Light-harvesting complex

A light-harvesting complex is one or more polypeptide chains containing photosynthetic pigments, which surrounds a photosynthetic reaction centre and focuses energy, attained from photons absorbed by the pigment, inward toward the reaction centre using resonance energy transfer or photons of a different wavelength (and lower energy level) emitted by fluorescent pigments.

The light reaction of photosynthesis
During the light reactions of photosynthesis the free energy carried by photons is captured by pigments held in large protein complexes called reaction centres. The energy of a photon causes electron excitation, raising an electron within the pigment to a higher energy level with a change of energy from upper to lower exactly equal to the energy of the photon (for example light with a wavelength of 680 nm will be carried by a photon with an energy of 1.73 eV) This results in electron transfer from the pigment molecule to a nearby electron acceptor. Elaborate arrangement of electron donors, pigments and amino acids within the reaction centre result in the pigment molecule being reduced, and hence neutralized, before the excited electron has a chance to return.

A light harvesting system that relied only on a single reaction centre would be rather inefficient for two reasons: the main photosynthetic pigment, chlorophyll a cannot absorb light at the peak solar output, between 450nm and 650nm; secondly since there is only one point within each reaction centre in which photon absorption takes place most photons of the correct wavelength pass through the complex without being absorbed. Light reaction centres exist to counter these two problems.

The mechanism of a light harvesting complex
Although absorption of a photon by a molecule can lead to electron excitation another result of photon absorption not leading to electron excitation is more common. Through electromagnetic interactions through space, the excitation energy can be transferred from one molecule to a nearby molecule. The rate of this process is called resonance energy transfer and depends strongly on the distance between the energy donor and energy acceptor molecules.

Energy transfer of this kind from a donor to an acceptor molecule can only occur if the acceptor is at an equal or lower energy state than the donor. The absorption of light in plant and bacterial reaction centres occurs in a dimer of chlorophyll molecules, often referred to the special pair. The excited state of this special pair is lower in energy than that of a single chlorophyll molecule; therefore resonance energy can be transferred from single chlorophyll molecules present in light harvesting complexes to the special pair present in the reaction centre.

Light harvesting complexes are wrapped around the reaction centre and use additional pigments, chlorophyll b, lycopene and β-carotene, to funnel absorbed energy to the special pair via resonance energy transfer. Lycopene and β-carotene are carotenoids; extended polymers that absorb light between 400nm and 500nm. Carotenoids serve a secondary function in plants, suppressing damaging photochemical reactions, particularly those including oxygen, which can be induced by bright sunlight. Laboratory tests have shown that plants lacking carotenoids quickly die on exposure to light and oxygen.

Light-harvesting complexes in plants
Chlorophylls and carotenoids are important in light-harvesting complexes present in plants. Chlorophyll b is almost identical to chlorophyll a except it has a formyl group in place of a methyl group. This small difference makes chlorophyll b absorb light with wavelengths between 400 and 500 nm more efficiently. Carotenoids are long linear organic molecules which have alternating single and double bonds along their length. Such molecules are called polyenes. Two examples of carotenoids are lycopene and β-carotene. These molecules also absorb light most efficiently in the 400 – 500 nm range. Due to their absorption region, carotenoids appear red and yellow and provide most of the red and yellow colours present in fruits and flowers.

The carotenoid molecules also serve a safeguarding function. Carotenoid molecules suppress damaging photochemical reactions, particularly those including oxygen, which exposure to sunlight can cause. Plants that lack carotenoid molecules quickly die upon exposure to oxygen and light.

The chlorophylls and carotenoids present in the light-harvesting complexes are referred to as accessory pigments. These accessory pigments are held inside the light harvesting proteins in a highly uniform fashion. The light-harvesting complexes are cylindrical in form and come in two sizes in alga: LH-1 which are large and completely surround the reaction centre and LH-2 which are smaller and are arranged in a ring structure around the LH-1 complex. The relative size and positioning of the proteins is shown in the image above.

In green plants the story is a little different, as the light-harvesting complexes do not form nice simple geometric shapes with respect to one another. Green plants contain chloroplasts which house the photosynthetic apparatus. Chloroplasts contain the thylakoid membrane, which holds the pigment protein complexes, known as LHC, light-harvesting complexes, I and II. The tightly spaced regions in the thylakoid membrane, known as grana, hold predominantly LHCII, which is the most abundant pigment-protein complex in green plants.

Phycobilisome


Little blue or red light reaches algae which reside at a depth of 1 metre or more in seawater, as this light is absorbed by seawater and fluorescent pigments of photosynthetic organisms above. A phycobilisome is a light-harvesting protein complex present in cyanobacteria, glaucocystophyta, and red algae. Fluorescent pigments, which are linked to the peptide chain absorb green light or red light. Other pigments which are present in the bacterial photosynthetic reaction centers, like bacteriochlorophyll and bacteriopheophytin do not absorb light in these regions. The fluorescent pigments which are present in the phycobilisome, such as phycocyanobilin and phycoerythrobilin re-emit the green light in regions which the other photosynthetic pigments can absorb.

The geometrical arrangement of a phycobilisome is very elegant and results in 95% efficiency of energy transfer. There is a central core of allophycocyanin which sits above the photosynthetic reaction center. There are phycocyanin and phycoerythrin subunits which radiate out from this center like thin tubes. This increases the surface area of the absorbing section and helps focus and concentrate light energy down into the reaction center. The energy transfer from exited electrons absorbed by pigments in the phycoercythrin subunits at the periphery of these antennas appears at the reaction centre in less than 100 ps.