Membrane systems

Endoplasmic reticulum (ER) consists of a series of membranes creating channels within the cytoplasm, which are continuous with the nuclear membrane. They also form sheets that enclose cellular spaces called cisternae. ER provides a large surface area for chemical reactions. Rough ER is studded with tiny granules (ribosome) that may be seen with an electron microscope. As a result, rough ER is most abundant in cells that either secrete proteins or are growing rapidly (eg cells in the pancreas that secrete insulin). Unlike rough ER, smooth ER has no ribosomes. Its function is to synthesize and store steroids as well as lipids. Hence it is abundant in cells that secrete steroid and lipid substances (eg cells in the sebaceous glands of the skin.) ER also forms a structural skeleton for maintaining cellular shape.
The Golgi apparatus consists of a series of parallel membranes that enclose flattened fluid filled spaces called cisternae. The ends of the membranes form vesicles that pinch off as Golgi vesicles. The Golgi apparatus is responsible for the manufacture of glycoproteins, the secretion of carbohydrates, the transport, modification and storage of materials such as lipids, the formation of lysosomes, and the post-translational modification of proteins such as enzymes in cells. (Think of it as the "warehouse" of the cell)
The cell surface membrane has numerous functions. It is a selectively permeable membrane that has a phospholipid bilayer that has proteins and cholesterol embedded within the bilayer, as well as extrinsic (peripheral proteins). The lipids and proteins on the membrane also can rotate about their own axis, hence it is fluid in nature. As a result, the cell membrane has a "fluid-mosaic model". It serves as a regulatory barrier to control the transport of substances in as well as out of the cell. The hydrophobic and hydrophilic portions control the entry and exit of fat soluble and water soluble molecules respectively, so it helps ot maintain ion concentrations. It provides shape and structural support to the cell. Cholesterol also prevents the phospholipids from packing tightly together, making the cell membrane more flexible and stable. The cell membrane also maintains the ionic balance in the cell with reference to its surroundings via active transport processes such as sodium potassium pumps (removing sodium ions while accumulating potassium ions via active transport). Water soluble ions are also transported by facilitated diffusion by channels across the membrane. Glycoproteins function as antigens to allow cells to be recognized by other agents, e.g. enzymes, hormones, and antibodies. Carbohydrate chains (glycolipids) attached to the surface of the membrane act as recognition sites.