NEET Biology — Plant Physiology Revision Notes

This comprehensive revision guide for NEET Biology covers the intricate principles of Plant Physiology, a high-yield chapter crucial for understanding the life processes within plants. It delves into essential topics ranging from transport mechanisms to photosynthesis, respiration, and plant growth regulators, providing a structured approach for effective revision and concept mastery.

NEET Biology: Plant Physiology Revision Notes

Plant Physiology is a cornerstone of NEET Biology, typically contributing a significant number of questions. Mastery of this chapter requires a deep understanding of complex biochemical pathways and physical processes. This revision note aims to consolidate all essential information, formulas, and concepts required to excel in this section.

Exam Weightage & Importance

Plant Physiology consistently accounts for 6-10% (approximately 4-6 questions) of the total Biology section in NEET. Questions often test conceptual clarity, understanding of biochemical pathways, and the ability to apply principles to different scenarios. High-scoring aspirants prioritize this unit due to its predictability and depth.

I. Transport in Plants

This section covers the mechanisms by which plants absorb, transport, and distribute water, minerals, and organic nutrients throughout their bodies.

1. Means of Transport

• Diffusion: Movement of molecules from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is a passive, slow process, independent of living systems.
  • Factors affecting diffusion: Permeability of membrane, temperature, pressure, concentration gradient, size of diffusing substance.
  • Significance: Gaseous exchange ($\text{O}_2$, $\text{CO}_2$) at stomata, movement of water from root hair to cortex.
• Facilitated Diffusion: Diffusion of hydrophilic substances across the membrane with the help of membrane proteins (channels or carriers). No energy expenditure.
  • Saturation: Maximum transport rate when all protein transporters are occupied.
  • Specificity: Transporters bind to specific molecules.
  • Sensitivity to inhibitors: Proteins are sensitive to inhibitors that bind to their active sites.
  • Porins: Large pores in the outer membranes of plastids, mitochondria, and some bacteria, allowing passage of molecules up to small proteins.
  • Aquaporins: Water channels made up of 8 different types of integral membrane proteins, facilitating water movement.
• Active Transport: Movement of molecules against a concentration gradient, requiring energy (ATP) and specific membrane proteins (pumps).
  • Features: Uphill transport, requires energy, specific, sensitive to inhibitors, saturable.
  • Example: $\text{Na}^+$/$\text{K}^+$ pump in animals, proton pumps in plants.

2. Plant-Water Relations

• Water Potential ($\Psi_w$): The potential energy of water per unit volume relative to pure water in reference conditions.
  • Pure water at standard temperature and pressure has $\Psi_w = 0$.
  • Formula: $\Psi_w = \Psi_s + \Psi_p + (\Psi_g)$ (Gravitational potential $\Psi_g$ is usually ignored in plant physiology calculations).
  • Units: Pascals (Pa), megapascals (MPa), or bars.
• Solute Potential ($\Psi_s$ / Osmotic Potential): Reduction in water potential due to the presence of solutes. It is always negative for solutions.
  • Formula (van't Hoff equation for dilute solutions): $\Psi_s = -iCRT$ (where $i$ is van't Hoff factor, $C$ is molar concentration, $R$ is gas constant, $T$ is temperature in Kelvin).
• Pressure Potential ($\Psi_p$ / Turgor Potential): Pressure exerted by protoplast against the cell wall. It is usually positive.
  • Turgor pressure: Important for maintaining cell shape, elongation, and stomatal opening.
• Osmosis: Diffusion of water across a semi-permeable membrane from a region of higher water potential to a region of lower water potential.
  • Endosmosis (towards inside): Water enters the cell, causing turgidity.
  • Exosmosis (towards outside): Water leaves the cell, causing plasmolysis.
  • Plasmolysis: Shrinkage of protoplast from the cell wall due to water loss in a hypertonic solution.
    • Incipient plasmolysis: Protoplast just begins to separate from the cell wall ($\Psi_p = 0$).
    • Complete plasmolysis: Protoplast fully separated, cell is flaccid.
  • Deplasmolysis: Reversal of plasmolysis when the cell is placed in a hypotonic solution.
• Imbibition: Absorption of water by solid particles (colloids), causing them to swell.
  • Examples: Seed germination, swelling of woody doors during rainy season.
  • Creates immense pressure (imbibition pressure).
• Water Potential Gradient: Water always moves from higher water potential to lower water potential.

3. Long Distance Transport of Water

• Apoplast Pathway: Movement of water through intercellular spaces and cell walls.
  • Faster, does not cross cell membrane, provides least resistance.
  • Blocked by Casparian strips in endodermis.
• Symplast Pathway: Movement of water through the cytoplasm of adjacent cells, connected by plasmodesmata.
  • Slower, involves crossing cell membranes, regulated by plasma membrane.
• Transmembrane Pathway (Vacuolar Pathway): Movement of water from vacuole to vacuole using symplast and apoplast pathways.
• Root Pressure Theory:
  • Root Pressure: Positive pressure developing in the xylem sap of roots.
  • Guttation: Loss of water in liquid form from leaf margins (especially at night) due to high root pressure, visible as drops.
  • Limitations: Insufficient pressure to account for water transport in tall trees; most effective at night.
• Cohesion-Tension-Transpiration Pull Theory (Dixon and Jolly): Most accepted theory for water ascent in plants.
  • Transpiration: Evaporative loss of water from stomata creates a negative pressure (tension) in the xylem.
  • Cohesion: Mutual attraction between water molecules due to hydrogen bonding.
  • Adhesion: Attraction of water molecules to the polar surfaces (xylem vessel walls).
  • Tensile Strength: Ability of a water column to resist breaking under tension.
  • Capillarity: Narrower the tube (xylem vessel), greater the capillary action.
  • The continuous column of water is pulled upwards by transpiration pull.

4. Transpiration

• Definition: Loss of water vapor from the aerial parts of plants, primarily through stomata.
• Types:
  • Stomatal transpiration (90-95%): Through stomata.
  • Cuticular transpiration (5-10%): Through cuticle.
  • Lenticular transpiration (<1%): Through lenticels.
• Mechanism of Stomatal Opening & Closing:
  • Potassium ion (K+) pump theory (Levitt):
    • Opening: Stomata open when turgor pressure in guard cells increases due to uptake of $\text{K}^+$ ions. $\text{K}^+$ uptake is balanced by influx of $\text{Cl}^-$ or synthesis of malate ions. Water follows by osmosis.
    • Closing: Stomata close when $\text{K}^+$ ions move out of guard cells, leading to water loss and decreased turgor. ABA plays a role in promoting stomatal closure during stress.
• Factors Affecting Transpiration:
  • External: Temperature (increases), light (increases), humidity (decreases), wind speed (increases), soil water (decreases if low).
  • Internal: Number and distribution of stomata, percentage of open stomata, water status of plant, canopy structure, leaf area.
• Significance of Transpiration:
  • Creates transpiration pull for water absorption and transport.
  • Supplies water for photosynthesis.
  • Transports minerals from soil to all parts of the plant.
  • Cools leaf surface (evaporative cooling).
  • Maintains cell turgidity.
• Transpiration - A Necessary Evil (Curtis): Essential for upward movement of water and minerals, but also leads to significant water loss.

