IGNOU BBYCT-137 Solved Question Paper PDF Download

IGNOU BBYCT-137 Previous Year Solved Question Paper in Hindi

IGNOU BBYCT-137 Previous Year Solved Question Paper in English

Q1. (a) State whether the following statements are True or False : (i) Phosphoenol Pyruvate (PEP) generates NADPH in place of NADP. (ii) The element sodium plays a key role in maintaining the turgor of plant cells. (iii) RuBP has a higher affinity for CO₂ than PEP. (iv) Ethylene is a gaseous hydrocarbon. (b) Fill in the blanks : (i) The upward movement of water is known as ______. (ii) Sugars are produced in ______ of C₄ plants. (iii) ______ protects nitrogenase against oxygen sensitivity in leguminous plants. (iv) ______ is the most widely distributed naturally occurring auxin. (c) Match the items given under Column A with those given in Column B: Column A (i) Coenzyme A, (ii) Haem, (iii) Sumner, (iv) Cobalt; Column B (1) Nitrate reductase, (2) Urease, (3) Catalase, (4) Vitamin derived coenzyme

Ans. (a) True or False:

• (i) False . Phosphoenol Pyruvate (PEP) does not directly generate NADPH. In C₄ plants, PEP’s role is to fix CO₂ to form oxaloacetate. Oxaloacetate can be converted to malate, and during the decarboxylation of malate, NADPH is generated. So, the process is indirect.
• (ii) False . The key element in maintaining the turgor of most plant cells, especially guard cells, is Potassium (K⁺) .
• (iii) False . The enzyme PEP carboxylase, which uses PEP, has a much higher affinity for CO₂ (as bicarbonate, HCO₃⁻) than the enzyme RuBisCO, which uses RuBP.
• (iv) True . Ethylene (C₂H₄) is a simple hydrocarbon that exists as a gas at biological temperatures and functions as a plant hormone.

(b) Fill in the blanks:

• (i) The upward movement of water is known as ascent of sap .
• (ii) Sugars are produced in bundle sheath cells of C₄ plants.
• (iii) Leghaemoglobin protects nitrogenase against oxygen sensitivity in leguminous plants.
• (iv) Indole-3-acetic acid (IAA) is the most widely distributed naturally occurring auxin.

(c) Match the columns:

• (i) Coenzyme A – (4) Vitamin derived coenzyme
• (ii) Haem – (3) Catalase
• (iii) Sumner – (2) Urease
• (iv) Cobalt – (1) Nitrate reductase ( Note: This match is by elimination; biologically, nitrate reductase contains molybdenum, not cobalt. The question may be flawed. )

Q2. Differentiate between the following pairs of terms : (a) Diffusion and Osmosis (b) C₃ and C₄ plants (c) Competitive and Non-competitive inhibition (d) Transpiration and Guttation

Ans. (a) Diffusion and Osmosis

• Diffusion: It is the net movement of any substance (solid, liquid, or gas) from an area of higher concentration to an area of lower concentration. It does not require a semi-permeable membrane.
• Osmosis: It is a special type of diffusion involving the movement of a solvent (usually water) across a semi-permeable membrane from a region of high solvent concentration (low solute concentration) to a region of low solvent concentration (high solute concentration).

(b)

C₃ and C₄ plants

• C₃ plants: The first stable product of carbon fixation is a 3-carbon compound (3-Phosphoglycerate). CO₂ is fixed by the enzyme RuBisCO. They exhibit a high rate of photorespiration. Examples include rice, wheat, and soybeans.
• C₄ plants: The first stable product is a 4-carbon compound (Oxaloacetate). They have a special ‘Kranz anatomy’. CO₂ is first fixed by PEP carboxylase in the mesophyll cells and then transported to bundle sheath cells where it is re-fixed by RuBisCO. Photorespiration is negligible, making them more efficient in hot, dry conditions. Examples include maize, sugarcane, and sorghum.

(c)

Competitive and Non-competitive inhibition

• Competitive inhibition: The inhibitor molecule resembles the substrate and competes for the same active site on the enzyme. This inhibition can be overcome by increasing the substrate concentration. It increases the Km but does not change the Vmax.
• Non-competitive inhibition: The inhibitor binds to the enzyme at a site other than the active site (an allosteric site). This binding alters the enzyme’s shape and reduces its activity. It cannot be overcome by increasing substrate concentration. It decreases the Vmax but does not change the Km.

