Transport in plants

📚The biology

Where the tissues are

The distribution of vascular tissue differs by organ, and Paper 3 asks you to draw it as a plan diagram — tissue outlines only, no individual cells.

• Root — a central star-shaped xylem core with phloem between its arms, surrounded by the endodermis and pericycle. Central position resists the pulling forces a root experiences.
• Stem — discrete vascular bundles arranged in a ring near the outside, with xylem on the inside of each bundle and phloem on the outside. A peripheral ring resists bending.
• Leaf — vascular bundles in the midrib and veins, again with xylem above (adaxial) and phloem below.

In every organ, the xylem lies on the side nearer the centre of the plant. If you can remember only one thing about the arrangement, remember that.

Cells for the job

Xylem vessel elements are dead, empty cells stacked end to end with the end walls completely broken down, forming a continuous hollow tube. The walls are thickened and waterproofed with lignin, laid down in rings or spirals so the vessel can withstand the negative pressure inside without collapsing while still allowing some flexibility. Pits in the walls let water move sideways. No cytoplasm, no nucleus and no end walls means no resistance to flow — the structure is a pipe, and every feature serves that.

Phloem sieve tube elements are living but stripped down: no nucleus, few organelles, a thin layer of cytoplasm, and end walls perforated into sieve plates through which sap flows. They cannot maintain themselves, so each is attached to a companion cell — a dense cell with a nucleus and many mitochondria, connected through plasmodesmata. The companion cell does the metabolic work, including the active loading of sucrose, and supplies the ATP.

Soil to xylem

Water enters root hair cells, which have a large surface area, by osmosis: the soil solution has a higher water potential than the cytoplasm. From there it crosses the root cortex by two routes:

• Apoplast pathway — through the cell walls and the spaces between them, without entering any cytoplasm. Water moves through the cellulose network, to which it adheres. This is the faster route and carries most of the water.
• Symplast pathway — through the cytoplasm of successive cells, passing from one to the next through plasmodesmata, driven by a water potential gradient.

At the endodermis the apoplast route is blocked. Endodermal cells have a band of waterproof suberin in their walls — the Casparian strip — which forces water out of the walls and into the cytoplasm. Everything entering the xylem must therefore pass through a cell surface membrane, so the plant can control which mineral ions get in, and can actively pump ions into the xylem to lower its water potential and draw water in behind them.

Transpiration and cohesion–tension

Transpiration is the evaporation of water from the internal surfaces of the leaf — the mesophyll cell walls — followed by diffusion of water vapour out through the stomata into the atmosphere. Note the two stages: evaporation then diffusion. Calling transpiration “water loss from leaves” earns nothing.

Evaporation lowers the water potential in the mesophyll cell walls, drawing water from the xylem. Because water molecules are polar and hydrogen-bond to each other, they stick together — cohesion — so pulling on the top of the column pulls the whole column up. The water is under tension, which is why xylem must be lignified against collapse and why the trunk of a tree measurably narrows on a hot day. Water molecules also adhere to the cellulose of the vessel walls, which helps support the column.

Rate of transpiration rises with light intensity (stomata open), temperature (more kinetic energy, steeper vapour gradient) and air movement (removes saturated air), and falls with humidity (shallower gradient).

Xerophytes

Adaptations that reduce transpiration, all working by trapping humid air or reducing exposed surface:

• Thick waxy cuticle — reduces evaporation through the epidermis.
• Sunken stomata in pits and hairs — trap water vapour, raising humidity outside the stoma and reducing the water potential gradient.
• Rolled leaves — enclose the stomata in a humid chamber; hinge cells cause the rolling.
• Reduced leaves or spines — smaller surface area for evaporation.
• Stomata on the lower surface only, and fewer of them.

Translocation

Assimilates — chiefly sucrose, and also amino acids — move dissolved in water through phloem sieve tubes from source to sink. A source makes or releases assimilate (a photosynthesising leaf, a storage organ in spring); a sink uses or stores it (a root, a growing bud, a fruit). Direction is therefore not fixed — the same organ can be a source at one time of year and a sink at another.

Loading at the source is active and is the part worth learning precisely:

1. The companion cell uses ATP to run proton pumps that actively transport H⁺ ions out of the cell into the cell wall.
2. This builds a concentration gradient of H⁺ across the membrane.
3. H⁺ ions diffuse back in through cotransporter proteins, and each brings a sucrose molecule with it — sucrose is dragged in against its own gradient.
4. Sucrose passes into the sieve tube through plasmodesmata.

The high solute concentration lowers the water potential in the sieve tube, so water enters from the xylem by osmosis, raising the hydrostatic pressure. At the sink, sucrose is removed and used, so water leaves and pressure falls. Sap therefore flows down a hydrostatic pressure gradient from source to sink — mass flow. The flow itself is passive; the pumping that creates the gradient is not.