医学英语新教程 王兰英 第一单元 课文原文+翻译.doc

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 provides greater 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。染色体是由DNA和蛋白质构成的。可记得我们早期的讨论，DNA就是细胞特征和活动的遗传密码。每一个细胞核中的DNA含有所有人类特征的全部遗传信息。在一些特殊的细胞中，实际上仅有少数的基因活跃。这些活的基因就是特殊类型细胞所需要的蛋白质的密码The nucleus is the control center of the cell because it contains the chromosomes (染色体). The 46 chromosomes of a human cell are usually not visible; they are long threads called chromatin (染色质). When a cell divides, however, the chromatin coils (twist into a winding shape) extensively into visible chromosomes. Chromosomes are made of DNA and protein. Remember from our earlier discussion that the DNA is the genetic code for the characteristics and activities of the cell.。Although the DNA in the nucleus of each cell contains all of the genetic information for all human traits, only a small number of genes are actually active in a particular cell. These active genes are the codes for the proteins necessary for the specific cell type. 细胞质和细胞器 Cytoplasm and cell organelles11 细胞质是水溶性物质，汽体和有机分子，它存在于细胞膜和细胞核之间。化学反应发生于细胞质中，而细胞器就存在此处。细胞器是细胞间物质，通常是由自己的膜界定，在细胞功能中有着特殊的作用。Cytoplasm is a watery solution of minerals, gases, and organic molecules that is found between the cell membrane and the nucleus. Chemical reactions take place within the cytoplasm, and the cell organelles are found here. Cell organelles are intracellular structures, often bounded by their own membranes, which have specific roles in cellular functioning 12 内质网状组织是膜小管的精密网，它从核膜扩展到细胞膜。内质网状组织糙面的外表有大量的核蛋白，而滑面一点核蛋白也没有。作为互相连接的小管网，内质网状组织用作细胞内，是细胞功能必需的传递通道。这些物质包括，内质网状组织核蛋白所合成的蛋白质，以及由内质网状组织滑面所合成的脂质。 The endoplasmic reticulum (ER) is an extensive network of membranous tubules that extend from the nuclear membrane to the cell membrane. Rough ER（粗面内质网） has numerous ribosomes on its surface, while smooth ER（滑面内质网） has no ribosomes at all. As a network of interconnected tunnels, the ER serves as a passageway for the transport of the materials necessary for cell function within the cell. These include proteins synthesized by the ribosomes on the rough ER, and lipids synthesized by the smooth ER.13核蛋白是非常小的物质，由蛋白质和核蛋白RNA构成。一些存在于内质网状组织的糙面，而另一些自由地漂浮在细胞质中。核蛋白就是蛋白质合成的场所。Ribosomes are very small structures made of protein and ribosomal RNA. Some are found on the surface of rough ER, while others float freely within the cytoplasm. Ribosomes are the site of protein synthesis. 14高尔基体是一系列扁平的膜囊，有点像众多的小碟子。碳水化合物就是在高尔基体中合成的，进行包装，和其他物质一道作为细胞的分泌物。为了分泌物质，高尔基体的小囊破裂，与细胞膜融合，并且把一些物质释放到细胞内。 The Golgi apparatus is a series of flat, membranous sacs, somewhat like a stack of saucers. Carbohydrates are synthesized within the Golgi apparatus, and are packaged, along with other materials, for secretion from the cell. To secrete a substance, small sacs of the Golgi membrane break off (decompose) and fuse（join together) with the cell membrane, releasing the substance to the exterior of the cell.15 线粒体，是椭圆形成球状的细胞器，存在于细胞质中，由双膜界定，内膜有很多皱襞叫做卵鞘脊。在线粒体内产生细胞呼吸的需氧反应。因此，线粒体是ATP产生的场所。需要大量ATP的细胞如，肌细胞，有许多线粒体来满足能量的需要。 Mitochondria（线粒体） are oval or spherical organelles within the cytoplasm, bounded by a double membrane. The inner membrane has folds called cristae. Within the mitochondria, the aerobic (oxygen-requiring) reactions of cell respiration take place. Therefore, mitochondria are the site of ATP (and hence energy) production. Cells that require large amounts of ATP, such as muscle cells, have many mitochondria to meet their need for energy. 16溶酶体，存在于细胞质中是单膜结构，含有许多消化酶。当一些白细胞吞食细菌的时候，这些细菌就会被溶酶体的消化酶消化和破坏。破落细胞和凋亡细胞也被这些酶消化，从而促使损伤组织的发炎。 