Plant Physiology is the study of the functional mechanisms, cellular processes, and metabolic pathways that govern plant life. Plants are autotrophic transducers of solar energy, synthesizing organic compounds from carbon dioxide and water. This energy is subsequently extracted by all living organisms via cellular respiration to drive metabolic work. Plant development, growth, and stress adaptation are coordinated by specialized signaling molecules called Plant Growth Regulators (PGRs).
Clinical and Daily Life Relevance:
NEET Relevance: This unit is high-yielding, contributing 4–5 questions annually. Focus areas include the Z-scheme, cyclic vs. non-cyclic photophosphorylation, Calvin (C3) and Hatch-Slack (C4) pathways, ATP bookkeeping in aerobic respiration, Complex IV/V mechanics in the Electron Transport System, and the agricultural applications of PGRs.
By the end of this unit, you should be able to:
Before starting, ensure you have reviewed:
NEET Priority: Very High
Photosynthesis consists of two phases: the light-dependent photochemical phase (grana) and the light-independent biosynthetic phase (stroma).
NEET Priority: Critical
The biosynthetic phase fixes \( \text{CO}_2 \) into carbohydrates in the stroma.
Occurs in the stroma of all photosynthetic plants. It has three main stages:
Adapted to high temperatures, high light intensity, and dry conditions. It exhibits Kranz Anatomy:
NEET Priority: Critical
Cellular respiration extracts chemical energy from substrates, conserving it as ATP.
Occurs in the cytoplasm. It is an anaerobic process consisting of 10 enzymatic steps:
In the absence of oxygen, pyruvate is reduced to regenerate NAD\( ^+ \):
In the presence of oxygen, pyruvate enters the mitochondrial matrix.
Occurs on the inner mitochondrial membrane (cristae). Electrons from NADH and \( \text{FADH}_2 \) are transferred through a series of carrier complexes:
NEET Priority: Very High
Plant development is regulated by five major classes of signaling molecules:
We account for ATP generated through substrate-level phosphorylation (SLP) and oxidative phosphorylation (OP) via coenzyme transport:
Example 1 (Photosynthesis Stoichiometry): How many molecules of ATP and NADPH are required to produce 3 molecules of glucose in a C3 plant compared to a C4 plant? Solution:
Example 2 (Z-Scheme electron source): Explain why photolysis of water is essential for non-cyclic photophosphorylation, and state its location. Solution:
Example 3 (Chemiosmosis pH gradient): In a chloroplast, the pH of the stroma is 8.0, and the pH of the thylakoid lumen is 5.0. Calculate the difference in proton concentration \( [\text{H}^+] \) across the membrane and explain its consequence. Solution:
Example 4 (Glycolysis net ATP calculation): If a cell is treated with a drug that inhibits the enzyme triose phosphate dehydrogenase (preventing glyceraldehyde-3-phosphate oxidation), what will be the net ATP yield from glycolysis? Solution:
Example 5 (RQ of Malic Acid): Calculate the Respiratory Quotient (RQ) for the oxidation of malic acid (\( \text{C}_4\text{H}_6\text{O}_5 \)). Solution:
Example 6 (Anaerobic energy yield): Why is the energy recovery from anaerobic fermentation so low compared to aerobic respiration? Solution:
Example 7 (Hormone bioassay): A student wants to identify a plant hormone in a sample. They perform an Avena curvature test and observe a curvature of \( 15^\circ \) in oat coleoptiles. What is the hormone, and what does this test measure? Solution:
Example 8 (Antagonistic hormones): Explain the antagonistic roles of Gibberellic Acid (GA) and Abscisic Acid (ABA) in seed germination. Solution:
Example 9 (Apical dominance mechanism): How does pruning a hedge promote lateral growth? Solution:
Example 10 (Cellular states): Explain the transition of parenchymal cells during the formation of cork cambium and cork. Solution:
Example 11 (Kranz anatomy efficiency): Why do C4 plants not exhibit photorespiration at high temperatures? Solution:
Example 12 (Proton translocation ETS): How many protons are pumped across the inner mitochondrial membrane per pair of electrons transferred from NADH and \( \text{FADH}_2 \)? Solution:
The primary electron donor in non-cyclic photophosphorylation is:
** Water is photolyzed by the oxygen-evolving complex associated with PS II, serving as the ultimate electron donor to replace electrons lost by PS II.
The enzyme responsible for ATP synthesis in chloroplasts is:
** The \( \text{CF}_0\text{-CF}_1 \) ATPase complex (Complex V of the thylakoid membrane) synthesizes ATP from ADP and \( \text{P}_i \) using the proton gradient.
