Class 11 Biology Chapter 11 Photosynthesis In Higher Plants

Key Concept: Calvin Cycle

c) It fixes carbon dioxide

[Solution Description]

RuBP, or ribulose bisphosphate, plays a crucial role as a substrate in the Calvin cycle. It reacts with CO$$_2$$ to form two molecules of PGA (3-phosphoglycerate) through an enzyme-driven process known as carbon fixation. This reaction is essential for the continuation of the Calvin cycle.

Key Concept: Experiment Analysis

c) Inability to receive light

[Solution Description] The purpose of covering a part of the leaf with foil is to block light, preventing photosynthesis in that area. Without light, the chlorophyll in the covered part cannot perform its function of converting CO_2 and water into glucose and oxygen. As a result, no starch is produced. Additionally, the presence of KOH indicates absorption of CO_2, further impairing photosynthesis. This demonstrates the necessity of light and CO_2 in the process.

Key Concept: Basic Concept

d) To carry electrons between complexes

[Solution Description] Cytochromes function as electron carriers within the electron transport chain. They facilitate the transfer of electrons from one molecule to another, aiding the process of energy conversion.

Key Concept: Electron Transport

c) Electron transport chain

[Solution Description] In the Z scheme of photosynthesis, electrons excited by light in Photosystem II travel through an electron transport chain before reaching Photosystem I. The primary function of this chain is to facilitate the transfer of electrons between the two systems.

Key Concept: Basic Concept

b) Provide electrons for PS II

[Solution Description] Water splitting in PS II provides electrons to replace those transferred from chlorophyll a during the electron transport chain, and as a result, oxygen is released as a byproduct.

Key Concept: Accessory Pigments

c) Chlorophyll b

[Solution Description]

Accessory pigments such as chlorophyll b, xanthophylls, and carotenoids do not directly participate in photosynthesis like chlorophyll a but help by capturing light energy and transferring it to chlorophyll a. Among the options provided, chlorophyll b serves this role.

Key Concept: Enzyme Function

c) PEP carboxylase

[Solution Description] In C$$_4$$ plants, the primary enzyme responsible for fixing CO$$_2$$ in the mesophyll cells is PEP carboxylase (PEPcase). This enzyme catalyzes the reaction where phosphoenol pyruvate (PEP) accepts CO$$_2$$, forming oxaloacetic acid (OAA), a four-carbon compound.

Key Concept: Comparative Analysis

c) Green sulfur bacteria – $H_2S$

[Solution Description] In green plants, the hydrogen donor is $H_2O$, and it is oxidized to release $O_2$. In some bacteria, such as purple sulfur bacteria, the hydrogen donor is $H_2S$, which gets oxidized to produce sulfur or sulfate as a by-product.

Key Concept: Basic Concept

b) To produce glucose

[Solution Description] The primary purpose of photosynthesis in green plants is to produce glucose, which serves as an energy source for the plant. Photosynthesis converts carbon dioxide and water into glucose and oxygen using light energy.

Key Concept: Basic Definition

d) ATP

[Solution Description] In cyclic photophosphorylation, electrons are cycled back to PS I and ATP is synthesized as a result. This process does not produce NADPH or oxygen.

Key Concept: Chlorophyll Amount

c) Amount of chlorophyll

[Solution Description] Chlorophyll amount determines the capacity of a plant to absorb light, which is crucial for photosynthesis.

Key Concept: Basic Definition

b) Synthesis of ATP from ADP and inorganic phosphate in the presence of light

[Solution Description] Photophosphorylation is the process in which ATP is synthesized from ADP and inorganic phosphate using energy derived from light, typically occurring during photosynthesis.

Key Concept: Glucose Storage Mechanism, Chloroplasts and Photosynthesis

c) Starch is more stable and less soluble than glucose, making it suitable for storage.

[Solution Description]

Julius von Sachs discovered that glucose is typically stored as starch because starch is less soluble and more stable than glucose, making it an ideal form for long-term storage. Chloroplasts, where chlorophyll resides, are the sites of photosynthesis. They convert light energy into chemical energy, producing glucose, which can then be polymerized into starch for storage. This storage mechanism ensures that plants have a reserve of energy-rich compounds.

Key Concept: ATP Utilization

c) Regeneration

[Solution Description] In the Calvin cycle, ATP is utilized in the regeneration phase to phosphorylate compounds and regenerate RuBP, ensuring the cycle can continue.