(d)

Transpiration and Guttation

• Transpiration: It is the loss of water from the plant in the form of water vapor , primarily through stomata on the leaves. It occurs mainly during the day and is a regulated process. The water lost is pure.
• Guttation: It is the loss of water in the form of liquid droplets from the margins of leaves through special pores called hydathodes. It typically occurs during the night or early morning when humidity is high and transpiration is low. The exuded water contains dissolved salts and sugars. It is not a regulated process.

Q3. (a) Explain water potential and its components. (b) Give a brief account on the photorespiration process.

Ans. (a) Water Potential and its Components Water potential (Ψw) is a fundamental concept in plant physiology that measures the potential energy of water in a system relative to pure, free water at the same temperature and pressure. It determines the direction of water movement; water always moves from an area of higher water potential to an area of lower water potential. It is expressed in units of pressure, such as megapascals (MPa). The total water potential is the sum of its component potentials:

• Solute Potential (Ψs): Also known as osmotic potential, it accounts for the effect of dissolved solutes. Solutes reduce the free energy of water, so the addition of solutes always lowers the water potential. Therefore, Ψs is always a negative value. The more solutes, the more negative the Ψs.
• Pressure Potential (Ψp): This component represents the physical hydrostatic pressure on the water. In a plant cell, this is the turgor pressure exerted by the cell content against the cell wall, which is typically positive . In the xylem, tension can lead to a negative pressure potential.
• Matric Potential (Ψm): This reflects the adhesion of water to surfaces, such as soil particles or cell walls. In fully hydrated plant cells, it is usually negligible and often omitted from the equation.

The relationship is summarized by the equation:

Ψw = Ψs + Ψp

(b) The Photorespiration Process Photorespiration is a light-dependent metabolic pathway that occurs in C₃ plants when the enzyme RuBisCO acts on oxygen (O₂) instead of carbon dioxide (CO₂). This “wasteful” process typically occurs under conditions of high temperature, high light intensity, and a high O₂ to CO₂ ratio, often when stomata close to conserve water. The process involves three organelles: the chloroplast, peroxisome, and mitochondrion.

1. Chloroplast: RuBisCO catalyzes the oxygenation of Ribulose-1,5-bisphosphate (RuBP), producing one molecule of 3-phosphoglycerate (3-PGA), which can enter the Calvin cycle, and one molecule of a 2-carbon compound, 2-phosphoglycolate.
2. Peroxisome: The 2-phosphoglycolate is dephosphorylated to glycolate in the chloroplast, which then moves to the peroxisome. Here, glycolate is oxidized to glyoxylate, producing hydrogen peroxide (H₂O₂), which is detoxified by catalase. Glyoxylate is then converted to the amino acid glycine.
3. Mitochondrion: Two molecules of glycine are transported into the mitochondrion. They are converted into one molecule of the amino acid serine, releasing one molecule of CO₂ and one molecule of ammonia (NH₃) in the process.

Photorespiration is considered wasteful because it consumes ATP and NADPH, and it releases previously fixed carbon as CO₂, thereby reducing the overall efficiency of photosynthesis by as much as 25%. However, it may play a role in protecting the plant from photo-oxidative damage under high light stress.

Q4. (a) Give a detailed account on the structure and functions of Photosystem-II. (b) Explain the chemiosmatic model mechanism for ATP synthesis.

Ans. (a) Structure and Functions of Photosystem II (PSII) Photosystem II is a large multi-protein complex embedded in the thylakoid membranes of chloroplasts, primarily in the grana stacks. It is the first complex in the light-dependent reactions of photosynthesis. Structure:

• Reaction Center Core: At its heart is the reaction center, which contains a special pair of chlorophyll a molecules known as P680 because they best absorb light at a wavelength of 680 nm. It also includes primary electron acceptors (like pheophytin) and is surrounded by the D1 and D2 proteins.
• Light-Harvesting Complex (LHCII): Surrounding the core is an antenna system composed of chlorophyll a, chlorophyll b, and carotenoids. These pigments capture light energy and funnel it to the P680 reaction center.
• Oxygen-Evolving Complex (OEC): Located on the lumenal side of the thylakoid membrane, this complex contains a cluster of manganese (Mn), calcium (Ca²⁺), and chloride (Cl⁻) ions. It is responsible for splitting water molecules.

