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Biochemistry-II - Photosynthesis

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    Study Guide Biochemistry - II – Photosynthesis 1. Light Reactions of Photosynthesis The light reactions are the first stage of photosynthesis. They convert light energy into chemical energy in the form of ATP and NADPH , which are later used in the dark reactions (Calvin cycle) to make carbohydrates. 1.1 Overview of the Light Reactions Photosynthesis begins when pigments in the light - harvesting complex absorb photons of visible light. When a pigment absorbs light, it becomes excited and transfers this energy to a special chlorophyll molecule in the reaction center . There are two types of reaction centers : • Photosystem II (PSII) • Photosystem I (PSI) Together, PSI and PSII carry out the light reactions. Electron flow through these systems creates: • A proton gradient (used to make ATP) • NADPH (used as reducing power) Unlike mitochondrial electron transport, the final electron acceptor is NADP ⁺ , not oxygen.

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    Study Guide 1.2 Redox and Energy Concepts The movement of electrons during the light reactions follows changes in free energy and reduction potential . The relationship between free energy change and reduction potential is: Δ G°′ = −n  E°′ This equation explains why excited chlorophyll molecules can donate electrons more easily than unexcited ones. 1.3 Photosystem II (PSII) Reaction Center P680 The reaction center chlorophyll of PSII is called P680 , named for its absorption of light at 680 nm . • When P680 absorbs light, it becomes excited ( P680* ) • P680* is a strong electron donor • It transfers an electron to plastoquinone (PQ)

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    Study Guide Figure 1 1.4 Role of Plastoquinone Plastoquinone behaves similarly to coenzyme Q in mitochondria: • It can accept one or two electrons • For each electron accepted, it also takes up a proton (H ⁺ ) from the stroma This proton uptake: • Makes the stroma more basic • Contributes to the proton gradient 1.5 Electron Transport and Proton Pumping After plastoquinone: • Electrons move through the cytochrome b ₆ f complex • Protons are pumped into the thylakoid lumen • Electrons are transferred to plastocyanin • Finally, electrons are delivered to Photosystem I

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    Study Guide The accumulation of protons in the thylakoid lumen drives ATP synthesis via ATP synthase, similar to oxidative phosphorylation. 1.6 Replacement of Electrons in PSII Once P680 loses an electron, it becomes oxidized and must be reduced before it can function again. This reduction comes from water . The reaction is: 2 H ₂ O + [Manganese center] oxidized → O ₂ + [Manganese center] reduced + 4 H ⁺ This reaction occurs at the manganese center , a metalloprotein complex: • Water is split • Oxygen (O ₂ ) is released • Protons are released into the thylakoid lumen • Electrons reduce oxidized P680 This step is the source of oxygen released during photosynthesis. 1.7 Photosystem I (PSI) Reaction Center P700 The reaction center of PSI is called P700 , which absorbs light at 700 nm . • Light excites P700 to P700* • P700* donates an electron to a series of carriers • The electron is ultimately transferred to ferredoxin 2 Fd reduced + NADP ⁺ + H ⁺ → 2 Fd oxidized + NADPH Formation of NADPH Reduced ferredoxin transfers electrons to NADP ⁺ , forming NADPH :

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    Study Guide NADPH provides the reducing power required for carbon dioxide fixation during the Calvin cycle. Regeneration of P700 After P700 donates an electron: • It becomes oxidized • It accepts an electron from the cytochrome b ₆ f complex • This electron originally came from PSII Once reduced, P700 is ready to absorb light again and repeat the cycle. Summary of the Light Reactions • Light energy excites chlorophyll in PSII and PSI • Electrons flow through an electron transport chain • A proton gradient is formed across the thylakoid membrane • ATP is produced by photophosphorylation • NADP ⁺ is reduced to NADPH • Water is split, releasing oxygen Key Takeaways • PSII generates ATP and releases oxygen • PSI produces NADPH • Both systems work together through electron flow • Light reactions convert solar energy into chemical energy 2. Cyclic Electron Flow Sometimes, the cell has plenty of NADPH but not enough NADP ⁺ . When this happens, a problem could occur during the light reactions. Normally, reduced ferredoxin passes its electrons to NADP ⁺

