3.1 Light absorption and photosynthetic pigments

Photosynthetic pigments are the unsung heroes of plant life. Chlorophylls and carotenoids work together to capture light energy, absorbing different wavelengths to maximize efficiency. These pigments are crucial for turning sunlight into usable energy for plants.

Light absorption and energy transfer are the first steps in photosynthesis. Photosystems and light-harvesting complexes team up to capture light and funnel that energy to reaction centers. This process kickstarts the chain of events that ultimately produces food for the plant.

Photosynthetic Pigments

Chlorophylls

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• Chlorophyll a is the primary photosynthetic pigment found in all oxygen-evolving photosynthetic organisms
• Chlorophyll a absorbs light most strongly in the blue and red regions of the visible spectrum but poorly in the green region
• Chlorophyll b is an accessory pigment that absorbs light in slightly different regions of the visible spectrum than chlorophyll a, allowing for a broader range of light absorption
• Chlorophyll b transfers the absorbed light energy to chlorophyll a for use in photosynthesis

Carotenoids

• Carotenoids are accessory pigments that absorb light in the blue and green regions of the visible spectrum
• Carotenoids transfer the absorbed light energy to chlorophyll a for use in photosynthesis
• Carotenoids also play a role in photoprotection by dissipating excess light energy as heat, preventing damage to the photosynthetic apparatus (xanthophyll cycle)
• Examples of carotenoids include beta-carotene (orange pigment in carrots) and lutein (yellow pigment in leaves)

Absorption Spectrum

• An absorption spectrum is a graph showing the amount of light absorbed by a pigment at different wavelengths
• Chlorophyll a has absorption peaks in the blue (around 430 nm) and red (around 660 nm) regions of the visible spectrum
• Chlorophyll b and carotenoids have absorption peaks that complement those of chlorophyll a, allowing for a broader range of light absorption
• The absorption spectra of photosynthetic pigments are adapted to the light environment in which the organism lives (aquatic plants absorb different wavelengths than terrestrial plants)

Light Absorption and Energy Transfer

Photosystems

• Photosystems are large protein complexes that contain photosynthetic pigments and are responsible for light absorption and energy transfer
• There are two types of photosystems in oxygenic photosynthesis: Photosystem I (PSI) and Photosystem II (PSII)
• Each photosystem has a reaction center chlorophyll a molecule (P700 in PSI and P680 in PSII) that initiates electron transfer when excited by light
• The reaction center is surrounded by light-harvesting complexes that contain additional chlorophyll and carotenoid molecules

Light-Harvesting Complexes

• Light-harvesting complexes (LHCs) are protein complexes that contain chlorophyll and carotenoid molecules
• LHCs are associated with the photosystems and function to absorb light energy and transfer it to the reaction center
• The arrangement of pigments in LHCs allows for efficient energy transfer through a process called resonance energy transfer
• Examples of LHCs include LHCI (associated with PSI) and LHCII (associated with PSII)

Action Spectrum and Excited State

• An action spectrum is a graph showing the relative effectiveness of different wavelengths of light in driving a photosynthetic process (oxygen evolution or carbon fixation)
• The action spectrum of photosynthesis closely resembles the absorption spectrum of chlorophyll a, indicating that chlorophyll a is the primary pigment driving photosynthesis
• When a pigment molecule absorbs a photon of light, it enters an excited state, where one of its electrons is promoted to a higher energy level
• The excited state is unstable, and the electron quickly returns to its ground state, releasing the absorbed energy as heat, fluorescence, or by transfer to another molecule (photochemistry)