Dissolved substances have to pass through the cell membrane to get into or out of a cell by Diffusion.
During diffusion particles move from a region where they are in high concentration to a region where they are in low concentration. Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration.
Osmosis is the movement of water from a less concentrated solution to a more concentrated solution through a partially permeable membrane.
Active transport is the process were dissolved molecules move across a cell membrane from a lower to a higher concentration. In active transport, particles move against the concentration gradient. This requires energy which is supplied by respiration.
Active transport uses carrier proteins pick up specific molecules and take them through the cell membrane against the concentration gradient.