Assertion: cell membrane's protein | /r/CapableOf | form a variety of transport systems
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Source sentences:
text: Transport of these vital substances is carried out by certain classes of intrinsic proteins that form a variety of transport systems: some are open channels, which allow ions to diffuse directly into the cell; others are “facilitators,” which help solutes diffuse past the lipid screen; yet others are “pumps,” which force solutes through the membrane when they are not concentrated enough to diffuse spontaneously.