Plant Physiology - Long Questions (5-6 Marks)-Biology-class-11
1.Question: Explain the Cohesion-Tension-Transpiration Pull Model for water transport in plants. Why is transpiration considered a necessary evil?
Answer: The Cohesion-Tension-Transpiration Pull Model (also known as the Cohesion-Tension model or Transpiration Pull theory) is the most widely accepted explanation for the long-distance transport of water in plants, particularly in tall trees. It relies on three key properties of water and the process of transpiration:
Transpiration Pull: Water evaporates from the aerial parts of the plant, primarily through stomata in the leaves. This creates a negative pressure (tension or pull) in the xylem vessels of the leaves. As water molecules are lost from the leaf surface, they pull on adjacent water molecules, creating a continuous column of water.
Cohesion: Water molecules exhibit strong cohesive forces, meaning they stick to each other due to hydrogen bonding. This strong intermolecular attraction allows water to form an unbroken, continuous column within the xylem vessels, extending from the roots to the leaves.
Adhesion: Water molecules also adhere (stick) to the hydrophilic walls of the xylem vessels. This adhesive force helps to counteract gravity and prevents the water column from breaking. The continuous column of water, held together by cohesion and adhesion, is pulled upwards by the tension created by transpiration from the leaves. This pull is strong enough to lift water to the highest parts of even very tall trees.
Transpiration as a "Necessary Evil": Transpiration is often called a "necessary evil" because while it results in a significant loss of water (an "evil" as water is a vital resource), it is "necessary" for several crucial plant processes:
Ascent of Sap: It provides the primary driving force (transpiration pull) for the upward movement of water and dissolved minerals from the roots to the leaves.
Cooling: Evaporation of water from the leaf surface has a cooling effect, helping to regulate leaf temperature and prevent overheating, especially under direct sunlight.
Mineral Distribution: The continuous flow of water due to transpiration helps distribute mineral nutrients absorbed by the roots throughout the plant.
Maintenance of Turgor: Transpiration helps to maintain the turgidity of cells, which is essential for cell expansion, growth, and the structural integrity of the plant. Without adequate transpiration, the plant may wilt. The "evil" aspect lies in the massive water loss, as plants lose about 95% of the water they absorb through transpiration. This makes plants vulnerable to drought stress, especially in arid environments.
2.Question: Describe the process of glycolysis, highlighting its end products and the net ATP gain. Where does this process occur in a eukaryotic cell?
Answer: Glycolysis (also known as the EMP pathway, after Embden, Meyerhof, and Parnas) is the first stage of cellular respiration, common to both aerobic and anaerobic pathways. It involves the partial oxidation of glucose into two molecules of pyruvate.
Process of Glycolysis (Key Steps): Glycolysis is a ten-step process, divided into two main phases:
Preparatory/Energy-Investment Phase (Steps 1-5):
Glucose is phosphorylated twice, consuming 2 ATP molecules, to form fructose-1,6-bisphosphate.
Fructose-1,6-bisphosphate is then cleaved into two molecules of 3-carbon compounds: glyceraldehyde-3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP). DHAP is rapidly converted to GAP. So, from this point onwards, the pathway proceeds with two molecules of GAP.
Pay-off/Energy-Generating Phase (Steps 6-10):
Each molecule of GAP is oxidized and phosphorylated to 1,3-bisphosphoglycerate, producing 1 NADH per GAP (total 2 NADH for one glucose).
A phosphate group is removed from 1,3-bisphosphoglycerate, forming 3-phosphoglycerate and producing 1 ATP per molecule (Substrate-Level Phosphorylation, total 2 ATP).
Through a series of rearrangements, the molecule is converted to phosphoenolpyruvate.
Finally, a phosphate group is transferred from phosphoenolpyruvate to ADP, forming pyruvate and another ATP molecule per molecule (Substrate-Level Phosphorylation, total 2 ATP).
End Products of Glycolysis (from one glucose molecule):
2 molecules of Pyruvic acid (pyruvate): A 3-carbon compound.
2 molecules of ATP (net gain): 4 ATP produced - 2 ATP consumed = 2 ATP net gain.
2 molecules of NADH: Reduced coenzymes that carry high-energy electrons to the Electron Transport System in aerobic respiration.
