Photosynthesis: Energy, Reactions, and Impact

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What proportion of the sunlight energy that falls on the Earth each day is absorbed by photosynthetic organisms and transformed into chemical energy?

Approximately 10%
Approximately 1% (correct)
About 50%
Less than 0.1%

In photosynthesis, where do the light reactions take place?

In the stroma of the chloroplast
In the mitochondrial matrix
Within the thylakoid membranes of the chloroplast (correct)
In the cytoplasm outside the chloroplast

During photosynthesis, which of the following substances is used in the dark reactions to generate hexose sugars?

Carbon dioxide, NADPH, and ATP (correct)
Oxygen and ATP
NADPH and water
Water and oxygen

Which of the following statements accurately describes the role of light reactions in photosynthesis?

They capture light energy to produce ATP and NADPH. (D) Signup and view all the answers

What is the name of the soluble portion of the chloroplast where the dark reactions occur?

Stroma (D) Signup and view all the answers

If a plant is exposed to a toxin that inhibits the function of the thylakoid membranes, which of the following processes would be most directly affected?

Light absorption and ATP synthesis (C) Signup and view all the answers

Consider a scenario where a plant is placed in an environment with abundant light but limited access to water. How would this condition primarily impact the process of photosynthesis?

It would inhibit the light reactions, reducing the production of ATP and NADPH. (C) Signup and view all the answers

What is the overall purpose of the light-dependent reactions in photosynthesis?

Absorb light energy to produce ATP and NADPH. (D) Signup and view all the answers

What is the primary function of plastoquinone (PQ) within the chloroplast membrane?

To shuttle electrons between Photosystem II (PSII) and the cytochrome b6f complex. (B) Signup and view all the answers

Which component transfers electrons from the cytochrome b6f complex to Photosystem I (PSI)?

Plastocyanin (D) Signup and view all the answers

What structural feature of the cytochrome b6f complex allows it to span the thylakoid membrane?

A series of 26 transmembrane α-helices. (A) Signup and view all the answers

What is the functional analogy between the cytochrome b6f complex in chloroplasts and Complex III in mitochondria?

Both use the energy of an oxidation-reduction reaction to create a proton gradient across a membrane. (C) Signup and view all the answers

In the R.viridis photosynthetic reaction center, which subunit does NOT appear to be involved in electron transfer?

M subunit (D) Signup and view all the answers

Plastoquinone A is similar to which of the following?

Coenzyme Q (D) Signup and view all the answers

What is the significance of solving the structure of the R.viridis photosynthetic reaction center?

It provided a detailed understanding of the molecular architecture of photosynthetic reaction centers. (A) Signup and view all the answers

Plastocyanin uses which metal to carry electrons in the photosynthetic electron transport chain?

Copper (A) Signup and view all the answers

What is the primary role of water in the photosynthetic reaction sequence in green plants?

To serve as the ultimate electron donor for NADP+ reduction, producing oxygen. (B) Signup and view all the answers

In the generalized equation for photosynthesis, CO2 + 2H2A → (CH2O) + 2A + H2O, what does 'H2A' represent?

A hydrogen donor (C) Signup and view all the answers

How does the aromaticity of chlorophyll contribute to its function in photosynthesis?

It enables chlorophyll to efficiently absorb light energy. (A) Signup and view all the answers

What is the role of the phytyl side chain in the structure of chlorophyll?

To anchor chlorophyll within the photosynthetic membrane. (D) Signup and view all the answers

How does the presence of both chlorophyll a and chlorophyll b in plants enhance photosynthesis?

It enables the plant to absorb a broader spectrum of light wavelengths. (A) Signup and view all the answers

What is the primary function of accessory light-harvesting pigments like β-Carotene?

To expand the range of light wavelengths that can be absorbed for photosynthesis. (A) Signup and view all the answers

Why is magnesium (Mg2+) important in the structure of chlorophyll?

It stabilizes the tetrapyrrole ring and affects the electron distribution for light absorption. (A) Signup and view all the answers

In photosynthetic bacteria, H2A (the hydrogen donor) can be H2S or isopropanol. How does this differ from green plants?

Green plants primarily use water (H2O) as their hydrogen donor. (C) Signup and view all the answers

What is the primary role of Photosystem I (PSI) in the photosynthetic electron transfer process?

