Chapter 11 Flashcards

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1
Q

What are the two main stages of photosynthesis?

A

Photosynthesis consists of two main stages:

Light Reactions: These occur in the thylakoid membranes of the chloroplasts. During the light reactions, light energy is captured by chlorophyll and other pigments, which is then used to split water molecules (photolysis), releasing oxygen (O₂) as a byproduct. The energy from light is also used to produce ATP and NADPH, which are energy carriers.
Calvin Cycle (Dark Reactions): This stage occurs in the stroma of the chloroplasts. The Calvin cycle uses the ATP and NADPH produced in the light reactions to convert carbon dioxide (CO₂) into glucose (C₆H₁₂O₆) through a series of enzyme-mediated steps. This process is also known as carbon fixation.

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2
Q

What are the key processes and products of the light reactions in photosynthesis?

A

The light reactions involve several key processes:

Water Splitting (Photolysis): Water molecules are split into oxygen, protons, and electrons. The oxygen is released as a byproduct.
Light Absorption by Chlorophyll: Chlorophyll and other pigments absorb light energy, which excites electrons to higher energy levels.
Electron Transport Chain (ETC): Excited electrons are transferred through a series of proteins in the thylakoid membrane, creating a proton gradient across the membrane.
Photophosphorylation: The energy from the proton gradient is used by ATP synthase to convert ADP and inorganic phosphate (Pi) into ATP. The main products of the light reactions are:
Oxygen (O₂): Released as a byproduct of water splitting.
ATP: Provides energy for the Calvin cycle.
NADPH: Provides reducing power (electrons) for the Calvin cycle

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3
Q

Describe the Calvin Cycle and its three main phases

A

The Calvin Cycle, also known as the dark reactions or light-independent reactions, occurs in the stroma of the chloroplasts and consists of three main phases:

Carbon Fixation: CO₂ is attached to a five-carbon sugar called ribulose bisphosphate (RuBP) by the enzyme rubisco. This reaction produces a six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).
Reduction: ATP and NADPH from the light reactions are used to convert the 3-PGA molecules into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. This phase involves the reduction of 3-PGA to G3P.
Regeneration of RuBP: Some of the G3P molecules are used to regenerate RuBP, enabling the cycle to continue. This phase requires ATP and involves a series of complex reactions that rearrange the carbon skeletons of G3P to form RuBP.

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4
Q

What is the role of chlorophyll in photosynthesis?

A

Chlorophyll is the primary pigment involved in photosynthesis, located in the thylakoid membranes of chloroplasts. Its main role is to absorb light energy, particularly in the blue and red wavelengths, and convert it into chemical energy. When chlorophyll absorbs light, its electrons become excited and are transferred to the electron transport chain, initiating the light reactions. Chlorophyll a is the main pigment, while chlorophyll b and carotenoids act as accessory pigments, broadening the spectrum of light that can be used for photosynthesis.

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5
Q

Explain the significance of the absorption spectrum and action spectrum in photosynthesis.

A

Absorption Spectrum: This spectrum shows the wavelengths of light absorbed by different pigments in the chloroplast, such as chlorophyll a, chlorophyll b, and carotenoids. It indicates which wavelengths are most effectively absorbed for photosynthesis.
Action Spectrum: This spectrum profiles the relative effectiveness of different wavelengths of light in driving photosynthesis. It is determined by measuring the rate of photosynthesis (e.g., oxygen production) at different wavelengths. The action spectrum typically shows peaks corresponding to the absorption peaks of chlorophyll and other pigments, demonstrating that these wavelengths are most effective for photosynthesis.

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6
Q

What are photosystems and their components?

A

Photosystems are complexes in the thylakoid membrane that capture light energy and convert it into chemical energy. Each photosystem has two main components:

Reaction-Center Complex: Contains a special pair of chlorophyll a molecules and a primary electron acceptor. The reaction center is where the initial light-driven electron transfer occurs.
Light-Harvesting Complexes: Consist of various pigments (chlorophyll a, chlorophyll b, and carotenoids) bound to proteins. These complexes capture light energy and transfer it to the reaction center, enhancing the efficiency of light absorption.

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7
Q

Differentiate between Photosystem II (PS II) and Photosystem I (PS I)

A

Photosystem II (PS II): Contains P680 chlorophyll a molecules, which absorb light best at 680 nm. PS II functions first in the light reactions and is responsible for splitting water molecules (photolysis) to release oxygen, protons, and electrons. The electrons are then passed to the electron transport chain.
Photosystem I (PS I): Contains P700 chlorophyll a molecules, which absorb light best at 700 nm. PS I functions second in the light reactions and receives electrons from the electron transport chain. It further excites the electrons and transfers them to NADP⁺, forming NADPH.