Location in a Eukaryotic Cell: Glycolysis occurs exclusively in the cytoplasm (or cytosol) of the eukaryotic cell. It does not require oxygen and can proceed under both aerobic and anaerobic conditions.
3..Question: Elaborate on the different phases of plant growth. Discuss the conditions necessary for optimal plant growth.
Answer: Phases of Plant Growth: Plant growth is a quantitative and irreversible increase in size, typically described in three sequential phases at the cellular level:
Meristematic (Formative) Phase:
This phase occurs in the apical meristems of the root and shoot, as well as the vascular cambium.
Cells in this region are characterized by being small, isodiametric (equally-sized), rich in protoplasm, having a large nucleus, and possessing a dense cytoplasm with few or no vacuoles.
The primary activity here is rapid and continuous cell division (mitosis), leading to an increase in cell number.
Elongation (Enlargement) Phase:
Cells proximal to the meristematic zone enter this phase.
The most characteristic feature is a rapid and significant increase in cell size due to extensive vacuolation (formation of a large central vacuole).
New cell wall material is deposited, and protoplasmic growth also occurs. This phase contributes significantly to the increase in length of the plant organ.
Maturation (Differentiation) Phase:
Cells further away from the apex, reaching their maximum size, enter the maturation phase.
During this phase, cells undergo differentiation, meaning they develop specialized structures (thickenings in cell walls, changes in cell shape) and acquire specific functions.
For example, cells might differentiate into xylem vessels for water conduction, phloem sieve tubes for food transport, epidermal cells for protection, or parenchyma cells for storage.
Conditions Necessary for Optimal Plant Growth: Plant growth is influenced by a combination of internal (hormonal) and external (environmental) factors. Optimal growth requires suitable conditions for all essential processes:
Water: Essential for photosynthesis, turgor maintenance, cell elongation, and as a solvent for nutrients. Water potential directly impacts growth.
Nutrients: Plants require both macronutrients (e.g., N, P, K, Ca, Mg, S) and micronutrients (e.g., Fe, Mn, Cu, Zn, B, Mo, Cl) in appropriate quantities for synthesis of protoplasm, enzymes, and structural components.
Optimal Temperature: Each plant species has an optimal temperature range for growth. Temperatures too low inhibit enzyme activity, while too high can denature enzymes and lead to desiccation.
Oxygen: Required for cellular respiration, which provides the energy (ATP) for growth processes. Aeration of the soil is crucial for root growth.
Light: Essential for photosynthesis, providing the energy for carbohydrate synthesis. Light intensity, quality (wavelength), and duration (photoperiod) influence growth, flowering, and development.
Gravity: Influences the direction of root (positive gravitropism) and shoot (negative gravitropism) growth.
Hormones/Growth Regulators: Endogenous plant hormones (auxins, gibberellins, cytokinins, ethylene, ABA) regulate and coordinate various aspects of growth and development.
4.Question: Describe the transport of food (sugars) in plants via the phloem, explaining the Mass Flow Hypothesis in detail.
Answer: The transport of food, primarily sugars (sucrose), in plants occurs through the phloem from regions of synthesis (source) to regions of utilization or storage (sink). This process is best explained by the Mass Flow Hypothesis (or Pressure Flow Hypothesis).
The Mass Flow Hypothesis:
Loading at the Source:
Sugars, primarily sucrose, are synthesized in the mesophyll cells of leaves (the source).
These sugars are then actively transported (requiring ATP) into the companion cells and subsequently into the sieve tube elements of the phloem. This process is called phloem loading.
As sucrose is loaded into the sieve tube, its concentration within the sieve tube elements at the source increases, lowering the water potential inside.
Osmotic Water Movement (Pressure Build-up):
Due to the lower water potential in the sieve tube elements at the source, water from the adjacent xylem vessels moves into the sieve tube elements by osmosis.
This influx of water increases the turgor pressure (hydrostatic pressure) within the sieve tube elements at the source end.
Mass Flow (Bulk Flow):
The high turgor pressure generated at the source creates a pressure gradient.
The sugar solution (phloem sap) then flows passively from the region of higher pressure (source) to the region of lower pressure (sink) through the sieve tubes. This bulk movement is known as mass flow.
Unloading at the Sink:
At the sink (e.g., roots, fruits, growing buds, storage organs), sugars are actively transported out of the sieve tube elements and into the surrounding sink cells. This is called phloem unloading.
