Describe the role of chloroplast pigments (chlorophyll a, chlorophyll b, carotene and xanthophyll) in light absorption in the thylakoid membranes.
Key Pigments in the Thylakoid Membrane
Pigment
Location in Thylakoid
Absorption Peaks (nm)
Primary Role
Chlorophyll a
Reaction centre (RC) of PSII and PSI
430, 662
Primary electron donor; converts light energy to chemical energy.
Chlorophyll b
Antenna complexes (LHCII, LHCI)
453, 642
Expands the range of absorbed light; transfers energy to chlorophyll a.
Carotene (β‑carotene)
Embedded in antenna complexes
450–500
Absorbs blue‑green light; protects against photo‑oxidative damage; transfers energy to chlorophyll a.
Xanthophyll (e.g., lutein, violaxanthin)
Embedded in antenna complexes
460–530
Absorbs excess blue light; dissipates excess energy as heat (non‑photochemical quenching).
How Pigments Work Together
The thylakoid membrane contains two photosystems (PSII and PSI) each with a reaction centre surrounded by an antenna of pigment‑protein complexes. Light energy is captured as follows:
Photons strike the antenna pigments (chlorophyll b, carotene, xanthophyll).
Energy is transferred by resonance energy transfer to chlorophyll a molecules in the antenna.
Excitation energy reaches the reaction‑centre chlorophyll a (P680 in PSII, P700 in PSI).
Excited chlorophyll a donates an electron to the primary electron acceptor, initiating the electron transport chain.
The complementary absorption spectra of the pigments ensure that the plant can utilise a broader portion of the solar spectrum. The combined absorption can be represented by the sum of the individual spectra:
where \$ci\$ is the concentration and \$\varepsiloni(\lambda)\$ the molar absorptivity of pigment \$i\$.
Protective Functions
Carotene quenches triplet chlorophyll and singlet oxygen, preventing oxidative damage.
Xanthophyll participates in the xanthophyll cycle, converting violaxanthin to zeaxanthin under high light to increase thermal dissipation.
Suggested diagram: Cross‑section of a thylakoid showing PSII and PSI with their associated antenna pigments (chlorophyll a, chlorophyll b, carotene, xanthophyll) and the direction of energy transfer.
Summary
Chlorophyll a is the sole pigment that drives charge separation in the reaction centre.
Chlorophyll b broadens the range of usable light and funnels energy to chlorophyll a.
Carotene captures blue‑green light and protects the photosystems from reactive oxygen species.
Xanthophylls act as a dynamic photoprotective shield, dissipating excess energy as heat.
The coordinated action of these pigments enables efficient conversion of solar energy into chemical energy under varying light conditions.