【正文】 and maintaining viable sperm (the male sex cell). It also produces the male sex hormone, testosterone, which regulates the development of a beard, pubic hair, a deep voice and other bodily characteristics of the adult male. 男、女性的生殖系統(tǒng)不同。它也分泌男性激素、睪酮，以此調(diào)節(jié)胡須、陰毛、深沉嗓音極其他成年男子身體發(fā)育的特征。女性生殖系統(tǒng)也分泌女性性激素——雌激素和孕酮，以此調(diào)節(jié)乳房及其他成熟女性身體發(fā)育的特征。s organs. It keeps out foreign substances and prevents excessive water evaporation. The nerves in the skin provide tactile information. The skin also helps keep the body39。C, heat is conserved by reducing blood flow through the skin or is expended by increasing blood flow and by evaporation of sweat from the skin. Hair and nails are accessory structures of the skin. 皮膚是保護(hù)肌體內(nèi)層結(jié)構(gòu)的完整層，也是機(jī)體的最大器官。皮膚上的神經(jīng)提供觸覺信息。通過皮膚的血流量降低時(shí)，熱量就被儲(chǔ)存起來，通過皮膚的血流增加及汗液蒸發(fā)時(shí)，熱量就散發(fā)。 Cells and Tissues細(xì)胞與組織In this passage, you will learn: body systems as a whole parts of a cell kinds of tissuesCells are organized into tissues, and tissues are arranged into organs, which in turn are grouped into systems. Each body system serves its specific functions. Bear in mind however that the body functions as a wholeno system is independent of the others. They work together to maintain the body’s state of internal stability, termed homeostasis. Now let’s begin our discussion with cells, the smallest unit of living matter that can exist by itself. 細(xì)胞構(gòu)成組織，組織構(gòu)成器官，器官又進(jìn)一步構(gòu)成系統(tǒng)?，F(xiàn)在，我們先討論細(xì)胞─能獨(dú)立存在活性物質(zhì)的最小單位。所有人體的功能都來自億萬個(gè)特定細(xì)胞活動(dòng)，有的動(dòng)植物僅由一個(gè)細(xì)胞構(gòu)成，其他生物則由億萬個(gè)細(xì)胞構(gòu)成。列如：有可能是立方形或扁形的。雞的蛋黃是一個(gè)非常大的單細(xì)胞。不管是細(xì)胞大小或形狀的不同，每個(gè)細(xì)胞都有“需要存活的機(jī)制”。The size of cells is usually measured in microns. A micron is a millionth of a meter, and about 25,000 microns equal one inch. The smallest bacteria are about micron in diameter. The average cell in the human bodyabout ten microns in diameteris a speck barely visible without the aid of a microscope. 細(xì)胞的大小是以“微米”為長度單位。人體正常細(xì)胞的評(píng)價(jià)圓徑約十微米，肉眼勉強(qiáng)可以看得到，不需要顯微鏡的幫助。在最微小水平針對(duì)細(xì)胞的結(jié)構(gòu)及功能的研究成為分子生物學(xué)。典型細(xì)胞的原生質(zhì)形成了三個(gè)重要部分，細(xì)胞膜、細(xì)胞質(zhì)和細(xì)胞核。大部分的化學(xué)反應(yīng)都在細(xì)胞核周圍，細(xì)胞質(zhì)完成的。the cells Membrane細(xì)胞膜 Cells can survive only in a liquid medium that brings in food and carries away waste. For onecelled organisms this fluid is an external body of waterthe ocean, a lake, or a stream. For manycelled plants and animals, however, the medium is part of the organismin plants, the sap。對(duì)單細(xì)胞生物來說，這就是細(xì)胞外的液體，如：大海里、河里或水流中。 The cell membrane is semipermeable, or differentially permeablesome substances can pass through it, but others cannot. This characteristic enables the cell to admit useful substances and to reject harmful substances from the surrounding fluid as well as to force out, or excrete waste products into the fluid. 細(xì)胞膜是半滲透性或區(qū)別性的滲透膜，即某些物質(zhì)能滲透。 