As sugars are removed from the sieve tube, the water potential inside the sieve tube at the sink increases.
Water Recirculation:
Due to the higher water potential, water moves out of the sieve tube elements at the sink and re-enters the xylem, where it can be recycled back to the leaves for transpiration or used elsewhere.
Key Points of Mass Flow Hypothesis:
Active Loading/Unloading: The movement of sugars into and out of the sieve tubes at source and sink is active (requires energy).
Passive Bulk Flow: The actual movement of sap within the sieve tubes is passive, driven by pressure differences.
Source-Sink Relationship: The direction of flow can be bidirectional, but always from a source (sugar-producing or releasing region) to a sink (sugar-consuming or storing region).
This hypothesis explains how sugars are efficiently translocated over long distances within the plant, meeting the metabolic demands of various tissues.
5.Question: Discuss the different types of plant growth regulators (PGRs) with an example of a specific physiological role for each.
Answer: Plant Growth Regulators (PGRs), also known as plant hormones or phytohormones, are small, simple molecules of diverse chemical composition that regulate various physiological processes in plants. They are typically produced in one part of the plant and transported to another part where they exert their effects. There are five major groups of PGRs:
Auxins:
Discovery: First discovered from human urine by F.W. Went (isolated from oat coleoptile tips). Indole-3-acetic acid (IAA) is the most common natural auxin.
Physiological Role:
Cell Elongation: Promote the elongation of cells, particularly in shoots, leading to primary growth.
Apical Dominance: The presence of the apical bud inhibits the growth of lateral (axillary) buds. Auxins produced in the apical meristem are responsible for this.
Root Initiation: Promote root formation in stem cuttings.
Fruit Development: Involved in the initiation of fruits and preventing fruit drop.
Herbicide: Synthetic auxins (e.g., 2,4-D) are used as herbicides to kill dicot weeds.
Gibberellins (GAs):
Discovery: Discovered from a fungal disease (bakanae disease of rice) caused by Gibberella fujikuroi. Over 100 gibberellins (GA1, GA2, GA3, etc.) are known, with GA3 being the most common and intensively studied.
Physiological Role:
Stem Elongation: Cause dramatic increases in stem length, particularly in genetically dwarf plants and in bolting (internode elongation) of rosette plants.
Seed Germination: Break seed dormancy and promote germination.
Fruit Growth: Lead to an increase in the size of fruits like grapes and apples.
Malting: Speed up the malting process in the brewing industry.
Cytokinins
:
Discovery: Identified as a cell division promoting substance. Kinetin was first discovered from herring sperm DNA. Zeatin is a natural cytokinin found in corn kernels and coconut milk.
Physiological Role:
Cell Division: Primary function is to promote cytokinesis (cell division).
Breaking Apical Dominance: Promote the growth of lateral buds, counteracting the effect of auxins.
Delaying Senescence: Delay the aging (senescence) of leaves and other organs.
Nutrient Mobilization: Help in the mobilization of nutrients.
Morphogenesis in Tissue Culture: Essential for shoot initiation and development in plant tissue culture.
Ethylene:
Discovery: A gaseous hormone, discovered as a volatile substance that causes fruit ripening.
Physiological Role:
Fruit Ripening: The most widely known effect is promoting the ripening of fruits (climacteric fruits like bananas, mangoes).
Senescence and Abscission: Promotes senescence (aging) and abscission (shedding) of leaves, flowers, and fruits.
Horizontal Growth of Seedlings: Causes horizontal growth of seedlings, swelling of the axis, and apical hook formation in dicot seedlings.
Flower Senescence: Promotes the fading of flowers.
Abscisic Acid (ABA):
Discovery: Initially isolated as dormin and abscisin II.
Physiological Role:
Stress Hormone: Often called the 'stress hormone' because its synthesis increases under adverse environmental conditions (drought, salinity).
Stomatal Closure: Induces stomatal closure to reduce water loss during drought stress.
Seed Dormancy: Promotes and maintains seed dormancy, ensuring seeds germinate only under favorable conditions.
Inhibitor of Growth: Generally acts as a growth inhibitor, counteracting the effects of growth-promoting hormones.
Abscission: Promotes abscission of leaves and fruits (though ethylene is also involved).
These five types of PGRs often act synergistically or antagonistically to regulate the complex processes of plant growth and development throughout the plant's life cycle.
