7.2 Transport Mechanisms

1. Root hairs provide a large absorption surface; water enters cells down a water-potential gradient.
2. Across cortex, water follows apoplast and/or symplast routes. Vacuoles are not part of the symplast definition.
3. Suberin in the Casparian strip of endodermal cell walls blocks the apoplast and prevents uncontrolled wall-space entry.
4. Water and ions cross an endodermal membrane into cytoplasm, enabling selective ion control.
5. The solution enters xylem. Lignin waterproofs and strengthens xylem vessel walls, supporting the conducting pathway.

1. Liquid water reaches mesophyll tissues from leaf xylem and moistens mesophyll cell walls.
2. Water evaporates from those cell-wall surfaces into the intercellular air spaces.
3. Water vapour diffuses through the air spaces and out through stomatal pores.
4. Net diffusion occurs from the more humid leaf interior to the less humid atmosphere while stomata provide an open route.

1. Transpiration removes water from leaf surfaces and lowers leaf water potential.
2. Water is drawn from nearby xylem, creating tension (negative pressure) in the water column.
3. Hydrogen bonds cause cohesion between water molecules, so pulling one part of the column transmits force through the column towards roots.
4. Hydrogen-bond adhesion between water and cellulose helps water remain in contact with xylem walls.
5. Lignified vessel walls resist collapse under tension, allowing upward bulk movement as transpiration pull continues.

1. Draw the section's large tissue boundaries and visible stomatal regions with clear unshaded lines and correct proportions.
2. Label only features present in the source, such as thick cuticle, rolled/folded lamina, sunken stomata, stomatal crypts, hairs, reduced air spaces or succulent water-storage tissue.
3. Extend each annotation beyond a name: state how the feature changes diffusion distance, exposed area, trapped humidity, air movement or the water-vapour gradient.
4. Use label lines ending on the exact feature and keep observation separate from mechanism.
5. Check that the annotation explains reduced water loss rather than merely calling the plant xerophytic.

• 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.

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.

1. Sucrose is loaded into sieve tubes at a source, lowering phloem water potential.
2. Water enters from nearby xylem by osmosis, raising hydrostatic/turgor pressure at the source.
3. The pressure difference drives bulk movement of water and dissolved assimilates through the sieve tube towards the sink.
4. Sucrose is unloaded at the sink for use or storage, raising phloem water potential.
5. Water leaves, hydrostatic pressure falls, and the source-to-sink pressure gradient is maintained.
6. Each sieve tube flows one way at a time, but different tubes may carry sap in opposite directions for different source-sink pairs.