7.2 Transport Mechanisms

Xylem sap is water containing dissolved mineral ions. Root hairs absorb the materials from the soil, and the solution then enters xylem vessels and is transported through the root, stem and leaf system.

• Root entry: Root hairs increase surface area. Mineral ions enter root-hair cells by diffusion or active transport, depending on the concentration conditions; ion uptake can lower cell water potential, so water enters by osmosis.
• Across the root: The absorbed solution moves through the root by two linked routes. The apoplast follows cellulose cell walls and their continuous spaces, so water and dissolved ions move without crossing a partially permeable membrane at each cell. The symplast follows cytoplasm, plasmodesmata and vacuoles, with water crossing membranes and moving between cells through plasmodesmata.
• Selective boundary: At the endodermis, the suberin Casparian strip blocks the apoplast route and forces water and dissolved ions into the symplast. This membrane-crossing step helps the root regulate which mineral ions reach the xylem.
• Xylem route: After entering xylem vessels, water and dissolved mineral ions form the xylem sap that moves from roots through stems to leaves. Xylem walls are lignified and waterproofed, supporting the conducting pathway.

The apoplast is not an osmosis pathway: water in cell walls moves through freely permeable spaces. The symplast involves cell membranes, cytoplasm, vacuoles and plasmodesmata. This card ends at root entry and the xylem pathway; the later transpiration/cohesion-tension cards explain long-distance driving mechanisms. Staff-only visual brief: a root hair→cortex→endodermis/Casparian strip→xylem route with apoplast and symplast branches; do not generate or bind an image.

Water moves from soil through root hair cells, the cortex and endodermis into the xylem. Mineral-ion uptake and membrane-controlled pathways make root entry selective rather than a simple open pipe.

• Root hair entry: Root hairs increase surface area. Mineral ions may enter by diffusion or active transport, depending on concentration conditions. Ion uptake can lower the water potential of root-hair cells, so water enters by osmosis.
• Apoplast route: Water moves through cellulose cell walls and their continuous spaces. It does not cross a partially permeable membrane at each cell, so this route is relatively direct and fast.
• Symplast route: Water enters cell cytoplasm/vacuoles across membranes and moves between cells through plasmodesmata. Membrane crossings make this route slower but allow cell-level control.
• Casparian boundary: The suberin Casparian strip in the endodermis blocks the apoplast. Water and dissolved minerals must enter the symplast before reaching the xylem, helping regulate which ions enter the vascular tissue.
• Destination: The selected solution enters xylem vessels. The later transpiration and cohesion-tension objectives explain long-distance driving; this card stops at radial transport from soil to xylem.

Path cue: soil → root hair → cortex → endodermis/Casparian strip → xylem.

Apoplast movement through cell walls is not osmosis; symplast movement includes membrane crossings and plasmodesmata. Do not use root pressure or cohesion-tension as the explanation for this radial pathway. Staff-only visual brief: show the root hair-to-xylem route with apoplast and symplast branches converging at the Casparian strip; do not generate or bind an image.

Transpiration is the loss of water vapour from a plant, mainly through stomata in the leaves. Water evaporates from moist mesophyll cell walls into the leaf air spaces, then the vapour diffuses out through open stomata.

• Leaf interface: Liquid water arrives in the leaf through xylem. It evaporates from mesophyll cell walls into the internal air spaces, creating water vapour there.
• Exit route: When stomata are open, water vapour diffuses from the humid internal air spaces to the atmosphere. Guard cells regulate the stomatal opening; loss of guard-cell water can close the stomata and reduce both transpiration and gaseous exchange.
• Supported influences: Lower external humidity and higher temperature can increase the water-vapour gradient between the leaf air spaces and the atmosphere. Stomatal state can override a simple factor prediction, so these are not guaranteed monotonic rate rules.
• Plant relevance: Evaporation provides evaporative cooling, and the associated transpiration stream helps mineral-ion uptake. The water-vapour loss itself is not the same thing as liquid-water movement through xylem.
• Process boundary: Transpiration is the vapour-loss process; the later water-pull objective explains how evaporation contributes to xylem transport without making transpiration alone equal to the whole cohesion-tension mechanism.

