Transport in Plants

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Transport in Plants — Key Facts for MDCAT

Why do plants need transport systems? Plants need to transport water, minerals, and organic compounds throughout their body. Since plants are stationary, they rely on specialized vascular tissues and physical forces for long-distance transport.

Two Major Vascular Tissues:

1. Xylem: Transports water and mineral salts from roots to aerial parts

   • Conduction: Passive process driven by transpiration pull
   • Conducting cells: Tracheids and vessel elements (dead cells, hollow)
   • Types: Protoxylem (first-formed) and Metaxylem (later-formed)
   • Tissue includes: Xylem parenchyma, xylem fibres, xylem ray

2. Phloem: Transports organic food (sugars, mainly sucrose) from leaves to all parts

   • Conduction: Active process (translocation)
   • Conducting cells: Sieve tube elements + companion cells
   • Sieve tubes: Living cells but lack nucleus at maturity; connected by sieve plates
   • Companion cells: Provide metabolic support to sieve tube elements

Transpiration: Loss of water vapour from aerial parts of plants (mainly through stomata).

• Transpiration pull creates the force for water movement in xylem (cohesion-tension theory)
• Cooling effect: Evaporative cooling prevents overheating
• Creates negative pressure (tension) in xylem columns
• Rate measured by porometer

Root Pressure: Pressure generated in roots when water is absorbed osmotically and pushed upward.

• Typical pressure: 0–2 atm (about 2–3 bars)
• Not sufficient to lift water to tops of tall trees
• Guttation (water droplets at leaf tips) occurs when root pressure > transpiration pull

Ascent of Sap: Water rises in xylem due to:

1. Root pressure (pushing from below)
2. Transpirational pull (pulling from above — major contributor)

⚡ Exam tip: Root pressure alone cannot explain water transport in tall trees (e.g., redwood trees > 100 m). The cohesion-tension theory states that water molecules are held together by hydrogen bonds (cohesion) and stick to xylem walls (adhesion), creating a continuous column that is pulled up by transpiration. Transpirational pull is the MAIN driver for ascent of sap. Root pressure becomes important when transpiration is low (at night, in humid conditions).

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students who want genuine understanding.

Transport in Plants — Complete Study Guide

Cohesion-Tension Theory (explaining ascent of sap):

1. Water evaporates from mesophyll cell walls into air spaces → vapour exits through stomata (transpiration)
2. This creates a meniscus (concave water surface) in leaf cell walls
3. Surface tension pulls water molecules inward → creates negative pressure (tension) in leaf xylem
4. This tension pulls water from petiole → stem → root xylem
5. Water molecules stick together (cohesion, ~350 kPa) and stick to xylem walls (adhesion)
6. The continuous water column does not break because of high tensile strength of water

Evidence supporting cohesion-tension theory:

• Negative pressure measured in xylem using pressure chamber
• Air bubbles (cavitation) interrupt water columns — can cause wilting
• A single cut through a stem causes air entry → water column breaks

Phloem Translocation:

• Organic compounds (mainly sucrose) are translocated from source (where produced) to sink (where used/stored)
• Sources: Mature leaves (photosynthesis), storage organs (during growth)
• Sinks: Roots, fruits, developing leaves, seeds, tubers
• Mechanism: Pressure flow hypothesis (mass flow hypothesis)
  • Sucrose actively loaded into sieve tubes at source → water follows osmotically → high turgor pressure
  • Sucrose actively unloaded at sink → water follows → low turgor pressure
  • Mass flow from high to low pressure

Active loading/unloading of sucrose:

• Sucrose is actively transported into companion cells/sieve tubes via proton-sucrose cotransport
• ATP is used to pump H⁺ out → creates H⁺ gradient → H⁺/sucrose symport

Uptake of Water and Minerals by Roots:

