Transport in Plants

🎯 Key Points

• Water movement is always passive, driven by a water potential gradient (water moves from higher to lower, i.e. less negative to more negative, water potential) — no ATP directly spent on water movement itself
• Apoplast pathway (through cell walls, fast, no membranes crossed) vs Symplast pathway (through cytoplasm via plasmodesmata, crosses membranes); the Casparian strip in root endodermis BLOCKS the apoplast route, forcing water through the symplast for selective ion control
• Ascent of sap (xylem, upward): root pressure (minor, pushes from below) + transpiration pull via the cohesion-tension theory (major, pulls from above) — cohesion between water molecules lets a continuous unbroken column be pulled up
• Stomatal opening: light activates a proton (H+) pump in guard cells → K+ flows in → water follows by osmosis → guard cells become turgid → stomata open; ABA reverses this to close stomata under stress
• Sugar transport (phloem): pressure flow / Munch hypothesis — an ACTIVE process, unlike water transport; sugars are loaded at the source (e.g. leaves), raising osmotic pressure there, which drives bulk flow of sap toward the sink (e.g. roots, fruits)

Water Absorption and the Root Pathways

• Water enters root hairs by osmosis, moving along a water potential gradient from soil (higher/less negative) to root xylem (lower/more negative)
• Apoplast pathway: water moves through cell walls and intercellular spaces without crossing any cell membrane — fast, but offers no opportunity for the plant to filter what passes through
• Symplast pathway: water moves from cell to cell through the cytoplasm, crossing the plasma membrane once and then travelling via plasmodesmata — slower, but allows selective control
• Casparian strip: a band of suberin (waterproof) in the cell walls of the root endodermis that completely blocks the apoplast pathway at that layer, forcing all water and dissolved minerals to pass through the symplast (crossing a selectively permeable membrane) before reaching the xylem — this is the plant's checkpoint for controlling which minerals actually enter the vascular tissue

Ascent of Sap (Xylem Transport)

• Root pressure: a positive pressure generated by active ion accumulation in the root xylem, drawing water in by osmosis; can push water a limited height (seen as guttation droplets on grass) but is far too weak alone to explain water rising to the top of tall trees
• Cohesion-Tension Theory (the dominant mechanism): transpiration from leaf surfaces creates a tension (negative pressure) that pulls water upward; this works only because water molecules are strongly cohesive (hydrogen-bonded to each other) and adhesive (to xylem walls), allowing an unbroken column of water to be pulled up as a continuous thread all the way from root to leaf, rather than the column simply breaking apart

Transpiration and Stomatal Regulation

• Transpiration is the evaporative loss of water vapour, mainly through stomata on leaves; it is essentially an "unavoidable cost" of having stomata open for gas exchange (CO2 in, O2 out)
• Stomatal opening mechanism: light activates a H+-ATPase pump in the guard cell plasma membrane, pumping protons out; this hyperpolarises the membrane, driving K+ influx through channels; water follows the K+ by osmosis, the guard cells become turgid, and because of their uneven wall thickening, turgid guard cells bow apart, opening the stomatal pore
• Stomatal closure: under water stress, ABA binds receptors on the guard cell, triggering a rise in cytosolic Ca²⁺, which activates anion efflux channels and inhibits K+ influx — guard cells lose turgor and the pore closes
• Transpiration also helps with mineral transport (the same water stream carrying dissolved minerals) and evaporative cooling of leaf surfaces

Phloem Transport: The Pressure Flow (Munch) Hypothesis

• Unlike xylem transport (passive, driven by transpiration), phloem transport is an ACTIVE, energy-requiring process
• At the source (e.g. a photosynthesising leaf), sucrose is actively loaded into sieve tube elements, lowering the water potential there and drawing water in by osmosis from the adjacent xylem — this raises turgor pressure at the source end
• This pressure difference drives bulk flow of the sugar-rich sap through sieve tubes toward the sink (e.g. a root or developing fruit), where sucrose is unloaded, raising the sink's water potential and causing water to exit back into the xylem
• Companion cells, connected to sieve tube elements via abundant plasmodesmata, are metabolically highly active and are responsible for the loading and unloading of sugars at both ends
• Unlike xylem (one-way, always upward), phloem transport can move in EITHER direction depending on which organ is currently the source and which is the sink (e.g. a storage root can become a source when the plant remobilises stored sugars for new growth)