Path cue: mesophyll cell wall → leaf air space → stomatal pore → atmosphere.

Do not call transpiration photosynthesis, guttation or the whole xylem pathway. The observable process defined here is evaporation at mesophyll surfaces followed by vapour diffusion through stomata. Staff-only visual brief: show mesophyll wall → leaf air space → stomatal pore → atmosphere, with guard-cell control and a separate liquid-xylem arrow; do not generate or bind an image.

The cohesion-tension theory explains how evaporation at leaves can help move a continuous column of water up xylem. Evaporation creates tension (negative pressure) in the xylem; cohesion transmits that pull through water molecules, while adhesion helps keep the column against the xylem walls.

1. Leaf event: Water evaporates from mesophyll surfaces into the leaf air spaces and exits by transpiration. This lowers water potential at the leaf end and creates tension in nearby xylem water.
2. Transmission: Hydrogen bonds create cohesion between adjacent water molecules. Because the molecules resist separating, the tension is transmitted down the continuous water column through the xylem towards the roots.
3. Wall support: Adhesion between water and cellulose in xylem walls helps stop the column pulling away from the vessel wall. Lignified xylem walls resist collapse under the pressure differences.
4. Long-distance result: The water-potential gradient between roots and leaves, maintained by root uptake and leaf water loss, supports upward movement through roots, xylem and leaves as the transpiration stream.

Causal cue: evaporation → xylem tension/transpiration pull → cohesion transmits pull → adhesion and lignified walls support the column → water moves upward.

Root pressure can assist entry and raise xylem pressure, but it is not the complete explanation for normal long-distance ascent.

Hydrogen bonding does not act as a pump by itself: evaporation supplies the initial pull, while cohesion and adhesion transmit/support it. This mechanism card explains the water column and xylem ascent; card 4590 defines the transpiration process, and card 4592 concerns xerophytic water-loss adaptations. Staff-only visual brief: show leaf evaporation, tension arrow down a continuous xylem water column, cohesion links, adhesion to cellulose wall and lignified-wall support; do not generate or bind an image.

Xerophytes are adapted to dry conditions. Their leaf features conserve water by reducing the exposed evaporating surface, increasing resistance to water-vapour diffusion, trapping humid air, or limiting the time when stomata are open.

• Thick waxy cuticle → longer diffusion barrier: A thicker cuticle increases the distance/resistance for water vapour leaving the leaf, so less water is lost through the surface.
• Rolled, curled or folded leaf → smaller exposed surface and humid interior: The outer surface exposed to dry air is reduced, while humid air is trapped inside the roll; the water-vapour gradient is therefore reduced.
• Sunken stomata and fine hairs → humid boundary layer: Stomata sit away from moving dry air and hairs trap moist air near the surface, reducing the diffusion gradient.
• Fewer stomata → fewer exit pores: Reducing pore number limits potential routes for water-vapour loss, although gas exchange capacity is also affected.
• Stomata closed by day and open at night → avoid the driest period: Opening at night and closing during daylight can reduce daytime water loss; the feature changes timing rather than making transpiration impossible.

Mechanism check: feature → altered exposed area, diffusion distance, humidity or stomatal opening → reduced water loss.

Not every xerophytic leaf has every listed feature. Identify the feature actually shown and explain its specific effect on the water-loss pathway; do not infer a complete xerophyte package from one adaptation. Water conservation can also constrain gaseous exchange, so “reduced transpiration” does not mean all plant exchange stops. Staff-only visual brief: annotated leaf cross-section with cuticle, sunken/hairy stomata and rolled humid space; do not generate or bind an image.

Phloem translocation is the transport of assimilates from a source, where they are available, to a sink, where they are stored or used. Phloem sap is a solution containing mainly sucrose as well as water and other dissolved substances such as amino acids, hormones and minerals.

• Source: A green leaf or green stem can produce sugars by photosynthesis and export them as sucrose. A storage organ can also act as a source when stored material is mobilised for new growth.
• Sink: A developing seed, fruit, root or other storage/growing region receives assimilates for use or storage. The same organ can change from source to sink as development or demand changes.
• Pathway: Sieve-tube elements carry the phloem sap. Companion cells support the associated sieve-tube elements and the loading/unloading interface; the detailed proton-gradient loading mechanism belongs to the next card.
• Direction: Assimilates can move upwards or downwards in phloem because direction follows the current source-to-sink relationship. A plant therefore cannot be classified as having one fixed “up” or “down” phloem direction.
• Energy boundary: Loading at the source and unloading at the sink require metabolic energy. That local active work is distinct from the subsequent bulk movement through the sieve tube.

