We propose a physical model of molecular clouds which is based on the idea that the back reaction of substructures of a cloud on the ambient medium maintains and stabilizes the cloud on larger scales: clumps, which are assumed to carry a magnetic moment, are coupled to the ambient medium by magnetic forces, hence continually inducing velocity fluctuations due to their random motion. The energy source is then the gravitational binding energy of the clumps in the global potential of the whole cloud.