Biology · BotanyBOT 06
Transport in Plants
Osmosis, plasmolysis, water potential, cohesion-tension, stomatal mechanism, phloem transport
Concept Core
Essential theory — everything NCERT tests on Transport in Plants
WATER POTENTIAL
ψ = ψs + ψp. Pure water ψ = 0. Solutes lower ψ (ψs always negative). Water moves from high ψ to low ψ. Turgid cell: ψp positive. Plasmolysed cell: ψp = 0.
OSMOSIS & PLASMOLYSIS
Osmosis: diffusion of water across semi-permeable membrane. Plasmolysis: cell in hypertonic solution loses water → membrane pulls from wall. Incipient plasmolysis = ψp just = 0. Reversible if returned to water.
COHESION-TENSION THEORY
Dixon & Joly: Transpiration creates tension at leaf; cohesion keeps water column intact; adhesion with xylem walls assists. Accounts for water rise in tall trees (>100m). Root pressure only works in small herbs.
CASPARIAN STRIP & APOPLAST/SYMPLAST
Apoplast: cell wall pathway (no membrane). Symplast: cytoplasm+plasmodesmata. Casparian strip (suberin) at endodermis blocks apoplast — forces symplast entry. Selective mineral absorption occurs here.
PHLOEM TRANSPORT — MÜNCH PRESSURE FLOW
Source (mesophyll) loads sucrose via H⁺-sucrose symporter → ψ drops → water enters → turgor pressure rises → mass flow to sink. Both upward and downward flow possible. Phloem sap: sugars + amino acids + hormones.
Fact & Formula Vault
High-yield facts, numbers, and formulas
ψ Formula
ψ = ψs + ψp
ψs = −iCRT
Turgid cell: ψp max
Plasmolysed: ψp = 0
Pathways
Apoplast: through cell walls
Symplast: through cytoplasm
Casparian strip: endodermis
Forces symplast at endodermis
Xylem vs Phloem
Xylem: dead, unidirectional
Phloem: living, bidirectional
Xylem: water + minerals
Phloem: sugars + amino acids
Worked Examples
NEET-style questions solved step-by-step
MEDIUMA cell has ψs = −10 bar, ψp = +4 bar. Solution ψ = −5 bar. Direction of water?▾
A cell has ψs = −10 bar, ψp = +4 bar. Solution ψ = −5 bar. Direction of water?
Cell ψ = −10 + 4 = −6 bar. Solution ψ = −5 bar. Water moves from high ψ (solution, −5) to low ψ (cell, −6). Water enters the cell.
HARDStomata open when guard cells accumulate K⁺. The immediate cause is:▾
Stomata open when guard cells accumulate K⁺. The immediate cause is:
Light activates H⁺-ATPase → H⁺ pumped out → membrane hyperpolarises → K⁺ channels open → K⁺ influx → water enters → turgid guard cells → stomatal opening. K⁺ accumulation drives osmotic water entry.
EASYGuttation in plants is due to:▾
Guttation in plants is due to:
Root pressure. Active mineral uptake creates osmotic gradient → water enters xylem under positive pressure → exuded through hydathodes at leaf margins, usually at night.
Mistake DNA
Common NEET traps for this chapter
⚠ ψs sign
ψs (solute potential) is ALWAYS negative. Adding solute always lowers ψ.
✓ Fix: ψs = −iCRT is always ≤ 0
⚠ Root pressure vs cohesion-tension
Root pressure only moves water a few metres. Tall trees rely on cohesion-tension.
✓ Fix: Tall tree = cohesion-tension (transpiration pull)
⚠ Phloem direction
Phloem is NOT only downward. It moves from any source to any sink.
✓ Fix: Source → Sink, not top → bottom
Chapter Intelligence
Exam data and last-minute strategy
NEET Frequency
1–2 Q/year. ψ calculations, stomatal K⁺ mechanism, Münch hypothesis, and Casparian strip are most tested.
High-Yield
ψ = ψs + ψp. K⁺ → stomata open. ABA → stomata close. Phloem: bidirectional. Guttation = root pressure. Cohesion-tension = tall trees.
Strategy
Practice ψ numericals. Draw xylem vs phloem table. Know Dixon & Joly for cohesion-tension.
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