Plant Physiology — Transport and Mineral Nutrition

Why This Chapter Matters

Plant physiology has 6-10 marks in NEET every year. Water relations (osmosis, imbibition, plasmolysis), mineral nutrition, and transpiration are frequently tested topics.

Core Concepts

1. Water Relations

Water potential (Ψ): Determines direction of water movement. Water moves from high to low water potential.

Ψ = Ψ_s + Ψ_p

Ψ_s = solute potential (negative — solutes reduce water potential)

Ψ_p = pressure potential (positive in turgid cells, negative in xylem during transpiration)

Pure water: Ψ = 0 (highest possible water potential)

Any solution: Ψ < 0 (negative)

Osmosis: Movement of water through semi-permeable membrane from high Ψ (dilute solution) to low Ψ (concentrated solution).

Turgor pressure: Pressure exerted by cell contents on cell wall when cell is fully turgid (fully hydrated).

Wall pressure: Counter pressure exerted by cell wall on cell contents (= turgor pressure in magnitude, opposite direction).

Plasmolysis: Loss of water → cell membrane pulls away from cell wall. Cell becomes flaccid.

Deplasmolysis: Water enters back → membrane returns to cell wall position.

Turgidity → Flaccidity: Loss of water, turgor pressure decreases.

Fully turgid cell: Ψ_p = positive, Ψ_s = negative, net Ψ lower than pure water.

Imbibition: Absorption of water by dry materials (seeds, wood). No solute — involves adsorption, generates imbibition pressure.

2. Mineral Nutrition

Essential elements:

•Macronutrients (needed in large amounts): C, H, O, N, P, K, Ca, Mg, S

•Micronutrients/trace elements (needed in small amounts): Fe, Mn, Zn, Cu, Mo, B, Cl, Ni

Roles of key minerals:

Element	Form absorbed	Function

|---|---|---|

Nitrogen	NO₃⁻ or NH₄⁺	Amino acids, proteins, nucleic acids, chlorophyll
Phosphorus	H₂PO₄⁻	ATP, nucleic acids, phospholipids
Potassium	K⁺	Stomatal movement (guard cells), enzyme activation
Calcium	Ca²⁺	Cell wall (middle lamella), cell signalling
Magnesium	Mg²⁺	Centre of chlorophyll, enzyme cofactor
Sulphur	SO₄²⁻	Amino acids (cysteine, methionine), proteins
Iron	Fe²⁺/Fe³⁺	Cytochromes, nitrogenase, ferredoxin

Deficiency symptoms:

Nitrogen: chlorosis (yellowing) of older leaves first

Iron: chlorosis of young leaves (interveinal chlorosis)

Magnesium: interveinal chlorosis (Mg is part of chlorophyll)

3. Nitrogen Fixation

Biological nitrogen fixation: Conversion of atmospheric N₂ to ammonia (NH₃).

Enzyme: Nitrogenase (requires Mo and Fe; anaerobic conditions)

Free-living N-fixing bacteria: Azotobacter (aerobic), Clostridium (anaerobic), Rhodospirillum (photosynthetic)

Symbiotic N-fixing: Rhizobium in root nodules of legumes

Cyanobacteria: Nostoc, Anabaena (also in Azolla symbiosis in rice fields)

Nitrification: NH₃ → NO₂⁻ → NO₃⁻ by Nitrosomonas, Nitrobacter

Denitrification: NO₃⁻ → N₂ by Pseudomonas, Thiobacillus

4. Transpiration

Loss of water vapour from aerial parts of plants (mostly leaves via stomata).

Types:

Stomatal: 90-95% (major) through stomata

Cuticular: through cuticle (5-10%)

Lenticular: through lenticels (minor)

Guard cell mechanism:

K⁺ ions move into guard cells in light → osmotic potential decreases → water enters → cells become turgid → stomata open

K⁺ move out in dark → cells lose turgor → stomata close

Factors affecting transpiration:

Light: increases (stomata open)

Temperature: increases (increases evaporation)

Humidity: decreases (if high humidity — reduces gradient)

Wind: increases (removes humid air from leaf surface)

CO₂: high CO₂ → stomata close

Cohesion-tension theory (Dixon and Jolly):

Water columns in xylem are continuous due to cohesion (H-bonds between water molecules)

Transpiration creates tension (negative pressure) which pulls water up

This creates the transpiration pull that moves water from roots to leaves

PYQs (NEET)

NEET 2023: Which element is part of the chlorophyll molecule?

Magnesium (Mg²⁺) — forms the centre of the porphyrin ring of chlorophyll.

NEET 2022: The enzyme responsible for biological nitrogen fixation:

Nitrogenase (contains Mo and Fe, works in anaerobic conditions)

NEET 2021: Water potential of pure water at standard conditions is:

Zero (0) — all solutions have negative water potential

NEET 2020: Transpiration is responsible for which function:

(A) Nutrient transport (B) Creation of transpiration pull to move water upward ✓ (C) Photosynthesis (D) Mineral absorption

MCQ Practice

Q1. Plasmolysis occurs when a cell is placed in:

(A) Hypotonic solution (B) Hypertonic solution ✓ (C) Isotonic solution (D) Distilled water

Q2. K⁺ ions enter guard cells in:

(A) Darkness (B) High CO₂ (C) Light ✓ (D) High temperature

Q3. Free-living aerobic nitrogen fixer:

(A) Rhizobium (B) Clostridium (C) Azotobacter ✓ (D) Anabaena

Q4 (Hard). A wilted plant has:

Flaccid cells (low turgor pressure). Ψ_p = 0 or even negative. Ψ_s unchanged (still negative). Net Ψ = Ψ_s (most negative) — this is the LOWEST water potential state in normal cells.

Revision Notes

WATER POTENTIAL: Ψ = Ψ_s + Ψ_p
Pure water: Ψ=0 | Any solution: Ψ<0
Water moves: high Ψ → low Ψ (from dilute to concentrated)

OSMOSIS: through semi-permeable membrane
TURGOR: cell full of water, Ψ_p positive
PLASMOLYSIS: hypertonic soln → water leaves → membrane pulls from wall

MINERAL NUTRITION:
Macronutrients: C,H,O,N,P,K,Ca,Mg,S
Micronutrients: Fe,Mn,Zn,Cu,Mo,B,Cl,Ni
Chlorophyll centre: Magnesium (Mg²⁺)
Nitrogenase cofactors: Mo and Fe

NITROGEN CYCLE:
N₂-fixation: Nitrogenase enzyme (Rhizobium, Azotobacter, Nostoc)
Nitrification: NH₃→NO₂→NO₃ (Nitrosomonas, Nitrobacter)
Denitrification: NO₃→N₂ (Pseudomonas)

TRANSPIRATION:
Mainly stomatal (90-95%)
Guard cells: K⁺ in → turgid → stomata open (light)
Cohesion-tension theory: Dixon & Jolly

Transport in Plants and Mineral Nutrition