The cell membrane is an extremely thin but tough band of protein and phospholipid molecules. Phospholipids are chemicals similar to stored fat. On the evidence of electron micrographs, biologists believe that it has pores through which certain small molecules pass intact. Substances probably pass through the cell membrane in several ways. Small chambers located on the cell membrane called caveolae gather chemical signals and channel them into the cells. The caveolae may also help distribute proteins throughout the cell. Large molecules enter the cell by a process called diffusion, in which they may be dissolved by substances in the cell membrane. They can then through the membrane without difficulty. Some cells take in large molecules by means of pinocytosis. In this process, the cell membrane forms a pocket around large molecules floating against it. The moleculeandfluidfilled pocket then breads loose from the membrane to bee a bubblelike vacuole, and the vacuole then drifts into the cytoplasm. Finally, the vacuole wall breaks up and the molecules are released into the cytoplasm. 細(xì)胞膜是一種非常薄但確以很堅(jiān)固的蛋白磷脂膜形成。通過電子顯微鏡，生物學(xué)家相信細(xì)胞膜有孔，使較小的物質(zhì)完全可以通過。細(xì)胞膜上的結(jié)構(gòu)，稱“小穴”，可接受化學(xué)信號(hào)再傳到細(xì)胞內(nèi)。大的分子可通過“滲濾”進(jìn)入細(xì)胞內(nèi)，或被細(xì)胞膜內(nèi)體分解，使它們更容易的進(jìn)入細(xì)胞內(nèi)。這個(gè)過程是通過細(xì)胞膜在大物質(zhì)形成了“帶”把它包圍著，將它以泡狀的“液泡”引入細(xì)胞質(zhì)，最后包圍解散，大物質(zhì)成功的進(jìn)入細(xì)胞內(nèi)。水分含量有百分之65到最高，百分之95。細(xì)胞質(zhì)是膠體性的。當(dāng)固體成分濃度增多，膜狀和纖維狀結(jié)果在細(xì)胞質(zhì)顯出。濃度的變化還可以表現(xiàn)細(xì)胞質(zhì)內(nèi)局部流動(dòng)的變化。 Most of the cell’s constant work of keeping alive is performed in the cytoplasm. Here food molecules are changed into the material needed for energy and materials for growth or stiffening of the cell membrane are manufactures needed by the rest of the organism. For example, plant cells containing chlorophyll manufacture glucosea plant foodfrom carbon dioxide and water. 大部分為了細(xì)胞存活的工作都在此處（細(xì)胞質(zhì)）完成的。多細(xì)胞植物的細(xì)胞質(zhì)還是生產(chǎn)了對(duì)其他生物提供食物的物質(zhì)。 One of the cytoplasm’s key energy transactions occurs in the sausageshaped mitochondria. Each mitochondrion has an inner and an outer membrane. Like the cell membrane, the membranes of the mitochondrion are semipermeable. Food molecules that pass into the cytoplasm are taken into the mitochondria and oxidized, or burned, for energy. 細(xì)胞質(zhì)能量轉(zhuǎn)換的要點(diǎn)發(fā)生在錐狀形的線粒體。像細(xì)胞膜一樣，線粒體的膜也是半滲透性。 The endoplasmic reticulum, a network of membranous tubes, runs through the cytoplasm. In the opinion of some biologists, this network is a continuous structure that begins at the cell membrane, twists through the cytoplasm, and ends at the membrane surrounding the nucleus. Located along the endoplasmic reticulum as well as elsewhere in the cytoplasm are numerous ribosomes. These tiny granules consist in part of ribonucleic acid (RNA). Proteins are manufactured at the ribosomes. The Golgi plex, or Golgi apparatus, is a membranous structure posed of stacks of thin sacs. Newly made proteins move from the endoplasmic reticulum to the Golgi plex, where they are stored for later secretion. 胞漿網(wǎng)，在細(xì)胞質(zhì)中由膜所形成管狀的網(wǎng)。分布于胞漿網(wǎng)包括細(xì)胞質(zhì)中某處的是核糖體。蛋白質(zhì)是在核糖體合成的。新合成的蛋白從胞漿網(wǎng)移動(dòng)到高爾基體儲(chǔ)存，為了后來分泌所用。溶酶體是一種有點(diǎn)像液泡的結(jié)構(gòu)，但較稠。每個(gè)溶酶體的膜內(nèi)都是添裝著消化酶。也認(rèn)為高爾基體對(duì)溶酶體的形成有關(guān)系。細(xì)胞核控制細(xì)胞的生長和分化。 Enclosed by a twolayered membrane, the nucleus contains a liquid called nucleoplasm as well as strands of deoxyribonucleic acid (DNA) covered with a coating of protein. A strand of DNA consists of a long series of genes, which are the units of heredity of plants and animals. Genes deter