What Absorbs Photons Of Light In Photosynthesis?

Photosynthesis is one of the most important biochemical processes on Earth. It is the process by which plants, algae, and certain bacteria convert sunlight into chemical energy. During photosynthesis, carbon dioxide and water are converted into glucose and oxygen using solar energy from the sun.

The glucose produced during photosynthesis provides a vital source of energy for nearly all life on Earth. In addition, the oxygen released as a byproduct sustains most aerobic organisms. Photosynthesis is the foundation of the food chain and the origin of fossil fuels like oil and coal.

Overall, photosynthesis is essential for life on Earth as we know it. Understanding the complex biochemical reactions behind photosynthesis has been an important endeavor for scientists working to maximize crop yields, develop renewable biofuels, and study climate change.

Photons of Light

Photons are particles that make up light. They are massless packets of electromagnetic energy that move at the speed of light. Photons have properties of both waves and particles. As particles, they have discrete energy levels that depend on their wavelength. Higher frequency photons have higher energy levels. Photons exhibit wave-particle duality, meaning they can behave as both particles and waves.

The photons used in photosynthesis come from sunlight. Sunlight contains all the wavelengths of visible light. However, plants primarily absorb photons in the blue and red regions of the visible light spectrum. Blue light has a wavelength of 450-495 nm and red light has a wavelength of 620-750 nm. Photons in these wavelengths contain the right amount of energy to power the light-dependent reactions of photosynthesis.

Light Absorption

Chlorophyll is the primary photosynthetic pigment in plants and algae. It is located in the thylakoid membranes within chloroplasts. When a photon of light hits chlorophyll, the energy from the photon causes an electron in chlorophyll to become “excited” and jump to a higher energy level. Chlorophyll specifically absorbs photons in the red and blue wavelength ranges, while reflecting green light (which is why plants appear green).

chlorophyll absorbs light energy during photosynthesis

There are several types of chlorophyll, but the most common are chlorophyll a and chlorophyll b. Both play critical roles in absorbing light energy during photosynthesis. Chlorophyll a has a central magnesium atom, while chlorophyll b has a central magnesium atom with an aldehyde group attached. This small difference allows chlorophyll b to absorb photons at slightly different wavelengths than chlorophyll a.

In summary, chlorophyll pigments contained in the chloroplasts of plant cells absorb photons of light. The energy from these light particles excites electrons in chlorophyll to higher energy levels, providing the energy needed to drive the reactions of photosynthesis.

The Light Reactions

The light reactions occur in the thylakoid membranes within chloroplasts. This is where photons from sunlight are absorbed by light-harvesting complexes and excite electrons from chlorophyll molecules. The excited electrons get passed along an electron transport chain, which creates a proton gradient across the thylakoid membrane through a process called chemiosmosis.

There are two possible pathways for the light reactions. In noncyclic photophosphorylation, electrons originate from the photolysis of water, get excited by photons, pass through the electron transport chain, and eventually reduce NADP+ to NADPH. The proton gradient powers ATP synthase to make ATP. In cyclic photophosphorylation, electrons get recycled and there is no production of NADPH or oxygen.

The light reactions can be summarized in a Z-scheme. Photons induce electron excitation from photosystem II up to a higher energy level. These electrons then get passed down the electron transport chain, losing energy along the way. When they reach photosystem I, another photon boosts the electrons to an even higher energy level. The electrons then complete the circuit, getting passed down further to ultimately reduce NADP+ and make NADPH.

Photolysis of Water

Photons of light energy are absorbed by the chlorophyll in Photosystem II, providing enough energy to break apart water molecules in a process called photolysis. This photolysis of water is the first major step in the light reactions of photosynthesis. When water molecules are split, they release electrons as well as oxygen gas (O2) as a byproduct.

The specific mechanism involves photons striking the chlorophyll molecules in Photosystem II, energizing their electrons. This energized chlorophyll molecule then passes its excited electron to the primary electron acceptor. The energized electron has enough energy to reduce the primary electron acceptor. Meanwhile, the loss of an electron leaves the chlorophyll oxidized. To regain its lost electron, the oxidized chlorophyll molecule extracts electrons from water molecules, resulting in the splitting of the water molecules into hydrogen ions, electrons, and oxygen gas which is released.

Overall, the photolysis of water initiates a flow of electrons while also producing oxygen as a byproduct of photosynthesis. This oxygen byproduct replenishes atmospheric oxygen and enables aerobic respiration in living organisms.

