Question

What determines if a particle can cross a cell membrane? *

Answer 1

Lipophilic (non-polar molecules), such as steroid hormones or O2 gas can pass... very small uncharged polar molecules (such as H2O, ethanol and urea) can also get through... otherwise you need a membrane protein to "facilitate" the transport of the molecule across the membrane, ion channels are used for water, or charged molecules; carriers/transporters are used for other molecules.