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For compounds to enter cells from the outside they must somehow penetrate the membrane (also outer membrane in gram-negative bacteria, and the cell wall if present. Cell membranes consist of lipid bilayers. Some simple rules about movement of molecules across cellular membranes:
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For compounds to enter cells from the outside they must somehow penetrate the membrane (also outer membrane in gram-negative bacteria, and the cell wall if present
Some simple rules about movement of molecules across cellular membranes: • Cell membranes represent physical barriers to the surrounding environment. But they • are not like a concrete wall and also not like the thin membrane in a soap-bubble. It is a • much more complex barrier that lets certain selected compounds through in a strictly • controlled manner. • Macromolecules and charged smaller molecules do not generally pass through • membranes passively (by diffusion). This for example means that organisms cannot use • proteins directly as food. They are instead partly degraded before uptake. • Some uncharged smaller molcules can enter cells by passive diffusion. • Many types of molecules can enter cells by various types of active transport systems. • These processes are compound-specific and may or may not require energy • consumption (i. e ATP).
