Beware, silly question:
How does anything just diffuse through the cell membrane, do substances just go though gaps in the cell membrane?
Even then wouldn't it face forces of attraction from the hydrophobic tails? E.g. in the case of lipid movement through the cell membrane. Wouldn't dispersion forces act between the lipid and hydrophobic tails therefore causing it to 'get stuck' for some periods of time
Not a silly question at all
So a lot of molecules like oxygen and water will freely diffuse across the plasma membrane, since they're relatively small.
For larger molecules like sugars and amino acids they diffuse via a process of facilitated diffusion, where they bind to membrane-spanning transport proteins and diffuse across through protein conformational changes; this doesn't require energy.
For ions moved against their concentration gradient, active transport is used - i.e. energy demanding. A classic example is of course the K+/Na+ ATPase pump.
I am not 100% here, but I don't know if lipid diffusion is in Biology 3/4; lipid diffusion is a bit more complex than the former.