文档介绍：细胞生物学第四章细胞质膜与细胞表面 MEMBRANE STRUCTURE AND FUNCTION Biological membranes surround a cell and separate partments. A membrane is a mosaic of lipids, proteins, and carbohydrates. One of the key functions of a membrane is selective permeability: the ability to allow some substances through but not others. This ability of a biological membrane to control the passage of substances is fundamental to life. Puncture a cell with a fine needle, and cytoplasm will not ooze out. Instead, the cell surface will appear to flow over the puncture and seal it. The membrane behaves as if it were fluid . This fluidity results from the properties of phospholipids , the most ponents in cell membranes. When placed in fluid, phospholipids cluster together to form what is called a lipid bilayer . The lipid bilayer is the structural basis of all cell membranes. A cell is separated from its environment by a selectively permeable plasma membrane . The plasma membrane monly described as a fluid mosaic . It is like a mosaic in having diverse protein molecules embedded in a matrix of phospholipids . ponent proteins and phospholipid molecules can move about, so it is quite fluid and flexible. The phospholipids in a membrane form a two-layer framework called a phospholipid bilayer . The steroid cholesterol helps stabilize the phospholipids . Carbohydrate molecules are attached to some of the molecules that make up the membrane.