Photosynthesis Overview and Mechanisms

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Questions and Answers

What occurs when an electron falls to a lower energy level?

A new electron is created from nothing.

The molecule becomes positively charged.

Energy is released and transferred to a nearby molecule. (correct)

The process stops and no further reactions happen.

Where does the synthesis of glucose from CO2 and H2O occur within the chloroplast?

In the stroma. (correct)

In the thylakoid membranes.

In the thylakoid lumen.

On the outer membrane.

What is the primary role of chlorophyll in photosynthesis?

To provide structure to the thylakoid membranes.

To synthesize glucose directly.

To decompose water molecules.

To capture light energy and initiate charge separation. (correct)

Which molecule is coordinated to the nitrogens of the pyrrole in chlorophyll?

Magnesium Signup and view all the answers

What do antenna chlorophylls do within the thylakoid membranes?

Absorb light energy and transfer it to the reaction center. Signup and view all the answers

What happens to the electron in chlorophyll after it is excited by light energy?

It is donated to an electron transport chain. Signup and view all the answers

Which statement accurately describes thylakoid membranes?

They are where light reactions of photosynthesis take place. Signup and view all the answers

How do chlorophyll a and b differ in terms of their absorption spectra?

They have different maximum wavelengths for light absorption. Signup and view all the answers

What is the special pair of chlorophyll molecules in Photosystem II called?

P680° Signup and view all the answers

What is the function of the Cytochrome b6f complex in the photosynthetic electron transport chain?

To release protons into the thylakoid lumen Signup and view all the answers

How does the Water-Oxidizing Complex (WOC) replenish the electron lost by the special pair in Photosystem II?

By stripping an electron from water Signup and view all the answers

What does the Z-scheme in photosynthesis illustrate?

The electron flow from H2O to NADP+ Signup and view all the answers

Which component is crucial in the Water-Oxidizing Complex (WOC) during photosynthesis?

Manganese Signup and view all the answers

Which molecule donates electrons back to Photosystem I after being reduced?

Plastocyanin Signup and view all the answers

What happens to the original chlorophyll molecule P680° during the excitation process?

It loses an electron Signup and view all the answers

What is one of the primary products generated along with the proton-motive force during photosynthesis?

Adenosine triphosphate (ATP) Signup and view all the answers

What significant event occurred around 2.4 billion years ago that influenced atmospheric composition?

First water-splitting photosynthesis releases O2 Signup and view all the answers

Which of the following is NOT a product of the light reactions in photosynthesis?

Glucose Signup and view all the answers

What is the primary role of the electron transport chain in the light reactions?

To create a proton-motive force for ATP synthesis Signup and view all the answers

Which type of organism is believed to have evolved oxygenic photosynthesis?

Eukaryotic photosynthetic cells Signup and view all the answers

Which process is necessitated by the rise of atmospheric oxygen concentration?

Oxidative phosphorylation Signup and view all the answers

What is the starting point of the light reactions in photosynthesis?

Photosystem II Signup and view all the answers

What is formed as electrons travel through the electron transport chain during the light reactions?

Proton gradient Signup and view all the answers

Early photosynthetic organisms used which substance as an electron source?

H2S Signup and view all the answers

What is primarily responsible for the proton-motive force in chloroplasts?

Proton gradient Signup and view all the answers

Which process occurs in Photosystem I during the light reactions of photosynthesis?

Reduction of ferredoxin Signup and view all the answers

What must occur for maximal activity of chloroplast ATP synthase?

Reduction of a disulfide bond Signup and view all the answers

What happens to ATP synthase in the absence of light?

The cystine bridge reforms Signup and view all the answers

What regulatory role does thioredoxin play in the function of ATP synthase?

It acts as a reductant Signup and view all the answers

How does Mg2+ contribute to electroneutrality in chloroplasts?

It moves into the stroma Signup and view all the answers

What is the role of the Cytochrome b6f complex in the light reactions?

Creating the proton gradient Signup and view all the answers

Why is the chloroplast CF1-CF0 complex significant?

It synthesizes ATP from the proton gradient Signup and view all the answers

What is the primary function of accessory pigments in photosynthesis?

To absorb light energy and transfer it to antenna chlorophylls. Signup and view all the answers

Which of the following statements is true about Photosystem I?

It produces NADPH as the final product. Signup and view all the answers

What replaces the electrons lost by Photosystem I?

Electrons from the photolysis of water. Signup and view all the answers

What is the role of ferredoxin in Photosystem I?

To mediate the transfer of electrons to NADP+. Signup and view all the answers

What is P700 in the context of Photosystem I?

A pair of chlorophyll molecules with an absorption maximum at 700 nm. Signup and view all the answers

How does the efficiency of photosynthesis increase?

