We say that membranes are selectivelypermeable as they act as a barrier to most substances: allows materials to be concentratein cells, excluded, or separated from outside environment (eukaryotic also compartmentalise inside organelles)
Molecules need to be able to enter and exit cells, there are 5 main ways substances can move across a cell membrane:
Simple Diffusion
Facilitated Diffusion
Active Transport
Osmosis
Endocytosis/Exocytosis
ATP in bulk transport: Provide energy for the vesicle movement along cytoskeletonmicrotubules (via motorproteins) and help fuse vesicle and plasma membrane together