Monday, December 29, 2008

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. 

Water Potential and Osmosis

Osmosis results in the net uptake or loss of water by the cell and depends on which component, the cell or extracellular fluids, has the highest water potential. Water potential is the free energy of water that is a consequence of solute concentration and applied pressure. Water potential is the physical property predicting the direction of water flow. Water will always move across the membrane from the solution with the higher water potential to the one with lower water potential. Pure water has a water potential of zero, and addition of solutes lowers water potential into the negative range. Increased pressure raises the water potential into the positive range. A negative pressure may also move water across a membrane; this bulk flow (movement of water due to pressure differences) is usually faster than movement caused by different solute concentrations. Plant cells will gain or lose water to intercellular fluids depending upon their water potential. 

A flaccid cell placed in a hyperosmotic solution will lose water by osmosis; the cell will plasmolyze (protoplast moves away from cell wall). A flaccid cell placed in a hypoosmotic solution will gain water by osmosis; the cell will swell and a turgor pressure develops; when pressure from the cell wall is equal to the osmotic pressure, equilibrium is reached and no net movement of water occurs. 

Transport Within Tissues and Organs

The symplast and apoplast both function in transport within tissues and organs. Lateral transport is usually along the radial axis of plant organs, and can occur by three routes in plant tissues and organs. These are:

1. Across the plasma membrane and cell walls. Solutes move from one cell to the next by repeatedly crossing plasma membranes and cell walls. 
2. The symplast route. A symplast is the continuum of cytoplasm within a plant tissue formed by the plasmodesmata, which passes through pores in the cell walls. Once water or a solute enters a cell by crossing a plasma membrane, the molecules can enter other cells by traveling through plasmodesmata. 
3. The apoplast route. An apoplast is the continuum between plant cells, which is formed between the continuous matrix of cell walls. Water and solute molecules can move from one area of a root or other organ via the apoplast without entering a cell.

Water and solute molecules can move laterally in a plant organ by anyone of these routes or by switching from one to another. 

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