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8
Q

What is the role of the electron transport chain in photosynthesis?

A

The electron transport chain (ETC) in photosynthesis is a series of protein complexes located in the thylakoid membrane. Its role is to transfer electrons from Photosystem II to Photosystem I, creating a proton gradient across the thylakoid membrane. As electrons move through the ETC, they release energy, which is used to pump protons from the stroma into the thylakoid space, generating a proton gradient. This gradient drives the synthesis of ATP through chemiosmosis, as protons flow back into the stroma through ATP synthase.

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9
Q

Describe the process of chemiosmosis in chloroplasts.

A

Chemiosmosis in chloroplasts involves the movement of protons (H⁺) across the thylakoid membrane, creating a proton gradient. During the light reactions, the electron transport chain pumps protons from the stroma into the thylakoid space, increasing the proton concentration inside the thylakoid. This creates a proton gradient with a higher concentration of protons inside the thylakoid than in the stroma. ATP synthase, an enzyme embedded in the thylakoid membrane, uses the energy from the flow of protons back into the stroma to convert ADP and inorganic phosphate (Pi) into ATP. This process is known as photophosphorylation.

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10
Q

What are the differences between cyclic and non-cyclic electron flow?

A

Non-Cyclic Electron Flow: Involves both Photosystem II (PS II) and Photosystem I (PS I). Electrons flow from water to NADP⁺, producing ATP and NADPH, and releasing oxygen as a byproduct. This pathway is the primary route for electron flow during the light reactions.
Cyclic Electron Flow: Involves only Photosystem I (PS I). Electrons are cycled back from ferredoxin (Fd) to the cytochrome complex and then to P700 chlorophyll in PS I. This pathway produces ATP but not NADPH and does not release oxygen. Cyclic electron flow helps balance the ATP/NADPH ratio needed for the Calvin cycle.

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11
Q

How do C4 and CAM plants minimize photorespiration?

A

C4 Plants: Use spatial separation of steps to minimize photorespiration. CO₂ is initially fixed into a four-carbon compound in mesophyll cells by the enzyme PEP carboxylase, which has a

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12
Q

What is photorespiration and why is it considered wasteful?

A

Photorespiration occurs when rubisco fixes O₂ instead of CO₂, leading to the consumption of ATP and release of CO₂ without producing sugar. It is wasteful because it reduces the efficiency of photosynthesis.

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13
Q

What are the main products of the Calvin Cycle?

A

The main product is glyceraldehyde-3-phosphate (G3P), which can be used to form glucose and other carbohydrates.

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14
Q

Explain the role of ATP and NADPH in the Calvin Cycle.

A

ATP provides the energy, and NADPH provides the reducing power needed to convert 3-phosphoglycerate into G3P during the Calvin Cycle.

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15
Q

What is the significance of the enzyme rubisco in photosynthesis?

A

Rubisco catalyzes the first step of the Calvin Cycle, the fixation of CO₂ to ribulose bisphosphate (RuBP). It is the most abundant enzyme on Earth.

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16
Q

How do environmental factors like light intensity and CO₂ concentration affect photosynthesis?

A

Increased light intensity and CO₂ concentration generally enhance the rate of photosynthesis up to a certain point, beyond which other factors may become limiting.

17
Q

What is the role of stomata in photosynthesis?

A

Stomata are pores on the leaf surface that allow CO₂ to enter and O₂ to exit. They also play a role in regulating water loss through transpiration.

18
Q

How do plants balance the need for CO₂ uptake with minimizing water loss?

A

Plants regulate the opening and closing of stomata to balance CO₂ uptake for photosynthesis with minimizing water loss through transpiration.

19
Q

What are the main differences between C3, C4, and CAM photosynthesis?

A

C3 Photosynthesis: The most common form, where CO₂ is directly fixed by rubisco in the Calvin Cycle.
C4 Photosynthesis: Involves an additional step where CO₂ is initially fixed into a four-carbon compound in mesophyll cells, then transported to bundle-sheath cells for the Calvin Cycle.
CAM Photosynthesis: CO₂ is fixed into organic acids at night and released for the Calvin Cycle during the day, allowing stomata to remain closed during the hottest part of the day.