Excitation of Chlorophyll

Chlorophyll is the main pigment that absorbs photons of light during photosynthesis. Within the chloroplasts of plant cells, chlorophyll is embedded in the membranes of the thylakoid discs that are stacked to form grana. When a photon of light is absorbed by a chlorophyll molecule, it causes one of the electrons of the chlorophyll to become “excited” to a higher energy state. This excited electron is more loosely bound and has potential energy that can be harvested for use in photosynthesis.

Specifically, chlorophyll absorbs photons mostly in the blue and red regions of the visible light spectrum. The absorbed energy from the photon raises an electron in the chlorophyll molecule to an excited state. This excitation provides the energy needed to ultimately drive ATP synthesis during the light-dependent reactions. The excited electron does not remain in the chlorophyll molecule for long. It is quickly captured by a primary electron acceptor and transported down an electron transport chain, which generates the proton gradient for ATP production.

So in summary, the absorption of light by chlorophyll and the resulting excitation of electrons is the initial step that converts light energy into a form that can be used to power the biochemistry of photosynthesis. The excited electrons are then channeled down energy-yielding pathways.

Electron Transport Chain

The electron transport chain (ETC) is a series of protein complexes and electron carrier molecules within the thylakoid membrane that shuttles electrons from photosystem II to photosystem I, ultimately generating ATP and NADPH. As electrons move down the ETC, they lose energy, which is used to pump hydrogen ions (H+) across the thylakoid membrane into the lumen. This creates an electrochemical gradient which drives the synthesis of ATP.

Specifically, as electrons pass from photosystem II to cytochrome b6f complex to plastocyanin and finally to photosystem I, the energy is used to pump H+ into the thylakoid lumen. This powers ATP synthase to attach free phosphates to ADP, forming ATP. Meanwhile, ferredoxin accepts energized electrons from photosystem I and uses them to reduce NADP+ to NADPH. The end result is the light reactions produce both ATP and NADPH, which will later be used in the Calvin cycle reactions of the dark reactions.

Cyclic vs Noncyclic Phosphorylation

There are two main pathways for phosphorylation in photosynthesis: cyclic and noncyclic. The key difference between these two pathways is that cyclic phosphorylation involves only photosystem I while noncyclic phosphorylation involves both photosystem I and photosystem II.

In noncyclic phosphorylation, electrons are transported from photosystem II to photosystem I. This creates a flow of electrons that generates ATP and NADPH. Water is split in the process, releasing oxygen as a byproduct.

In cyclic phosphorylation, only photosystem I is involved. Electrons recycled from photosystem I flow back into the electron transport chain rather than passing to NADP+. This cyclic pathway generates ATP but not NADPH or oxygen.

So in summary, noncyclic phosphorylation produces ATP, NADPH, and O2 while cyclic phosphorylation just produces ATP. The cyclic pathway helps generate extra energy when the cell has enough NADPH and doesn’t need more. Both pathways work together to provide energy for the Calvin cycle reactions.


Chemiosmosis is the process that uses the energy from the excited electrons to pump hydrogen ions (protons) across the thylakoid membrane in the chloroplast. As electrons move through the electron transport chain, energy is used to pump protons from the stroma into the thylakoid space. This creates a concentration gradient and generates a proton-motive force across the membrane.

Specifically, at two points during the light reactions, protons are actively transported:

  • When electrons are boosted to a higher energy level by photosystem II, protons are released into the thylakoid space.
  • As the electrons move down the electron transport chain, energy is used to pump additional protons into the thylakoid space.

This movement of protons generates an electrochemical gradient because there is both a proton concentration difference and an electrical charge difference across the thylakoid membrane. This proton-motive force then powers the synthesis of ATP, which is why chemiosmosis is called an energy-coupling mechanism.


Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organism’s activities. The light reactions of photosynthesis involve the absorption of photons from sunlight by pigments like chlorophyll and carotenoids in the chloroplasts. When these pigments absorb photons, they become excited and release electrons that are captured to generate ATP and NADPH. The electrons are moved through a series of proteins known as the electron transport chain. At the end of the chain, the electrons are used to produce NADPH while protons are pumped into the thylakoid space, generating a proton gradient. This gradient powers ATP synthase to produce ATP in a process called chemiosmosis. The chemical energy of ATP and NADPH will later be used to fix carbon dioxide into sugars in the Calvin cycle. In summary, light energy is converted into chemical energy through the light-dependent reactions of photosynthesis, providing the fuel that enables plants and other photoautotrophs to build complex molecules.

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