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    Active and Passive Transport in Plant Cells

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    The selective permeability of a plant cell's plasma membrane controls the movement of solutes between the cell and the extracellular solution. The movement of solutes across the cell's membrane can occur by active transport or passive transport. Active transport requires energy while passive transport does not require energy to move solutes.

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    Transport in Plants: Active and Passive

    Active Transport

    Active transport is the pumping of solutes across membranes against the gradient. It is called active because the cell must give out metabolic energy in the form of ATP to transport a solute "uphill".

    Active Transporters

    Proton pump: Hydrolyzes ATP and uses the released energy to pump hydrogen ions out of the cell. The result is a higher concentration of hydrogen ions outside the cell than inside the cell. The proton pump therefore moves positive charge outside of the cell, and generates a membrane potential. Proton pumping makes the inside of a plant cell negative in charge relative to the outside.
    Membrane potential: Voltage, a separation of opposite charges across a membrane. The charge separation is a form of potential (stored) energy used to perform cellular work.

    Plant cells use the stored energy in the proton gradient and the membrane potential to drive the transport of numerous different solutes.

    Other important terms:

    Cotransport: the coupling of the "downhill" diffusion of one substance to the "uphill" transport of another against its own concentration gradient.

    Chemiosmosis: the ability of certain membranes to use chemical energy to pump hydrogen ions and then use the energy stored in the hydrogen gradient to drive cellular work.

    Passive Transport

    Passive transport is the diffusion of a substance across a biological membrane. Passive transport occurs without the direct expenditure of metabolic energy by the cell. Therefore, most solutes diffuse slowly across the cell membrane unless they pass through transport proteins in the membrane.

    Transport proteins function as specific carrier proteins, which facilitate diffusion by binding selectively to a solute on one side of the membrane and releasing the solute on the other side. Transfer of the solute across the membrane consists of a conformational shape change by the carrier proteins. Overall, carrier proteins physically transport solutes, while other proteins function as a selective channel. For example, the membrane of most plant cells have potassium channels that allow potassium ions to pass through, but not other ions, such as sodium ions. There are some channels that are gated, which means certain environmental stimuli can cause channels to open or close.

    Campbell, N. A. (1996). Biology (4th ed). Benjamin/ Cummings: California.

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