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    Study Guide to form NADPH. However, if most of the NADP ⁺ has already been converted into NADPH, there is nowhere for the electrons to go . If electrons had no alternative path, they would accumulate in the photosystems . All the electron carriers would remain in a reduced state, and photosynthesis would come to a halt . Fortunately, this does not happen because plants have a backup pathway called cyclic electron flow . 2.1 How Cyclic Electron Flow Works In cyclic electron flow: • Electrons from reduced ferredoxin are redirected • Instead of reducing NADP ⁺ , the electrons are passed back to the cytochrome b ₆ f complex • From cytochrome b ₆ f, the electrons return to Photosystem I Because the electrons cycle back rather than leaving the system, no NADPH is produced in this pathway. 2.2 Importance of Cyclic Electron Flow Even though NADPH is not made, cyclic electron flow is still very important because: • Electron movement through cytochrome b ₆ f pumps protons into the thylakoid lumen • These protons increase the proton gradient • The proton gradient drives ATP synthesis by ATP synthase The ATP produced supplements the ATP made during normal (non - cyclic) electron flow involving Photosystem II. Key Takeaways • Cyclic electron flow occurs when NADP ⁺ is limited • Electrons are recycled back to Photosystem I • ATP is produced , but NADPH and O ₂ are not • This pathway prevents electron backup and keeps photosynthesis running efficiently

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    Study Guide 3. Z - Scheme of Photosynthesis Figure 1 The Z - scheme is a way to visualize the movement of electrons and energy changes during the light reactions of photosynthesis. It shows how electrons gain energy from light and then lose energy as they move through the electron transport chain. 3.1 Understanding the Vertical Axis In the Z - scheme diagram, the vertical axis represents reduction potential . • Molecules placed higher on the diagram have a more negative reduction potential • This means they are better electron donors • Molecules lower on the diagram are better electron acceptors This arrangement helps explain why electrons move in a specific direction during photosynthesis.

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    Study Guide 3.2 Electron Flow from Water to Photosystem II The process begins on the left side of the Z - scheme: • Water molecules are oxidized • Electrons are removed from water and passed to the oxidized form of P680 • This reaction releases: o Oxygen (O ₂ ) o Protons (H ⁺ ) into the thylakoid lumen This step replaces the electrons lost by Photosystem II. 3.3 Excitation and Electron Transfer in Photosystem II • When P680 absorbs a photon , it becomes excited to P680* • P680* moves to a much higher energy level in the Z - scheme • In this excited state, P680* is a strong reducing agent • It donates an electron to the quinone – cytochrome b ₆ f electron transport chain As electrons pass through this chain: • Protons are pumped into the thylakoid lumen • This contributes to the proton gradient needed for ATP synthesis 3.4 Transfer of Electrons to Photosystem I • Electrons from the cytochrome b ₆ f complex are transferred to Photosystem I • This reduces the PSI reaction center chlorophyll, P700 When P700 absorbs light , it becomes excited to P700* , reaching a high energy level in the Z - scheme. 3.5 Formation of NADPH or Cyclic Electron Flow From P700* , electrons can follow two possible paths :

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    Study Guide 1. Non - cyclic Electron Flow • Electrons are transferred to NADP ⁺ • NADPH is formed • NADPH provides reducing power for carbon fixation in the Calvin cycle 2. Cyclic Electron Flow • Electrons return to the cytochrome b ₆ f complex • This pathway produces ATP only • No NADPH is formed Summary of the Z - Scheme • Electrons start from water and end at NADP ⁺ • Light energy boosts electrons twice: once in PSII and once in PSI • Electron flow creates a proton gradient • The proton gradient drives ATP synthesis • NADPH is produced for use in the dark reactions Key Takeaways • The Z - scheme explains energy changes during the light reactions • PSII and PSI work together to move electrons uphill and downhill energetically • Light energy is converted into ATP and NADPH • Cyclic electron flow provides flexibility when ATP demand is high 4. ATP Synthesis in Photosynthesis ATP synthesis during the light reactions of photosynthesis is carried out by a large protein complex called ATP synthase . This complex is also known as the coupling factor because it couples proton movement to ATP formation.

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    Study Guide Figure 1 4.1 Structure of ATP Synthase ATP synthase has a distinctive mushroom - like shape : • The head (cap) of the enzyme faces the stroma • The stalk passes through the thylakoid membrane This structure allows ATP synthase to use the energy stored in a proton gradient to convert ADP and inorganic phosphate (Pi) into ATP . 4.2 Creation of the Proton Gradient During the light reactions: • Photosystem II (PSII) and Photosystem I (PSI) move protons into the thylakoid lumen • Water splitting at PSII adds additional protons to the lumen • Electron transport through the cytochrome b ₆ f complex also contributes to proton pumping

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