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Cell Structure
and Function
Biomembranes
Photosynthesis
Cell
Respiration
DNA Structure
and Replication
Contacts
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Biomembranes I:
Membrane Structure and Transport
Concept 3: Selective Permeability of
Membranes
Cell membranes are selectively
permeable. Some solutes cross the membrane freely, some cross with
assistance, and others do not cross at all.
A few lipophilic substances move freely across the cell membrane by
passive diffusion. Most small molecules or ions require the
assistance of specific protein carriers to transport them across the
membrane. Large molecules do not cross intact cell membranes, except
in certain special cases.
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