Key Concept: Enzyme Function

b) RuBisCO

[Solution Description] The enzyme responsible for catalyzing the reaction that leads to the formation of phosphoglycolate during photorespiration is RuBisCO. During this process, RuBisCO oxygenates ribulose-1,5-bisphosphate (RuBP) instead of carboxylating it.

Key Concept: Reaction Center Role

a) By absorbing light and transferring excited electrons to the electron acceptor

[Solution Description]

The reaction center chlorophyll in photosystem II, known as P680, absorbs light energy which excites electrons to a higher energy state. These high-energy electrons are then passed along the electron transport chain starting from an electron acceptor and eventually moving towards photosystem I. This process facilitates the flow of electrons and helps in creating a proton gradient used for ATP synthesis.

Key Concept: Experimental Design, Critical Thinking

c) The consistency of the light source and intensity.

[Solution Description] Designing a modern experiment to test plant-induced air purification involves simulating Priestley’s setup while considering variables such as light intensity, carbon dioxide levels, and temperature. Among these, controlling the light source is crucial because, according to Ingenhousz’s findings, plants need adequate sunlight to carry out photosynthesis and release oxygen. By keeping the light conditions consistent across different setups, one ensures that variations in air purification can be attributed to other tested environmental factors.

Key Concept: Basic Concept

b) Photosystem II (PS II)

[Solution Description] Water splitting is directly associated with Photosystem II (PS II). This process provides electrons for the electron transport chain.

Key Concept: Internal Factors

c) Internal CO$_2$ concentration

[Solution Description] Internal factors include conditions within the plant such as mesophyll cells, chloroplasts efficiency, and internal CO$_2$ concentration. These factors influence the plant’s ability to photosynthesize effectively.

Key Concept: Comparative Analysis

b) C4 plants require more ATP due to the additional steps in the Hatch-Slack pathway.

[Solution Description] In C3 plants, only the Calvin cycle occurs, requiring 18 ATP and 12 NADPH per glucose molecule. In C4 plants, an additional set of reactions, known as the Hatch-Slack pathway, is used for carbon fixation which requires extra ATP, making their total ATP demand higher compared to C3 plants.

Key Concept: Process Sequence

b) PS II $\rightarrow$ Electron transport chain $\rightarrow$ PS I $\rightarrow$ NADP\textsuperscript{+}

[Solution Description] The Z scheme involves the flow of electrons starting from Photosystem II (PS II), then moving through the electron transport chain to Photosystem I (PS I), and finally reducing NADP\textsuperscript{+} to form NADPH. Therefore, the correct sequence is PS II $\rightarrow$ Electron transport chain $\rightarrow$ PS I $\rightarrow$ NADP\textsuperscript{+}.

Key Concept: Long-term Effects, Real-World Application

c) Altered leaf anatomy reducing water loss

[Solution Description]

Chronic water stress leads to adaptations such as altered leaf anatomy, including thicker cuticles or smaller leaves, to minimize water loss. These changes reduce surface area for gas exchange, limiting photosynthetic capacity but allowing survival under limited water availability.

Key Concept: Enzyme Activity

c) Carboxylation and oxygenation

[Solution Description] RuBisCO exhibits dual activity: it acts as a carboxylase during the carboxylation process where CO2 is fixed into an organic molecule, and as an oxygenase when it binds to O2 instead of CO2.

Key Concept: Basic Concept

c) To capture light energy and split water molecules, releasing oxygen

[Solution Description] The primary role of Photosystem II is to absorb light energy which excites electrons and leads to their transfer to an acceptor molecule. This process also involves the splitting of water molecules, which provides electrons to replace those lost by PS I and results in the release of oxygen.

Key Concept: Electron Transport

a) Proton gradient

[Solution Description] The electrons moving through the electron transport chain lead to the pumping of protons across the thylakoid membrane, creating a proton gradient.

Key Concept: Transport Process

d) Bundle sheath cells

[Solution Description] C$$_4$$ compounds such as malic acid, formed in the mesophyll cells, are transported to the bundle sheath cells. Here, they release CO$$_2$$ for the Calvin cycle.

Key Concept: Chloroplast Alignment

b) During low light intensity

[Solution Description] Chloroplasts in mesophyll cells align themselves parallel to the cell wall during low light intensity to maximize light absorption, thus enhancing photosynthesis efficiency by exposing maximum surface area to available light.