By utilizing resonance energy transfer through antenna molecules. Signup and view all the answers

What is the primary product formed by ferredoxin-NADP+ reductase?

NADPH. Signup and view all the answers

Where is NADPH produced in the chloroplast?

On the stromal side of the thylakoid membrane. Signup and view all the answers

What happens to electrons in cyclic photophosphorylation?

Electrons flow from Photosystem I through cytochrome b6f to plastocyanin and return to P700. Signup and view all the answers

How many protons need to pass through ATP synthase to generate 3 ATP molecules?

12 protons Signup and view all the answers

Which herbicide inhibits Photosystem I?

Paraquat Signup and view all the answers

Where is Photosystem II located within the chloroplast structure?

In the stacked regions of the thylakoid membranes. Signup and view all the answers

What role does cytochrome b6f play in photosynthesis?

It contributes to the proton gradient. Signup and view all the answers

Which component of the thylakoid membranes is responsible for the synthesis of ATP?

ATP synthase Signup and view all the answers

What is produced as a result of eight photons being absorbed in photosynthesis?

One O2, two NADPH, and three ATP. Signup and view all the answers

What is the effect of reduced ferredoxin on ATP synthase?

It upregulates ATP synthase via reduced thoredoxin. Signup and view all the answers

Study Notes

Oxygenic Photosynthesis

Oxygenic photosynthesis evolved around 2.4 billion years ago.
Early photosynthetic organisms likely used H₂S as an electron source.
Earth's atmosphere was initially anaerobic.
Oxygenic photosynthesis, using H₂O as the electron source, evolved around 2.4 bya.
Atmospheric oxygen concentration subsequently rose, requiring the development of oxidative phosphorylation.

Photosynthesis: The Light Reactions

Photosynthesis utilizes light energy to elevate electrons from a low-energy state to a high-energy, excited state.
These excited electrons are employed to generate a proton-motive force that drives ATP synthesis.
High-energy electrons are also used to form NADPH, a crucial biosynthetic reducing agent.
The reactions powered by sunlight are designated as the light reactions.

Photosynthesis Initiation

Photosynthesis commences with light absorption by a photoreceptor molecule, also known as a pigment.
When a photon of appropriate energy is absorbed by a pigment, an electron within the pigment molecule transitions to a higher energy level.
The excited electron can return to its original state, releasing energy in the form of light or heat.

Resonance Energy Transfer

An electron can absorb energy from electromagnetic radiation with appropriate wavelength and elevate to a higher energy state.
When the excited electron returns to a lower energy level, the absorbed energy is released.
The released energy can be absorbed by an electron in a neighboring molecule, causing it to transition to a higher energy state.

Photoinduced Charge Separation

Another function of an excited electron is to migrate to a neighboring molecule possessing a lower excited state, a procedure termed electron transfer.
Electron transfer results in charge separation, with the initial molecule becoming positively charged and the recipient molecule becoming negatively charged.
Charge separation occurs at sites designated as reaction centers.

Photosynthesis in Eukaryotes

Photosynthesis in eukaryotes transpires within chloroplasts, which are double-membrane organelles.
The inner membrane encloses a compartment called the stroma, where glucose synthesis from CO₂ and H₂O occurs using ATP and NADPH produced in the light reactions.
Within the stroma, there are membranous structures called thylakoid membranes, which serve as the site of the light reactions of photosynthesis.

Chlorophyll

Chlorophyll is the primary light receptor in most photosynthetic systems.
It's a substituted tetrapyrrole bound to a large hydrophobic 20-carbon alcohol called phytol.
Chlorophyll is a highly conjugated molecule whose pyrrole nitrogens coordinate with magnesium.
Chlorophyll a and b exhibit distinct absorption spectra.

Light Absorption and Charge Separation

Light energy absorption by chlorophyll results in charge separation.
Excited chlorophyll donates electrons to an acceptor molecule, empowering the acceptor to function as a reducing agent.

Light Harvesting Molecules

Light-harvesting molecules and photosynthetic reaction centers are incorporated into the thylakoid membranes.
These molecules absorb light and transfer energy between molecules until it reaches the reaction center.
The pigments are arranged in light-harvesting complexes that surround the reaction center and funnel light energy to it through resonance energy transfer.

Photosystems in Plants

Photosynthesis in green plants involves two photosystems.
Photosystem I generates NADPH, a biosynthetic reducing agent.
Photosystem II replenishes the electrons in Photosystem I while simultaneously establishing a proton gradient used to synthesize ATP.
Water photolysis replaces missing electrons in Photosystem II.

Photosystem I Structure

Photosystem I is a sizable, membrane-spanning complex composed of a dozen proteins and numerous cofactors, including chlorophyll, quinones, and iron-sulfur clusters.
The core of photosystem I comprises an electron transfer chain of chlorophyll, phylloquinone, and iron-sulfur clusters.
The terminal electron acceptor is an iron-sulfur protein known as ferredoxin.

