How do molecules move across the plasma membrane?
Molecules move across the plasma/cell membrane through diffusion. If they are not small enough, they have to broken down by other substances, such as enzymes, which are biological catalysts.
If they are small enough, usually, the easiest way for them to move is through diffusion. This means that they will move from a region of higher concentration to a region of lower concentration.
But, this doesn't work for some nutrients, such as in the case of nitrates
Here are two pictures of the two processes: