Phase separation in a mixture of small and large colloidal particles and the interaction induced between a macromolecule (a large particle) and a cell membrane (a surface) immersed in smaller molecules (small particles), have been analyzed using an integral equation theory. Simplified models are employed to study the entropically driven formation of ordered structure, which is ascribed to the excluded volume effects (forces of increasing the system entropy by reduction of the total excluded volume for the small particles). In the analysis on the induced interaction, attention is focused on effects due to the packing fraction of the small particles, the diameter of the large particle, the surface curvature, presence of a small amount of particles slightly larger than the small particles, and attractive interaction between the small particles. A significant finding is that considerable specificity exists between the diameter of the large particle and the surface curvature. Works on the roles of the excluded volume effects in the "lock and key" steric interactions between macromolecules and protein folding in biological systems, which are in progress in our group, are also discussed.