Transport in Plants

Plants need to move water, minerals and food from one part to another. Over short distances, substances move by diffusion — the movement of molecules from a region of higher concentration to a region of lower concentration, without using energy. Diffusion is passive and slow, useful only for short-distance transport (for example, gases moving in and out of cells).

When carrier proteins help molecules cross a membrane down their concentration gradient without energy, it is facilitated diffusion. When molecules are pumped against their gradient using ATP, it is active transport.

Osmosis is a special kind of diffusion: the movement of water molecules across a semi-permeable membrane from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution). The direction and rate of osmosis depend on both pressure and the concentration of solutes, summed up as water potential.

• Pure water has the highest water potential (taken as zero).
• Adding solute lowers the water potential (makes it negative).
• Water always moves from higher to lower water potential.

Depending on the surrounding solution, a cell may be in a hypotonic solution (water enters, cell swells — turgid), an isotonic solution (no net movement), or a hypertonic solution (water leaves, the cell shrinks — plasmolysis).

Water and dissolved minerals are absorbed mainly by the root hairs — thin extensions of root epidermal cells that hugely increase the surface area. Water enters by osmosis and then moves across the root toward the centre, where it reaches the water-conducting tissue, the xylem.

From the roots, water travels all the way up to the leaves through the xylem — this is the ascent of sap. In a tall tree this is a rise of many metres, achieved without any pump. The main driving force is the transpiration pull, explained by the cohesion–tension theory:

• Transpiration — the loss of water vapour from the aerial parts of the plant, mostly through tiny pores in the leaves called stomata — pulls water upward.
• Cohesion — water molecules stick to one another (by hydrogen bonds), forming a continuous column.
• Adhesion — water molecules stick to the walls of the narrow xylem vessels.

Together, the pull at the top and the unbroken, cohesive water column drag water up the xylem like a rope being pulled from above.

Each stoma is bordered by two guard cells that open the pore when turgid and close it when flaccid, controlling how much water is lost. Although transpiration loses a lot of water, it also helps absorb and transport water and minerals and cools the plant (a kind of evaporative cooling).

The food made in the leaves by photosynthesis (mainly the sugar sucrose) must be carried to all other parts of the plant — growing tips, fruits, seeds and storage organs like roots and tubers. This transport of food is called translocation, and it takes place through the phloem.

Unlike water in the xylem (which moves only upward), food in the phloem can move in any direction — up or down — from a region where it is made or stored (the source) to a region where it is used or stored (the sink). For example, leaves are a source in summer, while a growing fruit is a sink; a storage root can be a sink while filling and a source when sprouting.

The widely accepted explanation is the pressure-flow (mass-flow) hypothesis:

• At the source, sugar is actively loaded into the phloem; this lowers the water potential, so water enters from the nearby xylem by osmosis, raising the pressure.
• At the sink, sugar is removed (used or stored); water leaves, lowering the pressure.
• The pressure difference pushes the sugary sap by bulk flow from the high-pressure source to the low-pressure sink.

Because loading sugar at the source needs energy, translocation in the phloem is an active process, unlike the passive ascent of water in the xylem.