Reasoning cue: identify the current source and sink → identify the assimilate in phloem sap → trace the sieve-tube pathway → state why direction and loading demand are context-dependent.

Phloem is not simply a downward sugar tube and is not the same transport system as xylem. Source/sink status is functional and can change; pressure-driven mass flow is explained in the later card. Do not use ATP as if it directly pushes every assimilate along the whole plant. No image is needed for this card.

Companion cells load assimilates such as sucrose into phloem sieve tubes. The route may be symplastic and passive, or—when sucrose takes the apoplastic route—may use ATP-dependent proton pumping and H⁺–sucrose co-transport.

1. Create the gradient: In a modified companion cell (transfer cell), a proton pump uses ATP to move H⁺ from the cytoplasm into the cell wall. This creates a high H⁺ concentration outside the cell.
2. Use the gradient: H⁺ moves back down its concentration gradient through a co-transporter protein. The same transport step carries sucrose into the companion cell against sucrose’s concentration gradient.
3. Enter the sieve tube: Sucrose moves from the companion cell into the associated sieve-tube element through plasmodesmata. Companion-cell membrane infoldings increase transport surface area, while many mitochondria supply ATP for the proton pump.
4. Recognise the alternative: If sucrose uses the symplastic route, it can move through cytoplasm and plasmodesmata by diffusion; the exact route is not presented as universal.
5. Pressure boundary: Loading raises the solute concentration in phloem and can lower its water potential, setting up water-entry and turgor consequences that the next mass-flow card connects to bulk movement. This card stops at loading/unloading rather than explaining the whole pressure-flow model.

Chain cue: ATP → proton pump → H⁺ gradient → H⁺–sucrose co-transport → companion cell → plasmodesmata → sieve tube.

Do not invent a named transporter or say ATP pushes every sucrose molecule along the entire plant. ATP powers the local proton pump; co-transport loads sucrose, while later mass flow is the long-distance process. Unloading occurs at sinks and may use apoplastic or symplastic routes, but its full pressure-flow consequence belongs to 4595. No image is generated for this card.

The pressure-flow hypothesis explains phloem mass flow: active sucrose loading at a source lowers the water potential of phloem sap, water enters by osmosis and raises source turgor pressure, and unloading at a sink lowers pressure there. Sap therefore moves in bulk down a hydrostatic pressure gradient.

1. Load at the source: Sucrose and other assimilates are loaded into sieve-tube elements at a source such as a photosynthesising leaf or storage organ. The higher solute concentration lowers the water potential of the phloem sap.
2. Raise source pressure: Water enters the sieve tube by osmosis, increasing hydrostatic/turgor pressure at the source.
3. Bulk movement: The pressure difference drives phloem sap—water carrying dissolved organic solutes—through the sieve tube by mass flow. This is bulk movement of the solution and is faster than diffusion of individual solute molecules.
4. Unload at the sink: Sucrose is removed at a sink for use or storage. Water follows by osmosis, so hydrostatic pressure falls at the sink and the source-to-sink pressure gradient is maintained.
5. Direction boundary: Each sieve tube carries sap in one direction at a time, but different tubes can carry sap upwards or downwards because direction follows the current source and sink. The source is not always a leaf and the sink is not always a root.

Chain cue: source loading → lower phloem water potential → water entry → high source turgor → mass flow → sink unloading → water exit/lower sink pressure.

Mass flow is not diffusion of sucrose molecules one by one, and ATP is used locally to establish loading/pressure differences rather than to push every molecule along the whole tube. This card explains the pressure-flow model; it does not claim exact pressure values or make all phloem movement globally one-way. Staff-only visual brief: source and sink with water arrows, pressure labels and a single-direction sieve-tube arrow; show a second tube in the opposite direction only to represent different source-sink pairs. Do not generate or bind an image.