BIOC*4520: Metabolic process Lec 6
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Questions and Answers

what is resonance energy transfer

the transfer of absorbed light energy from pigment to pigment in the antenna complexes until it reaches the reaction centre

what are the two Photo Systems that make up the reaction centre

photosystem 1 (P700 because it absorbs at 700nm) and photosystem 2 (P680 because it absorbs at 680nm)

What is the Emerson Effect, and what did it show about photosynthesis?

Shows that photosynthesis is more efficient when plants are exposed to light of two different wavelengths (red light and far-red light) at the same time, rather than just one. This showed that Photosystem I (PSI) and Photosystem II (PSII) work together and need different wavelengths of light to maximize photosynthesis efficiency.

What did Robert Emerson’s experiment reveal about photosynthesis when plants were exposed to both red (650 nm) and far-red light (700 nm)?

<p>Photosynthesis efficiency increased significantly.</p> Signup and view all the answers

Which of the following statements correctly describes the absorption of red and far-red light by Photosystem I (PSI) and Photosystem II (PSII)?

<p>PSI absorbs maximally in the far-red region (700 nm), while PSII absorbs maximally in the red region (680 nm).</p> Signup and view all the answers

write out the hill reaction and what is oxidant used

<p>2H2O + chloroplast -&gt; O2 + (4e-) + (4H+) in vitro ferricyanide is used as the oxidant to accept electrons.</p> <p>whereas in vivo NADP+ accepts the electrons to form NADPH, which is used in the Calvin cycle for synthesizing sugars.</p> Signup and view all the answers

broadly describe how Photosystems works in a Z scheme

<p>photosystem 2 (P680) is a strong oxidant, it splits the water molecule into protons, electrons, and oxygen. upon absorption of RED LIGHT, this caused PS2 to get oxidized and the e- to gain energy to be transported to PS1 (P700) by electron transport carriers, upon absorption of far red light by the PS1, excites the electron further and are transported to NADP+ to form NADPH</p> Signup and view all the answers

Explain the role of the cytochrome b6/f complex in the electron transport chain during photosynthesis. How does its position between the two photosystems influence its function?

<p>b6/f complex functions as an electron carrier between PS1 and PS2. It facilitates the transfer of electrons from plastoquinone to (Q) to Plastocyanin (pC). FOUND BETWEEN PS1 AND PS2</p> Signup and view all the answers

How does Duysens' finding that cytochrome f is reduced by red light (PSII) and oxidized by far-red light (PSI) help illustrate the flow of electrons between PSII and PSI during photosynthesis?

<p>e- generated by PS2 under red light transferred to b6/f complex reduce cyto f and under far-red light, PS1 becomes active, accepts the e-, oxidizing cyto f. this demonstrates the sequential flow of electrons in the electron transport chain during photosynthesis.</p> Signup and view all the answers

what metal does Plastocynanin

<p>blue copper</p> Signup and view all the answers

Mutants lacking plastocyanin are unable to photosynthesize. Based on its position in the electron transport chain, explain why plastocyanin is critical for electron transport in photosynthesis.

<p>Plastocyanin (PC) transfers electrons from cytochrome f to PSI. In mutants lacking PC, electron flow between cytochrome f and PSI is blocked, which prevents the reduction of NADP+ and the continuation of the electron transport chain. As a result, ATP and NADPH production are halted, and photosynthesis cannot proceed. This makes PC essential for the proper functioning of the electron transport chain in photosynthesis.</p> Signup and view all the answers

where does photochemistry take place

<p>the thylakoid of the chloroplast</p> Signup and view all the answers

describe everything that happens in the Z-scheme of photosynthesis

<p>PS2 absorbs red light, and as a strong oxidant, it causes the photolysis of water which results in the formation of protons, oxygen, and electrons which replace those lost by PS2 as it transfers excited electrons through the ETC. For this to take place, the electrons must be transferred to plastoquinone which then transfers the e- to cyto b6/f complex (it makes use of the energy generated in this process to pump protons from the chloroplast stroma into the thylakoid lumen to create a proton gradient for ATP synthesis) the electron is then transferred to plastocyanin which then transfers the electron to PS1 where the PS1 absorbs far-red light causing the excitation of the electron and its transfer to ferredoxin which delivers the electron to NADP+ with the help of NADP+ REDUCTASE, reducing it to NADPH</p> Signup and view all the answers

what are the 4 complexes embedded in the thylakoid membrane that are involved in the light reactions and what role do they play

<p>PS2 = photolysis of water, transfer e- to Plastoquinone (Q) and reduce it to PQH2 Cyto b6/f = Transfer e- between Q to PC, creates H+ gradient PS1 = accepts e- from PC and pass it to ferredoxin ATP synthase = generates ATP from the H+ gradient, ATP made out in the stroma</p> Signup and view all the answers

TK membrane is permeable to H+.

