Mass Transport in Plants

AQA spec ref: 3.3.5 - Mass transport in plants

Plants are large multicellular organisms that face the same fundamental challenge as animals: diffusion alone is too slow to move water, ions, and organic solutes over the distances required. They have evolved two specialised vascular tissues for mass transport: xylem (transports water and dissolved mineral ions from roots to leaves) and phloem (transports dissolved organic solutes - mainly sucrose - from sources to sinks). See Water and Transport Across Cell Membranes.

Water Movement in Roots - Apoplast and Symplast Pathways

Water enters roots from the soil by osmosis (soil water potential > root hair cell water potential). It then crosses the cortex to reach the xylem by two routes:

Apoplast Pathway

Water moves through the cell walls and intercellular spaces (the apoplast) without crossing any cell membranes. This is the path of least resistance - cell walls are permeable and hydrophilic. Water can move freely by mass flow down a water potential gradient.

The Casparian strip - a band of suberin (a waxy, waterproof material) that impregnates the cell walls of the endodermis (the innermost layer of the cortex). The Casparian strip completely blocks the apoplast pathway at the endodermis - water cannot pass through the endodermal cell walls.

This forces water to enter the cytoplasm of endodermal cells (cross the plasma membrane) before it can reach the xylem. This is critical because:

• It allows the plant to control which ions enter the xylem (selective transport across membranes)
• It prevents ions from leaking back out of the xylem into the cortex
• It generates the root pressure that assists upward water movement

Symplast Pathway

Water moves through the cytoplasm of adjacent cells, passing from one cell to the next via plasmodesmata (cytoplasmic connections through pores in cell walls). This pathway crosses cell membranes.

Movement into Xylem

After crossing the endodermis, water enters the xylem. Mineral ions are actively transported into xylem vessels, lowering the water potential → water follows by osmosis. This generates root pressure - a positive pressure that can push water up the stem (demonstrated by bleeding from a cut stem).

Xylem Structure

Xylem vessels are the primary water-conducting elements of the xylem. They are:

• Dead at maturity - the cell contents (cytoplasm, nucleus) are lost, leaving a hollow, empty lumen for unimpeded water flow
• Heavily lignified - secondary cell walls impregnated with lignin, a rigid polymer that:
• Prevents the vessel from collapsing under tension
• Makes the wall waterproof (water cannot leave through the walls)
• Provides mechanical support to the plant
• Joined end-to-end with perforated or absent end walls (perforation plates)→continuous tubes running the length of the plant
• Bordered pits - thin areas in the wall where lignification is absent; allow lateral movement of water between adjacent vessels

Transpiration and the Cohesion-Tension Theory

Transpiration is the loss of water vapour from the leaves (mainly through stomata). It is the driving force for the ascent of water in xylem.

Mechanism - Cohesion-Tension Theory

1. Evaporation - water evaporates from the wet cell walls of mesophyll cells into the intercellular air spaces → diffuses out through open stomata. This creates a water potential gradient: cell water potential falls.

1. Osmosis from xylem - mesophyll cells with lower water potential draw water from the leaf xylem by osmosis.

1. Tension (negative pressure) - as water is pulled from the top of the xylem column, it creates a tension (negative hydrostatic pressure) throughout the continuous water column in the xylem - like pulling on a rope. This pull is transmitted all the way down to the roots.

1. Cohesion - water molecules are strongly attracted to each other by hydrogen bonds (cohesion - see Water). This means the water column does not break - it is pulled up as a continuous column under tension.

1. Adhesion - water also adheres to the hydrophilic lignin walls of xylem vessels, preventing the column from pulling away from the walls.

1. Root uptake - the tension at the root draws water in from the soil by osmosis.

Summary of forces: transpiration pull (tension) > cohesion of water column + adhesion to xylem walls → continuous upward mass flow.

This is a passive process - no ATP is required at the xylem stage (though active transport is required to load ions into xylem and at stomatal guard cells).

Factors Affecting Transpiration Rate

Measuring Transpiration - The Potometer

A potometer measures the rate of water uptake by a cut plant shoot (not strictly transpiration rate, since some water is used in photosynthesis and metabolic processes, but these amounts are negligible). It works by:

• A cut shoot is attached to a sealed capillary tube submerged in water
• As the shoot transpires, water is drawn in through the capillary
• The movement of an air bubble along the capillary tube is measured over time→rate of water uptake

To compare transpiration rates under different conditions, the same shoot is used and one variable changed at a time. The bubble is reset using a syringe.

Phloem Structure

Phloem transports dissolved organic solutes (assimilates), mainly sucrose, from sources (where sucrose is produced or mobilised - leaves, storage organs) to sinks (where sucrose is used or stored - roots, growing buds, fruits, seeds).

