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Chapter
3 |
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Membrane
Transport
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| Membrane transport plays a fundamental role
in many biological processes in plant cells, including
generation of cell turgor, energy and signal transduction,
nutrient acquisition, waste product excretion, and metabolite
distribution and compartmentalization. Four fundamental
classes of transport system are present at all membranes.
Pumps catalyze transport of ions or complex organic molecules
against their thermodynamic gradients. At membranes other
than the ATP-synthesizing membranes of mitochondria and
chloroplasts, pumps are generally driven by ATP hydrolysis.
At all membranes, H+ pumps dominate the transport
characteristics, removing H+ from the cytosol
and generating a pmf across each membrane. Carriers translocate
a wide range of simple solutes, including ions, sugars,
and amino acids. Carriers are distinguished from pumps
by not executing scalar reactions such as ATP hydrolysis.
Solute transport through carriers is generally energized
through coupling of pmf-driven H+ transport
to the uphill flow of the solute. Ion channels are purely
dissipative with respect to catalysis of transport and
operate at very high turnover rates. Channels gate between
open and closed states, and gating is frequently controlled
either by membrane voltage or by a ligand, depending on
the function of the channel. Channels that are highly
selective for K+ and Ca2+ reside
in the vacuolar and plasma membranes. Less selective cation
and anion channels also are present in both membranes,
as are malate-selective channels in the tonoplast. Aquaporins
facilitate rapid transport of water across the plasma
membrane and tonoplast, bypassing an alternative pathway
for water transport that involves permeation through the
lipid bilayer. All classes of transport system have been
identified at a molecular level; in many cases, structural
aspects of the transport system can be related to solute
permeation and to control of transport system activity. |
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