The Cell Membrane

细胞膜

All living cells and many of the tiny organelles internal to cells are bounded by thin membranes. These membranes are composed primarily of phospholipids and proteins and are typically described as phospholipid bi-layers.

所有活细胞以及许多细胞内的微小细胞器都由薄膜所包围。这些膜主要由磷脂和蛋白质组成，通常被描述为磷脂双分子层。

In this sketch, the spheres represent the phosphate end, which is polar and water soluble (hydrophilic). The twin extensions represent the fatty acid components which are not water soluble (hydrophobic).

在这一示意图中，球体代表磷酸端，该端具有极性且可溶于水（亲水）。双延伸部分代表脂肪酸成分，这些成分不溶于水（疏水）。

Cell membranes also contain cholesterol in the phospholipid bilayer. In some membranes there are only a few cholesterol molecules, but in others there are as many cholesterols as phospholipids according to Audesirk & Audesirk. Cholesterol makes the bilayer stronger, more flexible but less fluid, and less permeable to water-soluble substances such as ions and monosaccharides.

细胞膜也含有胆固醇，位于磷脂双分子层中。在某些膜中只有少量的胆固醇分子，而在其他膜中，胆固醇的数量可能与磷脂分子相当，根据Audesirk & Audesirk的说法。胆固醇使双分子层更坚固、更灵活，但更不具有流动性，且对水溶性物质如离子和单糖的通透性更低。

The above illustration of a cell membrane was modified from Wikipedia. It shows that the cell membrane contains much more than just the phospholipid bi-layer.

上述细胞膜的示意图改编自维基百科。它显示细胞膜包含的远不止是磷脂双分子层。

Transport Across Cell Membranes

细胞膜两侧的运输

Essential and continuous parts of the life of a cell are the taking in of nutrients and the expelling of wastes. All of these must pass through the cell membrane.

细胞生命中的基本且持续的过程包括吸收营养物质和排出废物。所有这些物质都必须通过细胞膜。

Transport may occur by diffusion and osmosis across the membrane. It can also occur when a vescicle attaches to the cell membrane from the inside and then opens to form a pocket, expelling its contents to the outside. This may be called exocytosis. The cell membrane may also envelope something on the outside and surround it, taking it into the cell. This may be called endocytosis or phagocytosis.

物质可能通过扩散和渗透作用跨膜运输。当一个囊泡从内部附着到细胞膜上，然后打开形成一个口袋，将内容物排出细胞外时，这种运输方式也可以发生。这可能被称为胞吐作用。细胞膜也可能包围外部的物质并将其摄入细胞内，这种过程可能被称为胞吞作用或吞噬作用。

There are also examples where molecules move across a membrane from a region of low concentration to an region of high concentration, and this requires a source of energy to "pump" the molecules uphill in concentration. Such processes are called active transport.

还有些例子中，分子会从低浓度区域向高浓度区域跨膜移动，这一过程需要能量来‘泵’分子向上坡式移动。这种过程被称为主动运输。