5. Uptake and Transport of Mineral Nutrients

• Mineral Uptake:
  • Active absorption: Most mineral ions are absorbed actively (against concentration gradient), requiring ATP.
    • Specific carrier proteins present in root hair cell membranes.
    • Proton pumps $\text{H}^+$ ATPase create an electrochemical gradient.
  • Passive absorption: Some ions can be absorbed passively if there's a favorable concentration gradient.
• Translocation of Mineral Ions:
  • Transported primarily through the xylem along with the water stream (transpiration pull).
  • Specific tissues (e.g., young leaves, developing fruits, storage organs) act as sinks.
  • Mobilization: Redistributable elements (e.g., $\text{N}$, $\text{P}$, $\text{K}$) can be moved from senescent parts to younger parts.
  • Immobilized elements: (e.g., $\text{Ca}$, $\text{S}$) are not remobilized once incorporated into structural components.

6. Phloem Transport: Flow from Source to Sink

• Source: Region of synthesis (e.g., leaves for sugars).
• Sink: Region of storage or utilization (e.g., roots, fruits, developing buds).
• Nature of Phloem Sap: Mainly water and sucrose (a disaccharide, non-reducing sugar). Also hormones, amino acids.
• Pressure Flow (Mass Flow) Hypothesis (Münch): Most accepted mechanism.
  1. Loading at Source: Sucrose (and other sugars) is actively transported into sieve tube elements at the source. This is uphill transport requiring ATP.
  2. Water Movement: Increased solute concentration in sieve tube elements lowers water potential, causing water to move in from adjacent xylem by osmosis.
  3. Turgor Pressure Build-up: Water entry builds up hydrostatic pressure (turgor pressure) in the sieve tube.
  4. Mass Flow: The high-pressure sap moves towards regions of lower pressure (sink).
  5. Unloading at Sink: At the sink, sucrose is actively transported out of the sieve tubes and into storage cells or metabolizing cells.
  6. Water Recycles: Water potential in sieve tube at sink increases, causing water to move back into the xylem.
• Girdling Experiment: Removal of bark (including phloem) from a tree trunk. Accumulation of sugars above the girdle demonstrates downward translocation through phloem.

Worked Examples (Transport in Plants)

Example 1: Water Potential Calculation

A plant cell with $\Psi_s = -0.7 \text{ MPa}$ and $\Psi_p = +0.2 \text{ MPa}$ is placed in a solution with $\Psi_s = -0.5 \text{ MPa}$. a. Calculate the water potential of the plant cell. b. Calculate the water potential of the solution. c. In which direction will water move?

Solution:

a. Water potential of the plant cell: * $\Psi_w = \Psi_s + \Psi_p$ * $\Psi_w = -0.7 \text{ MPa} + 0.2 \text{ MPa}$ * $\Psi_w = -0.5 \text{ MPa}$

b. Water potential of the solution: * For an open beaker solution, $\Psi_p = 0$. * $\Psi_w = \Psi_s + \Psi_p$ * $\Psi_w = -0.5 \text{ MPa} + 0 \text{ MPa}$ * $\Psi_w = -0.5 \text{ MPa}$

c. Direction of water movement: * Since $\Psi_w$ of the cell ($-0.5 \text{ MPa}$) is equal to $\Psi_w$ of the solution ($-0.5 \text{ MPa}$), there will be no net movement of water. The cell is in an isotonic solution.

Example 2: Plasmolysis Observation

A student observes the cell wall of a palisade cell from a plant leaf. The cell is placed in $0.5 \text{ M}$ NaCl solution and quickly shows plasmolysis. What was the approximate water potential inside the cell wall region (apoplast) before plasmolysis occurred, relative to the external solution?

Solution:

• In plasmolysis, the protoplast shrinks because water moves out of the cell.
• This indicates that the external solution has a lower (more negative) water potential than the cell's cytoplasm.
• The apoplast is filled with water that is usually in equilibrium with the cell or slightly lower water potential than pure water due to dissolved substances.
• For water to move out of the cell and cause plasmolysis, the external solution must have significantly lower water potential than the apoplast and the cell's cytoplasm.
• Therefore, the water potential inside the cell wall (apoplast) region before plasmolysis was higher than the external $0.5 \text{ M}$ NaCl solution.

Example 3: Root Pressure & Transpiration

Explain why root pressure is ineffective in transporting water in tall trees, particularly during the day.

Solution:

Root pressure is generated by the active uptake of ions by root cells, leading to water influx and positive pressure in the xylem.

• Magnitude: Root pressure is relatively low, typically generating only about $0.1 \text{ MPa}$ to $0.2 \text{ MPa}$ of pressure. Tall trees can be over 100 meters high, requiring much higher pressures (around $2 \text{ MPa}$ or more) to lift water to that height against gravity.
• Transpiration during the day: During the day, stomata are open, and transpiration rates are very high. This intense evaporative pull creates a strong negative pressure (tension) in the xylem. This negative pressure effectively negates and often overcomes any positive root pressure, making transpiration pull the dominant force for water transport. Root pressure is more noticeable during periods of low transpiration, such as at night, leading to phenomena like guttation.

II. Photosynthesis in Higher Plants

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy, primarily in the form of glucose. This section details the mechanisms, sites, and factors influencing this vital process.

1. What is Photosynthesis?

• Overall Equation: $6\text{CO}_2 + 12\text{H}_2\text{O} \xrightarrow{\text{Light}} \text{C}6\text{H}{12}\text{O}_6 + 6\text{H}_2\text{O} + 6\text{O}_2$
• Site: Chloroplasts, specifically in the thylakoid membranes (light reactions) and stroma (dark reactions).
• Photosynthetic Pigments:
  • Chlorophyll a: (Bright/blue green) Primary pigment, directly involved in light reactions.
  • Chlorophyll b: (Yellow green) Accessory pigment.
  • Carotenoids: (Yellow/orange) Accessory pigments (carotenes, xanthophylls).
  • Functions of Accessory Pigments: Broaden the spectrum of light absorbed, protect chlorophyll a from photo-oxidation.

