All cells have a cell membrane that surrounds the cell to protect it and control what goes in or out。 Materials can move through the membrane by diffusion or osmosis。 Diffusion IS When materials movem or out of a cell from a place of high concentration to one of low concentration。 Osmosis is a special kind of diffusion that allows water to pass through the membrane。 However, In osmosis、 many other materials are not allow to pass through。 Plant cells have an extra Iayer called a cell wall that surrounds each cell's membrane。 The cell wall is much stiffer to help the plant's stems stand up and support leaves and flowers.