To produce 1 molecule of sucrose (\( \text{C}_{12}\text{H}_{22}\text{O}_{11} \)), a C3 plant requires:
** Sucrose is a disaccharide (\( \text{C}_{12}\text{H}_{22}\text{O}_{11} \)) composed of glucose and fructose. Synthesizing it requires twice the energy of a single glucose: \( 2 \times 18\ \text{ATP} = 36\ \text{ATP} \) and \( 2 \times 12\ \text{NADPH} = 24\ \text{NADPH} \).
Which of the following organelles is NOT involved in the photorespiratory pathway?
** Photorespiration involves the cooperative functioning of three organelles: chloroplasts, peroxisomes, and mitochondria. Lysosomes are not involved.
The conversion of fructose-6-phosphate to fructose-1,6-bisphosphate in glycolysis is catalyzed by:
** Phosphofructokinase (PFK) is the regulatory enzyme that phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate, consuming 1 ATP.
During the Krebs cycle, substrate-level phosphorylation occurs during the conversion of:
** Substrate-level phosphorylation in the Krebs cycle occurs when succinyl-CoA is cleaved to succinic acid, converting GDP to GTP (which is converted to ATP).
Complex IV of the mitochondrial Electron Transport System contains:
** Complex IV (Cytochrome \( c \) oxidase) contains cytochromes \( a \) and \( a_3 \) along with two copper centers.
The respiratory quotient (RQ) of tripalmitin is:
** Tripalmitin is a fat; its complete aerobic oxidation requires more oxygen relative to the \( \text{CO}_2 \) released, giving an RQ of approximately 0.7.
Which of the following hormones is commonly used to induce rooting in stem cuttings?
** Auxins (such as IBA or naphthaleneacetic acid, NAA) are applied to stem cuttings to promote root initiation.
Under conditions of water stress, which hormone mediates stomatal closure?
** Under water deficit, ABA is synthesized in leaves and acts on guard cells to close stomata, reducing water loss.
Assertion (A): Cyclic photophosphorylation does not produce NADPH or release oxygen.
Reason (R): Stroma lamellae membranes lack Photosystem II and the enzyme NADP reductase.
** Stroma lamellae lack PS II (so they cannot split water to release \( \text{O}_2 \)) and NADP reductase (so they cannot reduce NADP\( ^+ \) to NADPH). Both statements are true and R explains A.
Assertion (A): C4 plants are more productive than C3 plants under hot, dry conditions.
Reason (R): PEP Carboxylase has a high affinity for \( \text{CO}_2 \) and no affinity for \( \text{O}_2 \), allowing carbon fixation to proceed without photorespiration.
** PEPcase has high affinity for \( \text{CO}_2 \) and lacks oxygenase activity. It concentrates \( \text{CO}_2 \) inside the bundle sheath cells, suppressing RuBisCO's oxygenase activity and bypassing photorespiration. Both statements are true and R explains A.
Assertion (A): In the absence of oxygen, glycolysis yields a net of 2 ATP per glucose.
Reason (R): Fermentation regenerates NAD\( ^+ \) in the cytoplasm, allowing glycolysis to continue.
** In anaerobic conditions, fermentation reduces pyruvate to lactate or ethanol, regenerating NAD\( ^+ \) to allow glycolysis to continue producing 2 ATP per glucose. Both statements are true and R explains A.
Assertion (A): Abscisic Acid (ABA) is referred to as the stress hormone in plants.
Reason (R): ABA promotes seed germination and breaks bud dormancy during winter.
** The assertion is true: ABA is the stress hormone. The reason is false: ABA maintains dormancy and inhibits germination, acting as an antagonist to Gibberellins.
Assertion (A): 2,4-D is widely used as a weedicide in monocot cereal fields.
Reason (R): 2,4-D is a synthetic cytokinin that selectively stimulates dicotyledonous leaf division.
** The assertion is true: 2,4-D is a selective herbicide for broad-leaved weeds. The reason is false: 2,4-D is a synthetic auxin, not a cytokinin, and acts by causing uncontrolled cell elongation, not division, in dicots.
Statement I: Non-cyclic photophosphorylation involves both PS I and PS II.
Statement II: The splitting of water occurs on the outer surface of the thylakoid membrane, releasing protons into the stroma.
** Statement I is correct: non-cyclic photophosphorylation uses both photosystems. Statement II is incorrect: the photolysis of water occurs on the inner surface of the thylakoid membrane, releasing protons into the lumen.
Statement I: The Calvin cycle requires 18 ATP and 12 NADPH to produce one molecule of glucose.
Statement II: The Hatch-Slack pathway requires 30 ATP and 12 NADPH to produce one molecule of glucose.
