Profile
Settings
Refer your friends
Sign out
Introduction
Antenna pigments are extremely diverse, notably among algae. In comparison, there is evidence that the PSI and PSII reaction centre complexes are substantially conserved in terms of nature, composition, and function throughout all oxygenic photosynthetic species. PSI and PSII have distinct antenna systems in higher plants and algae that differ in colour and protein composition.
Composition of Photosynthetic Antenna Pigments
All functional pigments in photosynthetic membranes are complexed with proteins in a variety of ways. By and large, photosynthetic pigments and pigment-protein complexes perform two critical tasks. They serve as antennae or reaction centres. Plant antennae are composed of a vast number of pigment molecules coupled to proteins that collect photons and send their energy to the reaction centre.
Polypeptides that bind chlorophyll a, chlorophyll b, carotene, and xanthophyll produce these light-harvesting complexes.
When pigments are in close proximity to one another, it is possible for the quantum energy of an irradiation photon to be exchanged between them. A photon is a quantum of light energy, whereas an exciton is a quantum of excitation energy passed from one molecule to the next.
Plants have an inner and an outer antenna. The light is collected by the outside or peripheral component formed by the light-harvesting complex (LHC). The inner part of the antenna, which is composed of the core complex, is generally an integral component of the reaction centre, as it is responsible for the transport of excitons collected in the outer part of the antenna to the photosynthetic reaction centres.
Resonance transfer is the mechanism by which excitation energy is transferred from the light-harvesting chlorophyll to the reaction centre. This mechanism is sometimes referred to as Forster transfer, after the scientist who first characterised it.
Photons are not merely emitted by one molecule and absorbed by another in this process of resonance energy transfer in antenna complexes; rather, it is a non-radioactive mechanism by which excitation energy is transmitted from one molecule to another.
The Antennae can be divided into two groups
The antenna complexes at the core and at the periphery. In PSI, the core antenna is composed of around 100-120 Chl a and 15 β-carotene molecules covalently linked to the P700 reaction centre.
The PSII core antenna is made up of two pigment-protein complexes, CP 43 and CP 47, that exist independently of the PSII reaction centre complex. Each molecule binds between 20 and 25 chlorophyll and numerous β-carotene molecules. Among photosynthetic species, the pigment and protein components of the core antenna are identical.
The outer periphery antenna complexes are diverse. There is evidence that the size and content of peripheral antennas can be tailored to environmental conditions. In higher plants and green algae, the outer peripheral antennae of PSI and PSII are formed of two classes of Chl a/b binding proteins called LHCI and LHCII, which act as light harvesting complexes.
Individual peripheral antenna proteins have distinct characteristics, which can result in heterogeneous function, particularly in PSII. It has been demonstrated that the antennas of the PSII reaction centre contain four light harvesting complexes, designated LHCII a–d. However, the primary component is LHC IIb, which arises as a trimmer in the membrane.
The monomer is represented by a polypeptide that contains two lutein molecules. The major LHC II complexes are classified into two types: those that are covalently linked to PSII and those that can dissociate from PSII reversibly upon phosphorylation.
Indeed, phosphorylation of LHC II by ATP via a protein kinase can be used to modulate its activity, with one threonine residue on the polypeptide serving as the phosphorylation site. Additionally, PSII features three minor peripheral antenna complexes (CP29, CP26, and CP24) that connect the main LHC antenna to the PSII core.
Role of the Antenna in Photosynthesis
A. Light-harvesting Function
All photosynthetic pigments, whether they are found in the reaction centre or the antenna, have the ability to absorb sunlight directly. Under ordinary daily light intensities, the rate of light absorption by a reaction centre pigment is significantly less than the capacity of photosynthetic electron transport, and hence would not provide enough energy to drive the process.
Thus, successful photosynthesis requires the capture of energy from photons of varied wavelengths across a surface composed of hundreds of antenna pigments.
Due to the fact that auxiliary pigments such as Chl b and carotenoids belong to the antenna and absorb wavelengths of light that Chl can only absorbs weakly, the range of wavelengths over which light may be absorbed increases larger, hence increasing the effectiveness of light absorption.
B. Protection against Active Oxygen Species
The excited singlet state of chl a that emerges as a result of light absorption is unstable and will decay to the lowest energy level (ground state) via three intrinsic processes.