Providing reducing power in the form of NADPH. (C) Signup and view all the answers

In the Z scheme of photosynthesis, what is the role of cytochrome b6f complex?

It pumps protons from the stroma into the thylakoid lumen, contributing to ATP synthesis. (B) Signup and view all the answers

What would be the most immediate consequence if plastocyanin (PC) were non-functional in the Z scheme?

Electron transfer from PSII to PSI would be blocked. (A) Signup and view all the answers

Why does oxygen evolution peak every fourth flash of light in experiments with isolated chloroplasts?

Because PSII cycles through five oxidation states (S0-S4), requiring four light-dependent steps to release O2. (C) Signup and view all the answers

In the Z scheme, what is the original source of electrons that ultimately reduce NADP+ to NADPH?

Water ($H_2O$). (B) Signup and view all the answers

If a herbicide blocked the function of plastoquinone (PQ), which of the following would be the most direct consequence?

Inhibition of electron transport between PSII and cytochrome b6f. (D) Signup and view all the answers

The Z scheme illustrates the transfer of electrons in photosynthesis. How are the electron carriers arranged within this scheme?

According to their standard reduction potentials. (B) Signup and view all the answers

What is the net equation that represents the oxidation of water in PSII during the oxygen evolution process?

$2H_2O \rightarrow O_2 + 4H^+ + 4e^-$ (D) Signup and view all the answers

What is the primary role of Rubisco activase (RuA) in the carbon fixation process?

To detach RuBP from inactive Rubisco, thereby activating the enzyme. (D) Signup and view all the answers

During the ribulose bisphosphate carboxylase reaction, what role does $Mg^{2+}$ play at the active site?

It stabilizes the 2,3-enediol transition state and facilitates carbon-carbon bond cleavage. (B) Signup and view all the answers

Which of the following best describes the Calvin-Benson cycle's primary function?

To transform 3-phosphoglycerate into hexose sugars. (B) Signup and view all the answers

What is the ultimate fate of glucose-1-phosphate, the product of the Calvin-Benson cycle, in plants?

It serves as a precursor for the synthesis of starch and cellulose. (C) Signup and view all the answers

How does light indirectly activate the carbon dioxide fixation pathway?

By inducing pH changes in chloroplast compartments and generating reducing power. (C) Signup and view all the answers

How do light-induced pH changes in the chloroplast stroma contribute to the activation of Rubisco and Rubisco activase?

The stroma becomes more alkaline, favoring enzyme activity. (A) Signup and view all the answers

Besides pH changes, how else does light regulate the Calvin cycle?

By reducing ferredoxin, which then reduces thioredoxin, activating NADPH-using enzymes. (B) Signup and view all the answers

How does the movement of $Mg^{2+}$ ions from the thylakoid lumen into the stroma affect the Calvin cycle?

It increases the activity of Mg++ dependant enzymes in the stroma. (D) Signup and view all the answers

How does the C-4 pathway enhance CO2 fixation efficiency, especially in tropical grasses?

By acting as a CO2 delivery system, transporting CO2 from mesophyll cells to bundle sheath cells, where Rubisco is shielded from O2. (B) Signup and view all the answers

In crassulacean acid metabolism (CAM) plants, what is the primary reason for separating the intake and fixation of CO2 temporally?

To reduce water loss by opening stomata only at night when temperatures are lower and humidity is higher. (A) Signup and view all the answers

During photorespiration, Rubisco catalyzes a reaction with O2 instead of CO2. What immediate effect does this have on the products of the reaction?

It produces 3-phosphoglycerate and phosphoglycolate, leading to a loss of fixed carbon as CO2. (D) Signup and view all the answers

If the $K_m$ of Rubisco for O2 is four times higher than for CO2, why does photorespiration still occur?

Because the concentration of O2 in the atmosphere is significantly higher than that of CO2. (D) Signup and view all the answers

Which of the following series of conversions occur during CAM photosynthesis at night?

CO2 + PEP → oxaloacetate → malate (A) Signup and view all the answers

Photosynthesis

The process where solar energy is captured and transformed into chemical energy, then used to create organic molecules from carbon dioxide.

CO2 Fixation

The process of converting carbon dioxide and water into sugar and oxygen using sunlight.