Phloem consists of:

• Sieve tube elements - living cells but unusual: no nucleus, few organelles (to allow unimpeded flow). Connected end-to-end by sieve plates - porous end walls with large pores (sieve pores) through which solutes flow. Cytoplasm of adjacent sieve tube elements is continuous through the sieve pores.

• Companion cells - closely associated with each sieve tube element (derived from the same precursor cell). They have a nucleus, many mitochondria, and are metabolically very active. They support the sieve tube element and are involved in loading/unloading sucrose. Connected to sieve tubes by numerous plasmodesmata.

Translocation - The Mass Flow Hypothesis

The mechanism of transport in phloem is described by the mass flow hypothesis (Münch, 1930):

Loading at the Source

In leaves, sucrose is produced by photosynthesis (or mobilised from starch in storage organs). It is actively loaded into companion cells and then into sieve tube elements by H⁺-sucrose cotransport:

1. H⁺-ATPase on the companion cell membrane actively pumps H⁺ out of the cell (using ATP) → creates a H⁺ gradient (high [H⁺] outside, low inside)
2. Sucrose-H⁺ cotransporter uses the energy of H⁺ moving back in down its gradient to simultaneously move sucrose into the companion cell against its concentration gradient
3. Sucrose passes from companion cell→sieve tube element via plasmodesmata

This loading of sucrose raises the solute concentration in the sieve tube → water potential falls → water enters sieve tubes from surrounding xylem by osmosis → hydrostatic pressure in sieve tube rises at the source end.

Unloading at the Sink

At sinks (roots, fruits), sucrose is removed from sieve tubes (by active transport or facilitated diffusion, depending on the sink) → sucrose concentration in sieve tube falls → water potential rises → water leaves sieve tubes by osmosis (back into xylem or surrounding cells) → hydrostatic pressure falls at the sink end.

Mass Flow

The pressure difference between source (high pressure) and sink (low pressure) drives a bulk flow of solution (sucrose dissolved in water) along the sieve tubes from source to sink. This is mass flow - the whole solution moves, not just sucrose molecules diffusing.

Evidence For and Against Mass Flow

Supporting evidence:

• Sieve tube contents are under pressure (demonstrated by aphid stylus experiments - aphids feed by inserting their stylet into phloem; when the head is cut off, sap continues to exude under pressure)
• Sucrose concentration is higher at source than sink
• Metabolic inhibitors (e.g. DNP, which blocks ATP synthesis) reduce translocation - confirming active loading at the source

Weaknesses/problems:

• Sieve plates would create high resistance - mass flow would need very high pressures to overcome this
• Different solutes can move at different speeds in the same sieve tube (incompatible with pure mass flow)
• Some observations suggest bidirectional transport can occur within the same tissue (though not the same sieve tube)

Summary

• Apoplast = cell wall route; blocked at endodermis by Casparian strip (suberin)
• Symplast = cytoplasm route via plasmodesmata; must cross plasma membranes at endodermis
• Xylem: dead, lignified, hollow tubes; carries water + minerals upward
• Cohesion-tension: transpiration pull → tension → cohesion holds column → water drawn up
• Transpiration affected by temperature, humidity, wind, light, water availability
• Phloem: sieve tubes (no nucleus, sieve plates) + companion cells (many mitochondria, nucleus); carries sucrose source→sink
• Mass flow: sucrose loading (H⁺ cotransport, ATP) → high pressure at source; unloading at sink → low pressure → bulk flow source to sink

AQA Exam Tips

• Casparian strip: "a band of suberin in the cell walls of the endodermis that blocks the apoplast pathway, forcing water through the symplast (across cell membranes) → plant can control ion entry to xylem." All three elements needed for full marks.
• Cohesion-tension: describe the mechanism in sequence: (1) transpiration → water lost from leaf (2) water pulled from xylem → tension created (3) cohesion holds water column together (4) water drawn up from roots. AQA mark schemes require all steps.
• Why xylem is dead: dead cells → no cytoplasm → no obstruction to water flow → unimpeded mass flow. Also allows lignification of the entire cell wall.
• Active loading in phloem: requires ATP (H⁺-ATPase). Metabolic inhibitors reduce translocation - evidence for active loading. This is a common source of data in exam questions.
• Source and sink: be ready to identify sources and sinks in different contexts (seed germination: cotyledon = source, embryo = sink; spring: root storage = source, growing leaves = sink).
• Potometer measures water uptake, not transpiration: strictly speaking, transpiration includes only water lost as vapour. A tiny amount of water uptake is used in photosynthesis. Always state "rate of water uptake" not "transpiration rate" unless asked.