图解泰国电影鬼屋

The cell membrane
All cells are enclosed by a boundary structure, the plasma membrane, which provides a barrier to other cells and the external environment. Although the membrane serves to contain the cell contents, unicellular organisms (bacteria included) usually have extra material on the outside and plant cells are characterized by a rigid cell wall of cellulose ( Figure 4 ). Over a century of investigation has shown membranes to be an incredibly complicated and dynamic mixture of lipid (fat)molecules and proteins. Although the basic structure of the plasma membrane is extremely thin-just a couple of lipid molecules thick, forming a lipid bilayer-it is extremely tough and flexible, and also permeable to allow for the constant exchange of molecules between the cell and its surroundings.This is achieved via the water that constantly enters and exits in a controlled manner bringing soluble molecules like oxygen(needed as a fuel), and exporting waste products such as carbon dioxide. Large external material can be physically engulfed by the membrane, a process known as phagocytosis. The reverse process is exocytosis, in which a membrane-bound vacuole of material destined for export reaches the cell surface, at which point the membranes fuse and open to the outside, releasing the contents without breaching the overall integrity of the membrane. The dynamic nature of the cell membrane is such that the entire plasma membrane is ‘turned over’ (replaced) on an hourly basis.