2. Light Reactions (Photochemical Phase)

• Location: Thylakoid membranes.
• Key Processes: Light absorption, water splitting, oxygen release, formation of ATP and NADPH.
• Photosystems: Pigment-protein complexes.
  • Photosystem I (PSI): Reaction center $\text{P}_{700}$ (absorbs max at $700 \text{ nm}$). Located mainly on stromal side of grana and stromal lamellae.
  • Photosystem II (PSII): Reaction center $\text{P}_{680}$ (absorbs max at $680 \text{ nm}$). Located mainly on inner surface of thylakoid membrane facing lumen.
• Electron Transport Chain:
  • Non-cyclic Photophosphorylation (Z-scheme): Involves both PSI and PSII.
    1. Light excites $\text{P}_{680}$ (PSII), electrons are released.
    2. Water splitting (Photolysis): $\text{H}_2\text{O} \rightarrow 2\text{H}^+ + 2\text{e}^- + \frac{1}{2}\text{O}2$. Electrons replace those lost by $\text{P}{680}$. Oxygen is released.
    3. Electrons from PSII pass through an electron transport chain: Plastoquinone ($\text{PQ}$) $\rightarrow$ Cytochrome $b_6f$ complex $\rightarrow$ Plastocyanin ($\text{PC}$). This downhill movement generates a proton gradient.
    4. Electrons reach PSI ($\text{P}{700}$), replacing those lost by $\text{P}{700}$ upon light excitation.
    5. Electrons from PSI pass to Ferredoxin ($\text{Fd}$) $\rightarrow$ $\text{NADP}^+$ reductase $\rightarrow$ $\text{NADP}^+$ is reduced to $\text{NADPH} + \text{H}^+$.
    6. ATP synthesis (Chemiosmosis): Proton gradient across thylakoid membrane drives ATP synthase, leading to ATP formation.
    • Products: ATP, NADPH, $\text{O}_2$.
  • Cyclic Photophosphorylation: Involves only PSI.
    1. Light excites $\text{P}_{700}$ (PSI), electrons are released.
    2. Electrons from PSI pass to $\text{Fd} \rightarrow$ Cytochrome $b_6f$ complex $\rightarrow$ $\text{PC} \rightarrow$ back to $\text{P}_{700}$.
    3. Only ATP is produced (no NADPH, no $\text{O}_2$). Occurs under low $\text{CO}_2$ conditions or high light intensity when $\text{NADPH}$ is abundant.
• Chemiosmotic Hypothesis (Mitchell): Explains ATP synthesis in chloroplasts (and mitochondria).
  1. Proton Gradient: Formed due to water splitting (inside thylakoid lumen), proton pump activity (moving $\text{H}^+$ from stroma to lumen by $\text{PQ}$), and $\text{NADP}^+$ reduction (removes $\text{H}^+$ from stroma).
  2. ATP Synthase (CF0-CF1 complex): CF0 is embedded in membrane, CF1 protrudes into stroma. Protons flow from lumen (high concentration) to stroma (low concentration) through CF0, causing conformational change in CF1, leading to ATP synthesis from ADP and $\text{Pi}$.

3. Dark Reactions (Biosynthetic Phase)

• Location: Chloroplast stroma.

• Uses: ATP and NADPH from light reactions to fix $\text{CO}_2$ into carbohydrates.

• $\text{C}_3$ Pathway (Calvin Cycle):

  • Discovery: Melvin Calvin.
  • First stable product: 3-phosphoglycerate (3-PGA), a 3-carbon compound.
  • $\text{CO}_2$ acceptor: Ribulose-1,5-bisphosphate (RuBP), a 5-carbon sugar.
  • Enzyme: RuBP carboxylase-oxygenase (RuBisCO) – most abundant enzyme on Earth.
  • Three stages:
    1. Carboxylation: $\text{CO}_2$ combines with RuBP, catalyzed by RuBisCO, forming two molecules of 3-PGA.
    2. Reduction: 3-PGA is converted to triose phosphate (Glyceraldehyde-3-phosphate). Requires 2 ATP for phosphorylation and 2 NADPH for reduction per $\text{CO}_2$ fixed.
    3. Regeneration: RuBP is regenerated from triose phosphates. Requires 1 ATP per RuBP.
  • Net requirement for 1 glucose ($6 \text{ CO}_2$): $18 \text{ ATP}$ and $12 \text{ NADPH}$.

• $\text{C}_4$ Pathway (Hatch-Slack Pathway):

  • Characteristics: Occurs in plants adapted to hot, dry climates (e.g., maize, sugarcane). Shows Kranz anatomy.
  • Kranz Anatomy: Bundle sheath cells (large, thick-walled, no intercellular spaces, many chloroplasts) surrounding vascular bundles, with mesophyll cells encircling them.
  • First stable product: Oxaloacetic acid (OAA), a 4-carbon compound.
  • $\text{CO}_2$ acceptor in mesophyll: Phosphoenolpyruvate (PEP), a 3-carbon compound.
  • Enzyme: PEP carboxylase (PEPCase) – highly efficient for $\text{CO}_2$ fixation, especially at low $\text{CO}_2$ concentrations, no oxygenase activity.
  • Pathway Steps:
    1. Mesophyll cells: $\text{CO}_2$ fixated by PEPCase to PEP, forming OAA.
    2. OAA converted to malate or aspartate (4C acids).
    3. Malate/Aspartate transported to bundle sheath cells.
    4. Bundle sheath cells: Malate undergoes decarboxylation, releasing $\text{CO}_2$.
    5. The released $\text{CO}_2$ enters the Calvin cycle (C3 pathway) in bundle sheath cells, where RuBisCO is present.
    6. The 3-carbon molecule (e.g., pyruvate) remaining after decarboxylation returns to mesophyll cells to regenerate PEP, requiring 1 ATP.
  • Advantages: Efficient $\text{CO}_2$ fixation, minimizes photorespiration.
  • Net requirement for 1 glucose ($6 \text{ CO}_2$): $30 \text{ ATP}$ and $12 \text{ NADPH}$. (Extra 12 ATP for PEP regeneration).

4. Photorespiration

• Definition: Wasteful process in $\text{C}_3$ plants where RuBisCO binds with $\text{O}_2$ instead of $\text{CO}_2$.
• Enzyme involved: RuBisCO (has both carboxylase and oxygenase activity).
• Conditions that favor it: High $\text{O}_2$ concentration, low $\text{CO}_2$ concentration, high temperature, high light intensity.
• Process:
  1. RuBisCO binds $\text{O}_2$ with RuBP, forming 1 molecule of 3-PGA and 1 molecule of 2-phosphoglycolate (2C compound).
  2. 2-phosphoglycolate is metabolized in peroxisomes and mitochondria, leading to loss of fixed $\text{CO}_2$ without ATP or NADPH synthesis.
• Consequences: No ATP or NADPH synthesis, leads to $25%$ to $50%$ loss of photosynthetically fixed carbon.
• $\text{C}_4$ plants minimize photorespiration: Due to spatial separation, $\text{CO}_2$ concentration is high in bundle sheath cells, favoring RuBisCO's carboxylase activity.