<p>False</p> Signup and view all the answers

ATP synthase allows protons to move from the lumen to the stroma.

<p>True</p> Signup and view all the answers

The net movement of protons occurs from the stroma into the lumen during photosynthesis.

<p>False</p> Signup and view all the answers

The cytochrome b6f complex plays a role in generating a proton gradient.

<p>True</p> Signup and view all the answers

Photosystem I (PSI) does not participate in the net movement of protons.

<p>False</p> Signup and view all the answers

The dissipation of the H+ gradient is responsible for the driving force of ATP synthesis.

<p>True</p> Signup and view all the answers

ATP synthesis involves the accumulation of H+ ions in the stroma.

<p>False</p> Signup and view all the answers

ATP synthase facilitates the movement of protons into the thylakoid lumen to drive ATP production.

<p>False</p> Signup and view all the answers

The reversal of hydrolysis is part of the ATP synthesis process.

<p>True</p> Signup and view all the answers

The experiment by Andre Jagendorf proved that ATP could be synthesized in chloroplasts without light by creating a pH gradient across the thylakoid membrane.

<p>True</p> Signup and view all the answers

Peter Mitchell's chemiosmotic hypothesis is not related to ATP synthesis in chloroplasts.

<p>False</p> Signup and view all the answers

A pH gradient across the thylakoid membrane is unnecessary for ATP synthesis in the absence of light.

<p>False</p> Signup and view all the answers

ATP synthesis in chloroplasts can only occur when light is present.

<p>False</p> Signup and view all the answers

The thylakoid membrane is crucial for establishing a pH gradient necessary for ATP production.

<p>True</p> Signup and view all the answers

Jagendorf's experiment showed that ATP could be synthesized using a pH gradient created by acidifying the thylakoid lumen and then increasing the external pH.

<p>True</p> Signup and view all the answers

The dissipation of the proton gradient in Jagendorf's experiment inhibited ATP synthesis.

<p>False</p> Signup and view all the answers

ATP synthesis can occur in chloroplasts even without the presence of light if a sufficient pH gradient is established.

<p>True</p> Signup and view all the answers

The creation of a pH gradient across the thylakoid membrane is insignificant for the chemiosmotic theory.

<p>False</p> Signup and view all the answers

In Jagendorf's experiment, ADP and Pi were added after establishing the proton gradient to facilitate ATP synthesis.

<p>True</p> Signup and view all the answers

The CF₁CF₀ ATPase is located in the thylakoid membrane of mitochondria.

<p>False</p> Signup and view all the answers

Protons flow from the stroma to the thylakoid lumen during the function of CF₀.

<p>False</p> Signup and view all the answers

The rotation of the CF₁ portion is driven by the flow of protons through the CF₀ channel.

<p>True</p> Signup and view all the answers

ATP synthesis occurs during the light-independent reactions of photosynthesis.

<p>False</p> Signup and view all the answers

The CF₁ subunit is responsible for converting ATP into ADP and Pi in the ATP synthesis process.

<p>False</p> Signup and view all the answers

Why is no oxygen (O₂) produced during cyclic photophosphorylation?

<p>Since Photosystem II (PSII) is not involved, water is not split, and no oxygen is produced.</p> Signup and view all the answers

How does cyclic photophosphorylation contribute to ATP production?

<p>Using the energy generated as electrons are transported from ferredoxin to the cytochrome b6f complex, protons (H⁺) are pumped into the thylakoid lumen, creating a proton gradient. This proton gradient then powers ATP synthase, which synthesizes ATP from ADP and inorganic phosphate (Pi) through the process of chemiosmosis.</p> Signup and view all the answers

Study Notes

Resonance Energy Transfer

  • A process where energy is transferred from an excited molecule to a nearby molecule.
  • No electron transfer occurs.
  • Happens when molecules are close enough for their electron clouds to overlap.

Photosystems

  • Photosystem I (PSI): Absorbs light best at wavelengths around 700nm, associated with chlorophyll a.
  • Photosystem II (PSII): Absorbs light best at wavelengths around 680nm, also associated with chlorophyll a, but with a different arrangement than PSI.

