lipid molecule face the interior of the membrane and the hydrophilic and small organic molecules such as nutrients, through the activity of
ends face outwards. Most proteins are embedded within, or float in, the membrane transport proteins. However, lipid-soluble substances can
lipid bilayer as a fluid mosaic. Some proteins, because of extensive pass directly through the membrane so that, for example, steroid hor-
hydrophobic regions of their polypeptide chains, span the entire width mones enter the cytoplasm freely. Their receptor proteins are either
of the membrane (transmembrane proteins), whereas others are only cytosolic or nuclear, rather than being located on the cell surface.
superficially attached to the bilayer by lipid groups. Both are integral Plasma membranes are able to generate electrochemical gradients
(intrinsic) membrane proteins, as distinct from peripheral (extrinsic) and potential differences by selective ion transport, and actively take up
membrane proteins, which are membrane-bound only through their or export small molecules by energy-dependent processes. They also
association with other proteins. Carbohydrates in the form of oligosac- provide surfaces for the attachment of enzymes, sites for the receptors