Answer: When you mix sand and water there are no reactions. The sand sits at the bottom of the water because the sand is heavier than the water is.
Cell membrane is a semipermable, which means that certain molecules will be able to pass through the membrane. It is contains phospholipids. But for smaller molecules such as CO2 and Water, it can pass through the membrane through simple diffusion. Cell membrane controls the movement of the materials into and out of the cell.
