Light is necessary for photosynthesis in plants, but the supply of light in natural environments is not constant. The incident of light can vary under different conditions, the way the plants maintain a balance is through regulation of photosynthetic light harvesting complexes. The photosynthetic antenna system is organised to accumulate and distribute excited state energy by means of excitation and transfer them to reaction centre
- Brief Introduction to the Antenna systems in photosynthesis.
- Antenna are crucial to photosynthesis, they enhance the capturing and processing of photosynthesis much more efficient, in a sense they act as biological catalysts by enhancing and speeding up the processes by many folds.
- The light-harvesting complexes II (LH2) are integral membrane proteins that form ring-like structures, oligomers of αβ-heterodimers, in the photosynthetic membranes of purple bacteria
- Rb sphaeroides were used to study LH2
- The Photosystem of Rb sphaeroides is composed of two light-harvesting complexes, the first type of light-harvesting antenna complex is designated as LH1 and the second as LH2, these have varying spectral characteristics and arrangements
- These LH1 and LH2 components are highly organized to ensure effective harvesting of light and energy transfer.
- The major difference between LH1 and LH2 are the absorption band; LH1 has 875nm range in the spectrum, whilst LH2 has between 800-850nm
- For the purpose of this essay I will only focus on the structure and composition of LH2 complex. LH2 is mostly referred to as peripheral light-harvesting complex. It is not in direct contact with the reaction center but transfers energy to the reaction center via LH1 light harvesting complex.
- Research has shown that LH2 does not occur in fixed stoichiometry with the reaction center, but instead the amount of LH2 found in the membrane depends on the growth conditions such as light intensity and temperature.
- The content of LH2 can be varied; this allows the bacteria to regulate the amount of photosynthesis and to optimize their light harvesting capacity to prevailing light intensity. During low light, the amount of LH2 can increase to about 3 times the amount of LH1.
- The direction of energy transfer is from LH2 to LH1 (800nm→ 850nm→ 875nm) and then to the reaction center. In this process, the light-harvesting system acts as a ‘funnel' to capture excitation energy and then transfer it to reaction center with about more than 95 % efficiency and little energy loss. With respect to their absorption maxima, they are referred to as the B875 (LH1), B800-850 (LH2)
- LH2 has two light harvesting pigments; carotenoids and bacteriochlorophylls. The excited energy is absorbed and transferred by these light-harvesting pigments.
- The high efficiency of energy transfer is made possible by these amazing parameters such as the distance between pigments, the relative geometric arrangement of the transition dipole moments of the pigments, and the excited state lifetimes. Research has shown that these parameters are highly dependent on the interactions between the pigments and the protein scaffold.
- LH2 is built up from minimal units consisting of a heterodimer of two protein subunits, known as the alpha and beta peptides, along with three molecules of bacteriochlorphyll and one molecule of carotenoid.
- The ring-like arrangement of the αβ heterodimers of LH2 is an elegant and simple construction to allow delocalization of the excitation energy and its very efficient transfer to LHI and the RC
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Photosynthesis begins when an antenna pigment absorbs light. This pigment can be a bacteriochlorophylls or carotenoids. A wide range of different antenna complexes is found in different photosynthetic systems. An antenna permit an organism to increase greatly the absorption range for light without having to build an entire reaction center and associated electron transfer system for each pigment, which would be very costly in terms of cellular resources for both plants and bacteria. The capture of light energy during the initial stage of bacterial photosynthesis is mediated by light-harvesting or antenna complexes known as LH1 and LH2, which are integral membrane proteins. The energy of the photons is then very rapidly transferred to the photochemical reaction centre. Where a charge separation followed by electron transport across the membrane takes place.
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LH2 Complexes are not directly associated with reaction centres; however they transfer energy to the reaction centres via LH1. LH2 is a heterodimer compose of alpha & beta subunits. Each heterodimer bind three bacteriochlorophylls a molecules and at least one molecule of carotenoids.
Overall the antenna system is designed in such a ways that it act likes a “funnel” that increases the surface area and effectiveness absorption of photon. The composition and structures of LH2 could not have been engineered in a better way, they are arranged in the peripheral near LH1 antenna complex, this is to aid the transfer of energy from LH2 to LH1 and then to the reaction center. The amount of LH2 and their pigments can be varied; this allows the bacteria to regulate the amount of photosynthesis and to optimize their light harvesting capacity to prevailing light intensity. During low light, the amount of LH2 can increase to about 3 times to compensate for the low light intensities.
The need for a photosynthetic antenna for both plants and bacteria is absolutely crucial, just in the same way that a television requires antenna to broadcast images, plants need photosynthetic antenna for living and they adjust their antenna composition according to inhabitants and to changes in light intensity, in order to achieve an efficient use of light energy, they have pigments that are able to absorb light in a range of spectrums (maximizing efficiencies). Further more an effective antenna is particularly important for growth at low light intensity areas.