Abstract
By considering the inner regions of living cells' membranes, P systems with inner regions are introduced. Then, a new type of membrane computing systems are considered, called $K$-subset transforming systems with membranes, which can treat nonintegral multiplicities of objects. As an application, a K-subset transforming system is proposed in order to model the light reactions of the photosynthesis. The behaviour of such systems is simulated on a computer.