【文章內(nèi)容簡(jiǎn)介】 t the energy of the electrons, and they are accepted by ferredoxin. Ferredoxin is then reoxidized, and the coenzyme NADP+ is reduced to the NADPH. The ATP generated previously and the NADPH then take part in the light independent 。當(dāng)光激活光系統(tǒng)Ⅱ的光反應(yīng)中心時(shí)，通過(guò)一系列的氧化還原反應(yīng)實(shí)現(xiàn)能量的傳遞。/這一包裝過(guò)程從光線到達(dá)光合系統(tǒng)II的活性中心P680這一步驟開(kāi)始，包括了一系列氧化還原反應(yīng)反應(yīng)。在第一個(gè)步驟中，水分子被分解，放出氧氣，提供電子。電子首先傳遞給質(zhì)體醌，然后通過(guò)一系列載體形成的電子傳遞鏈。每傳遞4個(gè)電子，形成2個(gè)ATP。鏈的最終受體是光合系統(tǒng)I的P700 活性中心。此處光子激活電子，電子傳遞給鐵氧還蛋白。鐵氧還蛋白再氧化，并且輔酶NADP+還原成NADPH。前面產(chǎn)生的ATP和NADPH進(jìn)入暗反應(yīng)。The production of ATP from the transport of electrons excited by light energy down an electron transport chain is termed photophosphorylation. The oneway flow of electrons through photosystems II and I is called noncyclic photophosphorylation。 plants also derive additional ATP through cyclic photophosphorylation, in which some electrons are shunted back through the electron transport chain between photosystems Ⅱ and Ⅰ.由光能激發(fā)產(chǎn)生的電子沿電子傳遞鏈傳遞并產(chǎn)生ATP 的過(guò)程稱(chēng)為光合磷酸化。電子通過(guò)光合系統(tǒng)Ⅱ流經(jīng)光合系統(tǒng)Ⅰ的電子路徑稱(chēng)非循環(huán)式光合磷酸化；植物通過(guò)循環(huán)式光合磷酸化獲得額外的ATP，在這一過(guò)程中一些電子在光合系統(tǒng)Ⅰ和Ⅱ之間的電子傳遞鏈中回流。In the lightindependent reactions of photosynthesis, which are driven by ATP and NADPH, C02 is converted to carbohydrate. The reactions are also known as the CalvinBenson cycle. Atmospheric CO2, is fixed as it reacts with ribulose biphosphate (RuBP), a reaction that is catalyzed by the enzyme ribulose biphosphate carboxylThe reduction Of C02 to carbohydrate (fructose diphosphate several more steps of the cycle. Finally, RUBP is regenerated so that the cycle may ，CO2轉(zhuǎn)化成碳水化合物。即卡爾文本森循環(huán)。在二磷酸核酮糖羧化酶的催化下，二磷酸核酮糖固定空氣中的CO2。CO2通過(guò)循環(huán)中的幾步反應(yīng)還原成為糖類(lèi)（二磷酸果糖），最終核酮糖二磷酸再生，這樣循環(huán)能夠持續(xù)進(jìn)行。High levels of oxygen in plant cells can disrupt photosynthesis and can also cause photorespirationan inefficient fun of the dark reactions in which 02 is fixed rather than C02 and no carbohydrate is ，同時(shí)也能夠引起光呼吸，這是一種效率差的暗反應(yīng)， 02而不是CO2被固定，不產(chǎn)生糖類(lèi)。 Most plants are C3 plants。 they experience decreased carbohydrate production under hot, dry conditions as a result of the effects of photorespiration. Among C4 plants, however, special leaf anatomy and a unique biochemical pathway enable the plant to thrive in and conditions. Thus C4 plants lessen photorespiration by carrying out photosynthesis only in cells that are insulated from high levels of CO2. They also possess a novel mechanism for carbon ，在高溫干旱條件下，由于光呼吸作用而使碳水化合物的合成降低。而在大多數(shù)的C4 植物中，由于葉脈的特殊構(gòu)造和獨(dú)特的化學(xué)路徑使植物依然很茂盛。這樣C4 植物并非在細(xì)胞中存在高濃度的CO2條件下才能進(jìn)行光合作用，這是碳固定的一個(gè)新機(jī)制。The cell nucleus is the main repository of genetic information. Within the nucleus are the chromosomes tightly coiled strands of DNA and clusters of associated proteins. Long stretches of the continuous DNA molecule wind around these clusters of proteins, or histones, forming beadlike plexes known as nucleosomes. More coiling and supercoiling produces a dense chromosome structure. Each long strand of DNA bines with histones and nonhistone proteins to make up the substance 。DNA盤(pán)繞成螺旋線以及相關(guān)的成簇蛋白質(zhì)。DNA螺旋線纏繞成簇的組蛋白形成珠鏈狀的核小體。這些螺旋或超螺旋形成致密的染色體組結(jié)構(gòu)。每個(gè)長(zhǎng)鏈DNA與組蛋白和非組蛋白一起構(gòu)成染色質(zhì)。