** Both statements are correct: C3 plants require 18 ATP and 12 NADPH per glucose, while C4 plants require an additional 12 ATP (totaling 30 ATP) to run the PEP regeneration shunt.
Statement I: Glycolysis does not consume oxygen and occurs in both aerobic and anaerobic organisms.
Statement II: The Krebs cycle takes place in the outer mitochondrial membrane.
** Statement I is correct: glycolysis is anaerobic and occurs in the cytoplasm of all cells. Statement II is incorrect: the Krebs cycle takes place in the mitochondrial matrix, not the outer membrane.
Statement I: The Respiratory Quotient (RQ) is greater than 1.0 when organic acids are used as respiratory substrates.
Statement II: The RQ of carbohydrates is exactly 1.0 under aerobic conditions.
** Both statements are correct: organic acids contain more oxygen in their structure, requiring less oxygen for combustion (RQ > 1.0). Carbohydrates require equal volumes of \( \text{O}_2 \) and \( \text{CO}_2 \) (RQ = 1.0).
Statement I: Dedifferentiation is the process by which mature cells lose their specialized structures and regain the ability to divide.
Statement II: The formation of cork cambium from cortical parenchyma cells is an example of redifferentiation.
** Statement I is correct. Statement II is incorrect: the formation of cork cambium from differentiated parenchyma cells is an example of dedifferentiation.
Match the plant growth regulator in Column I with its classic biological effect or application in Column II:
| Column I | Column II |
|---|---|
| A. Auxin | I. Rosette plant bolting / stem elongation |
| B. Gibberellin | II. Selective weedicide for dicots |
| C. Cytokinin | III. Stomatal closure during drought |
| D. Abscisic Acid | IV. Nutrient mobilization / delay of senescence |
** Matches hormones with their functions:
Match the respiratory enzyme or complex in Column I with its location or cofactor in Column II:
| Column I | Column II |
|---|---|
| A. Complex I | I. Succinate dehydrogenase (inner membrane) |
| B. Complex II | II. NADH dehydrogenase (inner membrane) |
| C. Complex IV | III. Cytochrome \( c \) oxidase (contains copper cofactors) |
| D. ATP Synthase | IV. \( \text{F}_0\text{-F}_1 \) complex (Complex V) |
** Matches respiratory complexes with their cofactors and locations.
During non-cyclic photophosphorylation, the movement of electrons down the transport chain leads to:
** The movement of electrons down the thylakoid ETC drives protons from the stroma to the lumen, building the proton gradient.
The number of carbon dioxide molecules fixed to produce one molecule of glucose is:
** 6 molecules of \( \text{CO}_2 \) must be fixed through 6 turns of the Calvin cycle to produce 1 molecule of glucose (\( \text{C}_6\text{H}_{12}\text{O}_6 \)).
The first stable product of carbon fixation in C4 plants is:
** The primary product of carbon fixation in C4 plants is the 4-carbon organic acid oxaloacetic acid (OAA), formed in mesophyll cells.
The total number of decarboxylation steps in one turn of the Krebs cycle (excluding the link reaction) is:
** In one turn of the Krebs cycle, decarboxylation occurs at two steps: conversion of isocitrate to \( \alpha \)-ketoglutarate, and conversion of \( \alpha \)-ketoglutarate to succinyl-CoA.
Which complex in the inner mitochondrial membrane does not act as a proton pump?
** Complex II (Succinate dehydrogenase) transfers electrons from succinate to ubiquinone without pumping protons across the inner membrane.
The hormone that acts as a gaseous regulator of fruit ripening is:
** Ethylene is the gaseous hormone that regulates fruit ripening.
In tissue culture, shoot differentiation from parenchyma cells is induced by a:
** In plant tissue culture, a high cytokinin to auxin ratio promotes shoot differentiation, while a high auxin to cytokinin ratio promotes root differentiation.
During cellular respiration, the terminal electron acceptor is:
** Oxygen is the terminal electron acceptor in aerobic respiration, accepting electrons at Complex IV to form water.
| Parameter / Setup | Key Relation / Equation | Critical Pitfall |
|---|---|---|
| C3 glucose cost | 18 ATP + 12 NADPH | Do not confuse with C4 cost (30 ATP). |
| C4 glucose cost | 30 ATP + 12 NADPH | Requires more energy, but bypasses photorespiration. |
| Glycolysis SLP | 4 ATP generated (2 net) | Substrate-level phosphorylation generates 4 ATP total. |
| Krebs Cycle SLP | 2 GTP/ATP per glucose | Produced during succinyl-CoA conversion. |
| Oxygen terminal role | Reduces to water at Complex IV | Oxygen acts as the final electron acceptor. |
| PGR antagonist | ABA is antagonistic to GA | ABA promotes dormancy, GA promotes germination. |
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