Plasma membranes surround cells, (plant cells also have a cell wall). Membranes are selectively permeable (only allow certain molecules through) and are roughly 10 nm thick.
Membranes are made of phospholipid and protein, arranged in a fluid mosaic. There are two layers of phospholipid, with large protein molecules floating in this oily layer (most proteins go all the way across the membrane). "Fluid" means that the membrane is liquid. "Mosaic" refers to the proteins scattered around like mosaic tiles on a floor. Diagram
Unsaturated fatty acids make the membrane fluid.
Saturated fatty acids make the membrane viscous (so it does not change shape easily).
Cholesterol is added to some membranes as an antifreeze; organisms living in cold climates can survive temperatures below freezing because their membranes stay fluid.
Proteins in the membrane have many functions including : transport, enzymes, receptors for hormones and joining cells together.
Carbohydrates (oligosaccharides) on the outside of the membrane let the immune system identify the cell.
These carbohydrates are important in organ transplants: the donor organ has to match (as closely as possible) the recipient.
Passive transport : the cell uses no energy. Passive transport includes diffusion and osmosis.
Diffusion : the movement of molecules from high concentration to low concentration ( liquid or gas ).
Cells get a lot of materials like glucose and oxygen by diffusion.
Osmosis : the diffusion of water through a selectively permeable membrane.