The photosynthetic light reactions in plants involve several key processes, primarily occurring in the thylakoid membranes of chloroplasts. A common misconception is that the ultimate electron acceptor in these reactions is O2. In reality, the primary electron acceptor is NADP+, which is reduced to NADPH, a crucial molecule that carries electrons to the Calvin cycle for sugar synthesis.
During the light reactions, there are two main pathways: linear electron flow and cyclic electron flow. Linear electron flow involves both Photosystem II and Photosystem I, where water is split to release oxygen. In contrast, cyclic electron flow only utilizes Photosystem I, producing ATP without generating NADPH or oxygen. This process recycles electrons back to the proton pump via ferredoxin, emphasizing the distinction between the two pathways.
Additionally, archaea possess rhodopsins, which can generate a proton motive force using light, but these proteins also have other functions, such as visual perception in the eyes and chloride pumping. In the chloroplasts, photosystems contain numerous chlorophyll molecules that act as antennas, transferring excitation energy to the reaction center chlorophyll, which is essential for initiating the photosynthetic process.
To quantify the energy of photons involved in photosynthesis, one can use the Planck-Einstein equation, which relates energy to wavelength. The equation is given by:
E = \frac{hc}{\lambda}
where E is the energy of a photon, h is Planck's constant (approximately 6.626 x 10-34 J·s), c is the speed of light (approximately 3 x 108 m/s), and λ is the wavelength in meters. For a wavelength of 680 nm (or 680 x 10-9 m), the energy of a single photon can be calculated. After performing the calculation, the energy for one photon is found to be approximately 2.925 x 10-19 J. To find the energy per mole of photons, this value is multiplied by Avogadro's number (6.022 x 1023), resulting in approximately 176 kJ/mol. This energy is critical for driving the reactions of photosynthesis.