A pictorial display of an organism39。s chromosomes in the coiled, condensed state is known as a karyotype. Karyotype reveal that in most cells all but sex chromosomes are present as two copies, referred to as homologous pairs. Nonsex chromosomes are called whose cells contain two sets of parental chromosomes are called diploid。 those with cells containing a single set of parental chromosomes are called 、濃縮狀態(tài)有機(jī)體染色體，稱(chēng)為染色體組型。除了性染色體外，大多數(shù)細(xì)胞的染色體組成對(duì)出現(xiàn)，稱(chēng)同源染色體對(duì)。非性染色體稱(chēng)常染色體。生物細(xì)胞含有兩套親本染色體的有機(jī)體稱(chēng)為二倍體；含有單套染色體的有機(jī)體稱(chēng)單倍體。The cell cycle is a regular sequence in which the cell grows, prepares for division, and divides to form two daughter cells, each of which then repeats the cycle. Such cycling in effect makes singlecelled organisms immortal. Many cells in multicellular organisms, including animal muscle and nerve cells, either slow the cycle or break out of it ：細(xì)胞生長(zhǎng)、準(zhǔn)備分裂、分裂形成兩個(gè)子細(xì)胞，子細(xì)胞再循環(huán)。此循環(huán)使得單細(xì)胞生物永生。多細(xì)胞生物中的許多細(xì)胞，包括動(dòng)物肌肉和神經(jīng)細(xì)胞，能夠使細(xì)胞周期時(shí)間延長(zhǎng)或完全脫離細(xì)胞周期。 The normal cell cycle consists of four phases. The first three include G1, the period of normal metabolism。 S phase, during which normal synthesis of biological molecules continues, DNA is replicated, and histones are synthesized。 and G2, a brief period of metabolism and additional growth. Toget G1, S, and G2 phases are called interphase. The fourth phase of the cell cycle is M phase, the period of mitosis, during which the replicated chromosomes condense and move and the cell divides. It is believed that properties of the cell cytoplasm control the cell cycle, along with external stimulators and inhibitors such as 。前3步包括G1(正常的代謝階段)；S期（持續(xù)進(jìn)行生物分子的正常合成，DNA復(fù)制，組蛋白合成）；G2期（代謝和再次生長(zhǎng)的短階段）。G1期、S期和G2期合起來(lái)稱(chēng)為間期。細(xì)胞周期的第四步為M 期（進(jìn)行有絲分裂的階段），在這一階段，復(fù)制的染色體濃縮、運(yùn)動(dòng)，細(xì)胞分裂。被認(rèn)為是細(xì)胞質(zhì)的特性和外部的刺激因子和抑制劑例如抑素控制了細(xì)胞周期。Biologists divide the mitotic cycle into four phases. At the beginning of prophase the chromosomes each consist of two highly condensed chromatids attached to each other at a centromere. As prophase ends and metaphase begins, the condensed chromosomes bee associated with the spindle. Eventually thein a plane (called the metaphase plate) at a rig spindle fibers. Next, during anaphase , the two sister chromatids of each chromosome split, and one from each pair is drawn toward each pole of the cell. During telophase nuclear envelopes begin to form around each set of chromosomes, and division of the cytoplasm takes 。分裂前期，每一條染色體包括兩條高度濃縮的染色單體，兩個(gè)染色單體通過(guò)著絲粒連接在一起。當(dāng)前期結(jié)束時(shí)，分裂中期開(kāi)始，濃縮的染色體與紡錘體相連，最后以與紡錘絲成垂直角度排列在赤道板上。在分裂后期，每個(gè)染色體上的兩個(gè)姊妹染色單體分開(kāi)，分別移向細(xì)胞兩極。在分裂末期，圍繞每一套染色體開(kāi)始形成核膜，然后開(kāi)始細(xì)胞質(zhì)的分裂。 As mitosis proceeds, the spindle microtubules play a crucial role in ensuring that both paired and separated chromatids move in the right directions at the proper times. Each half of the spindle forms as micros extend from each pole of a dividing cell to the region of the metaphase plate. During prophase, other microtubules, the centromeric fibers, extend outward from the spindle poles to structures on the chromosomes called kinetochores. During anaphase the fibers begin to shorten, and the chr