Which aspects contribute to variation in membrane composition?

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Membranes vary because they’re built from two broad sets of components: lipids and proteins. The lipid portion can differ in which lipids are present (different phospholipid head groups, tail saturation, cholesterol content), and these differences affect membrane fluidity, thickness, and curvature. The protein portion can vary in how many proteins are present and which kinds they are (channels, transporters, receptors, enzymes), shaping transport, signaling, and overall membrane function. Carbohydrate attachments are added to lipids and proteins, forming glycoproteins and glycolipids, and they influence surface interactions, but they don’t represent a separate category of membrane composition beyond the lipids and proteins themselves. Since both lipid makeup and the repertoire of membrane proteins can change, variation in membrane composition reflects changes in both aspects.