The document discusses cell boundaries and membranes. It defines that cell membranes are a thin, flexible barrier made of a lipid bilayer that controls what moves in and out of cells. The membrane contains proteins and carbohydrates that help transport molecules and identify cells. Cell walls exist outside the membrane in plants, algae, fungi and prokaryotes to provide support and protection. They are made of cellulose in plant cells. The membrane and diffusion allow substances to move into and out of cells as concentrations change on either side.