5. Factors Affecting Photosynthesis

• Blackman's Law of Limiting Factors: "When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor."
• Light:
  • Light intensity: Increases rate up to saturation point. Beyond saturation, other factors become limiting.
  • Light quality: Red and blue light are most effective (action spectrum).
  • Duration: Longer duration means more photosynthesis, up to a point.
• Carbon Dioxide Concentration:
  • The major limiting factor for photosynthesis, especially in $\text{C}_3$ plants.
  • Increasing $\text{CO}_2$ concentration increases rate, up to saturation. $\text{C}_4$ plants saturate at lower $\text{CO}_2$ concentrations (around $360 \text{ ppm}$) than $\text{C}_3$ plants (around $450 \text{ ppm}$).
• Temperature:
  • Affects dark reactions (enzymatic). Optimal temperature varies for different plants (e.g., $\text{C}_4$ plants have higher optimum than $\text{C}_3$ plants).
  • Light reactions are also temperature-sensitive but to a lesser extent.
• Water:
  • Water stress causes stomata to close (reducing $\text{CO}_2$ availability), wilting, and reduced leaf area. Indirectly limits photosynthesis.
• Other Factors: Oxygen concentration (inhibits in $\text{C}_3$ plants due to photorespiration), mineral nutrients (e.g., Mg for chlorophyll, N for enzymes).

Worked Examples (Photosynthesis)

Example 4: ATP and NADPH Quantities

Calculate the total number of ATP and $\text{NADPH}_2$ required to produce 10 molecules of glucose in a $\text{C}_3$ plant.

Solution:

• For 1 molecule of glucose in a $\text{C}_3$ plant:
  • Calvin cycle requires $18 \text{ ATP}$ and $12 \text{ NADPH}_2$.
• For 10 molecules of glucose:
  • ATP required = $18 \text{ ATP/glucose} \times 10 \text{ glucose} = 180 \text{ ATP}$
  • $\text{NADPH}_2$ required = $12 \text{ NADPH}_2\text{/glucose} \times 10 \text{ glucose} = 120 \text{ NADPH}_2$

Example 5: Photorespiration in $\text{C}_4$ Plants

Why do $\text{C}_4$ plants show very little or no photorespiration?

Solution:

$\text{C}_4$ plants have evolved a mechanism to effectively suppress photorespiration due to their specialized anatomy and biochemistry.

1. Kranz Anatomy: They possess Kranz anatomy, characterized by bundle sheath cells surrounding the vascular bundles, and mesophyll cells around them.
2. Initial $\text{CO}_2$ Fixation by PEPCase: In the mesophyll cells, $\text{CO}_2$ is initially fixed by the enzyme PEP carboxylase (PEPCase) into a 4-carbon acid (OAA). PEPCase has a very high affinity for $\text{CO}_2$ and no affinity for $\text{O}_2$. This allows efficient $\text{CO}_2$ capture even at low atmospheric $\text{CO}_2$ concentrations.
3. $\text{CO}_2$ Concentration in Bundle Sheath Cells: The 4-carbon acid is then transported to the bundle sheath cells. Inside these cells, the 4-carbon acid is decarboxylated, releasing a high concentration of $\text{CO}_2$.
4. RuBisCO Activity: This elevated $\text{CO}_2$ concentration within the bundle sheath cells ensures that RuBisCO, which is present there, primarily functions as a carboxylase (binding $\text{CO}_2$) rather than an oxygenase (binding $\text{O}_2$).
5. Spatial Separation: Effectively, the mesophyll cells act as a $\text{CO}_2$ pump, concentrating $\text{CO}_2$ around RuBisCO in the bundle sheath cells, thereby minimizing the chances of photorespiration.

Example 6: Limiting Factor

A plant is kept in a growth chamber with ample water, optimal temperature, and sufficient light intensity. If the $\text{CO}_2$ concentration is gradually increased, the rate of photosynthesis initially increases but then plateaus. Explain why.

Solution:

This observation is best explained by Blackman's Law of Limiting Factors.

• Initial Increase: As $\text{CO}_2$ concentration increases, it acts as a limiting factor. With sufficient light, water, and temperature, the availability of $\text{CO}_2$ directly determines the rate of carbon fixation in the Calvin cycle, and thus the overall rate of photosynthesis.
• Plateau: The rate of photosynthesis plateaus because $\text{CO}_2$ is no longer the limiting factor. At this point, another factor has become limiting. Despite increasing $\text{CO}_2$, the rate cannot increase further because the process is now limited by the availability of other components like:
  • Light-dependent reactions: The rate of ATP and NADPH production might be capped by the maximum efficiency of light absorption or the electron transport chain, even if light intensity itself is considered "sufficient."
  • Enzyme saturation: The enzymes involved in the Calvin cycle (e.g., RuBisCO) might be working at their maximum capacity, and their turnover rate cannot be increased further, even with a higher substrate ($\text{CO}_2$) concentration.
  • Mineral nutrient deficiency: If there's a deficiency of essential minerals like nitrogen (for enzymes) or magnesium (for chlorophyll), this can also limit the overall photosynthetic capacity.

Therefore, the plateau indicates that the process has become limited by a factor other than $\text{CO}_2$.

III. Respiration in Plants

Plant respiration is the process of breaking down organic substances (food) to release energy for cellular activities.

1. Do Plants Breathe?

• Yes, plants breathe, but not in the same sense as animals. They do not have specialized respiratory organs.
• Each plant part takes care of its own gas exchange.
• Gas exchange occurs through stomata in leaves, lenticels in stems, and general surface of roots.

2. Glycolysis (EMP Pathway)

• Location: Cytoplasm.
• Definition: Partial oxidation of glucose to two molecules of pyruvate.
• Independent: Does not require oxygen (anaerobic).
• Steps:
  1. Phosphorylation: Glucose $\rightarrow$ Glucose-6-phosphate $\rightarrow$ Fructose-6-phosphate $\rightarrow$ Fructose-1,6-bisphosphate (consumes 2 ATP).
  2. Cleavage: Fructose-1,6-bisphosphate splits into Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde-3-phosphate (GAP). DHAP isomerizes to GAP. So, two molecules of GAP are formed.
  3. Oxidative & ATP-generating: GAP is oxidized and phosphorylated to 1,3-Bisphosphoglycerate. This step forms 2 NADH.
  4. Substrate-level phosphorylation: 1,3-Bisphosphoglycerate $\rightarrow$ 3-Phosphoglycerate (forms 2 ATP).
  5. Remaining steps lead to Pyruvate (forms 2 ATP during conversion of PEP to Pyruvate).
• Net products per glucose: 2 Pyruvate, 2 ATP (net), 2 NADH.

3. Fermentation (Anaerobic Respiration)

• Occurs when oxygen is unavailable after glycolysis.
• Purpose: Reoxidize NADH to $\text{NAD}^+$ to allow glycolysis to continue.
• Types:
  • Alcoholic Fermentation: Carried out by yeast and some bacteria.
    • Pyruvic acid $\rightarrow$ Acetaldehyde + $\text{CO}_2$ (catalyzed by Pyruvic acid decarboxylase).
    • Acetaldehyde $\rightarrow$ Ethanol (catalyzed by Alcohol dehydrogenase, uses NADH).
  • Lactic Acid Fermentation: Carried out by some bacteria and animal muscle cells (during strenuous exercise).
    • Pyruvic acid $\rightarrow$ Lactic acid (catalyzed by Lactate dehydrogenase, uses NADH).
• Yield: Very little energy (2 ATP per glucose). Much less efficient than aerobic respiration.