The Emerson Effect

  • Demonstrates the interdependence of two photosystems in photosynthesis
  • When red light (650 nm) and far-red light (700 nm) are provided simultaneously, photosynthesis rates are higher than the sum of rates when each light is applied alone.
  • This suggests that both red and far-red light need to be absorbed to get the full effect of photosynthesis.

Robert Emerson's Experiment

  • Robert Emerson’s experiment showed that the rate of photosynthesis was higher when plants were exposed to both red and far-red light than when they were exposed to either light alone.
  • This supported the idea that two different photosystems were involved in photosynthesis.

Photosystem Light Absorption

  • PSII: Absorbs red light (680nm) better than far-red light (700nm)
  • PSI: Absorbs far-red light (700nm) better than red light (680nm)

Hill Reaction

  • Reaction: 2H2O + 2A → O2 + 2AH2
  • Description: Photosynthesis's light-dependent reactions produce oxygen from water.
  • Oxidant used: A is an electron acceptor (e.g., ferricyanide)
  • Significance: Explains the role of PSII in generating oxygen and reducing an electron acceptor

Photosystems in the Z scheme of electron transport

  • PSII absorbs low-energy light (red light)
  • PSII Excites electrons to a high energy level
  • The excited electrons flow through an electron transport chain
  • The electron transport chain releases energy to power proton pumping, creating a proton gradient
  • PSI absorbs higher-energy light (far-red light)
  • PSI boosts the energy of electrons even higher.
  • The high-energy electrons reduce NADP+ to NADPH

Cytochrome b6/f Complex

  • Role: Transfers electrons between PSII and PSI.
  • Position: Located between PSII and PSI in the thylakoid membrane
  • Function: Uses the energy from electron transport to create a proton gradient across the thylakoid membrane, contributing to ATP production.

Duysens' Finding

  • Cytochrome f is reduced by red light (PSII) and oxidized by far-red light (PSI)
  • This observation shows that the cytochrome b6/f complex is involved in the electron flow from PSII to PSI.

Plastocyanin

  • A copper-containing protein
  • Acts as a mobile electron carrier between the cytochrome b6/f complex and PSI

Importance of Plastocyanin

  • Plastocyanin is essential for electron transport in photosynthesis because it facilitates the transfer of electrons from the cytochrome b6/f complex to PSI.
  • This is crucial for generating the high-energy electrons needed to reduce NADP+ to NADPH.

Photochemistry Location

  • The thylakoid membrane, within the chloroplast

The Z-scheme of Photosynthesis

  • Light Absorption: Light is absorbed by PSII and PSI, energizing electrons.
  • Electron Transport: Excited electrons from PSII flow through the electron transport chain, releasing energy to pump protons across the thylakoid membrane, generating a proton gradient.
  • Proton Motive Force: The proton gradient creates a proton motive force used by ATP synthase to produce ATP.
  • Water Splitting: PSII oxidizes water, releasing oxygen as a byproduct.
  • NADPH Production: PSI transfers high-energy electrons to NADP+ reducing it to NADPH.

Complexes in the Thylakoid Membrane

  • Photosystem II (PSII): Absorbs light energy, splits water, and releases oxygen.
  • Cytochrome b6/f complex: Transfers electrons between PSII and PSI, generating a proton gradient.
  • Photosystem I (PSI): Absorbs light and transfers electrons to ferredoxin.
  • ATP Synthase: Uses the proton gradient across the thylakoid membrane to produce ATP

Thylakoid Proton Movement

  • TK Membrane: The thylakoid membrane is permeable to protons (H+).
  • ATP Synthase: ATP synthase allows protons to move from the lumen to the stroma.
  • Net movement: The net movement of protons occurs from the stroma into the lumen during photosynthesis.
  • Cytochrome b6f complex: Significant contributor to the proton gradient.
  • PSI: Does not participate in the net movement of protons.

Resonance Energy Transfer

  • A process where energy is transferred between molecules without the emission of light.
  • Occurs when a molecule absorbs light energy and becomes excited.
  • The excited molecule can then transfer its energy to a nearby molecule, causing it to become excited.
  • The efficiency of energy transfer depends on the distance between the molecules and the overlap of their energy levels.