Functions:

1. Light Absorption: The antenna pigments of LHCII absorb photons, and the excitation energy is transferred to the P680 reaction center.
2. Charge Separation: The energy excites P680, causing it to donate an electron to a primary electron acceptor, pheophytin. This leaves P680 in an oxidized, highly reactive state (P680⁺).
3. Water Splitting (Photolysis): To replace its lost electron, the powerful oxidant P680⁺ extracts an electron from water. The OEC catalyzes this reaction: 2H₂O → 4H⁺ + 4e⁻ + O₂ . This process releases electrons for the transport chain, protons into the thylakoid lumen, and is the source of nearly all atmospheric oxygen.
4. Electron Transport: The energized electron from P680 is passed along an electron transport chain (to plastoquinone, cytochrome b₆f complex, and plastocyanin) towards Photosystem I.

(b)

Chemiosmotic Model for ATP Synthesis

The chemiosmotic model, proposed by Peter Mitchell, explains how the energy released from electron transport is used to generate ATP. This mechanism is central to both photosynthesis (photophosphorylation) and cellular respiration (oxidative phosphorylation).

The mechanism involves two key steps:

1. Establishment of a Proton Motive Force: During the light-dependent reactions of photosynthesis, the movement of electrons through the electron transport chain (from PSII to PSI) provides energy to pump protons (H⁺) from the chloroplast stroma into the thylakoid lumen. Additionally, the splitting of water by PSII releases protons into the lumen, and the reduction of NADP⁺ in the stroma consumes protons. This results in the accumulation of a high concentration of H⁺ inside the lumen relative to the stroma, creating a strong electrochemical gradient known as the proton-motive force . This force consists of a chemical gradient (difference in H⁺ concentration) and an electrical gradient (charge difference across the membrane).
2. ATP Synthesis via ATP Synthase: Embedded in the thylakoid membrane is a large enzyme complex called ATP Synthase . This enzyme has two main components:
   • F₀ unit: A transmembrane channel that allows protons to flow down their electrochemical gradient from the lumen back into the stroma.
   • F₁ unit: A catalytic “knob” that protrudes into the stroma.

   The flow of protons through the F₀ channel causes the F₁ unit to rotate. This rotational energy induces a conformational change in the F₁ subunit, which drives the synthesis of
   
   ATP
   
   from ADP and inorganic phosphate (Pi). The energy stored in the proton gradient is thus converted into the chemical energy of ATP.

Q5. (a) Briefly describe the radial movement of ions in roots. (b) What is oxidative stress and how does it affect plants?

Ans. (a) Radial Movement of Ions in Roots The radial movement of ions describes their transport from the soil solution, across the root, and into the central vascular cylinder (xylem). This journey involves crossing several layers of cells: the epidermis, cortex, endodermis, and pericycle. There are two primary pathways for this movement:

1. The Apoplast Pathway: In this pathway, water and dissolved ions move through the non-living parts of the root, which include the cell walls and the intercellular spaces. This is a rapid, non-selective pathway. However, the apoplastic route is blocked at the endodermis by the Casparian strip , a band of waterproof, suberin-impregnated cell wall material.
2. The Symplast Pathway: In this pathway, ions move through the living part of the cells, from the cytoplasm of one cell to the next via intercellular connections called plasmodesmata . To enter the symplast, ions must first cross the plasma membrane of an epidermal or cortex cell, making this a selective and regulated process.

Overall Process:

Ions can travel via either pathway through the cortex. Upon reaching the endodermis, the Casparian strip forces any ions traveling in the apoplast to cross the endodermal plasma membrane and enter the symplast. This ensures that the plant has selective control over which substances enter the xylem. Once past the endodermis, ions are actively loaded from the pericycle cells into the xylem vessels for long-distance transport to the shoot.

(b)

Oxidative Stress and its Effects on Plants

Oxidative stress

is a state of physiological stress caused by an imbalance between the production of reactive oxygen species (ROS) and a plant’s ability to detoxify these reactive products or repair the resulting damage.

Reactive Oxygen Species (ROS)

are highly reactive molecules and free radicals derived from molecular oxygen. Common ROS include the superoxide radical (O₂⁻), hydrogen peroxide (H₂O₂), and the hydroxyl radical (•OH). While they are normal by-products of metabolic processes like photosynthesis and respiration, their production can dramatically increase under adverse environmental conditions such as drought, salinity, high light, and extreme temperatures.

Effects on Plants:

When the production of ROS overwhelms the plant’s antioxidant defense systems, they can cause significant damage to cellular components.

