Rapid Review

Pathways by which Substances Cross Membranes

	Diffusion		Mediated transport
	Through lipid bilayer	Through protein channel	Facilitated diffusion	Active transport
Direction of net flux	High conc to low	High conc to low	High conc to low	Low conc to high
Use of integral membrane protein	No	Yes	Yes	Yes
Saturatable	No	No	Yes	Yes
Chemical specificity	No	Yes	Yes	Yes
Energy input required	No	No	No	Yes
Examples of substances	O2, CO2, fatty acids	Na+, K+, Ca2+	Glucose	H+, amino acids, glucose, Na+, K+, Ca2+