4. Aerobic Respiration

• Requires oxygen.
• Location: Mitochondria.
• Steps:
  • A. Oxidative Decarboxylation of Pyruvate (Link Reaction):
    • Location: Mitochondrial matrix.
    • Pyruvic acid (3C) $\rightarrow$ Acetyl $\text{CoA}$ (2C) + $\text{CO}_2$ (catalyzed by Pyruvate dehydrogenase complex).
    • Produces 1 NADH per pyruvate. So, 2 NADH per glucose.
  • B. Krebs Cycle (TCA Cycle / Citric Acid Cycle):
    • Location: Mitochondrial matrix.
    • Starts with: Acetyl $\text{CoA}$ combines with Oxaloacetic acid (OAA, 4C) to form Citric acid (6C).
    • Series of cyclic reactions involving oxidative decarboxylations and dehydrogenations.
    • Products per Acetyl $\text{CoA}$ (half glucose): 3 NADH, 1 $\text{FADH}_2$, 1 $\text{GTP}$ (converted to ATP), 2 $\text{CO}_2$.
    • Products per glucose (2 Acetyl $\text{CoA}$): 6 NADH, 2 $\text{FADH}_2$, 2 ATP ($\text{GTP}$), 4 $\text{CO}_2$.
  • C. Electron Transport System (ETS) and Oxidative Phosphorylation:
    • Location: Inner mitochondrial membrane.
    • ETS: Electrons from NADH and $\text{FADH}_2$ (generated from glycolysis, link reaction, and Krebs cycle) are passed down a chain of electron carriers (Complex I-IV).
      • Complex I (NADH dehydrogenase): Transfers electrons from NADH to Ubiquinone (UQ).
      • Complex II (Succinate dehydrogenase): Transfers electrons from $\text{FADH}_2$ to UQ.
      • Complex III (Cytochrome $bc_1$ complex): Transfers electrons from UQ to Cytochrome c.
      • Complex IV (Cytochrome oxidase complex): Transfers electrons from Cytochrome c to $\text{O}_2$ (final electron acceptor), forming water.
    • Oxidative Phosphorylation: As electrons move through ETS, protons ($\text{H}^+$) are pumped from the mitochondrial matrix into the intermembrane space, creating a proton gradient.
    • ATP Synthase (Complex V): Protons flow back into the matrix through $\text{F}_0\text{F}_1$ ATP synthase, driving the synthesis of ATP.
    • ATP Yield:
      • 1 NADH $\rightarrow$ 3 ATP (approx. 2.5 ATP with recent understanding)
      • 1 $\text{FADH}_2 \rightarrow$ 2 ATP (approx. 1.5 ATP with recent understanding)

5. Respiration Balance Sheet

• Total ATP yield per glucose (theoretical maximum):
  • Glycolysis: 2 ATP (net) + 2 NADH ($\rightarrow$ 6 ATP, via malate-aspartate shuttle or 4 ATP via glycerol phosphate shuttle)
  • Link Reaction: 2 NADH ($\rightarrow$ 6 ATP)
  • Krebs Cycle: 2 ATP ($\text{GTP}$) + 6 NADH ($\rightarrow$ 18 ATP) + 2 $\text{FADH}_2$ ($\rightarrow$ 4 ATP)
  • Total: 36 or 38 ATP.
    • 38 ATP: If NADH from glycolysis generates 6 ATP (e.g., in liver, heart kidney cells via malate-aspartate shuttle).
    • 36 ATP: If NADH from glycolysis generates 4 ATP (e.g., in skeletal muscle, brain cells via glycerol phosphate shuttle).
  • Recent values (considering 2.5 ATP per NADH and 1.5 ATP per FADH2): Around 30-32 ATP.
  • Factors for actual lower yield: Energy used for transport, proton leakage.

6. Amphibolic Pathway

• Respiration is an amphibolic pathway (both catabolic and anabolic).
  • Catabolism: Breakdown of glucose, fats, proteins to release energy.
  • Anabolism: Intermediate compounds of respiration are used in the synthesis of other biomolecules (e.g., Acetyl $\text{CoA}$ for fatty acids, OAA for amino acids, alpha-ketoglutarate for amino acids).

7. Respiratory Quotient (RQ)

• Definition: Ratio of volume of $\text{CO}_2$ evolved to the volume of $\text{O}_2$ consumed during respiration.
• Formula: $RQ = \frac{\text{Volume of } \text{CO}_2 \text{ evolved}}{\text{Volume of } \text{O}_2 \text{ consumed}}$
• Values:
  • Carbohydrates: $RQ = 1$ (e.g., Glucose: $\text{C}6\text{H}{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$)
  • Fats: $RQ < 1$ (e.g., Tripalmitin: $2\text{C}{51}\text{H}{98}\text{O}_6 + 145\text{O}_2 \rightarrow 102\text{CO}_2 + 98\text{H}_2\text{O}$, $RQ = 102/145 \approx 0.7$)
  • Proteins: $RQ \approx 0.9$
  • Organic Acids: $RQ > 1$ (e.g., Oxalic acid: $\text{C}_2\text{H}_2\text{O}_4 + \frac{1}{2}\text{O}_2 \rightarrow 2\text{CO}_2 + \text{H}_2\text{O}$, $RQ = 2/0.5 = 4$)
  • Anaerobic Respiration: $RQ = \infty$ (infinity) because $\text{O}_2$ consumed is zero.

Worked Examples (Respiration)

Example 7: ATP Yield Calculation

Calculate the net ATP gain if a molecule of 2-phosphoglycerate (3-PGA) directly enters glycolysis at step 8 through the conversion to phosphoenolpyruvate (PEP), bypassing the initial steps. Assume aerobic respiration and that 1 NADH yields 2.5 ATP and 1 $\text{FADH}_2$ yields 1.5 ATP.

Solution:

Let's trace the remaining steps from 2-phosphoglycerate (2-PGA):

1. 2-PGA $\rightarrow$ PEP: No ATP/NADH.
2. PEP $\rightarrow$ Pyruvate: This step produces 1 ATP by substrate-level phosphorylation. Since 2-PGA is a 3-carbon molecule, we assume it's one half of the glucose pathway, so we consider one molecule leading to one pyruvate.
   • ATP from substrate-level phosphorylation: 1 ATP
3. Pyruvate $\rightarrow$ Acetyl CoA (Link Reaction):
   • Produces 1 NADH.
   • ATP from NADH: $1 \times 2.5 \text{ ATP} = 2.5 \text{ ATP}$
4. Acetyl CoA in Krebs Cycle:
   • Produces 3 NADH, 1 $\text{FADH}_2$, 1 ATP (from GTP).
   • ATP from NADH: $3 \times 2.5 \text{ ATP} = 7.5 \text{ ATP}$
   • ATP from $\text{FADH}_2$: $1 \times 1.5 \text{ ATP} = 1.5 \text{ ATP}$
   • ATP from GTP: $1 \text{ ATP}$

Total Net ATP Gain: $1 \text{ (from PEP to Pyruvate)} + 2.5 \text{ (from Link Reaction NADH)} + 7.5 \text{ (from Krebs NADH)} + 1.5 \text{ (from Krebs FADH}_2) + 1 \text{ (from Krebs substrate-level)}$ $= 1 + 2.5 + 7.5 + 1.5 + 1 = 13.5 \text{ ATP}$

So, approximately 13.5 ATP would be gained from one molecule of 2-phosphoglycerate entering glycolysis at this stage.