• Lipid Peroxidation: ROS attack polyunsaturated fatty acids in cell membranes, leading to a loss of membrane fluidity and integrity, causing leakage of cellular contents.
• Protein Damage: ROS can oxidize amino acids, causing proteins to lose their function. This can inactivate critical enzymes and damage structural proteins.
• DNA Damage: ROS can cause breaks in DNA strands and chemical modifications of bases, leading to mutations and compromising the plant’s genetic integrity.
• Inhibition of Photosynthesis: The photosynthetic machinery, especially Photosystem II, is highly susceptible to damage by ROS, leading to a decline in photosynthetic efficiency.

Severe oxidative stress can ultimately trigger programmed cell death and lead to the death of the plant. Plants have evolved complex antioxidant systems (both enzymatic and non-enzymatic) to scavenge ROS and mitigate this damage.

Q6. (a) Discuss the various factors that affect the enzyme activity. (b) Describe how auxins affect plant growth.

Ans. (a) Factors Affecting Enzyme Activity Enzymes are highly specific biological catalysts, and their activity is influenced by several factors:

1. Temperature: Each enzyme has an optimal temperature at which its activity is maximal. As temperature increases towards the optimum, the reaction rate increases. However, temperatures above the optimum cause the enzyme to denature (lose its three-dimensional structure), leading to a rapid loss of activity. Low temperatures decrease activity but usually do not cause irreversible damage.
2. pH: Every enzyme has an optimal pH range for its activity. Extreme pH values (highly acidic or alkaline) can alter the charge on the amino acids in the active site and disrupt the enzyme’s tertiary structure, leading to denaturation and inactivation. For instance, pepsin in the stomach works best at a very low pH (~2), while trypsin in the small intestine has an optimal pH of around 8.
3. Enzyme Concentration: Assuming the substrate is not limiting, the rate of reaction is directly proportional to the concentration of the enzyme. The more enzyme molecules available, the more active sites there are to bind with the substrate, leading to a faster reaction rate.
4. Substrate Concentration: As the concentration of the substrate increases (with a fixed enzyme concentration), the reaction rate increases until it reaches a maximum velocity (Vmax). At this point, the enzyme is said to be saturated with the substrate, and further increases in substrate concentration do not increase the reaction rate.
5. Inhibitors: The presence of certain chemical substances, called inhibitors, can reduce or block enzyme activity. These can be competitive (compete for the active site) or non-competitive (bind to an allosteric site).
6. Activators: Some enzymes require the presence of non-protein molecules called cofactors (e.g., metal ions like Mg²⁺, Zn²⁺) or coenzymes (organic molecules, often derived from vitamins) to be active.

(b)

How Auxins Affect Plant Growth

Auxins, with Indole-3-acetic acid (IAA) being the most common natural example, are a class of plant hormones that play a crucial role in regulating plant growth and development.

• Cell Elongation: This is the most well-known effect of auxins. According to the Acid Growth Hypothesis , auxin promotes cell elongation primarily in stems and coleoptiles. It stimulates proton pumps (H⁺-ATPases) in the plasma membrane, which pump protons into the cell wall region. The resulting decrease in pH activates enzymes called expansins , which loosen the bonds holding the cell wall components together. This increases wall extensibility, allowing the cell to expand under the force of turgor pressure.
• Apical Dominance: Auxin produced in the apical (terminal) bud diffuses downwards and inhibits the growth of the lateral (axillary) buds. This phenomenon ensures that the plant invests its energy in growing taller to compete for light. If the apical bud is removed, the source of auxin is eliminated, and the lateral buds begin to grow.
• Root Initiation: While high concentrations of auxin can inhibit root elongation, it is crucial for initiating the formation of lateral and adventitious roots. This property is exploited commercially, as synthetic auxins are used as rooting powders to propagate plants from cuttings.
• Tropisms: Auxin mediates plant responses to environmental stimuli, such as phototropism (growth towards light) and gravitropism (growth in response to gravity). In phototropism, unilateral light causes auxin to migrate to the shaded side of the stem, promoting greater cell elongation there and causing the stem to bend towards the light source.
• Fruit Development: Auxins are involved in the growth and development of fruits and can be used to induce the development of seedless (parthenocarpic) fruits.

