Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

4. Photosynthesis
4.4 Light Absorption and Chlorophyll Excitation

An electron is a charged particle that orbits the atomic Nucleus. It possesses a magnetic moment vector, known as spin, which reflects the direction of the electron's rotation around its own axis. In its stable state, valence electrons occupy the lowest energy levels and are distributed among them in accordance with the Pauli exclusion principle: two electrons with antiparallel spins per orbital. This state of the molecule is referred to as the ground singlet state S0. Upon absorption of a photon by a pigment molecule, the electron is promoted to a higher orbital utilizing the photon's energy. If the spin direction of the excited electron is preserved, the molecule is considered to be in the excited singlet state S*. If the excited electron reverses its spin, this excited state is called a triplet state T*.

Absorption of a red light quantum by a chlorophyll molecule results in the excited singlet state S*₁. When a blue light quantum of higher energy level is absorbed, the electron transitions to a higher S*2 orbital. The lifetime of the electron at S*₂ is no more than 10^-12 seconds. Within such a short timeframe, the electronic excitation energy cannot be utilized, and the electron returns to S*₁, losing a minor amount of energy as heat. The lifetime of the electron in the S*1 state is 10^-9. If the pigment molecule does not participate in a photochemical reaction, the electron returns to its stable state. In the process, part of the energy is lost as heat, and a light quantum with a longer wavelength compared to the absorbed photon is emitted—a phenomenon known as fluorescence.

The lifetime of the electron in the T* state is 10^-4 -10^-2 seconds. From the triplet excited state, the molecule can return to the stable state by emitting, In addition to heat, a light quantum of an even longer wavelength than during fluorescence. This weak emission is called phosphorescence. If the molecule in the excited state participates in Photochemical reactions involving electron transfer to an acceptor, neither fluorescence nor phosphorescence occurs.

To more fully utilize the light incident on the leaf, photon energy is captured by 200–400 pigment molecules of the light-harvesting (antenna) complex and transferred to a single chlorophyll a molecule acting as the reaction center, which directly participates in photochemical reactions. Energy transfer among pigment molecules occurs via inductive Resonance. A pigment molecule that has absorbed a photon generates an alternating electric field that enhances electron oscillation in a neighboring molecule. This is facilitated by the overlap of vibrational frequencies and the very short distance between pigment molecules. Furthermore, upon leaf illumination, thylakoids and grana shift and pack tightly together, causing METABOLISM/14.html">Chloroplasts to decrease in volume. Energy transfer by pigment molecules proceeds with high efficiency. Specifically, approximately 90% of the absorbed Energy is transferred from chlorophyll b to chlorophyll a, and 40% from carotenoids to chlorophyll a.

The assembly of the light-harvesting complex and the reaction center constitutes a photosystem. The hypothesis regarding the existence of Two Photosystems in chloroplasts was proposed by R. Emerson in 1957 while studying The Effect of light on the quantum yield of Photosynthesis in the alga Chlorella. The quantum yield of photosynthesis is The amount of oxygen evolved or carbon dioxide fixed per quantum of absorbed energy. He established that simultaneous illumination of Chlorella with short-wavelength (650 nm) and long-wavelength (700 nm) red light produces a greater effect than the sum of the quantum yields driven by each of these red wavelengths applied separately. This phenomenon was named the Emerson enhancement effect. Later, R. Emerson's hypothesis concerning the presence of two photosystems received experimental confirmation.