Example 8: Respiratory Quotient (RQ)

Calculate the Respiratory Quotient (RQ) for a hypothetical substrate $C_{10}H_{10}O_5$ during aerobic respiration.

Solution:

The general equation for aerobic respiration of a carbohydrate-like substrate is: $C_xH_yO_z + (x + y/4 - z/2)O_2 \rightarrow xCO_2 + (y/2)H_2O$

For $C_{10}H_{10}O_5$: $x=10, y=10, z=5$

1. Calculate $\text{O}_2$ consumed: $\text{O}_2 \text{ consumed} = x + y/4 - z/2 = 10 + 10/4 - 5/2 = 10 + 2.5 - 2.5 = 10$

2. Calculate $\text{CO}_2$ evolved: $\text{CO}_2 \text{ evolved} = x = 10$

3. Calculate RQ: $RQ = \frac{\text{Volume of } \text{CO}_2 \text{ evolved}}{\text{Volume of } \text{O}_2 \text{ consumed}} = \frac{10}{10} = 1$

Therefore, the RQ for the hypothetical substrate $C_{10}H_{10}O_5$ is 1. This suggests it is a carbohydrate or a substance with a similar $\text{C}:\text{O}$ ratio.

IV. Plant Growth and Development

This section covers the organized and coordinated changes that occur throughout a plant's life cycle, from germination to senescence.

1. Growth

• Definition: Irreversible increase in size, dry weight, or volume of a cell, organ, or whole organism.
• Characteristics:
  • Determinate vs. Indeterminate: Most plants show indeterminate growth (grow throughout life); some show determinate growth (e.g., fruits, leaves).
  • Meristems: Regions of active cell division (apical and lateral meristems).
  • Growth is measurable: Parameter's include length, area, cell number, fresh weight, dry weight.
• Measurement of Growth:
  • Arithmetic Growth: Rate of increase is constant; one daughter cell divides, other differentiates.
    • Plotting length/weight against time yields a linear curve.
    • Formula: $L_t = L_0 + rt$ (where $L_t$ is length at time $t$, $L_0$ is initial length, $r$ is growth rate).
  • Geometric Growth: All daughter cells continue to divide; initial phase is slow (lag phase), then rapid (exponential phase), then slows down (stationary phase).
    • Plotting against time yields a sigmoid (S-shaped) curve.
    • Formula: $W_1 = W_0 e^{rt}$ (where $W_1$ is final weight, $W_0$ is initial weight, $r$ is relative growth rate, $t$ is time, $e$ is base of natural logarithms).
• Growth Rates:
  • Absolute Growth Rate (AGR): Total growth per unit time.
  • Relative Growth Rate (RGR): Growth per unit time per unit initial parameter (e.g., per unit initial dry weight). Often more informative for comparing growth in different sized organisms.

2. Differentiation, Dedifferentiation, and Redifferentiation

• Differentiation: Cells derived from meristems mature to perform specific functions, losing the capacity to divide.
• Dedifferentiation: Differentiated cells regain the capacity to divide under certain conditions (e.g., parenchyma cells forming cork cambium or wound meristem).
• Redifferentiation: Dedifferentiated cells again lose the capacity to divide and mature to perform specific functions.

3. Development

• Definition: Sum of all changes an organism goes through during its life cycle, including growth and differentiation.
• Plasticity: Ability of plants to form different kinds of structures in response to environment (e.g., heterophylly in cotton, coriander, larkspur; buttercup).

4. Plant Growth Regulators (PGRs) / Phytohormones

Organic compounds naturally produced in plants that influence physiological processes even in small concentrations.

A. Growth Promoters:

• 1. Auxins: (e.g., Indole-3-acetic acid (IAA) - natural; NAA, 2,4-D - synthetic)
  • Discovery: First isolated from human urine. Went, F.W. (1928) isolated from oat coleoptile tips.
  • Site of Synthesis: Apical meristems of shoots and roots, young leaves.
  • Functions:
    • Apical dominance (inhibits lateral bud growth).
    • Cell elongation (especially in roots, stimulates shoot growth).
    • Initiates rooting in stem cuttings.
    • Parthenocarpy (fruit development without fertilization, e.g., tomatoes).
    • Prevents premature abscission of young fruits.
    • Responsible for tropisms (phototropism, gravitropism).
    • 2,4-D: Synthetic auxin, widely used as a herbicide to kill dicot weeds in monocot fields.
• 2. Gibberellins (GAs): (e.g., GA3 - most studied)
  • Discovery: Identified from fungus Gibberella fujikuroi (causing 'bakanae' or foolish seedling disease in rice).
  • Site of Synthesis: Young leaves, roots, shoot tips, developing seeds.
  • Functions:
    • Stretching of internodes (dramatically increases stem length, especially in genetically dwarf plants).
    • Bolting (internode elongation) in rosette plants.
    • Delay senescence.
    • Breaks seed dormancy, promotes germination (e.g., in barley seeds, stimulates $\alpha$-amylase production).
    • Promotes fruit growth (e.g., increasing size of grapes).
    • Used to speed up malting process in brewing industry.
• 3. Cytokinins: (e.g., Kinetin - synthetic; Zeatin - natural from maize kernel, coconut milk)
  • Discovery: Skoog and Miller discovered its role in cell division. Letham isolated zeatin.
  • Site of Synthesis: Actively dividing regions (root apices, developing shoot buds, young fruits).
  • Functions:
    • Promotes cell division (mitosis).
    • Helps overcome apical dominance (promotes lateral bud growth).
    • Promotes nutrient mobilization, delays leaf senescence (Richmond-Lang effect).
    • Promotes chloroplast development.

B. Growth Inhibitors:

• 4. Abscisic Acid (ABA): (Stress hormone)
  • Discovery: Identified as abscisin II and dormin independently.
  • Site of Synthesis: Chloroplasts (e.g., leaves, fruits, seeds).
  • Functions:
    • Antagonist to GAs (promotes dormancy in seeds and buds).
    • Promotes abscission of leaves and fruits.
    • Causes stomatal closure under water stress (stress hormone).
    • Inhibits seed germination.
    • Important for seed development and maturation.
• 5. Ethylene (Gaseous hormone):
  • Discovery: Cousin confirmed its role in fruit ripening.
  • Site of Synthesis: Produced in large amounts by tissues undergoing senescence and ripening fruits.
  • Functions:
    • Promotes fruit ripening (climacteric fruits: respiration rate increases).
    • Enhances senescence and abscission of leaves and flowers.
    • Breaks seed and bud dormancy.
    • Promotes rapid internode/petiole elongation in deep-water rice.
    • Promotes root growth and root hair formation, increasing absorption surface.
    • Initiates flowering in pineapples.
    • Most widely used PGR in agriculture (e.g., Ethephon, releases ethylene).