Key Concept: Light Intensity

c) It increases initially and then saturates

[Solution Description] At low light intensities, the rate of photosynthesis increases linearly with an increase in light intensity. However, as the light intensity continues to increase and approaches saturation point (10\% of full sunlight), the rate no longer increases significantly.

Key Concept: Comparison

b) Both spectra have peaks primarily in blue and red regions.

[Solution Description] The action spectrum closely resembles the absorption spectrum of chlorophyll a and b as both show peaks in the blue and red regions of the spectrum, indicating these are the most effective wavelengths for photosynthesis.

Key Concept: Photorespiration Pathway

b) Phosphoglycerate and Phosphoglycolate

[Solution Description] During photorespiration in C3 plants, RuBisCO catalyzes the reaction between RuBP and O$$_2$$, leading to the formation of phosphoglycerate and phosphoglycolate. This process does not result in sugar production or ATP synthesis but instead utilizes ATP, making it energetically costly. Therefore, the direct products of this reaction are phosphoglycerate and phosphoglycolate.

Key Concept: Pathway Comparison

c) It takes place in the bundle sheath cells rather than mesophyll cells

[Solution Description] In C₄ plants, the Calvin cycle takes place in the bundle sheath cells where RuBisCO operates under high CO₂ concentration due to decarboxylation processes. In C₃ plants, this cycle occurs directly in the mesophyll cells without such compartmentalization or prior CO₂ concentration mechanisms.

Key Concept: ATP Role

b) It provides energy for phosphorylation.

[Solution Description] ATP provides the energy needed for phosphorylation reactions in the reduction phase. Specifically, it converts 3-phosphoglycerate into glycerate-1,3-bisphosphate.

Key Concept: Pigment Function

c) Absorb light and transfer energy to chlorophyll a

[Solution Description] Accessory pigments such as carotenoids and xanthophylls absorb additional wavelengths of light and transfer the energy to chlorophyll a for use in photosynthesis.

Key Concept: Enzyme Function

d) It fixes CO2 by forming oxaloacetate from PEP.

[Solution Description]

PEP carboxylase is responsible for the initial CO2 fixation in mesophyll cells of C4 plants. It catalyzes the reaction where phosphoenol pyruvate (PEP) reacts with CO2 to form oxaloacetate, which is then converted to C4 acids like malic acid or aspartic acid.

Key Concept: Synthesis of Ideas, Complex Reactions

b) They solidify the idea that chlorophyll absorbs light and facilitates glucose and oxygen production.

[Solution Description] Priestley’s work demonstrated that plants restore air quality by producing oxygen. Ingenhousz showed that light is necessary for this process, implying sunlight’s role in photosynthesis. Sachs further clarified this by demonstrating glucose production and identifying chlorophyll within chloroplasts as essential for capturing light energy. Collectively, these findings highlight chlorophyll’s critical function in absorbing light to drive the conversion of carbon dioxide and water into glucose and oxygen, central to photosynthesis.

Key Concept: Light Quality vs. Intensity, Limiting Factor Dynamics

c) The light intensity is too low for saturation.

[Solution Description] Light saturation for photosynthesis usually occurs at 10\% of full sunlight intensity. If a plant is exposed to light at only 5\%, it’s below the threshold needed to saturate the photosystems fully. Even though temperature and CO$$_2$$ levels are optimal, insufficient light limits the electron transport chain’s effectiveness and ultimately the ATP and NADPH production required for the Calvin Cycle, preventing maximum photosynthesis efficiency.

Key Concept: Phase Identification

b) Reduction

[Solution Description]

ATP and NADPH are used in the reduction phase of the Calvin cycle to convert 3-phosphoglyceric acid into glyceraldehyde-3-phosphate. Hence, the answer is Reduction.

Key Concept: CO₂ Concentration Basics

b) 0.03% – 0.04%

[Solution Description]

The atmospheric concentration of CO₂ typically ranges between 0.03% and 0.04%. This concentration is essential for understanding how CO₂ levels can limit photosynthesis.

Key Concept: Calvin Cycle Continuation

b) It continues for some time and then stops

[Solution Description] The Calvin cycle continues for some time after light becomes unavailable because it relies on the ATP and NADPH produced during the light reactions. However, once these stored products are exhausted, the cycle will stop.