Q7. Write short notes on any four of the following : (a) Phycobilins (b) Biological Nitrogen fixation (c) Synthetic Growth Hormones (d) Photoperiodism (e) Carrier proteins

Ans. (a) Phycobilins Phycobilins are water-soluble accessory photosynthetic pigments found in cyanobacteria and red algae (Rhodophyta). They are proteins that are covalently linked to light-absorbing chromophores. The entire pigment-protein complex is called a phycobiliprotein. The two main types are phycoerythrin (a red pigment) and phycocyanin (a blue pigment). These pigments absorb light in the green, yellow, and orange regions of the spectrum (approx. 500-650 nm), wavelengths that are poorly absorbed by chlorophylls. This allows these organisms to thrive in aquatic environments at depths where blue-green light penetrates. The energy captured by phycobilins is efficiently transferred to the chlorophyll a in the reaction centers of the photosystems. (b) Biological Nitrogen Fixation Biological nitrogen fixation is the process by which atmospheric nitrogen gas (N₂), which is largely inert and unavailable to most organisms, is converted into ammonia (NH₃) or related nitrogenous compounds. This vital process is carried out by a specialized group of prokaryotes called diazotrophs. The conversion is catalyzed by an enzyme complex called nitrogenase , which is extremely sensitive to oxygen. There are two main types:

• Symbiotic fixation: Bacteria like Rhizobium form nodules on the roots of leguminous plants (e.g., peas, beans), fixing nitrogen in exchange for carbohydrates. A protein called leghaemoglobin maintains the low-oxygen environment needed for nitrogenase to function.
• Non-symbiotic fixation: Performed by free-living bacteria in the soil, such as the aerobic Azotobacter and the anaerobic Clostridium , and by some cyanobacteria like Nostoc .

The process is highly energy-intensive, requiring 16 ATP molecules to fix one molecule of N₂.

(c)

Synthetic Growth Hormones

Synthetic growth hormones are man-made chemical compounds designed to mimic the effects of natural plant hormones (phytohormones). They are often used in agriculture and horticulture because they can be more potent, stable, and cost-effective than their natural counterparts. Examples include:

• Synthetic Auxins (e.g., Naphthaleneacetic acid (NAA), 2,4-Dichlorophenoxyacetic acid (2,4-D)): Used as rooting agents for cuttings, to prevent premature fruit drop, and, at high concentrations, as selective herbicides against broadleaf weeds (e.g., 2,4-D).
• Synthetic Cytokinins (e.g., Kinetin, Benzyladenine): Used in plant tissue culture to promote cell division and shoot formation, and to delay senescence in leafy vegetables and cut flowers.
• Ethephon: A compound that releases ethylene gas upon application. It is used to promote uniform fruit ripening (in tomatoes, bananas), induce flowering in pineapples, and stimulate latex flow in rubber trees.

(d)

Photoperiodism

Photoperiodism is the physiological response of plants to the relative lengths of day and night, particularly concerning the induction of flowering. Based on their photoperiodic requirements, plants are classified into three main groups:

1. Short-Day Plants (SDPs): These plants flower only when the day length is shorter than a certain critical duration. In reality, they are long-night plants, as they require an uninterrupted period of darkness that exceeds a critical length. Examples include chrysanthemums and soybeans.
2. Long-Day Plants (LDPs): These plants flower when the day length exceeds a critical duration. They are essentially short-night plants. Examples include spinach, lettuce, and wheat.
3. Day-Neutral Plants (DNPs): Flowering in these plants is not controlled by day length but by other factors such as developmental maturity or temperature. Examples include tomatoes, maize, and cucumbers.

The pigment responsible for perceiving the photoperiod is

phytochrome

.

(e)

Carrier Proteins

Carrier proteins, also known as transporters or permeases, are integral membrane proteins that facilitate the movement of specific ions and small molecules (like sugars and amino acids) across a biological membrane. Unlike channel proteins that form a simple pore, carriers operate by binding to the specific solute they transport. This binding induces a

conformational change

in the protein’s structure. This change in shape effectively moves the solute from one side of the membrane to the other, where it is released. Key characteristics of carrier-mediated transport include:

• Specificity: Each carrier protein is highly specific for the molecule(s) it transports.
• Saturation: The rate of transport reaches a maximum (Vmax) when all carrier proteins are occupied, a phenomenon similar to enzyme kinetics.

Carrier proteins are involved in both

facilitated diffusion

(movement down a concentration gradient, no energy required) and

active transport

(movement against a concentration gradient, requires energy).