The Role of Chlorophyll in Harnessing Light Energy (College Board AP Biology)

• Chloroplasts contain several different photosynthetic pigments within the thylakoids, which absorb different wavelengths of light
• There are two types of pigment: chlorophylls and carotenoids
  • Accessory pigments that surround the primary pigment absorb both similar and different wavelengths of light to chlorophyll, this expands the wavelength range that can be absorbed from light for use in photosynthesis

Chloroplast Pigments Table

Pigment group	Name of pigment	Color of pigment
Chlorophylls	Chlorophyll a Chlorophyll b	Yellow-green Blue-green
Carotenoids	β-carotene Xanthophyll	Orange Yellow

• Chlorophylls absorb wavelengths in the blue-violet and red regions of the light spectrum
  • They reflect green light, causing plants to appear green
• Carotenoids absorb wavelengths of light mainly in the blue-violet region of the spectrum

Absorption Spectra of Photosynthetic Pigments Graph

Chlorophyll and carotenoids absorb light across the visible light spectrum to use in the light-dependent reaction of photosynthesis

Pigments and Photosystems

• Within chloroplasts, thylakoids stack up to form structures known as grana (singular – granum)
• The thylakoid membrane system provides a large number of pigment molecules in an arrangement that ensures as much light as necessary is absorbed
• The pigment molecules are arranged in light-harvesting clusters known as photosystems
• In a photosystem, the different pigment molecules are arranged in funnel-like structures in the thylakoid membrane
• Each pigment molecule passes energy down to the next pigment molecule in the cluster until it reaches the primary pigment reaction centre
• There are two different photosystems, each with a specific form of chlorophyll a
  • Photosystem 1 (PSI), often referred to as P700
    • The chlorophyll a in this system has a maximum absorption of light at 700 nm
  • Photosystem 2 (PSII), often referred to as P680
    • The chlorophyll a in this system has a maximum absorption of light at 680 nm
• The two photosystems are connected by the transfer of electrons through an electron transport chain in the thylakoid membrane

Photosystems Within a Chloroplast Diagram

An example of a photosystem in a chloroplast thylakoid membrane: a light-harvesting cluster of photosynthetic pigments involved in the light-dependent stage of photosynthesis