5. Photoperiodism

• Definition: Response of plants to relative lengths of day and night (photoperiod), influencing flowering.
• Critical Photoperiod: A specific duration of light/dark period required for flowering.
• Phytochrome: Pigment responsible for perceiving light signals. Exists in two interconvertible forms: $\text{P}r$ (red light absorbing, $660 \text{ nm}$) and $\text{P}{fr}$ (far-red light absorbing, $730 \text{ nm}$). $\text{P}_{fr}$ is the physiologically active form.
• Classification of Plants:
  • Long Day Plants (LDPs): Flower when day length is longer than critical photoperiod (e.g., wheat, oats, spinach). Require Pfr to accumulate.
  • Short Day Plants (SDPs): Flower when day length is shorter than critical photoperiod (e.g., tobacco, soybean, chrysanthemum). Require continuous long dark period, Pfr inhibits flowering.
  • Day Neutral Plants (DNP): Flowering is unaffected by photoperiod (e.g., tomato, maize, cucumber).

6. Vernalisation

• Definition: Promotion of flowering by a period of low temperature treatment (chilling).
• Site of perception: Apical meristem.
• Examples: Winter varieties of wheat and barley, biennial plants (e.g., sugar beet, cabbage, carrots).
• Mechanism: Chilling stimulus causes formation of a florigen-like substance (hypothetical hormone).
• Prevents precoecious reproductive development.

7. Seed Dormancy

• Definition: State where seeds fail to germinate even under favorable conditions.
• Causes: Impermeable seed coat, chemical inhibitors (ABA, phenolic acids, ferulic acid), immature embryo, low oxygen availability.
• Methods to break dormancy:
  • Scarification: Mechanical abrasion or acid treatment to break hard seed coat.
  • Stratification: Cold/moist conditions to overcome physiological dormancy (mimics winter).
  • Chemical treatments: Gibberellins, nitrates, thiourea.
  • Light: For some seeds (e.g., lettuce, required light; onion, inhibited by light).

Worked Examples (Plant Growth and Development)

Example 9: Auxin and Apical Dominance

A gardener removes the apical bud from a potted plant. a. What physiological phenomenon is the gardener trying to overcome? b. What change would be observed in the plant's growth pattern? c. Which plant hormone's function is primarily responsible for this phenomenon?

Solution:

a. The gardener is trying to overcome apical dominance. Apical dominance is the phenomenon where the apical bud (shoot tip) inhibits the growth of lateral (axillary) buds.

b. After removing the apical bud (decapitation), the inhibition on the lateral buds is released. As a result, the lateral buds will start growing, leading to the development of side branches. This makes the plant bushier and denser.

c. Auxin is the plant hormone primarily responsible for apical dominance. Auxins are produced at the shoot apex and transported downwards. High concentrations of auxin in the stem inhibit the growth of lateral buds, while promoting the growth of the apical bud. Removal of the apical bud removes the source of this inhibitory auxin.

Example 10: Photoperiodism

A plant flowers only when the day length is shorter than a critical value. What type of plant is this, and what would happen if it receives a flash of light during its long night period?

Solution:

• Type of plant: This plant is a Short Day Plant (SDP). SDPs require a photoperiod shorter than a critical duration, which essentially means they need a continuous long dark period to flower.

• Effect of a flash of light during the night: If a flash of red light (which converts $\text{P}r$ to $\text{P}{fr}$, the active form) is given during the continuous long dark period, it will interrupt the critical dark period. This interruption will effectively shorten the continuous dark period below the required threshold for an SDP, causing the plant to not flower or to delay flowering. The $\text{P}_{fr}$ formed by the light flash will act as an inhibitor for flowering in SDPs during the dark period. If this flash is followed by a far-red light, then the effect of red light is reversed, and flowering may occur.

Common Mistakes & How to Avoid Them

Memory Tricks / Mnemonics

• PGRs Functions:
  • Auxins: Apical dominance, Absence of fruit drop, All-round growth (stem elongation).
  • Gibberellins: Giant plants (stem elongation), Germination, Grape elongation.
  • Cytokinins: Cell division, Cytoplasm dividing (Cytokinesis), Counters apical dominance.
  • ABA: Absent growth, Absent water (stress hormone, closes stomata), Absent germination (dormancy).
  • Ethylene: Ethylene for Eating (fruit ripening), Elongation prevention (root growth), Ethylene for Everything (versatile effects).
• Kranz Anatomy (C4 Plants): "Kranz means wreath, like a wreath of bundle sheath cells around vascular bundles."
• Photosynthesis ETS (Non-cyclic): "P(Q)BC (PSII $\rightarrow$ Plastoquinone $\rightarrow$ Cytochrome $b_6f$ complex) P(S)F(dNADP) (Plastocyanin $\rightarrow$ PSI $\rightarrow$ Ferredoxin $\rightarrow$ $\text{NADP}^+$ reductase)." Or simply 'PQ-Cyt-PC-Fd-NADH'.
• Glycolysis Steps for ATP/NADH: "Girl Guides Find Fantastic Girls Girls Don't Buy Pickles, Prepare Plenty Pastries!" (Glucose, G6P, F6P, F1,6BP, GAP, DHAP, 1,3BPG, 3PG, 2PG, PEP, Pyruvate). Focus on points of ATP/NADH.
  • ATP used: $1^{\text{st}}$ and $3^{\text{rd}}$ phosphorylation steps.
  • NADH produced: Oxidation of GAP.
  • ATP produced: Substrate-level phosphorylation twice.
• Krebs Cycle (mnemonic for intermediates): "Citrate Is Kreb's Starting Substrate, Fumarate Malate Oxaloacetate." (Citrate, Isocitrate, $\alpha$-Ketoglutarate, Succinyl $\text{CoA}$, Succinate, Fumarate, Malate, Oxaloacetate).
• Photoperiodism:
  • LDP (Long Day Plant): "Long light, short night."
  • SDP (Short Day Plant): "Short light, long night."
  • What matters most for triggering flowering is the length of the continuous dark period.

