Transport in plantsEdexcel A-Level Biology B: Revision notes

Xylem carries water and mineral ions from roots to shoots. Mature xylem vessels are dead, hollow tubes with no end walls and no cell contents, so water flows in a continuous column. Walls are thickened with lignin, which is waterproof and strong so the vessels do not collapse under tension. Gaps in the lignin, called pits, let water move sideways between vessels, bypassing blockages.

Phloem carries organic solutes such as sucrose (translocation). Sieve tube elements are living cells joined end to end by perforated sieve plates, with few organelles and little cytoplasm to leave room for flow. Each is supported by a companion cell, which has a nucleus and many mitochondria to supply ATP for loading sucrose.

Water enters root hair cells by osmosis and crosses the cortex by two routes.

The apoplast pathway is through the cell walls and the spaces between cells. Water moves in the porous cellulose walls without crossing a membrane, and little resistance is met.

The symplast pathway is through the cytoplasm of connected cells, passing between them through plasmodesmata. Water crosses the cell surface membrane once, then moves down a water potential gradient.

At the endodermis a waterproof band of suberin (the Casparian strip) blocks the apoplast route, so water must enter the symplast before it reaches the xylem. This means the plant controls which substances enter the xylem.

Water evaporates from the surface of mesophyll cell walls and diffuses out of the leaf through the stomata (transpiration). This lowers the water potential of the leaf cells, so water is drawn out of the xylem. Because the water column is continuous, the pull is transmitted down the xylem, creating tension (negative pressure) which draws water up from the roots.

The column does not break because of cohesion (hydrogen bonds between water molecules) and adhesion (attraction between water and the walls of the vessels). Evidence: tree trunks shrink in diameter in the day when transpiration is fast, and air is drawn in if a xylem vessel is cut. Transpiration therefore drives the transpiration stream, with no energy from the plant required.

Transpiration rate depends on the water vapour gradient between the air spaces in the leaf and the air outside, and on whether the stomata are open.

• Temperature: higher temperature gives water molecules more kinetic energy, so faster evaporation and diffusion; warm air also holds more vapour, which keeps the gradient steep.
• Light intensity: light causes stomata to open for photosynthesis, so more vapour escapes.
• Humidity: high humidity reduces the gradient, so transpiration is slower; low humidity speeds it up.
• Air movement: wind removes humid air from around the stomata, which steepens the gradient and increases the rate.

The mass-flow hypothesis explains movement of sucrose from a source (such as a leaf) to a sink (such as a root or fruit). Companion cells actively load sucrose into the sieve tube. This lowers the water potential, so water enters by osmosis from the xylem, raising the hydrostatic pressure. At the sink sucrose is unloaded, water potential rises and water leaves, so pressure falls. The contents flow down this pressure gradient.

Strengths: sap flows out under pressure from a cut aphid stylet, with greater pressure near the source; sucrose concentration is higher in leaves than roots; metabolic inhibitors reduce flow, which supports active loading; ringing a stem makes sucrose accumulate above the ring.

Weaknesses: sieve plates would resist flow; different solutes can move at different rates and in opposite directions; it does not fully explain why sieve plates exist.

A potometer measures the rate at which a shoot takes up water, which is used to estimate transpiration. Cut the shoot underwater at a slant, and assemble the apparatus underwater so no air enters the xylem. Seal joints with petroleum jelly and dry the leaves. Introduce an air bubble into the capillary tube, note its start position and measure the distance it moves in a set time. A reservoir is used to reset the bubble.

Rate of uptake = volume ÷ time, where volume = $\pi r^2 d$. Example: a bubble moves 42 mm in 5 minutes in a tube of radius 0.5 mm, so volume = $\pi \times 0.5^2 \times 42 = 33$ mm³, and the rate = 6.6 mm³ min⁻¹.

Vary one factor (such as fan or light), and keep the others constant. Repeat and take a mean. Limitation: water uptake is not exactly the same as water lost, because some water is used in photosynthesis and to keep cells turgid.