What Molecules In The Plants Absorb The Light Energy?

Plants and other photosynthetic organisms like algae and some bacteria need light energy to drive the chemical process of photosynthesis. During photosynthesis, plants absorb light energy from the sun, carbon dioxide from the air, and water from the soil to produce carbohydrates like glucose. The carbohydrates provide energy for the plant and the oxygen is released as a byproduct. Photosynthesis is an essential biological process that supports nearly all life on Earth. The light energy from the sun fuels the synthesis of carbohydrates, which plants use for energy and convert into proteins, fats, and other molecules they need to grow and function.


chlorophyll is the main light-absorbing molecule in plants that enables photosynthesis.
Chlorophyll is the main light-absorbing molecule in plants and is crucial for photosynthesis. Located in the chloroplasts of plant cells, chlorophyll absorbs light energy from the sun to drive the light-dependent reactions of photosynthesis. The green color of chlorophyll makes it an efficient absorber of light in the blue and red regions of the visible spectrum, which are the most abundant wavelengths that reach Earth from the sun.

Different types of chlorophyll exist, but the most common are chlorophyll a and chlorophyll b. They have very similar chemical structures but differ slightly, allowing them to absorb light from complementary parts of the spectrum. This helps plants absorb a broader range of light wavelengths for photosynthesis. The chemical structure of chlorophyll also allows it to convert the absorbed light energy into a usable form of chemical energy that can drive the biosynthesis of carbohydrates and other organic compounds in plants. Overall, chlorophyll serves as the primary interface between light energy and plant life.

Structure of Chlorophyll

Chlorophyll is a green pigment found in plants, algae and cyanobacteria. There are several different forms of chlorophyll, but plants primarily contain chlorophyll a and chlorophyll b. The chemical structure of chlorophyll is composed of a porphyrin ring coordinated to a magnesium ion. The porphyrin ring contains several side chains, which differ between chlorophyll a and b.

Chlorophyll a has a methyl group on one ring and a long hydrophobic phytol chain. The phytol chain anchors chlorophyll a in the thylakoid membrane inside chloroplasts. Chlorophyll b contains an aldehyde group instead of the methyl group found on chlorophyll a. This slight difference in structure allows chlorophyll a and b to absorb light at slightly different wavelengths. Both are essential for photosynthesis.

Light Absorption

Chlorophyll is optimized to absorb certain wavelengths of visible light very efficiently. The chlorophylls found in green plants, algae and cyanobacteria absorb light primarily in the violet, blue and red regions of the spectrum. This is why plants appear green.

Specifically, chlorophyll a absorbs wavelengths from around 400 to 450 nm (violet) and 650 to 700 nm (red). Chlorophyll b absorbs wavelengths from 450 to 500 nm (blue) and 600 to 650 nm (orange-red). The absorption spectra of chlorophylls overlap to ensure that a wide range of visible light is captured.

When a chlorophyll molecule absorbs a photon of light, an electron in the molecule becomes excited to a higher energy state. This excitation provides the energy to drive photosynthesis and sugar production.


Photosynthesis takes place in specialized cell organelles called chloroplasts. Inside the chloroplast are stacks of disc-shaped structures called thylakoids. It is within the thylakoid membranes that the light reactions of photosynthesis occur through photosystems. There are two types of photosystems involved: Photosystem I and Photosystem II.

Photosystem II (PSII) is the first complex involved in the light-dependent reactions. It absorbs light energy at the wavelength of 680 nm using chlorophyll a at its reaction center. This excitation of electrons leads to the splitting of water molecules, releasing oxygen as a byproduct. The energized electrons are transported through the electron transport chain, which generates ATP.

Photosystem I (PSI) contains a chlorophyll a molecule at its reaction center that absorbs light energy at 700 nm wavelength. The excitation of PSI results in the generation of NADPH. The electron transport chain between PSII and PSI generates the energy needed to produce ATP.

Together, PSII and PSI are able to convert light energy into chemical energy that is stored in ATP and NADPH. The two photosystems work in cooperation, connected through the flow of electrons in what is known as the Z-scheme. This movement of electrons energized by light allows photosynthesis to occur.

Antenna Complexes

Within the thylakoid membranes of chloroplasts are pigment-protein complexes called antenna complexes. There are two main types of antenna complexes: light-harvesting complex I (LHCI) and light-harvesting complex II (LHCII).

