Sunday, December 28, 2008

Internal Transport in Plants

Land plants require a transport system because unlike their aquatic ancestors, photosynthetic plant organs have no direct access to water and minerals. The internal transport of plant system involves the lifting of water and minerals to great heights to the tips of all branches and leaves, against the gravitational pull.

Three levels of transport occur in plants:

1. Uptake of water and solutes by individual cells.
2. Short-distance, cell-to-cell transport at the level of tissues and organs.
3. Long-distance transport of sap in xylem and phloem at the whole-plant level.

Transport at the cellular level depends on the selective permeabilities of membranes.

Active and Passive Transport of Solutes

The plasma membrane's selective permeability controls the movement of solutes between a plant cell and the extracellular fluids. Solutes may move by passive or active transport.

Passive transport occurs when a solute molecule diffuses across a membrane down a concentration gradient with no direct expenditure of energy by the cell. Transport proteins embedded in the cell membrane may increase the speed at which solutes cross. Transport proteins may facilitate diffusion by serving as carrier proteins or forming selective channels. Carrier proteins bind selectively to a solute molecule on one side of the membrane, undergo a conformational change, and release the solute molecule on the opposite side of the membrane. Selective channels are passageways by which selective molecules may enter and leave a cell; some gated selective channels are stimulated to open or close by environmental conditions.

Active transport occurs when a solute molecule is moved across a membrane against a concentration gradient. It is an energy-requiring process. The proton pump is an active transporter important to plants.

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