Water potential (Ψw = Ψs + Ψp) determines the direction of water movement: from higher to lower Ψw across semipermeable membranes. The apoplast pathway moves water through cell walls rapidly but non-selectively, while the symplast pathway moves water through plasmodesmata selectively. The Casparian strip — a suberin band in the endodermis — blocks the apoplast pathway, ensuring all minerals are filtered through the selective symplast. Root pressure causes guttation (liquid water secreted through hydathodes at night), while the cohesion-tension theory (Dixon and Joly) explains water ascent in tall plants via transpiration pull. Transpiration is predominantly stomatal (90–95%), with guard cells regulating aperture through $K^{+}$ flux and ABA signalling. Plants require 17 essential elements: 9 macronutrients and 8 micronutrients, established by Arnon and Stout's three criteria of essentiality. Mobile elements (N, P, K, Mg) show deficiency in older leaves first; immobile elements (Ca, Fe, Mn, B) show deficiency in younger leaves first. Nitrogenase — a Mo-Fe protein — fixes $N_{2}$ to $NH_{3}$ using 16 ATP under strictly anaerobic conditions, with leghemoglobin scavenging $O_{2}$ in legume nodules. Nitrification proceeds in two steps: Nitrosomonas converts $NH_{3}$ to $NO_{2}^{-}$, and Nitrobacter converts $NO_{2}^{-}$ to $NO_{3}^{-}$; denitrification by Pseudomonas returns $NO_{3}^{-}$ to atmospheric $N_{2}$. Key free-living nitrogen fixers are Azotobacter (aerobic) and Clostridium (anaerobic), while Anabaena and Nostoc fix nitrogen in specialised heterocyst cells.