LHCII is the most abundant antenna complex. It contains chlorophyll a, chlorophyll b, and xanthophyll pigments that collectively absorb light energy across a broad range of wavelengths. This broad absorption spectrum allows LHCII to efficiently harvest light energy.

The absorbed light energy is transferred via resonance energy transfer to a special pair of chlorophyll a molecules in the reaction center. From there, the excitation energy is transferred to the electron transport chain, leading to photophosphorylation and ultimately photosynthesis.

LHCI contains chlorophyll a, chlorophyll b, and carotenoid pigments. It absorbs light at slightly different wavelengths than LHCII. LHCI transfers the absorbed energy to the core complex, helping funnel excitation energy into the reaction center.

By having antenna complexes with different pigments tuned to various wavelengths, plants are able to maximize light absorption and optimize photosynthesis efficiency.

Electron Transport

The electron transport chain is a series of electron carrier molecules that shuttle electrons from photosystem II to photosystem I. As electrons move through this chain, they lose energy, which is used to pump hydrogen ions (H+) across the thylakoid membrane into the lumen. This creates an electrochemical gradient that will be used later to power ATP synthase and produce ATP.

A key function of the electron transport chain is to produce NADPH. This starts when electrons from photosystem I are excited by light energy. These energized electrons are picked up by the electron acceptor ferredoxin and used to reduce NADP+ to NADPH. NADPH is a vital energy carrier molecule that provides the hydrogen (H+) and electrons needed for the Calvin cycle reactions. This system of using light energy to reduce NADP+ into NADPH is called non-cyclic photophosphorylation.

So in summary, the electron transport chain produces NADPH by using photons from photosystem I to energize electrons, transporting them through a series of carriers, and finally using them to reduce NADP+ into NADPH. This NADPH provides the crucial energy and reducing power needed to fix CO2 into sugar in the Calvin cycle.


Photophosphorylation is the process by which light energy is converted into chemical energy in the form of ATP. This takes place in the thylakoid membrane inside the chloroplasts. As electrons pass through the electron transport chain, they create a proton gradient across the thylakoid membrane. This gradient powers the ATP synthase enzyme to produce ATP from ADP. The production of ATP is called photophosphorylation because phosphorylation (the addition of a phosphate group to ADP to make ATP) is driven directly by photons of light.

There are two types of photophosphorylation:

– Cyclic photophosphorylation involves only photosystem I and produces ATP but no NADPH or oxygen.

– Noncyclic photophosphorylation involves both photosystems and produces ATP, NADPH, and oxygen.

In both cases, light energy is converted into chemical bond energy in ATP. The ATP can then be used to power other cellular reactions and processes. Photophosphorylation is a key mechanism for photosynthetic organisms like plants to capture and harness the energy in sunlight.

Other Pigments

In addition to chlorophyll, plants contain other pigments that absorb light energy and pass it on to the photosynthetic machinery. Two important classes of pigments are carotenoids and phycoerythrin.

Carotenoids are yellow, orange, and red pigments found in plants. They help absorb blue and green light that chlorophyll cannot capture, broadening the spectrum of light that can drive photosynthesis. Carotenoids also protect chlorophyll from photodamage by absorbing excess light energy. The most common carotenoids are beta-carotene, lutein, zeaxanthin, and lycopene.

Phycoerythrin is a red pigment found in red algae and cryptomonads. It absorbs blue and green light strongly, complementing the absorption of chlorophyll. By having phycoerythrin in addition to chlorophyll, these organisms can photosynthesize deeper in the water column where blue and green light still penetrate.

So in summary, other plant pigments like carotenoids and phycoerythrin absorb different wavelengths of light than chlorophyll. This allows more of the visible light spectrum to be utilized for photosynthesis.


In summary, plants contain several light-absorbing molecules that allow them to convert light energy into chemical energy through photosynthesis. The main light-absorbing molecule is chlorophyll, which exists in several forms but always contains a porphyrin ring structure. Chlorophyll absorbs light mostly in the blue and red regions of the visible spectrum. The absorption of light leads to excitation of electrons in the chlorophyll molecules. These excited electrons are shuttled through photosystems and electron transport chains, resulting in the production of ATP and NADPH. Plants also contain various carotenoid and phytochrome pigments that absorb light and help regulate growth and development. Together, these light-harvesting molecules allow plants to efficiently capture solar energy and power the process of photosynthesis.

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