Intersystem crossing to the triplet state, radiation decay (fluorescence), and thermal emission are examples of these processes (heat). The yield of triplet chloride produced in the antenna is significantly more than that produced in the reaction centre. Through triplet energy transfer, the
The singlet oxygen created is extremely reactive and has the ability to oxidise a wide variety of critical biological components, most notably lipids. This difficulty is solved in all photosynthetic organisms by the presence of carotenoids in the antenna complexes, which rapidly extinguish Chl triplet states. The quenching reaction entails the transfer of triplet energy from Chl a to the carotenoid, followed by its thermal dissipation on the carotenoid.
Thus, carotenoid pigments in photosynthetic antennae are critical for photoprotection because they limit the creation and accumulation of singlet oxygen. This is why plants lacking carotenoids are unable to grow and thrive under bright light.
C. Regulation of Light Utilisation
The rate of photosynthesis is linear with the incident light intensity at limiting light levels. As light intensity increases, photosynthesis gets saturated and eventually becomes light-independent. It’s worth noting that photosynthesis is considered to reach saturation due to a constraint in the capacity of dark reactions, not in photosynthetic electron transport.
On the other hand, as light intensity increases, the rate of photon absorption stays linear. As a result, plants growing under excessive light intensities constantly absorb more light energy than their photosynthetic potential for CO2 fixation can employ. Thus, excessive light absorption is an issue that plants regularly encounter in the field.
When we examine the coupling between photosynthesis’s light and dark reactions, it becomes clear that a limitation of photosynthetic dark reactions is produced by slower NADP+, ADP, and inorganic phosphate regeneration, which eventually results in an inhibition of electron transport.
Following the restriction of PSII reaction sites, enhanced fluorescence and triplet formation occur, resulting in the creation of singlet O₂ and associated oxidative damage.
Simultaneously, the scarcity of NADP⁺ permits molecular O₂ to compete for an acceptor from PSI with ferredoxin, resulting in the formation of superoxide and other free radicals that also cause oxidative damage. The antenna pigment-protein complexes are critical in regulating how absorbed light energy is used.
This is accomplished through the transfer of excited state energy between antenna pigments, while the damage caused by excess light absorption is minimised either through energy transfer to functional reaction centres (photochemical quenching) or through non-destructive dissipation of excess light energy as heat (non-photochemical quenching).
Thus, the structure and composition of the antenna dictate the optimal light collecting effectiveness under low light circumstances.
A mechanism has been proposed to account for the quenching reaction’s components in the antenna and the location of the quenching.
This is performed by exchanging excited state energy across antenna pigments, while the damage caused by excess light absorption is minimised either through energy transfer to functional reaction centres (photochemical quenching) or by non-destructive dissipation of surplus light energy as heat (non-photochemical quenching).
Thus, the antenna’s form and composition dictate its best light collection efficiency in low light conditions. A mechanism has been presented to account for the components of the quenching process in the antenna and its position.
D. The Regulation of Energy Distribution from the PSII to the PSI
The two photosystems’ respective light harvesting capacity must be regulated in order to achieve the greatest electron transport rates at a given light intensity (Allen, 1992). This is another mechanism in the antenna complex that regulates how light energy is used in photosynthesis.
When NADP⁺ becomes limiting due to high light or dark reaction limits, the rate of PQH₂ oxidation by PSI reduces due to the slower electron transport through PSI.
Reduced PQ activates a protein kinase, which phosphorylates a pool of LHC II antenna components, resulting in the lateral transfer of phospho-LHC II from PSII in appressed areas to stroma-exposed parts of the thylakoid membrane.
Phospho-LHC II can then interact with and excite PSI at the expense of PSII. When the PQ pool oxidises, the kinase is inactivated and the process is reversed by a phosphatase enzyme via dephosphorylation of phospho-LHC II, causing the resultant LHC II to move back into the antenna’s appressed membrane areas.
Conclusion
At the centre and on the periphery, there are antenna complexes. The core antenna in PSI is made up of 100-120 Chl a and 15-carotene molecules that are covalently connected to the P700 reaction centre.
There are many roles of antenna in photosynthesis:
- The Regulation of Energy Distribution from the PSII to the PSI
- Regulation of Light Utilization
- Light-harvesting Function
- Protection against Active Oxygen Species