Key Concept: Real-world Application, Comparative Analysis

a) Utilizing LED lights to mimic sunlight spectrum for indoor farming.

[Solution Description] Ingenhousz’s demonstration that sunlight is necessary for oxygen evolution can guide modern agriculture by emphasizing controlled light exposure for optimized plant growth. By combining this understanding with current techniques like artificial lighting and genetic modification, practitioners can enhance photosynthetic rates and crop yield.

Key Concept: Product Formation

b) Two

[Solution Description] During the carboxylation phase of the Calvin cycle, one molecule of CO$$_2$$ reacts with ribulose bisphosphate (RuBP) to form an unstable 6-carbon compound that quickly splits into two molecules of 3-phosphoglycerate (3-PGA).

Key Concept: Photosystem Identification

b) PS II

[Solution Description] Photosystem II is characterized by the presence of the P680 chlorophyll molecule, which is responsible for capturing photons used in the initial phase of the light reaction.

Key Concept: Redox Potential

d) Electrons move downhill from a higher to a lower redox potential.

[Solution Description] In the electron transport system, electrons are excited in Photosystem II (PS II) by absorbing light at 680 nm. These high-energy electrons move through an electron transport chain, losing energy as they transfer to molecules with a greater redox potential until they reach Photosystem I (PS I). In PS I, electrons absorb additional light energy at 700 nm before continuing along the Z-scheme.

Key Concept: Pathway Identification

c) Phosphoglycerate and Phosphoglycolate

[Solution Description] In C3 plants, during photorespiration, RuBP binds with O2 instead of CO2, leading to the formation of phosphoglycerate and phosphoglycolate as products.

Key Concept: Photosystem Function

c) Both PS I and PS II

[Solution Description] Non-cyclic photophosphorylation involves both Photosystem II (PS II) and Photosystem I (PS I). Electrons flow from water through PS II to PS I and finally reduce NADP+ to NADPH.

Key Concept: Molecule Count

c) 3

[Solution Description] For each CO2 molecule fixed, 3 ATP molecules are used in the Calvin Cycle.

Key Concept: CO2 Concentration

d) CO2 concentration

[Solution Description] CO2 concentration is considered a major limiting factor for photosynthesis because its availability directly affects the photosynthetic rate. In many environments, increasing the amount of CO2 can significantly enhance photosynthesis until other factors become limiting.

Key Concept: Enzyme Activity, Energy Calculation

d) It increases energy demand to 24 ATP and 15 NADPH due to recycling.

[Solution Description] When the concentration of oxygen is high, RuBisCO’s oxygenation activity increases, leading to photorespiration instead of photosynthesis. This process reduces efficiency as it consumes O$_2$ and releases CO$_2$, without producing sugar. Normally, the Calvin Cycle requires 18 ATP and 12 NADPH for glucose synthesis; however, with increased photorespiration, more ATP and reducing power are needed to recycle glycolate produced during photorespiration back into the cycle.

Key Concept: Chief Pigment

d) Chlorophyll a

[Solution Description] The chief pigment responsible for photosynthesis is chlorophyll a. It plays a central role in capturing light energy and converting it into chemical energy.

Key Concept: Light Reaction Components

b) ATP and NADPH

[Solution Description] The light reactions in photosynthesis occur in the chloroplasts and produce ATP and NADPH, which are used in the Calvin cycle to fix CO$_2$ into carbohydrates.

Key Concept: Photosynthesis Equation

a) $6CO_2 + 6H_2O \xrightarrow{Light} C_6H_{12}O_6 + 6O_2$

[Solution Description] The empirical equation for photosynthesis represents how carbon dioxide and water are transformed into glucose and oxygen in the presence of light. It can be represented as:

$6CO_2 + 6H_2O \xrightarrow{Light} C_6H_{12}O_6 + 6O_2$

In this equation, six molecules of carbon dioxide (CO\textsubscript{2}) react with six molecules of water (H\textsubscript{2}O) to produce one molecule of glucose (C\textsubscript{6}H\textsubscript{12}O\textsubscript{6}) and six molecules of oxygen (O\textsubscript{2}).

Key Concept: Proton Movement

a) Thylakoid lumen

[Solution Description] During electron transport in photosynthesis, protons are moved from the stroma to the thylakoid lumen, creating a proton gradient across the thylakoid membrane.

