What Happens To Excited Electrons In Photosystem 2?

Photosynthesis is the process plants and some bacteria use to convert sunlight into chemical energy in the form of glucose. This process is incredibly important for life on Earth as it provides the food, oxygen, and replenishes the atmosphere.

In plants, algae and cyanobacteria, photosynthesis uses the green pigment chlorophyll to absorb light energy. Using this light energy, photosynthesis converts carbon dioxide and water into oxygen and energy-rich carbohydrates like sugars and starches. The overall general equation for photosynthesis is:

6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2

This means that for every 6 molecules of CO2 and H2O, 1 molecule of glucose sugar (C6H12O6) and 6 molecules of oxygen (O2) are produced. The oxygen is then released into the atmosphere while the energy-rich glucose molecules fuel the plant’s activities.

Photosynthesis takes place in two sequential stages: the light dependent reactions and the light independent/dark reactions. In the light reactions, light energy is absorbed by chlorophyll and that energy is stored in energy carriers like ATP and NADPH. In the dark reactions, the stored energy is used to fix CO2 into glucose.

Photosystem 2

Photosystem 2 (PS2) is located in the thylakoid membrane of chloroplasts inside plant cells. The thylakoid membrane forms an interconnected system of flattened sacs inside the chloroplast where the light-dependent reactions of photosynthesis take place. PS2 functions as a light-harvesting complex that captures photons and uses their energy to drive the extraction of electrons from water.

Structurally, PS2 consists of antenna proteins as well as a reaction center composed of chlorophyll a molecules. The antenna proteins contain hundreds of chlorophyll a and b molecules as well as carotenoid pigments that can absorb light energy. When a photon is absorbed by one of these pigments, the energy is transferred along to the reaction center. Here, a special chlorophyll a molecule donates an excited electron to the first electron carrier in the electron transport chain. Meanwhile, the reaction center becomes oxidized.

PS2 gets its name because it is the first complex to receive excited electrons during the light-dependent reactions. The excitation energy originates from photons captured by the antenna pigments. PS2 then facilitates the extraction of electrons from water to replace those lost through excitation. This replenishes the reaction center so that PS2 can continue capturing photons and generating excited electrons.

How Light Energy is Captured

The main pigment involved in photosynthesis is chlorophyll. Chlorophyll molecules are specifically designed to absorb light energy. When light shines on a leaf, the chlorophyll pigment absorbs the light energy. The energy from the absorbed light causes electrons in the chlorophyll molecules to become excited and jump to a higher energy state.

There are two types of chlorophyll – chlorophyll a and chlorophyll b. Both types absorb light in the red and blue regions of the visible light spectrum. This gives leaves their green color, as the green light is reflected. Chlorophyll a plays a primary role in photosynthesis by absorbing light energy and using it to drive electron transport. Chlorophyll b is an accessory pigment that transfers the absorbed light energy to chlorophyll a.

In addition to chlorophyll, plants also contain various carotenoid pigments such as beta-carotene and xanthophylls. Carotenoids absorb light energy and transfer it to chlorophyll. They also help protect the photosynthetic machinery from damage when light energy levels are excessive.

So in summary, light energy is captured by pigments like chlorophyll, causing the electrons in them to become excited to a higher energy state. This initial step of photosynthesis is critical for providing the energy needed for subsequent reactions.

Excited Electrons

excited electrons from photosystem 2 initiate the electron transport chain.
When light energy is captured by chlorophyll in photosystem 2, the energy is used to boost electrons to a higher energy state. These electrons become “excited” as they absorb photons and move to a higher orbital. This excitation provides the energy needed to move the electrons through an electron transport chain. Without light energy exciting electrons in chlorophyll, the electron transport chain would not function.

The boosted electrons are now primed to travel through a series of proteins and mobile electron carriers. As they flow through the electron transport chain, they will move from higher to lower energy states, releasing energy that will eventually be used to produce ATP. So in summary, the light-excited electrons in photosystem 2 provide the starting point for the electron transport chain and energy production in photosynthesis. Their excitation enables the rest of the process.

Electron Transport Chain

After absorbing light energy, electrons in the photosystem become excited to a higher energy state. These excited electrons then travel down an electron transport chain located in the thylakoid membrane, which is stacked into grana inside the chloroplast. As the electrons move down the electron transport chain, they lose energy in small increments which is used to pump hydrogen ions (H+) across the thylakoid membrane into the lumen. This creates a proton gradient and membrane potential across the thylakoid membrane.