P700 in Photosystem I

The reaction center in photosystem I is denoted as P700, comprised of a pair of chlorophyll molecules.
Electrons from excited P700 travel down an electron transport chain to the iron-sulfur protein ferredoxin.
Ferredoxin-NADP+ reductase facilitates electron transfer from ferredoxin to produce NADPH, a crucial biosynthetic reducing agent.
NADPH synthesis occurs on the stromal side of the thylakoid membrane.

Ferredoxin-NADP+ Reductase

Ferredoxin-NADP+ reductase acts as a mediator between the one-electron transfer from ferredoxin and the two-electron transfer to NADP+.
It comprises FAD, which mediates electron transfers.

Photosystem II

Photosystem II transfers electrons to Photosystem I and generates a proton gradient for ATP synthesis.
The proteins D2 and D1 form a scaffold for more than 20 cofactors and other proteins.
The water-oxidizing complex performs the crucial task of replacing electrons in Photosystem II through water photolysis, releasing oxygen as a byproduct.

P680 in Photosystem II

The reaction center in photosystem II is P680, a chlorophyll pair.
Excited P680 transfers electrons through components including pheophytin, plastoquinone, and the cytochrome b6f complex.
The cytochrome b6f complex pumps protons into the thylakoid lumen, creating a proton-motive force.
Plastocyanin delivers electrons to oxidized P700 in photosystem I.

Cytochrome b6f

Cytochrome b6f releases protons from plastoquinol into the thylakoid lumen and pumps protons.
The mechanism is analogous to the Q cycle of complex III in cellular respiration.
Reduced plastocyanin donates electrons to Photosystem I, which is a crucial component of the electron transport chain of photosynthesis.

Water-Oxidizing Complex (WOC)

The water-oxidizing complex of photosystem II replenishes electrons lost from the special pair.
This process involves water photolysis at the WOC, using manganese as a crucial component.
- This process separates water molecules into oxygen, protons, and electrons.

The Z-scheme

The electron flow from H₂O to NADP⁺ is referred to as the Z-scheme of photosynthesis.

Variations in Electron Donors

Some organisms employ electron donors other than water.
- Some bacteria use substances like hydrogen sulfide (H₂S), hydrogen (H₂), or various organic compounds instead of water as electron sources.

Proton-Motive Force

The flow of electrons from H₂O to NADP⁺ generates a proton-motive force.
Most of the energy in chloroplasts stems from the proton gradient, with minimal contribution from the membrane potential.
Electroneutrality is maintained by Mg²⁺ moving into the stroma while H⁺ is pumped into the thylakoid lumen.
ATP synthase in chloroplasts (CF₁-CF₀ complex) is comparable to mitochondrial F₁-F₀ complex.
Newly synthesized ATP is released into the stroma, where it's utilized in carbohydrate synthesis.

Herbicides

Certain herbicides inhibit specific components of the light reactions of photosynthesis.
- Diuron and atrazine inhibit photosystem II.
- Paraquat inhibits photosystem I and can generate reactive oxygen species.

Regulation of Chloroplast ATP Synthase

Chloroplast ATP synthase activity is governed by the availability of reducing agents and the proton-motive force.
Maximal activity requires a disulfide bond in the γ subunit to be reduced to two cysteines.
Thioredoxin, formed from ferredoxin in photosystem I, acts as the reductant.
The δ subunit of ATP synthase is sensitive to changes in proton-motive force, impacting disulfide bond reduction and ATP synthesis regulation.

Cyclic Photophosphorylation

Cyclic photophosphorylation diverts electron flow to ATP synthesis in the absence of NADPH production or oxygen generation.
Electrons from Photosystem I flow through the cytochrome b6f complex and plastocyanin to recycle back to Photosystem I.
Protons pumped by cytochrome b6f are utilized to synthesize ATP.

Photo Yields in Photosynthesis

Eight photons result in one O₂, two NADPH, and three ATP molecules.
This process involves the pumping of 12 protons into the thylakoid lumen via electron transport and their subsequent passage through ATP synthase during the synthesis of 3 ATP.
Cyclic photophosphorylation, using two photons, results in one ATP molecule.

Organization of Photosynthetic Components

Thylakoid membranes are structured into stacked and unstacked regions.
Photosystem II is preferentially situated in stacked regions, while Photosystem I and ATP synthase are located in unstacked regions to facilitate easy access to products and substrates.

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Description

This quiz explores the evolution and mechanisms of oxygenic photosynthesis, detailing its historical development and biochemical processes. It covers the light reactions, including the role of light energy in electron elevation and ATP synthesis. Test your understanding of how these processes shape our atmosphere and support life.