Diffusion through the cell membrane

All substances, including the cell cytoplasm and substances within it, are made up of particles. Particles of some substances diffuse into and out of a cell by moving through small holes in the cell membrane. The particles that make up some substances are too big to diffuse through the holes in the cell membrane. The cell membrane is selectively-permeable because only some substances can diffuse through it. The net movement of particles through a cell membrane is down a concentration gradient.