The electron transport chain contains a series of electron carrier molecules such as plastoquinone and cytochromes. Each carrier molecule passes the excited electron to the next carrier, and the electron loses energy during each transfer. The electron eventually returns to the ground state, with the energy being used to pump H+ ions and produce ATP. This cyclic process constantly generates proton motive force across the membrane.

Splitting Water

An important part of photosynthesis is the splitting of water molecules by excited electrons from photosystem 2. This process takes place at a special cluster of four manganese atoms and one calcium atom known as the oxygen-evolving complex (OEC). The OEC cycles through five different states, S0 through S4, with each state representing a different arrangement of the manganese and calcium atoms.

When two excited electrons are present in the OEC (state S1), they can be used to split a water molecule into one oxygen atom and two hydrogen atoms. This releases the oxygen gas as a byproduct and provides the hydrogen atoms needed to create energy carriers like NADPH. As the OEC cycles through its five states, it can split multiple water molecules, generating more oxygen gas which is released into the atmosphere.

The splitting of water is thus a key step in photosynthesis. It provides the electrons needed to replace those lost from photosystem 2 and also generates oxygen as a byproduct of photosynthesis. Without the water-splitting reaction driven by excited electrons, photosynthesis would not be able to occur.

Electron Carriers

In the electron transport chain, electrons need to be shuttled between the protein complexes embedded in the thylakoid membrane. Specialized electron carrier molecules facilitate this transport process.

One key electron carrier is plastoquinone. This small mobile molecule diffuses freely within the thylakoid membrane. Plastoquinone picks up electrons from photosystem II and transports them to the cytochrome complex.

The cytochromes are protein complexes with iron-containing heme groups that alternate between reduced and oxidized states as they accept and donate electrons. There are two major cytochromes in the electron transport chain – cytochrome b6f and cytochrome c. Cytochrome b6f passes electrons from plastoquinone to plastocyanin.

Plastocyanin is a copper-containing protein that can accept electrons from cytochrome b6f. It diffuses through the thylakoid lumen and transports electrons to photosystem I. This plastoquinone – cytochrome – plastocyanin electron transport shuttle is crucial for linking the two photosystems.

ATP Synthase

The flow of electrons through the electron transport chain generates a proton gradient across the thylakoid membrane. This proton gradient powers the enzyme ATP synthase to produce ATP. As protons flow down the concentration gradient into the stroma through ATP synthase, the enzyme harnesses the energy to phosphorylate ADP, producing ATP.

Specifically, the flow of protons causes the rotary mechanism of ATP synthase to spin. This rotation catalyzes the reaction between ADP and inorganic phosphate to synthesize ATP. For each turn of the enzyme, multiple ATP molecules can be produced. Overall, the proton gradient generated by the electron transport chain is coupled to ATP synthesis by ATP synthase.

Cyclic vs Noncyclic Phosphorylation

Photosynthesis involves two main stages – the light-dependent reactions and the light-independent reactions. The light-dependent reactions convert light energy into chemical energy and include photophosphorylation, which occurs via cyclic or noncyclic pathways.

In noncyclic photophosphorylation, electrons from photosystem II are energized by light and passed along the electron transport chain. The electron transport chain pumps H+ ions into the thylakoid space, creating a proton gradient. ATP synthase allows H+ to flow back into the stroma, and this powers the phosphorylation of ADP to make ATP. This process is called noncyclic because the electrons start at PSII, end at PSI, and do not recycle back to PSII.

In cyclic photophosphorylation, only photosystem I is involved. Excited electrons from PSI are passed down an electron transport chain and return to PSI. This recycles the electrons in a loop. The electron transport chain still pumps H+ to generate ATP using the proton gradient and ATP synthase, but no NADPH is produced since the electrons are recycled. This process is called cyclic because the electrons cycle from PSI back to PSI.

In summary, noncyclic photophosphorylation involves both photosystems and generates both ATP and NADPH, while cyclic photophosphorylation only involves PSI and generates just ATP. Both pathways work together to provide the energy and reducing power needed for the light-independent reactions of photosynthesis.


In summary, light energy is captured in photosystem 2 through the excitation of electrons in chlorophyll molecules. These excited electrons are passed through an electron transport chain, generating a proton gradient that drives ATP synthase to produce ATP. The electron transport chain culminates with electrons extracted from water being used to replace the excited electrons from chlorophyll. This splitting of water molecules provides the electrons needed to repeat the light reactions.

This pathway of excited electrons in photosystem 2 is essential for plant photosynthesis. It allows light energy to be converted into chemical energy in the form of ATP and NADPH, which are then used to fix carbon dioxide into carbohydrates in the Calvin cycle. Without this efficient process of capturing and utilizing light energy, photosynthetic organisms would not be able to synthesize the organic compounds they need to survive.

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