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Chapter 3

Membrane Transport

 
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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