第一课细胞结构及细胞转运机制1 所有的生物都是由细胞和细胞外基质构成的。
这种的简单的论述叫做细胞理论，是150年前第一次提出来的。
我们可以把这种理论叫做猜测或者假设，有时候也确实如此，但是证据证实了细胞理论的正确性。
All living organisms are made of cells and cell products. This simple statement, called the Cell Theory, was first proposed over 150 years ago. You may think of a theory as a guess or hypothesis, and sometimes this is so. But a theory is actually the best explanation of all the available evidence. All of the evidence science has gathered so far supports the validity of the Cell Theory.2 细胞是多细胞生物最小活的亚单位，比如人。
细胞是复杂的化学排列；是活体；并且进行着特殊的活动。
微生物如变形虫、细菌是单细胞生物，其细胞有着独立功能。
然而，人类细胞必须相互依赖，共同作用。
内环境稳定取决于所有不同类型细胞的作用。
Cells are the smallest living subunits of a multicellular organism such as a human being. A cell is a complex arrangement of the chemicals; is living; and carries out specific activities. Microorganisms, such as amoebas and bacteria, are single cells which function independently. Human cells, however, must work together, and function interdependently. Homeostasis depends upon the contributions of all of the different kinds of cells.3人类细胞在大小、形状和功能上有所不同。
大多数人类细胞非常小，以至于借助显微镜才能看到，其测量单位叫微米。
人类的卵细胞除外，它的直径为一毫米，用肉眼就能看得见。
有一些神经细胞，尽管其直径非常小，但是相当长，他们分布在胳膊和腿上，至少有两英尺长。
Human cells vary in size, shape, and function. Most human cells are so small they can only be seen with the aid of a microscope, and are measured in units called microns. One exception is the human ovum or egg cell, which is about one millimeter in diameter, just visible to the unaided eye. Some nerve cells, although microscopic in diameter, may be quite long. Those in our arms and legs, for example, are at least two feet long.4关于其形状，人类细胞变化极大，一些是圆的，还有一些是矩形的，其他是不规则形的。
而白细胞的形状是随着它的运动而变化的。
With respect to shape, human cells vary greatly. Some are round or spherical, others rectangular, still others irregular. White blood cells even change shape as they move.细胞结构Cell Structure5 尽管它们有很多不同之处，但是人类细胞有很多相同的结构特征。
如，细胞膜、细胞质、细胞器和细胞核。
红细胞除外，因为当它们成熟的时候，就没有细胞核。
细胞膜形成了细胞的外层，包裹着细胞质、细胞器和细胞核。
Despite their many differences, human cells have several similar structural features: a cell membrane, cytoplasm and cell organelles, and a nucleus. Red blood cells are an exception since they have no nuclei when mature. The cell membrane forms the outer boundary of the cell, and surrounds the cytoplasm, organelles, and nucleus.细胞膜Cell Membrane6 细胞膜也叫做质膜，细胞是由磷脂、胆固醇和蛋白质构成的。
磷脂是脂溶性物质，通过扩散，经细胞膜进出细胞。
胆固醇为细胞膜提供了极大的稳定性。
蛋白质有很多作用，一些形成小孔使一些物质通过。
还有一些是酶，有助于物质进入细胞。
而另外一些蛋白质外边有很多低聚糖是抗原，标明了它可以鉴别自身的细胞。
然而，还有一些蛋白质用作激素的受体部位。
通过第一次和细胞的特殊受体相结合，许多激素产生了特殊的作用。
然后，这种结合激发了细胞膜内或细胞内的化学反应。
Also called the plasma membrane, the cell membrane is made of phospholipids, cholesterol, and proteins. The phospholipids permit lipid-soluble materials to easily enter or leave the cell by diffusion through the cell membrane. Cholesterol providesgreater stability to the membrane. The proteins have several functions: some form pores or openings to permit passage of materials; others are enzymes that also help substances enter the cell. Still other proteins, with oligosaccharides （低聚糖,寡糖）on their outer surface, are antigens, markers that identify the cells of an individual as “self”. And yet another group of proteins serves as receptor sites for hormones. Many hormones bring about their specific effects by first bonding (linking) to a particular receptor on the cell membrane. This bonding then triggers chemical reactions within the cell membrane or the interior of the cell.7 尽管细胞膜是细胞的外层，但是我们应该明白，它不是固定的、像墙一样的外层。
而相当活跃，充满朝气。
细胞膜有选择性的通透，也就是说，一些物质可以通过，而一些物质不可以通过。
Although the cell membrane is the outer boundary of the cell, it should already be apparent to you that it is not a static (motionless) or wall-like boundary, but rather an active, dynamic one. The cell membrane is selectively permeable, that is certain substances are permitted to pass through and others are not.细胞核Nucleus8 除了成熟的红细胞之外，所有的人类细胞都有细胞核。
这些细胞核漂浮在细胞质中，有双层的核膜界定，并且有很多小孔。
With the exception of mature red blood cells, all human cells have a nucleus. The nucleus floats in the cytoplasm, and is bounded by a double-layered nuclear membrane with many pores.9 细胞核仁是小球状，有DNA、RNA和蛋白质构成。
核仁形成了特殊类型的RNA ，叫做核蛋白RNA，它是核蛋白的一部分（细胞器），并且涉及蛋白质的合成。
A nucleolus (核仁) is a small sphere made of DNA, RNA, and protein. The nucleoli form a type of RNA called ribosomal RNA, which becomes part of ribosomes 核糖体(a cell organelle) and is involved in protein synthesis.10 细胞核是细胞的控制中心，因为它含有染色体。
人类细胞中的46 个染色体通常是看不见的，它们是长线型，叫做染色质，当细胞分裂的时候，染色体激烈的环状旋转，就形成了看得见的染色体。