Transport in Plants

Osmosis is the diffusion of water across a selectively permeable membrane, from a region of lower solute concentration (more free water) to a region of higher solute concentration (less free water). Root hair cells use active transport, which requires energy from ATP, to pump mineral ions such as nitrate and potassium from the soil into their cytoplasm. This keeps the inside of the root more concentrated in solutes than the soil water, so water keeps moving into the root by osmosis. Root hairs matter because they give a huge membrane surface area for both processes.

• Once inside, water crosses the cortex toward the centre of the root, travelling partly through cell walls and the spaces between cells, and partly from cell to cell through the cytoplasm.
• The innermost layer of the cortex, the endodermis, has a waxy band called the Casparian strip in the walls between its cells. Water cannot leak around the cells through the walls here, so it must pass through an endodermal cell membrane. This acts as a checkpoint that lets the plant control which minerals enter the xylem.
• Ions pumped into the xylem draw more water in behind them, creating root pressure. Root pressure can push water a short way up the stem and explains guttation, the droplets seen on the edges of some leaves early in the morning. It is far too weak to lift water to the top of a tree.

Transpiration is the evaporation of water from the leaves, mostly out through open stomata. Water evaporates from the moist walls of mesophyll cells into the leaf's air spaces, then diffuses out of the stomata into drier air. Each molecule that leaves is replaced by water drawn from the nearest xylem, which creates a pulling force, or tension, in the water column. The cohesion-tension theory explains why that pull reaches all the way down to the roots:

1. Tension: evaporation at the leaf pulls on the water in the xylem, like sucking on a very long, very thin straw.
2. Cohesion: water molecules are polar and attract one another through hydrogen bonds, so the pull at the top is passed down the whole unbroken column.
3. Adhesion: water molecules are also attracted to the walls of the narrow xylem tubes, which helps the column resist gravity. Lignin keeps the tubes from collapsing under tension.

Stomata are a compromise. A leaf must open them to let in the carbon dioxide needed for photosynthesis, but every open stoma also lets water vapour out. Each stoma is bordered by two guard cells. When guard cells actively take in potassium ions, water follows by osmosis and the cells swell and become firm (turgid). Their walls are thicker and less stretchy on the side facing the pore, so as they swell they bow outward and the pore opens. When they lose ions and water, they go limp and the pore closes. In drought, the hormone abscisic acid signals guard cells to close the stomata, saving water at the cost of slowing photosynthesis. Most plants open their stomata in daylight and close them at night, although some desert plants such as cacti do the reverse to lose less water in the heat of the day.

Sugars made in the leaves travel through phloem in a process called translocation. The best-supported explanation is the pressure-flow hypothesis. At a source (a place that makes or releases sugar, such as a mature leaf, or a storage root in early spring), companion cells use ATP to load sucrose into the sieve tubes. The high sugar concentration draws water in from the neighbouring xylem by osmosis, raising the pressure. At a sink (a place that uses or stores sugar, such as roots, growing tips, developing seeds and fruit), sucrose is unloaded, water leaves, and the pressure drops. Sap flows from high pressure to low pressure, so it moves from source to sink, which can be upward or downward.