Previous Year Question (PYQ) Patterns

Analysis of past NEET papers reveals recurring themes and question types from Plant Physiology:

1. Water Potential Calculations & Concepts: Direct calculations of $\Psi_w$, $\Psi_s$, $\Psi_p$, or conceptual questions about the direction of water movement, plasmolysis, turgidity.
2. Photosynthesis - Pathways Comparison ($\text{C}_3$ vs $\text{C}_4$): Questions on Kranz anatomy, first stable products, primary $\text{CO}_2$ acceptors, enzymes involved, photorespiration presence/absence, number of ATP/NADPH required.
3. Photosynthesis - Light Reactions: Electron flow (cyclic vs non-cyclic photophosphorylation), products of light reaction, site of $\text{O}_2$ evolution, chemiosmotic hypothesis steps, components of ETS in chloroplasts.
4. Respiration - ATP Yield: Calculation of total ATP from a glucose molecule (aerobic), or specific stages (glycolysis, Krebs cycle). Understanding the net gain in glycolysis.
5. Respiration - Krebs Cycle & Glycolysis Details: Identification of intermediate compounds, enzymes, products at various steps. Location of processes.
6. Plant Growth Regulators (PGRs): Functions of individual hormones (auxin's apical dominance, gibberellin's bolting, cytokinin's cell division, ABA's stress response, ethylene's ripening). Applications in agriculture.
7. Transpiration Mechanisms & Factors: Cohesion-tension theory, stomatal mechanism (K+ theory), factors affecting transpiration rate.
8. RQ Values: Calculating RQ for different substrates or identifying the substrate based on RQ.
9. Photoperiodism & Vernalisation: Definitions, types of plants, roles of phytochrome, practical applications.

Typical Question Formats:

• Direct Recall: "Which enzyme is responsible for initial $\text{CO}_2$ fixation in $\text{C}_4$ plants?"
• Application-based: "A seed is failing to germinate despite favorable conditions. Which PGR could help overcome this?"
• Diagram-based: Identify parts of chloroplast/mitochondria or label steps in a pathway diagram.
• Match the column: Matching hormones with functions or pathways with their unique features.
• Assertion-Reason: Testing conceptual understanding and cause-effect relationships.

Revision Checklist

Going through this checklist will help ensure you've covered all critical areas for Plant Physiology.

I. Transport in Plants

• Diffusion, Facilitated Diffusion, Active Transport (differences, examples)
• Water Potential ($\Psi_w, \Psi_s, \Psi_p$) - definition, formula, calculation, influencing factors.
• Osmosis, Plasmolysis, Imbibition - concepts and significance.
• Long-distance water transport - Apoplast, Symplast pathways.
• Root Pressure, Guttation - mechanism, limitations.
• Cohesion-Tension-Transpiration Pull Theory - components, mechanism.
• Transpiration - types, stomatal mechanism (K+ theory), factors, significance.
• Mineral Uptake & Transport - active/passive, mobilization.
• Phloem Transport - Pressure Flow Hypothesis (source-sink, loading/unloading, components).

II. Photosynthesis in Higher Plants

• Overall equation, site (chloroplast structure - thylakoid, stroma).
• Photosynthetic pigments - types, absorption spectra, accessory pigments function.
• Light Reactions -
  • PS I & PS II ($\text{P}{700}, \text{P}{680}$) - location, function.
  • Non-cyclic (Z-scheme) - electron flow, products (ATP, NADPH, $\text{O}_2$), photolysis.
  • Cyclic - electron flow, product (ATP only), conditions.
  • Chemiosmotic hypothesis - proton gradient, ATP synthase.
• Dark Reactions -
  • $\text{C}_3$ Pathway (Calvin Cycle) - location, RuBisCO, RuBP, 3-PGA, steps (carboxylation, reduction, regeneration), ATP/NADPH cost for 1 glucose.
  • $\text{C}_4$ Pathway (Hatch-Slack) - Kranz anatomy, PEPCase, OAA, $\text{CO}_2$ "pump", spatial separation, advantages, ATP/NADPH cost for 1 glucose.
• Photorespiration - definition, mechanism, enzyme, conditions, consequences, why C4 avoids it.
• Factors affecting photosynthesis - Law of Limiting Factors (light, $\text{CO}_2$, temperature, water), their effects.

III. Respiration in Plants

• Glycolysis - location, anaerobic, steps, net products (ATP, NADH).
• Fermentation - types (alcoholic, lactic acid), products, purpose (regenerate $\text{NAD}^+$).
• Aerobic Respiration -
  • Link Reaction - location, products (Acetyl $\text{CoA}$, $\text{CO}_2$, NADH).
  • Krebs Cycle - location, starting/ending compounds, products per Acetyl $\text{CoA}$ (NADH, $\text{FADH}_2$, ATP, $\text{CO}_2$).
  • ETS & Oxidative Phosphorylation - location (inner mitochondrial membrane), electron carriers, proton gradient, ATP synthase, final electron acceptor ($\text{O}_2$), ATP yield per NADH/FADH2.
• Respiration Balance Sheet - total ATP yield per glucose (theoretical).
• Amphibolic pathway - definition, examples.
• Respiratory Quotient (RQ) - formula, values for different substrates.

IV. Plant Growth and Development

• Growth - definition, characteristics, types (arithmetic, geometric), growth rates (AGR, RGR).
• Differentiation, Dedifferentiation, Redifferentiation - concepts, examples.
• Development, Plasticity - definition, examples.
• Plant Growth Regulators (PGRs) -
  • Auxins - functions (apical dominance, rooting, parthenocarpy, 2,4-D).
  • Gibberellins - functions (stem elongation/bolting, dormancy break, fruit growth, malting).
  • Cytokinins - functions (cell division, lateral bud growth, delay senescence).
  • Abscisic Acid (ABA) - functions (dormancy, abscission, stomatal closure, stress hormone).
  • Ethylene - functions (fruit ripening, senescence, abscission, root growth).
• Photoperiodism - definition, critical photoperiod, phytochrome ($\text{P}r$ vs $\text{P}{fr}$), LDP, SDP, DNP.
• Vernalisation - definition, mechanism, examples.
• Seed Dormancy - causes, methods to break dormancy.

Last-Day Strategy

The day before the exam, focused and calm revision is key. Avoid learning new topics.

1. Review Mnemonics & Tables: Quickly go through all the memory tricks and comparative tables (C3 vs C4, PGR functions, RQ values, ATP yield). These are high-yield information and easy to forget.
2. Flowchart Visualization: Mentally (or quickly sketch) the flowcharts for:
   • Water movement (root to leaf).
   • Light reaction (Z-scheme, cyclic).
   • Calvin cycle steps.
   • Glycolysis, Link reaction, Krebs cycle, ETS. Focus on inputs, outputs, key enzymes, and ATP/NADH production/consumption.
3. Hormone Functions (Quick Scan): Read through all the functions of each PGR. Remember specific examples (e.g., 2,4-D for dicot weeds, Ethephon for ripening).
4. Key Definitions and Concepts: Briefly glance at definitions for water potential, photoperiodism, vernalization, photorespiration, etc. Ensure you understand the underlying principles.
5. Previous Year Question Patterns: Revisit the PYQ patterns and your identified common mistakes. This helps you anticipate question types and avoid silly errors.
6. Don't attempt new problems: Stick to reviewing concepts. Solving new complex problems might cause unnecessary anxiety if you get stuck.
7. Relax and Sleep Well: Adequate rest is crucial for optimal cognitive function during the exam. Trust your preparation!

By systematically revisiting these core concepts and utilizing the provided resources, you can effectively master Plant Physiology for NEET. Good luck!