2}$O into glucose catalyzed by the enzyme RuBisCo, and this constitutes carbon fixation and makes it the enzymatic phase.

Chlorophyll present in the plants can absorb the the wavelength of both blue and red regions of VIBGYOR which ranges from 400-500 nm and 600-700 nm respectively. It reflects the green light range of 500-600 nm hence the leaves appear green. 

The light-dependent reactions start in photosystem II (PSII). When the pigment in the reaction centre of PS II i.e, P$ _{680}$ absorbs a photon, an electron in this molecule gets excited and transferred to a primary electron acceptor, Pheophytin and then go through molecules in a series of redox. The electron flow goes from PSII to cytochrome b6f complex than to PSI. In PSI, the electron is finally accepted by NADP. Thus photosystem I in light-dependent reaction directly transfers electrons to NADP.