Key Concept: Temperature Range

d) 30°C – 40°C

[Solution Description] C4 plants are well adapted to higher temperatures compared to C3 plants. They have an optimal photosynthetic activity at a temperature range typically between 30°C and 40°C.

Key Concept: Photosynthesis Equation

d) Oxygen

[Solution Description] In the given equation, $$6O_2$$ represents oxygen, which is released into the atmosphere as a by-product of photosynthesis.

Key Concept: Absorption Peaks

c) 680 nm

[Solution Description] Chlorophyll a shows maximum absorption at 680 nm in the red region. This is a characteristic peak for chlorophyll a, associated with its efficiency in capturing light energy for photosynthesis.

Key Concept: Accessory Pigment Function, Light Absorption

b) By expanding the range of wavelengths absorbed

[Solution Description] Accessory pigments, such as carotenoids and phycobilins, play an essential role in enhancing photosynthetic efficiency by broadening the spectrum of light that can be absorbed. Chlorophyll primarily absorbs blue and red wavelengths, while accessory pigments can absorb additional portions of the visible spectrum, capturing energy that would otherwise go unused. By transferring this captured energy to chlorophyll, they maximize the overall absorption of light, leading to improved photosynthetic output.

Key Concept: Energy Transfer

c) They absorb light and pass energy to chlorophyll a.

[Solution Description] Carotenoids absorb light and transfer this energy to chlorophyll a. This process is essential because it allows for a broader range of light wavelengths to be used in photosynthesis, enhancing the efficiency of energy capture.

Key Concept: CO₂ and Light Interaction

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description] The assertion states that C4 plants have a higher rate of photosynthesis at lower CO₂ concentrations, which is true because C4 plants are more efficient in fixing CO₂ even when it is scarce. The reason states that C3 plants have a higher saturation point for CO₂ than C4 plants, which is also true as C3 plants saturate beyond 450 µL/L while C4 plants saturate at about 360 µL/L. However, the reason does not explain why C4 plants have a higher rate of photosynthesis at low CO₂ concentrations. Instead, it’s due to their different mechanisms of CO₂ fixation.

Key Concept: Enzyme Affinity, Photorespiration Impact

c) It would increase photorespiration, reducing CO₂ fixation efficiency.

[Solution Description] As RuBisCO can act both as a carboxylase and an oxygenase, an increase in O₂ concentration relative to CO₂ would enhance its oxygenase activity. This leads to increased binding of O₂ instead of CO₂. In C3 plants, this results in higher rates of photorespiration, reducing overall CO₂ fixation efficiency.

Key Concept: Enzyme Comparison

b) PEPcase is in mesophyll cells, RuBisCO in bundle sheath cells

[Solution Description] In C₄ plants, PEPcase is responsible for the initial fixation of CO₂ into oxaloacetic acid in mesophyll cells, while RuBisCO is located in bundle sheath cells where it facilitates the Calvin cycle by acting primarily as a carboxylase.

Key Concept: Kranz Anatomy Function

c) To concentrate CO2 around RuBisCO

[Solution Description]

The primary function of Kranz anatomy in C4 plants is to concentrate CO2 around the enzyme RuBisCO, reducing photorespiration. This structural adaptation involves bundle sheath cells surrounding the vascular bundles with a high concentration of chloroplasts, making them impervious to gaseous exchange and enabling efficient photosynthesis.

Key Concept: Wavelength and Photosynthesis

c) Blue and red

[Solution Description] Chlorophyll a has two primary absorption peaks: one in the blue region around 430 nm and another in the red region around 662 nm. These peaks correspond to the areas where photosynthesis is most efficient because they match the action spectrum of photosynthesis.

Key Concept: Photosynthesis Requirement

c) ATP

[Solution Description] ATP is produced in the light reactions and used in the Calvin cycle to provide energy for sugar synthesis.

Key Concept: Enzyme Affinity

b) Relative concentration of CO$_2$ and O$_2$

[Solution Description] The enzyme RuBisCO can catalyze the reaction of both CO$_2$ and O$_2$ with ribulose bisphosphate (RuBP). The relative concentration of these gases in the leaf determines which molecule RuBisCO will more likely bind to. Higher concentration of CO$_2$ favors carboxylation, while higher concentration of O$_2$ promotes oxygenation.