Outline the roles of membranes at the surface of cells and within cells (10 marks)
Both the cell surface membrane and the membranes surrounding certain organelles have the same basic structure and similar functions. There are 5 major roles of membranes including separating cell contents from the outside environment; separating cell components from cytoplasm; cell recognition and signalling; holding the components of some metabolic pathways in place and regulating the transport of materials into or out of cells.
The phospholipid bilayer is the basic structural component of all biological membranes. A bilayer is formed when phospholipid molecules are completely surrounded by water. The hydrophilic (water-loving) heads stick in the water, while the hydrophobic (water-hating) fatty acid tails point towards each other in a mirror image. The formation of the tails creates a barrier to many molecules, separates the cell contents from the outside world and gives stability. Mechanical stability is supported by cholesterol in the bilayer. This steroid molecule fits between fatty acid tails and helps make the barrier more complete, so substances like water molecules and ions cannot pass easily and directly through the membrane. The structure of the membrane is called the fluid mosaic model because the membrane is fluid (the phospholipid molecules are not actually bonded) and because of the mosaic arrangement of the protein molecules.
Inside the fluid mosaic model are various protein molecules floating in the phospholipid bilayer; some completely freely, some bound to other components or to structures within the cell. Extrinsic proteins are partially embedded in the bilayer whereas intrinsic proteins completely span the bilayer. Some of the phospholipid molecules and proteins have small carbohydrate parts attached to them. The carbohydrate part attached to phospholipid