Transport in Plants

Water uptake by root hair cells.

• Root hair cells are epidermal cells with a long, thin extension into the soil.
• Their value is surface area — many root hairs hugely increase the area for absorption.
• Water enters by osmosis: soil water has a higher (less negative) water potential than the cell cytoplasm, so water moves down the water-potential gradient into the cell.
• Mineral ions (e.g. nitrate) are taken up by active transport, lowering the cell's water potential and keeping osmosis going.

Xylem structure — built for transport.

• Xylem vessels are dead at maturity: no cytoplasm or end walls, so they form continuous hollow tubes with low resistance to flow.
• Walls are thickened with lignin — waterproof and strong, preventing the vessel collapsing under tension (negative pressure).
• Pits (gaps in the lignified wall) let water move sideways between vessels and into surrounding cells.

The transpiration stream is the continuous flow of water from roots → stem → leaves, replacing water lost by transpiration.

Cohesion-tension theory explains how water rises tens of metres against gravity with no living pump.

1. Transpiration — water evaporates from the moist cell walls of the spongy mesophyll and diffuses out through open stomata. This loss generates a pull (tension) at the top of the xylem.
2. Cohesion — water molecules stick to one another by hydrogen bonds, so they behave as a continuous, unbroken water column.
3. Adhesion — water molecules are attracted to the lignified xylem walls, helping the column resist gravity.
4. The tension at the leaf is transmitted all the way down the continuous column, drawing water up from the roots — and ultimately pulling more water into the root from the soil.

Factors affecting transpiration rate (they alter how fast water evaporates from the leaf):

• Light ↑ → stomata open for photosynthesis → rate ↑.
• Temperature ↑ → faster evaporation and diffusion → rate ↑.
• Humidity ↑ → smaller water-vapour gradient between leaf and air → rate ↓.
• Wind / air movement ↑ → removes the humid layer at the leaf surface → steeper gradient → rate ↑.

Measuring transpiration with a potometer.

• A potometer measures the rate of water uptake by a cut shoot, used as a proxy for transpiration.
• An air bubble is introduced into a capillary tube; as the shoot transpires, water is drawn up and the bubble moves.
• Rate = distance moved ÷ time (e.g. mm min⁻¹), or volume ÷ time if the tube's cross-section is known.
• A reservoir tap resets the bubble for repeat readings; you change one factor (e.g. add a fan) and keep others constant.

Stomata and guard cells.

• Stomata are pores (mainly on the lower epidermis) bordered by two guard cells.
• When guard cells take up water and become turgid, their unevenly thickened walls bow apart and the stoma opens; when they lose water and become flaccid, the stoma closes.
• The trade-off: open stomata let CO₂ in for photosynthesis but also let water vapour out. Plants must balance carbon gain against water loss — many close stomata in hot, dry conditions to conserve water.

Phloem and translocation.

• Phloem is living tissue. It contains sieve tube elements (joined end-to-end through perforated sieve plates, with little cytoplasm and no nucleus) and adjacent companion cells (full of mitochondria; they support the sieve tubes).
• Translocation = movement of dissolved organic solutes (mainly sucrose) through phloem from source to sink.
  • Source = where sugar is made or stored and released, e.g. photosynthesising leaves.
  • Sink = where sugar is used or stored, e.g. growing roots, fruits, storage organs.
• Active loading (mass flow / pressure-flow hypothesis):
  1. Sucrose is actively loaded into sieve tubes at the source (using ATP, via companion cells).
  2. This lowers the water potential there, so water enters by osmosis → high hydrostatic (turgor) pressure builds at the source.
  3. At the sink, sucrose is removed (used/stored), water leaves, and pressure falls.
  4. Sap flows down the pressure gradient from high-pressure source to low-pressure sink — carrying sucrose with it.

Xerophyte adaptations (plants adapted to dry habitats reduce transpiration):

• Thick waxy cuticle — reduces evaporation through the epidermis.
• Reduced number of stomata and/or stomata sunk in pits — trap humid air, lowering the gradient.
• Rolled or needle-like leaves — reduce exposed surface area; trap moist air inside.
• Hairs (trichomes) around stomata — trap a humid boundary layer.