Light Harvesting Complex: meaning, diagram

What Is A Light Harvesting Complex (LHC)?

The Light Harvesting Complex is a protein complex with pigments in the thylakoid membranes, performing one of the central roles in photosynthesis. The activity here is to capture light and then transfer the collection of energy to the reaction centres of Photosystem I and Photosystem II, where this acquired energy is exploited to power the light-dependent reactions of photosynthesis.

The maximum efficiency of harvesting available light by LHC employed by plants, algae, and cyanobacteria should be optimised to perform the available light utilisation fully maximised by other photons. Thus, it would make this process obvious and essential for the creation of the fuel that will drive the synthesis of ATP and NADPH, which further comprises not only the growth but also the energy needs of the total biomass.

Structure Of Light-Harvesting Complex

The Light Harvesting Complex is a combination of pigments, proteins, and cofactors arranged in a manner to have the highest absorbance and hence the most efficient energy transfer from light.

Basic Structure

The basic structure mainly consists of chlorophyll molecules, carotenoids, and some proteins that play a role in the stabilization and activity of the complex.

Pigments: The key among the pigments are chlorophyll a and chlorophyll b, which serve to trap the light. Other pigments include carotenoids such as β-carotene and lutein. They help in the absorption of the light and help in photoprotection.

The Proteins: These proteins help in providing a scaffold in which the pigments are placed in a very specific orientation and ergo help in the efficient transfer of energy.

The Cofactors: They act as the molecules that help in the transfer of both energy and electrons in the LHC.

Types Of Light Harvesting Complexes

The different types of light-harvesting complexes are:

LHC I (Photosystem I)

This complex captures light mainly for Photosystem I, which has its maximal absorption peak around 700 nm. LHC I is responsible for driving the formation of NADPH.

LHC II (Photosystem II)

This complex captures the light reaching Photosystem II, which has a maximum absorption of around 680 nm. It is accordingly very important for abstracting the water-splitting reaction responsible for the production of ATP.

Function Of Light-Harvesting Complex

The Light Harvesting Complex fulfils a pivotal part in the first process or step of photosynthesis called light absorption.

Role In Light Absorption

The mechanism comprises pigments absorbing photons and getting excited from their ground state to an excited state. Chlorophyll molecules optimally positioned and abundant in the LHC absorb extremely strong energy of light. Carotenoids complement this by further absorption of wavelengths and photoprotection due to the dissipation of over-absorbed energy.

Energy Transfer Mechanism

After trapping the light energy, it must efficiently be transferred to the reaction centre where photochemical reactions occur. During the mechanism of the energy transfer, a process called resonance energy transfer (RET) is conducted.

In the RET mechanism, the energy of an excited pigment molecule is transferred non-radiatively to an acceptor molecule located nearby. It occurs through dipole-dipole interaction while the energy jumps from one molecule, going close to the reaction centre, to another. It is finally transferred to the attached chlorophylls in the reaction centre.

Explanation of resonance energy transfer

Resonance energy transfer is an effective process over a small distance (1 – 10 nm), as it enables the capability of the LHC to funnel light energy captured towards the reaction centre with very minimal loss. Because the process of RET is effective, it ensures that most of the absorbed light energy is used in the photochemical reaction process to trigger ATP and NADPH production needed for the Calvin cycle and other biosynthetic processes.

Light Harvesting Complex in Different Organisms

The light-harvesting complex in different organisms is explained below:

Plants

In both terrestrial and aquatic plants, the pigment is primarily found associated with Photosystem I (LHC I) and Photosystem II (LHC II). For example in the higher plants, of which one such example here is Arabidopsis thaliana, the LHC II serves the function of trapping the light and transferring the energy to Photosystem II which subsequently inducts the photosynthesis process. The normal Growth, development, and acclimation to shifting light environments in most plant types result from the effective operation of a group of LHCs.

Algae

Similarities and differences with plant LHCs: The algae are similar to the plant LHCs, but an atypical photosynthetic alga, Chlamydomonas reinhardtii, shows that the LHCS LIMIT group can be quite different, even though the complexes can be finely tuned with unique components to adapt to various aquatic environments.

These have adaptive features like chlorophyll c and other pigments that enable algae to have effective light absorption at any depth and quality of light within water. While the absorption of light and transfer of energy among all of them share a common basic description, the changes in pigment and structure of the protein give the algae a better fit in the prevailing conditions that support their living.

Bacteria

Photosynthetic bacteria, such as cyanobacteria, have significantly different LHCs compared to plants and algae. Cyanobacteria use phycobilisomes, which are large pigment-protein complexes that are Phycobilisomes. These structures capture the light energy and funnel it to the reaction centres of Photosystem II.

Phycobilisomes are structurally similar to LHCs in plants and algae; however, in this case, they contain phycobilins, such as phycocyanin and phycoerythrin and these pigments allow such bacteria to capture light efficiently in different spectral regions, particularly in low light or shaded environments. This type of structural and functional divergence is a good indicator of the diversity of different LHC adoptions in different photosynthetic organisms

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