• Apoplast pathway: Water moves through cell walls and intercellular spaces without crossing membranes (up to endodermis)
• Symplast pathway: Water moves through cytoplasm of cells via plasmodesmata
• Transmembrane pathway: Water enters cell, exits opposite side (crosses membranes twice)

Casparian Strip:

• Waxy band in the endodermal cell walls
• Blocks the apoplast pathway at the endodermis
• Forces water and dissolved minerals through the symplast pathway
• This allows selective uptake of minerals (active transport into stele)

⚡ Common mistakes: Confusing xylem (water transport, dead cells, one direction root→leaf) with phloem (food transport, living cells, bidirectional). Thinking root pressure is the main force for water ascent — actually transpirational pull is dominant. For phloem translocation, the pressure flow hypothesis requires energy for loading and unloading, not for the actual flow.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Transport in Plants — Advanced Notes

Transpiration Pull — Physical Basis: The evaporation of water from leaf surfaces creates a water potential gradient. Water potential ($\Psi$) is measured in MPa. $$\Psi_{total} = \Psi_w = \Psi_s + \Psi_p + \Psi_g + \Psi_m$$ Where: $\Psi_s$ = solute potential (always negative), $\Psi_p$ = pressure potential, $\Psi_g$ = gravitational potential, $\Psi_m$ = matric potential.

At the leaf-air interface, $\Psi_w$ is very low (very negative) due to low humidity. Water moves from high $\Psi_w$ (soil, ~-0.1 MPa) to low $\Psi_w$ (atmosphere, can be -100 MPa in dry air).

Stomatal Mechanism:

• Stomata are pores flanked by two guard cells
• Mechanism of opening: Active uptake of K⁺ and Cl⁻ into guard cells → water follows osmotically → guard cells swell → stomatal pore opens
• Mechanism of closing: K⁺ and Cl⁻ leave guard cells → water leaves → guard cells shrink → pore closes
• Blue light photoreceptors stimulate H⁺-ATPases in guard cell membranes
• Stomatal opening is maximal in morning, minimal at night
• Abscisic acid (ABA) promotes stomatal closure during drought stress

Wilting: Temporary wilting: Stomata close, transpiration stops, turgor is recovered overnight Permanent wilting: Soil water potential is too low, even overnight recovery cannot restore turgor, plant dies

Guttation:

• Occurs when root pressure forces water out of leaf tips/hydathodes (special openings)
• Seen in mornings in humid conditions (e.g., grass blades, tomato leaves)
• Distinguish from dew (condensation from air) — guttation water has dissolved substances

Cavitation and Embolism in Xylem:

• Formation of air bubbles (cavitation) breaks the water column
• Cavitation can be caused by: freeze-thaw cycles, drought stress, high transpiration rates
• Refilling of embolised vessels: positive root pressure can refill during low transpiration periods

Mineral Transport in Xylem:

• N, P, K, Mg: Mobile in xylem (can be redistributed to young leaves)
• Ca, Fe, B: Immobile in xylem (deficiency symptoms appear first in young leaves)

Mineral Redistribution (Phloem mobility): Mobile in phloem: N, P, K, Mg, S Immobile in phloem: Ca, Fe, B, Mn, Zn This determines which elements can be recycled from older to younger leaves.

Plant Transport Disorders:

• Blossom end rot (tomato): Calcium deficiency due to insufficient xylem transport
• Tip burn (lettuce): High transpiration rate overwhelming calcium supply
• Chlorosis: Yellowing due to iron or magnesium deficiency (iron deficiency first appears in young leaves because Fe is immobile in xylem)

MDCAT Question Patterns: MDCAT Pakistan transport in plants questions frequently test: (1) distinguishing xylem and phloem structure and function, (2) cohesion-tension theory components, (3) transpiration vs guttation vs root pressure, (4) apoplast vs symplast pathways and the role of Casparian strip, (5) source-sink relationship in phloem translocation, (6) stomatal opening/closing mechanism, (7) water potential components. 2–3 questions per paper. Cohesion-tension theory is high-yield.

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