Means of Transport in Plants

• Diffusion: passive movement of substances down a concentration gradient; no energy is spent and no transport proteins are required; it is slow and effective only over short distances, and is the sole means by which gases move within the plant body
• Facilitated diffusion: still passive (down the gradient, no ATP) but assisted by membrane transport proteins; hydrophilic or larger molecules cross through specific channels or carrier proteins; it is highly selective, can be saturated, and includes uniport, symport and antiport arrangements
• Active transport: uses ATP to pump molecules AGAINST their concentration gradient via membrane pump proteins; it is selective, saturable and sensitive to specific inhibitors, and lets a cell accumulate ions to concentrations far higher than in the surrounding soil solution

Plant–Water Relations: Water Potential, Osmosis and Plasmolysis

• Water potential (Ψ_w): the potential energy of water in a system; pure water has the highest water potential, taken as zero; adding solutes or applying tension lowers it (makes it more negative), and water always moves from a region of higher (less negative) to lower (more negative) water potential
• Ψ_w = Ψ_s + Ψ_p: water potential is the sum of solute potential (Ψ_s, always negative, lowered by dissolved solutes) and pressure potential (Ψ_p, usually positive, e.g. the turgor pressure of a cell)
• Osmosis: net diffusion of water across a selectively permeable membrane along a water-potential gradient; a cell in a hypotonic solution gains water (endosmosis, becomes turgid), while in a hypertonic solution it loses water (exosmosis)
• Plasmolysis: in a hypertonic solution the protoplast loses water and shrinks away from the cell wall; the stage at which it just begins to pull away is incipient plasmolysis; the process is reversible (deplasmolysis) when the cell is returned to pure water or a hypotonic solution

Imbibition

• Imbibition is a special type of diffusion in which a solid colloid (the imbibant) adsorbs water and swells in volume, e.g. dry seeds swelling on soaking or wooden doors swelling in the rainy season
• It requires both a water-potential gradient between the adsorbent and the water AND an affinity between them; it generates enormous imbibition pressure, which is how germinating seeds and growing roots can crack rocks and break through hard soil

Uptake and Transport of Mineral Nutrients

• Unlike water, minerals cannot be absorbed passively; most ions are taken up by active transport against their concentration gradient using membrane carrier proteins and ATP, which is why mineral uptake is selective and energy-dependent
• Ions enter through the root epidermis (partly passive, partly active), reach the endodermis where the Casparian strip forces a selective, active passage, and are then loaded into the xylem for long-distance transport
• Minerals are carried upward chiefly in the transpiration stream of the xylem; they are also remobilised — mobile elements such as nitrogen, phosphorus, sulphur and potassium can be exported from older, senescing leaves through the phloem to young growing parts, which is why the deficiency symptoms of these mobile elements appear in older leaves first

🚀 NEET Advanced Edge

Why phloem transport, unlike xylem, requires active loading: Xylem transport relies purely on a passive physical gradient (transpiration tension), but phloem sap must move against varying and sometimes unfavourable pressure differences depending on plant needs — active sucrose loading at the source artificially creates the pressure gradient needed, which is why phloem transport (but not xylem transport) is blocked by metabolic inhibitors that stop ATP production.

Why a ring of bark removal (girdling) kills a tree slowly, not instantly: Removing a ring of bark (which contains phloem, lying just outside the xylem) cuts off the downward sugar supply from leaves to roots; the roots can survive for a while on stored reserves, but eventually starve since they cannot receive new photosynthate — while water and minerals continue moving upward normally through the unaffected xylem, which is why the tree doesn't wilt immediately, just dies back gradually.

Worked reasoning: A plant cell is placed in a solution and neither gains nor loses water. Stating that the cell's water potential equals the solution's water potential is sufficient to explain this — true or false, and why? Answer: True. Net water movement always follows the water potential gradient; if there is no net movement, the two water potentials must be equal (the cell is at "incipient plasmolysis" equilibrium with that specific external solution), even though the cell's own internal pressure and solute potential components might individually still be nonzero.