We can now see that all living things are made of cells: small, membrane-enclosed units filled with a concentrated aqueous solution of chemicals and endowed with the extraordinary ability to create copies of themselves by growing and then dividing in two. Because cells are the fundamental units of life, it is to cell biology—the study of the structure, function, and behavior of cells—that we must look for answers to the questions of what life is and how it works.

我们现在能够看到，一切生物都由细胞构成：细胞是被膜包围的小单位，内部充满浓缩的化学物质水溶液，并被赋予了一种非凡的能力——通过生长而后一分为二来复制自己。正因为细胞是生命的基本单位，我们必须求助于细胞生物学——即研究细胞的结构、功能与行为的学科——来回答生命是什么、生命如何运作这些问题。

The individual cell is the minimal self-reproducing unit of life. A cell consists of a self-replicating collection of catalysts, enclosed in a plasma membrane. All cells operate as biochemical factories, driven by the free energy released in a complicated network of chemical reactions. Central to a cell’s ability to reproduce is the transmission of its genetic information to its progeny cells when it divides.

单个细胞是能够自我复制的最小生命单位。一个细胞由一套能自我复制的催化剂组成，并被一层质膜包围。所有细胞都作为生化工厂运转，由一个复杂化学反应网络所释放的自由能驱动。细胞之所以能够繁殖，其核心在于分裂时把自身的遗传信息传递给子代细胞。

Proteins are polymers of amino acid subunits and are the catalysts for almost all the cell’s chemical reactions. They are also responsible for the selective import and export of molecules across the plasma membrane that surrounds each cell. The specific shape and function of each protein depend on its amino acid sequence, which is specified by the nucleotide sequence of a corresponding segment of the DNA—the gene that codes for that protein. In this way, the DNA of the cell determines the cell’s chemistry, which is fundamentally similar in all cells, reflecting their ultimate origin from a common ancestor cell that existed on Earth more than 3.5 billion years ago.

蛋白质是氨基酸亚基的聚合物，是细胞几乎全部化学反应的催化剂。它们还负责分子跨越包围每个细胞的质膜的选择性输入与输出。每种蛋白质的特定形状与功能取决于它的氨基酸序列，而后者由 DNA 上相应片段——即编码该蛋白质的基因——的核苷酸序列所规定。就这样，细胞的 DNA 决定了细胞的化学，而这种化学在所有细胞中都基本相同，反映出它们最终都源自 35 亿年前存在于地球上的一个共同祖先细胞。

What limits the dimensions of a cell? The lower limit is probably set by the minimum number of each type of biomolecule required by the cell. The smallest cells, certain bacteria known as mycoplasmas, are 300 nm in diameter and have a volume of about 10−14 mL. A single bacterial ribosome is about 20 nm in its longest dimension, so a few ribosomes take up a substantial fraction of the volume in a mycoplasmal cell. The upper limit of cell size is probably set by the rate of transport of nutrients into the cell and waste products out. As the size of a cell increases, its surface-to-volume ratio decreases.

是什么限制了细胞的尺寸？下限大概是由细胞所需的每一类生物分子的最少数目决定的。最小的细胞——某些被称为支原体的细菌——直径为 300 nm，体积约 10⁻¹⁴ mL。一个细菌核糖体最长的一维约为 20 nm，因此在支原体细胞中，仅仅几个核糖体就占去了相当可观的一部分体积。细胞尺寸的上限，大概由养分进入细胞、废物排出细胞的转运速率决定。随着细胞体积增大，它的表面积/体积比会下降。

Like bacteria, the archaea we know most about are small and lack the internal, membrane-bound organelles that distinguish the eukaryotes. But they differ from bacteria in many ways, including the chemistry of their cell walls, the kinds of lipids that make up their membrane, and the range of biochemical reactions that they can carry out. Another surprising conclusion came from genome comparisons: although archaea resemble bacteria in their outward appearances, their genomes are much more closely related to eukaryotes than to bacteria (see Figure 1–9).

