Provide means of transportation

Simple diffusion – movement of small or lipophilic molecules (e.g. O2, CO2, etc.)
Osmosis – movement of water molecules (dependent on solute concentrations)
Facilitated diffusion – movement of large or charged molecules via membrane proteins (e.g. ions, sucrose, etc.)

OR

Endoplasmic reticulum (ER), in biology, a continuous membrane system that forms a series of flattened sacs within the cytoplasm of eukaryotic cells and serves multiple functions, being important particularly in the synthesis, folding, modification, and transport of proteins