Photosystems

  • Photosystem II (PSII) absorbs light energy at a wavelength of 680 nm.
  • Photosystem I (PSI) absorbs light energy at a wavelength of 700 nm.
  • The two photosystems work together to capture light energy and convert it to chemical energy.
  • PSII is responsible for the primary photochemistry of photosynthesis, splitting water molecules and releasing oxygen.
  • PSI is involved in the generation of NADPH, which is a high-energy electron carrier used to reduce carbon dioxide.

The Emerson Effect

  • The Emerson effect is the observation that the rate of photosynthesis is higher when plants are exposed to both red light (650 nm) and far-red light (700 nm) than when they are exposed to either light alone.
  • The Emerson effect is explained by the presence of two photosystems in the chloroplast, which can work together to maximize the efficiency of photons being absorbed by the plants.

Robert Emerson’s Experiment

  • Robert Emerson’s experiment showed that plants exposed to both red and far-red light had a higher rate of photosynthesis than those exposed to either light alone.
  • This is because the two wavelengths of light are absorbed by different photosystems in the plant (PSII and PSI).
  • PSII absorbs red light, while PSI absorbs far-red light.

Light Absorption by Photosystems

  • Photosystem II (PSII) primarily absorbs red light (680 nm).
  • Photosystem I (PSI) primarily absorbs far-red light (700 nm).
  • Both photosystems can absorb other wavelengths of light, but their absorption maxima are at these specific wavelengths.

The Hill Reaction

  • The Hill reaction is the light-dependent reaction in photosynthesis that produces oxygen.
  • The Hill reaction is named after Robert Hill, who discovered it in 1937.
  • The equation for the Hill reaction is: H2O + 2H+ + 2e → 1/2O2 + 2H2O
  • The oxidant used in the Hill reaction is plastoquinone (PQ).

The Z-Scheme

  • The Z-scheme is a model of the electron transport chain in photosynthesis.
  • The Z-scheme describes the flow of electrons from water to NADPH, which is driven by the absorption of light energy by PSII and PSI.
  • The Z-scheme is named for its shape, which resembles the letter "Z."
  • The process starts with light being absorbed by PSII, which excites electrons to higher energy level and eventually leads to the splitting of water molecules.
  • The energized electrons travel down an electron transport chain, through plastoquinone and the cytochrome b6f complex until they reach PSI.
  • PSI absorbs photons and the electrons are re-energized and move to ferredoxin.
  • The electrons from ferredoxin are then used to reduce NADP+ to NADPH.
  • NADPH can then go on to drive the Calvin cycle in the stroma of the chloroplast for carbon dioxide fixation, forming glucose.

The Cytochrome b6/f Complex

  • The cytochrome b6/f complex is located between PSII and PSI in the thylakoid membrane of chloroplasts (photosynthetic membrane).
  • Together with PSI and PSII it plays a central role in photophosphorylation, the process of ATP formation.
  • The cytochrome b6/f complex uses the energy of electrons to pump protons (H+) across the thylakoid membrane, creating a proton gradient that drives ATP synthesis.
  • The cytochrome b6/f complex uses the energy gained from electrons to transport protons from the stroma to the thylakoid lumen, contributing to the proton gradient that drives ATP synthesis.

Duysens' Finding

  • Duysens' observation that cytochrome f is reduced by red light (PSII) and oxidized by far-red light (PSI) supports the flow of electrons from PSII to PSI during photosynthesis.
  • PSII absorbs red wavelengths of light, and the energy captured is used to reduce cytochrome f.
  • PSI absorbs far-red wavelengths of light, and the energy captured causes the oxidation of cytochrome f.

Plastocyanin

  • Plastocyanin is a small, copper-containing protein that shuttles electrons from the cytochrome b6/f complex in the thylakoid membrane to PSI in photosynthesis.
  • Plastocyanin is critical for electron transport in photosynthesis.

Mutants Lacking Plastocyanin

  • Mutants lacking plastocyanin cannot perform photosynthesis because the electron transport chain is disrupted, thus, unable to produce ATP and NADPH.

Photochemistry

  • Photochemistry takes place within the complexes embedded within the thylakoid membrane of chloroplasts.

The Z-Scheme Summarized

  • The Z-scheme is a series of reactions that involve the absorption of light energy by two separate photosystems (PSII and PSI) within chloroplasts, which leads to the production of ATP and NADPH in the thylakoid membranes of chloroplasts.

