Question

What happens to chlorophyll a from the light harvesting complex after absorbing light? What happens to chlorophyll a from the reaction center after absorbing light?

Answer 1

The absorption of light energy and its conversion into chemical energy occurs in multiprotein complexes, called photosystems, located in the thylakoid membrane. A photosystem has two closely linked components, an antennacontaining light-absorbing pigments and a reaction center comprising a complex of proteins and two chlorophyll amolecules. Each antenna (named by analogy with radio antennas) contains one or more light-harvesting complexes(LHCs). The energy of the light captured by LHCs is funneled to the two chlorophylls in the reaction center, where the primary events of photosynthesis occur.

Chlorophylls are very effective photoreceptors because they contain networks of alternating single and double bonds. Such compounds are called polyenes. They have very strong absorption bands in the visible region of the spectrum. The peak molar absorption coefficient of chlorophyll a is higher than 10⁵ M^-1 cm^-1.

The energy from the light excites an electron from its ground energy level to an excited energy level. For most compounds that absorb light, the electron simply returns to the ground state and the absorbed energy is converted into heat. However, if a suitable electron acceptor is nearby, the excited electron can move from the initial molecule to the acceptor. This process results in the formation of a positive charge on the initial molecule (due to the loss of an electron) and a negative charge on the acceptor and is, hence, referred to as photoinduced charge separation. The site where the separational change occurs is called the reaction center.The electron, extracted from its initial site by absorption of light, can reduce other species to store the light energy in chemical forms.

The absorption of a quantum of light of wavelength ≈680 nm causes a chlorophyll a molecule to enter the first excited state. The energy of such photons increases the energy of chlorophyll a by 42 kcal/mol. This differs from the ground (unexcited) state largely in the distribution of electrons around the C and N atoms of the porphyrin ring. Excited states are unstable, and will return to the ground state by one of several competing processes.

When the same chlorophyll a is bound to the unique protein environment of the reaction center, this excited-state energy is used to promote a charge separation across the thylakoid membrane: an electron is transported from a chlorophyll molecule to the primary electron acceptor, the quinone Q, on the stromal surface of the membrane, leaving a positive charge on the chlorophyll close to the luminal surface. The reduced primary electron acceptor becomes a powerful reducing agent, with a strong tendency to transfer the electron to another molecule. The positively charged chlorophyll, a strong oxidizing agent, will attract an electron from an electron donor on the luminal surface. These potent biological reductants and oxidants provide all the energy needed to drive all subsequent reactions of photosynthesis: electron transport, ATP synthesis, and CO₂ fixation.