Transport of Materials across Membranes

Whereas a major function of biological membranes is to maintain the status quo by preventing loss of vital materials and entry of harmful substances, membranes must also engage in selective transport processes. Living cells depend on an influx of phosphate and other ions, and of nutrients such as carbohydrates and amino acids. They extrude certain ions, such as Na+, and rid themselves of metabolic end products. How do these ionic or polar species traverse the phospholipid bilayer of the plasma membrane How do pyruvate, malate, the tricarboxylic acid citrate and even ATP move between the cytosol and the mitochondrial matrix (see figs. 13.15 and 14.1) The answer is that biological membranes contain proteins that act as specific transporters, or permeases. These proteins behave much like conventional enzymes They bind substrates and they release products. Their primary function, however, is not to catalyze chemical reactions but to move materials from one side of a membrane to the other. In this section we discuss the general features of membrane transport and examine the structures and activities of several transport proteins. 

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Transport of Materials across Membranes The plasma membrane keeps proteins and other essential... 

In this chapter we discussed the structure and function of biological membranes. First we considered their structure starting with an examination of the constituents of membranes. Then we turned to questions concerning the transport of materials across membranes. 

Permeases a term applied to carrier proteins involved in the transport of materials across membranes. P. is a misnomer and should be avoided. 

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