Lysosomes are single membrane structures within the cytoplasm that contain digestive enzymes. When certain white blood cells engulf bacteria, the bacteria are digested and destroyed by these lysosomal enzymes. Worn-out cell parts and dead cells are also digested by these enzymes, which contributes to the process of inflammation in damaged tissues. 17中心粒，是杆状结构，互相垂直。其功能就是在细胞分裂期间整理紡垂丝纤维。Centrioles are a pair of rod-shaped structures perpendicular（垂直的）to one another. Their function is to organize the spindle fibers during cell division. 18纤毛和鞭毛穿过细胞膜是个活动的凸生物。纤毛的功能就是清除细胞表面的物质。它们通常比鞭毛短，有个别的细胞才有很多纤毛。比如,覆在输卵管上的细胞用纤毛把卵细胞扫向子宫。只有人类细胞才有鞭毛，它们是精子细胞，这些鞭毛为精子细胞提供了动力。 细胞的转运机制 Cilia and flagella （纤毛和鞭毛）are mobile threadlike projections through the cell membrane. Cilia serve the function of sweeping materials across the cell surface. They are usually shorter than flagella, and an individual cell has many of them. Cells lining the fallopian tubes（输卵管）, for example, have cilia to sweep the egg cell toward the uterus. The only human cell with a flagellum is the sperm cell. The flagellum provides motility, or movement, for the sperm cell.19活的细胞持续不断的和它周围的血液，或组织液相互作用，吞食一些物质，分泌和排泄另一些物质。有一些转运机制使细胞把物质带进或带出细胞：扩散、渗透、易化扩散、主动转运、滤过、吞食作用以及胞饮作用。其中一些由细胞完成，而无需能量消耗。但，另外一些则需能量，三磷酸腺苷。接下来将描述每一机制，并举例说明该机制对人体的重要性。 Living cells constantly interact with the blood or tissue fluid around them, taking in some substances and secreting or excreting others. There are several mechanisms of transport that enable cells to move materials into or out of the cell: diffusion, osmosis, facilitated diffusion（易化扩散）, active transport（主动转运）, filtration, phagocytosis, and pinocytosis（胞饮作用）. Some of these take place without the expenditure of energy by the cells. But, others do require energy, in the form of ATP. Each of these mechanisms is described below and an example is included to show how each is important to the body. 扩散Diffusion 20 扩散就是分子运动，从浓度高的地方到浓度低的地方。扩散的发生是因为分子有很多自由能，也就是说，它们总处在运动之中。固体中的分子运动相当慢，而液体中的分子运动非常快，气体中的分子运动的仍然很快。如，当冰吸收热能时，它就会融化被蒸发。当糖溶解时，糖分子就会互相碰撞。这种碰撞使糖分子扩散，直到最终扩散到大分子中，这些分子仍然在运动，但是一些上升到了顶端，而另一些下沉到低端，从而达到了平衡。Diffusion is the movement of molecules from an area of greater concentration to an area of lesser concentration. Diffusion occurs because molecules have free energy, that is, they are always in motion. The molecules in a solid move very slowly; those in a liquid move faster, and those in a gas move faster still, as when ice absorbs heat energy, melts, and then evaporates（turn into vapor）. As the sugar dissolves, the sugar molecules collide (strike) with one another. These collisions spread out the sugar molecules until they are evenly (equally) dispersed among the water molecules. The molecules are still moving, but as some go to the top others go to the bottom, and so on. Thus, an equilibrium (steadiness) is reached. 21 体内，通过扩散氧气和二氧化碳进行运动。例如，在肺里，肺泡中的氧气浓度很高，而在肺毛细血管周围的血液中的氧气度却很低。这与二氧化碳恰恰相反。如，肺泡中空气中的二氧化碳的浓度很低，而肺毛细血管中的二氧化碳浓度却很高。这些气体向相反的方向扩散，都是从浓度高的地方向浓度低的地方运动。氧气的扩散从空气到血液再扩散到全身。二氧化碳的扩散从血液到空气直到呼出。 Within the body, the gases oxygen and carbon dioxide move by diffusion. In the lungs, for example, there is a high concentration of oxygen in the alveoli (air sacs), and a low concentration of oxygen in the blood in the surrounding pulmonary capillaries. The opposite is true for carbon dioxide: a low concentration in the air in the alveoli and a high concentration in the blood in the pulmonary capillaries. These gases diffuse in opposite directions, each moving from where there is more to where there is less.Oxygen diffuses from the air to the blood to be circulated throughout the body. Carbon dioxide diffuses from the blood to the air to be exhaled 渗透 Osmosis22 渗透简单定义为水通过选择性通透膜进行扩散。也就是说，水将从水多的地方流到水少的地方。换句话说，就是水将自然的流向更多溶解物的地方。如，盐或糖。如果把2%的盐溶物和6%盐溶物用一个膜隔开，是水而不是盐才能通过它。水将会从2%的盐溶物流向6%的盐溶物，其结果就是2%的盐溶物浓度变得很高，而6%的盐溶物却更加稀释。