How can molecules cross a cell membrane?

Semi-permeable membranes in the cell will let some molecules through but not others. There are three main ways in which these molecules can move. 1) Active transportAs the name suggests, here, molecules are actively moved across the cell membrane using energy gained from respiration. Unlike the two methods below, molecules can be moved in this way from a region of low concentration to a region of higher concentration (so against a concentration gradient) using carrier proteins. 2) DiffusionDiffusion is simpler than active transport, and doesn't require energy. Molecules in liquids and gases are constantly moving and vibrating, and because of this, they naturally spread (just as a smell may diffuse across a room). In a cell, when molecules are in a high concentration in one region, they will naturally spread down a concentration gradient to where they are less concentrated, before reaching an equilibrium (i.e. they reach a balance). 3) OsmosisOsmosis is simply the diffusion of water molecules. Water moves from where there is a lot of it (a dilute solution) to where there is less (less dilute solution).