Transport in plants encompasses both short-distance movement across individual cells and long-distance movement through specialised vascular tissue. Water and solutes obey definite physical and physiological laws, and mastering these laws is essential for scoring in NEET Biology.

Water Potential and Osmosis

Water potential (Ψw) is the chemical potential of water in a system relative to pure water and is expressed as Ψw = Ψs + Ψp. The solute potential (Ψs), also called osmotic potential, is always negative because dissolved solutes lower the free energy of water. The pressure potential (Ψp) is positive in turgid cells and zero at the point of incipient plasmolysis. Pure water at standard temperature and pressure has a water potential of zero. Water always moves from a region of higher water potential to a region of lower water potential across a semipermeable membrane — this is osmosis. When a plant cell is placed in a hypertonic solution, water exits and plasmolysis occurs: the protoplast shrinks away from the cell wall, leaving the cytoplasm detached from the wall. Imbibition is a special form of water adsorption by hydrophilic colloids such as cellulose and starch — dry seeds absorbing water and swelling generate immense imbibition pressure, powerful enough to crack seed coats and split rocks.

Pathways of Water Absorption

Water enters the root and travels toward the xylem by two routes. In the apoplast pathway, water moves through the continuous network of cell walls and intercellular spaces without crossing any plasma membrane — this route is fast but non-selective. In the symplast pathway, water moves through the cytoplasm of adjacent cells connected by plasmodesmata — this route is slower but selective because it crosses plasma membranes. At the endodermis, the Casparian strip — a band of waterproofing suberin deposited on the radial and transverse walls of endodermal cells — physically blocks the apoplast pathway. This forces all water and dissolved minerals into the symplast, ensuring that only selectively transported ions reach the xylem. The Casparian strip is therefore the gatekeeper of mineral absorption.

Root Pressure and Guttation

Root pressure is a positive hydrostatic pressure generated in the xylem of roots due to osmotic entry of water when transpiration is minimal, typically at night or early morning. It can push water upward short distances. A visible consequence is guttation: water droplets secreted in liquid form from leaf tips and margins through specialised pores called hydathodes. Students frequently confuse guttation (liquid water, hydathodes, root pressure, night-time) with transpiration (water vapour, stomata/cuticle, transpiration pull, daytime). Root pressure alone cannot explain water ascent in tall trees.

Cohesion-Tension Theory

The dominant mechanism of long-distance water transport is the cohesion-tension (transpiration pull) theory proposed by Dixon and Joly. Transpiration from mesophyll cells of leaves creates a negative pressure (tension) in the xylem. Cohesion — the strong hydrogen bonding between water molecules — keeps the water column intact. Adhesion — attraction between water molecules and the hydrophilic walls of xylem tracheids and vessels — prevents the column from collapsing laterally. The result is a continuous, unbroken water column stretching from root hair to leaf cell, driven entirely by the evaporative demand at the leaf surface. Transpiration occurs via three routes: stomatal (90–95%, regulated by guard cells), cuticular (5–10%, through the waxy cuticle), and lenticular (negligible, through lenticels in bark). Guard cell opening is triggered by light (blue light activates $H^{+}$-ATPase pump driving $K^{+}$ influx), low $CO_{2}$ concentration, and high humidity; closing is triggered by darkness, high $CO_{2}$, water stress, and abscisic acid (ABA).

Mineral Nutrition and Essentiality Criteria

Plants require 17 essential elements. Arnon and Stout (1939) defined three criteria of essentiality: (1) the element must be absolutely required for normal growth and reproduction; (2) its deficiency cannot be compensated by supplying any other element; (3) the element must be directly involved in plant metabolism — not merely a corrective agent for some other toxic substance. The 9 macronutrients (required in millimolar quantities) are C, H, O, N, P, K, Ca, Mg, and S. The 8 micronutrients (required in micromolar quantities) are Fe, Mn, Zn, Cu, Mo, B, Cl, and Ni.

Deficiency symptoms depend on nutrient mobility in the phloem. Mobile elements (N, P, K, Mg) are retranslocated from older leaves to actively growing younger tissues; therefore their deficiency appears first in older (lower) leaves as chlorosis or necrosis. Immobile elements (Ca, Fe, S, Mn, B) cannot be remobilised; therefore their deficiency appears first in younger (upper) leaves and meristematic regions. Key specific symptoms: Mo deficiency causes whiptail disease in cauliflower; B deficiency causes internal cork in apple and hollow stem in celery; Zn deficiency causes little leaf disease and khaira disease of rice.

Biological Nitrogen Fixation

The nitrogenase enzyme complex (a Mo-Fe protein) catalyses the reduction of atmospheric $N_{2}$ to $NH_{3}$: $N_{2}$ + 8$H^{+}$ + 8$e^{-}$ + 16 ATP → 2$NH_{3}$ + $H_{2}$ + 16 ADP + 16 Pᵢ. This reaction is irreversibly inhibited by oxygen. In legume root nodules, the pink oxygen-scavenging protein leghemoglobin maintains the strictly microaerobic environment required. Rhizobium is the primary symbiotic nitrogen fixer in legume nodules; Frankia fixes nitrogen in non-legume species such as Alnus and Casuarina. Free-living nitrogen fixers include Azotobacter (aerobic soil bacterium), Clostridium (anaerobic soil bacterium), and cyanobacteria Anabaena and Nostoc, which fix nitrogen within thick-walled anaerobic cells called heterocysts.

The Nitrogen Cycle

Nitrogen cycles through four major processes: (1) Fixation — conversion of $N_{2}$ to $NH_{3}$ by nitrogen-fixing organisms; (2) Nitrification — chemoautotrophic oxidation of $NH_{3}$ to $NO_{2}^{-}$ by Nitrosomonas, then $NO_{2}^{-}$ to $NO_{3}^{-}$ by Nitrobacter; (3) Assimilation — uptake of $NO_{3}^{-}$ by plant roots and its reduction to amino acids; (4) Denitrification — conversion of $NO_{3}^{-}$ back to $N_{2}$ by anaerobic bacteria Pseudomonas and Thiobacillus, returning nitrogen to the atmosphere and completing the cycle.