Osmosis may be simply defined as the diffusion of water through a selectively permeable membrane or barrier. That is, water will move from an area with more water present to an area with less water. Another way to say this is that water will naturally tend to (incline to) move to an area where there is more dissolved material, such as salt or sugar. If a 2% salt solution and a 6% salt solution are separated by a membrane allowing water but not salt to pass through it, water will diffuse from the 2% salt solution to the 6% salt solution. The result is that the 2% solution will become more concentrated and the 6% solution will become more dilute. 23 在体内，位于小肠的细胞吸收通过渗透的消化物质。这些细胞首先吸收的是盐，成为多盐体。而水随着盐进入细胞。渗透过程也发生于肾，重新吸收大量的水，防止由于尿而脱水。 In the body, the cells lining the small intestine absorb water from digested food by osmosis. These cells have first absorbed salts, have become more “salty”, and water follows salt into the cells. The process of osmosis also takes place in the kidneys, which reabsorb large amounts of water to prevent its loss in urine. 易化扩散 Facilitated Diffusion24 Facilitate这个词意思为帮助或者有助于。在易化扩散中，分子的运动是通过膜从浓度高的地方到浓度低的地方，但是它需要帮助才能完成。 The word facilitate means to help or assist. In facilitated diffusion, molecules move through a membrane from an area of greater concentration to an area of lesser concentration, but they need some help to do this.25 在体内，我们的细胞必须吸收葡萄糖，把它用作ATP的产生。然而葡萄糖本身不能穿过大多数的细胞膜进行扩散。即使细胞外边比里边多也不行。葡萄糖扩散到所有的细胞需要载体酶，蛋白质是细胞的一部分，葡萄糖和载体酶相结合，这样做在细胞膜磷脂中才会溶解。葡萄糖载体分子通过膜进行扩散，把葡萄糖释放进细胞。In the body, our cells must take in glucose to use for ATP production. Glucose, however, will not diffuse through most cell membranes by itself, even if there is more outside the cell than inside.Diffusion of glucose into most cells requires carrier enzymes, proteins that are part of the cell membrane. Glucose bonds to (links to) the carrier enzymes, and by doing so becomes soluble in the phospholipids of the cell membrane. The glucose-carrier molecule diffuses through the membrane and glucose is released to the interior of the cell. 主动转运 Active Transport26 主动转运需要ATP能量，使分子从浓度低的地方到浓度高的地方。注意这和扩散正相反，在扩散中，分子自由能向浓度低的地方运动。因此我们说，主动转运就是其运动与浓度梯度相反。Active transport requires the energy of ATP to move molecules from an area of lesser concentration to an area of greater concentration. Notice that this is the opposite of diffusion, in which the free energy of molecules causes them to move to where there are fewer of them. Active transport is therefore said to be movement against a concentration gradient. 27 在体内，神经细胞和肌细胞都有钠泵，把钠离子转移到细胞外面。细胞外面有丰富的钠离子，他们不断扩散到细胞中，到浓度低的地方去。没有钠泵把它们排到外面，那么进入的钠离子就会引起不必要的神经冲动，或肌肉收缩。神经细胞和肌细胞不断地产生ATP能量来保持钠泵不停的工作，来防止本能的冲动。 In the body, nerve cells and muscle cells have “sodium pumps” to move sodium ions (Na+) out of the cells. Sodium ions are more abundant outside the cells, and constantly diffuse into the cell, their area of lesser concentration. Without the sodium pumps to return them outside, the incoming sodium ions would bring about (cause, produce, trigger) an unwanted nerve impulse or muscle contraction. Nerve and muscle cells constantly produce ATP to keep their sodium pumps working and prevent spontaneous impulses.28 另一个主动转运的例子是通过位于小肠的细胞对葡糖糖和氨基酸的吸收。这些细胞使用ATP能量从消化的食物中吸收营养物质，即使细胞内的浓度高于细胞外的浓度。Another example of active transport is the absorption of glucose and amino acids by the cells lining the small intestine. The cells use ATP to absorb these nutrients from digested food, even when their intracellular concentration becomes greater than their extracellular concentration. 过滤 Filtration29 滤过的过程也需能量，但是所需的能量不是来自ATP 。它是一种机械能。滤过指的是水和溶解物被迫地通过膜从压力高的地方到压力低的地方。The process of filtration also requires energy, b