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 makes this process essential for the creation of the fuel that will drive the synthesis of ATP and NADPH, which further comprises growth as well as the energy needs of the total biomass.

Structure of Light-Harvesting Complex

The light-harvesting complex is a combination of biological pigments, proteins, and cofactors arranged in a manner to have the highest absorbance and hence the most efficient energy transfer from light.

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 pigments are chlorophyll including 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 of extra wavelengths and help in photoprotection.

Proteins

These proteins help in providing structural support for the pigments which are placed in a very specific orientation and ensure the efficient transfer of energy.

Cofactors

Cofactors 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 important as it transfers energy for the water-splitting reaction and 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 (RET)

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:

In 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, example is Arabidopsis thaliana, the LHC II serves the function of trapping the light and transferring the energy to Photosystem II which subsequently induces 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.

In Algae

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 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.

In Bacteria (Cyanobacteria)

Photosynthetic bacteria, such as cyanobacteria, have significantly different LHCs compared to plants and algae. Cyanobacteria use phycobilisomes, which are large pigment-protein complexes. 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.

Importance of LHC

The importance of light-harvesting complex includes:

• The LHC helps to maximize the capture of light energy and transfer efficiency.

• It helps to adapt to various light environment conditions.

• It drives the formation of energy i.e., ATP and NADPH formation in photosynthesis.

• It enhances the photoprotection and photosynthetic efficiency in plants and algae.

Light Harvesting Complex NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Structure of LHC

• Types of LHC

• Functions of LHC

Practice Questions for NEET

Q1. Which molecule acts the safeguarding function in Light-harvesting complexes?

1. Chlorophylls

2. Carotenoid

3. Phycobilisome

4. Thylakoids

Correct answer: 2) Carotenoid

Explanation:

The carotenoid molecules serve a safeguarding function. Carotenoid molecules inhibit harmful photochemical reactions, especially those involving oxygen that can be triggered by exposure to sunlight. Plants that lack carotenoid molecules die quickly when exposed to oxygen and light.

Hence, the correct answer is option 2) Carotenoid

Q2. Choose the incorrect statement regarding the photosystem II.

1. It is present in the non-appressed part of the thylakoid.

2. The reaction center of photosystem II is P700.

3. Photosystem II obtains electrons through the photolysis of water.

4. Photosystem II is involved in only non-cyclic photophosphorylation.

Correct answer: 2) The reaction center of photosystem II is P700.

Explanation:

Regarding Photosystem II, the phrase "Reaction center of Photosystem II is P700" is inaccurate. This is untrue since Photosystem II's response center is P680, not P700. Photosystem I's response center is P700. Whereas Photosystem I absorbs light at 700 nm, Photosystem II absorbs light best at 680 nm.

The other claims are true. In contrast to Photosystem I, which is found in the appressed sections of the thylakoid membrane, Photosystem II is found in the non-appressed part, or the area where the membranes are not firmly stacked. The process of photolysis, in which water molecules split to release electrons, protons, and oxygen, is another way that photosystem II gets electrons. Lastly, Photosystem II participates in non-cyclic photophosphorylation, a process that creates ATP and NADPH—two vital components for the plant's energy requirements—by allowing electrons to pass through both Photosystem II and Photosystem I.

Hence, the correct answer is option 2) The reaction center of photosystem II is P700.

Q3. Energy required for ATP synthesis in PS II comes from

1. Electron gradient

2. Reduction of glucose

3. Oxidation of glucose

4. Proton gradient

Correct answer: 4) Proton Gradient

Explanation:

On absorption of the photons, the antenna molecules get excited, and electrons are pushed into the outer orbitals. The excited antenna molecules hand over their energy to core molecules by resonance and come to the ground state. The energy picked up by the core molecules is supplied to the reaction center. On absorption of energy, the photo center gets excited and extrudes an electron after which it comes to the ground state to repeat the cycle. The frequency of excitation of the reaction center is very high. It cannot be met by its direct absorption and it cannot absorb a shorter wavelength. Therefore, the reaction center needs the help of harvesting molecules in the absorption of light energy. The energy required for ATP synthesis in PS II comes from Proton Gradient

Hence, the correct answer is option 4) Proton gradient.

Also Read:

• MCQs on Light Harvesting Complex

• An Overview of Photosystem

• C4 and CAM Pathway