The membranous compartments of eukaryotic cells are called organelles. All eukaryotic cells have many organelles and structures in common; the most obvious is the cell nucleus. But they also have some differences. For example, many plant cells have chloroplasts that perform photosynthesis. Each type of organelle has a specific role: some organelles have been characterized as factories that make specific products, whereas others are like power plants that take in energy in one form and convert it to a more useful form. These functional roles are defined by the chemical reactions that occur within the organelles. Ion concentrations differ from one organelle to another. For instance, the pH (a reflection of the concentration on H⁺) is 7.4 inside the nucleus, but 4.5 (a concentration 1,000 times higher) inside another organelle, the lysosome.