The 4 Complexes of the Light Reactions

  • Photosystem II (PSII) - Captures light energy and uses it to split water molecules, releasing oxygen as a byproduct.
  • Cytochrome b6f Complex - Uses the energy of electrons to pump protons (H+) across the thylakoid membrane, creating a proton gradient that drives ATP synthesis.
  • Photosystem I (PSI) - Transfers the electrons to ferredoxin, a protein that can accept electrons.
  • ATP Synthase - A protein complex that uses the energy from the proton gradient to synthesize ATP.

Proton Movement in the Thylakoid Membrane

  • The thylakoid membrane is permeable to hydrogen ions (H+); however, during photosynthesis, the movement of protons from the stroma to the lumen is favored.
  • This movement is driven by the proton motive force, which is generated by the electron transport chain.
  • The cytochrome b6f complex facilitates the movement of electrons from PSII to PSI, which is essential for generating the proton gradient across the thylakoid membrane.
  • The dissipation of the H+ gradient across the thylakoid membrane provides the energy for ATP synthesis.
  • ATP is synthesized using the energy from the proton gradient by ATP synthase, a rotary motor protein located in the thylakoid membrane complex which allows protons to move from the lumen to the stroma.

Andre Jagendorf's Experiment

  • Jagendorf's experiment focused on chloroplasts, which are organelles responsible for photosynthesis in plants.
  • The experiment demonstrated that ATP (adenosine triphosphate), the primary energy currency of cells, could be synthesized without light.
  • This was achieved by establishing a pH gradient across the thylakoid membrane.
  • The thylakoid membrane is a key component of chloroplasts that contains the machinery for light-dependent reactions of photosynthesis.
  • The pH gradient was created by using a solution of acidic pH on one side of the membrane and a solution of neutral pH on the other side.
  • The movement of protons (H+) across the membrane, driven by the pH difference, provided the energy for ATP synthesis.

Chemiosmotic Hypothesis

  • This experiment provided strong evidence for Peter Mitchell's chemiosmotic hypothesis, which proposes that the energy for ATP synthesis comes from the electrochemical potential gradient of protons across a membrane.
  • This gradient is established by the movement of protons from one side of the membrane to the other, driven by the energy from light or other sources.
  • The energy stored in this gradient is then used to drive ATP synthesis by an enzyme called ATP synthase.
  • The experiment demonstrates that the presence of light is not essential for ATP synthesis but rather the existence of a proton gradient.

Jagendorf's Experiment

  • Demonstrated ATP synthesis without light
  • Used an artificial pH gradient across the thylakoid membrane
  • Acidified the thylakoid lumen (inside)
  • Added NaOH to raise the external pH
  • Created a steep proton gradient (high concentration inside)
  • Added ADP and Pi (ingredients for ATP)
  • Dissipation of the proton gradient drove ATP synthesis
  • Supported the chemiosmotic theory (ATP synthesis driven by proton gradient)

Chloroplast ATP Synthase

  • The CF₁CF₀ ATPase, also known as chloroplast ATP synthase, is an enzyme complex located in the thylakoid membrane of chloroplasts.

  • It plays a crucial role in photosynthesis, specifically in the light-dependent reactions, where it synthesizes ATP.

  • The enzyme consists of two main parts: CF₀ and CF₁.

  • CF₀ is embedded in the thylakoid membrane and forms a channel for proton (H⁺) flow.

  • This channel allows protons to move down their concentration gradient from the thylakoid lumen into the stroma.

  • The movement of protons through the CF₀ channel causes the CF₁ portion, which extends into the stroma, to rotate.

  • This rotation powers the catalytic activity of the CF₁ subunit.

  • The CF₁ subunit then converts ADP and inorganic phosphate (Pi) into ATP.

Cyclic Photophosphorylation

  • ATP synthesis process in photosynthesis
  • Involves the cyclic flow of electrons
  • Occurs in the thylakoid membranes of chloroplasts
  • Requires Photosystem I (PSI), electron transport chain (ETC) and ATP synthase

Process Steps

  • PSI absorbs light energy which excites electrons
  • Excited electrons move into the ETC
  • Proton gradient is created as electrons move through the ETC (protons are pumped into the thylakoid lumen)
  • Proton gradient drives ATP synthesis through ATP synthase
  • Electrons return to PSI to complete the cycle

Function

  • Cyclic photophosphorylation only produces ATP
  • Non-cyclic photophosphorylation produces both ATP and NADPH
  • Provides energy for metabolic processes within the plant

Importance

  • Important for energy production in photosynthetic organisms
  • Supports cellular processes that require ATP

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