和细菌一样，我们了解最多的那些古菌也很小，并且缺少把真核生物区别开来的内部膜结合细胞器。但它们在许多方面与细菌不同，包括细胞壁的化学组成、构成其膜的脂类种类，以及它们所能进行的生化反应的范围。另一个令人意外的结论来自基因组比较：尽管古菌在外观上像细菌，它们的基因组却与真核生物的亲缘关系远比与细菌的近（见图 1-9）。

Although not themselves living cells, viruses often serve as vectors for gene transfer between cells. A virus will replicate in one cell, emerge from it with a protective wrapping, and then enter and infect another cell, which may be of the same or different species. Often, the infected host cell is killed by the massive proliferation of virus particles inside it, but sometimes the viral DNA, instead of directly generating new virus particles, may persist in its host for many cell generations as a relatively innocuous passenger—either as a separate intracellular fragment of DNA, known as a plasmid, or as a DNA sequence inserted into the cell’s own genome.

病毒本身虽然不是活细胞，却常常充当细胞之间基因转移的载体。一种病毒会在一个细胞中复制，带着保护性外壳从中出来，然后进入并感染另一个细胞——后者可能是同一物种的，也可能是不同物种的。被感染的宿主细胞常常因体内病毒颗粒的大量增殖而被杀死；但有时病毒 DNA 并不直接产生新的病毒颗粒，而是作为一个相对无害的“搭客”在宿主中存留许多个细胞世代——或作为一段独立的胞内 DNA 片段（即质粒），或作为一段插入细胞自身基因组的 DNA 序列。

A species that has one of the smallest known genomes is the bacterium Mycoplasma genitalium, which causes a common, sexually transmitted, human disease (Figure 1–8). This organism lives as a parasite in mammals, where the environment provides it with many of the small molecules it needs ready-made. Nevertheless, it still has to make all the large molecules—DNA, RNAs, and proteins. It has 525 genes, most of which are essential.

已知基因组最小的物种之一，是引起一种常见性传播人类疾病的细菌——生殖支原体（图 1-8）。这种生物以寄生方式生活在哺乳动物体内，那里的环境为它现成地提供了所需的许多小分子。尽管如此，它仍然必须自己合成全部大分子——DNA、各种 RNA 和蛋白质。它有 525 个基因，其中大多数是必需基因。

Most living organisms are single cells. Others, such as us, are like vast multicellular cities in which groups of cells perform specialized functions that are linked by intricate systems of intercellular communication. But even for the aggregate of more than 1013 cells that makes up a human body, the whole organism has been generated by cell divisions from a single cell. The single cell therefore contains all of the hereditary information that defines a species (Figure 1–1).

大多数生物就是单个细胞。另一些，比如我们，则像一座座庞大的多细胞城市，其中成群的细胞执行各自专门的功能，并由精巧的细胞间通讯系统联系起来。但即便是构成人体的那 10^13 个以上细胞的集合体，整个个体也是由一个细胞经细胞分裂产生的。因此，单个细胞就含有界定一个物种所需的全部遗传信息（图 1-1）。

Chloroplasts (Figure 1–28) perform photosynthesis in plant cells and algae, using the energy of sunlight to synthesize their own “food” (in the form of carbohydrates) from atmospheric CO2 and water. Like mitochondria, they are enclosed in double membranes, have their own “circular” genomes, and reproduce by dividing. They almost certainly evolved from a symbiotic photosynthetic bacterium that was captured by an ancient eukaryotic cell that already possessed mitochondria.

叶绿体（图 1-28）在植物细胞和藻类中进行光合作用，利用日光的能量，由大气中的 CO2 和水合成它们自己的“食物”（以碳水化合物的形式）。和线粒体一样，叶绿体由双层膜包被，拥有自己的“环状”基因组，并通过分裂来增殖。它们几乎肯定是由一种共生的光合细菌演化而来——这种细菌被一个已经拥有线粒体的古真核细胞所捕获。

Viruses are small packets of genetic material that have evolved as parasites that depend on the reproductive and biosynthetic machinery of the host cells they infect. Viruses are not strictly alive, because they depend on the machinery of their host cells for their reproduction. Although we now know that viruses are the most abundant—in terms of sheer numbers—of all the biological entities on this planet, they are too small to be seen in the light microscope.

病毒是一小包遗传物质，它们作为寄生物演化而来，依赖于所感染宿主细胞的复制与生物合成机器。病毒并非严格意义上的活物，因为它们的繁殖依赖宿主细胞的机器。虽然我们现在知道，就绝对数量而言病毒是这个星球上最多的生物实体，但它们太小，在光学显微镜下看不到。