They link to carrier (integral) proteins that change their own shape in order to move the molecule through the semipermeable membrane. This very important feature, called specificity, is in great importance and causes that only one kind of polar molecule (or group of structurally related ones) can pass. Because by both simple and carrier-facilitated diffusions the movements are always along the concentration gradient (from a region with a higher concentration to one with lower), no energy is needed for these kinds of transport. The movement that requires energy is the active transport, mostly used by bigger molecules and ions moving against concentration gradients (from places with lower concentration to ones with higher). During the active transport, a set of proteins in the plasma membrane breaks down energy in ATP and uses it for binding the cargo, changing shape and releasing it outside or inside the cell. Some carriers move single cargo in a single direction and therefore are called uniports. Others move two cargoes simultaneously in the same direction – those are the symports. They make the co-transport (coupled transport), used for example by sugars and amino acids, possible. That is very important, because neither the sugar nor the amino acid can be transported alone. The third group, the antiports (for example the K-Na-ATPase pump), are most complex – they move two cargoes simultaneously in opposite ways.
Transporting materials into and out of cells, the plasma membrane and its components play a great role. They are the very important not only for cell nutrition but also excretion, protection and communication between cells.