Transport in Plants

Figure 36.1 The Pathways of Water and Solutes in the Plant Plants need water for photosynthesis, transporting solutes, temperature control, and developing the internal pressure that supports the plant body. Plants usually obtain water and minerals from the soil via the roots, which in turn obtain carbohydrates and other important materials from the leaves. Water enters the plant through osmosis, but the uptake of minerals requires transport proteins.

Uptake and Movement of Water and Solutes For osmosis to occur, a membrane must be permeable to water but not to the solutes. Plant cells have a rigid cell wall. As water enters the cell, the plasma membrane presses against the cell wall, restricting expansion. The opposing force exerted by the rigid cell wall as water enters is called the pressure potential, or turgor pressure. Water enters a plant cell until the pressure potential exactly balances the solute potential. The cell is then called turgid.

Uptake and Movement of Water and Solutes Water always moves across a semipermeable membrane toward the region of more negative (lower) water potential. The structure of many plants is maintained by the pressure potential in their cells. Loss of pressure potential causes wilting.

Uptake and Movement of Water and Solutes Mineral ions generally require transport proteins in order to cross membranes. When the concentration of ions is greater in the soil than in the plant, the plant can take up ions by facilitated diffusion, a passive process. If the concentration of ions is lower in the soil than in the plant, however, ion uptake requires energy.

Uptake and Movement of Water and Solutes Where bulk flow of water is occurring, dissolved minerals are carried along. When movement is less, minerals move by diffusion. Minerals must be actively transported across certain membranes. The cells at the surface of the root hairs actively transport ions. Water moves into the cells of the root because the root cells have more negative water potential than the soil solution.

Transport of Water and Minerals in the Xylem In the xylem, water and minerals constitute the xylem sap.

Transport of Water and Minerals in the Xylem The transpiration–cohesion–tension mechanism: The concentration of water vapor is higher inside the leaf than outside, so water diffuses out of the leaf through the stomata. This process is called transpiration. This creates a tension in the mesophyll that draws water from the xylem of the nearest vein into the apoplast surrounding the mesophyll cells. The removal of water from the veins, in turn, establishes tension on the entire volume of water in the xylem, so the column is drawn up from the roots.

Figure 36.8 The Transpiration–Cohesion–Tension Mechanism

Transport of Water and Minerals in the Xylem Hydrogen bonding between water molecules results in cohesion, the tendency of water molecules to stick to one another. The narrower the tube, the greater the tension the water column can stand. The water column is also maintained by adhesion of water molecules to the walls of the tube.

Transport of Water and Minerals in the Xylem The key elements in water transport in xylem: Transpiration Tension Cohesion The transpiration–cohesion–tension mechanism does not require energy. These things makes water flow up the stems to the leaves. At each step, water moves passively toward a region with a more negative water potential.

Transport of Water and Minerals in the Xylem Mineral ions in the xylem sap rise passively with the solution. Transpiration also contributes to the plant’s temperature regulation, cooling plants in hot environments.

Transpiration and the Stomata Leaf and stem epidermis has a waxy cuticle that is impermeable to water, but also to CO2. Stomata, or pores, in the epidermis allow CO2 to enter by diffusion. Guard cells control the opening and closing of the stomata. In most plants, the stomata are open during the day (when light is intense enough to maintain photosynthesis) and closed at night (to prevent water loss).

Translocation of Substances in the Phloem Sugars, amino acids, some minerals, and other solutes are transported in phloem and move from sources to sinks. A source is an organ such as a mature leaf or a starch-storing root that produces more sugars than it requires. A sink is an organ that consumes sugars, such as a root, flower, or developing fruit. Sources ↓ Sinks

Translocation of Substances in the Phloem Translocation (movement of organic solutes) stops if the phloem is killed. Sieve tube cells at the source have a greater sucrose concentration that surrounding cells, so water enters by osmosis. This causes greater pressure potential at the source, so that the sap moves by bulk flow towards the sink. At the sink, sucrose is unloaded by active transport, maintaining the solute and water potential gradients.

Figure 36.14 The Pressure Flow Model