Krebs 循环周期 - 高级

What type of acid do these fruits contain?
::这些水果含有哪种酸?

Citric acid. Citric acid is also the first product formed in the Krebs cycle, and therefore this acid occurs in the metabolism of virtually all living things.
::酸也是Krebs循环中形成的第一个产品,因此,这种酸存在于几乎所有生物的代谢中。

Aerobic Respiration
::有氧呼吸

Enticing clues - volcanic gases, vast iron ore sediments, and bubbles of ancient air trapped in amber – suggest dramatic changes during the history of Earth's atmosphere . Correlating these clues with the leads to two major conclusions: that early life evolved in the absence of oxygen, and that oxygen first appeared between 2 and 3 billion years ago ( Figure  ) because of by the blue-green , cyanobacteria . The chemistry of reflects this history. Its first stage, , is universal and does not use oxygen.
::引人入胜的线索 — — 火山气体、巨大的铁矿石沉积物和被困在琥珀中的古老空气泡沫 — — 表明地球大气史上发生了巨大变化。 将这些线索与导致两大结论的线索相混淆:早期生命在没有氧气的情况下演变,而氧气在20亿到30亿年前首次出现(图 ) , 其原因就是蓝绿色、氰球菌。 其化学反应反映了这一历史。 其第一阶段是普遍性的,并不使用氧气。

Absolutely dependent on oxygen gas, we find it difficult to imagine that its appearance must have been disastrous for the anaerobic organisms that evolved in its absence. But oxygen is highly reactive, and at first, its effect on evolution was so negative that some have named this period the “oxygen catastrophe.” However, as oxygen gradually formed a protective ozone layer , life rebounded. After the first organisms evolved to use oxygen to their advantage, the diversity of aerobic organisms exploded. According to the Theory of Endosymbiosis , engulfing of some of these aerobic bacteria led to eukaryotic cells with , and multicellularity , the evolution of multicellular eukaryotic organisms, followed. Today, we live in an atmosphere that is 21% oxygen, and most of life follows glycolysis with the last two, aerobic stages of cellular respiration.
::氧气完全依赖氧气,我们很难想象它的外观对在没有氧气的情况下进化的厌氧生物体来说是灾难性的。 但氧是高度反应性的,最初,它对进化的影响是消极的,以至于一些人将这一时期命名为“氧灾难 ” 。 然而,随着氧逐渐形成保护臭氧层,生命反弹了。 在第一批生物进化后,氧气对其有利的使用,有氧生物体的多样性爆炸了。根据“内多氧生物论 ” ( Endoosymbiocis),这些有氧细菌中的一部分被吞没,导致透氧细胞细胞细胞细胞,多细胞细胞的进化。 之后,我们生活在一个高达21%的氧气的大气中,大部分生命都是在细胞呼吸的最后两个有氧分解阶段。

Recall the purpose of cellular respiration: to release energy from glucose to make ATP , the universal molecule of energy for cellular work. The following equation describes the overall process, although it summarizes many individual chemical reactions .
::回顾细胞呼吸的目的:释放葡萄糖的能量,使ATP成为细胞工作通用能量分子。下面的方程式描述了整个过程,尽管它总结了许多单独的化学反应。

Once again, the first stage of this process, glycolysis, is ancient, universal, and anaerobic. In the cytoplasm of most , glycolysis breaks each 6-carbon molecule of glucose into two 3-carbon molecules of pyruvate . Chemical energy, which had been stored in the now broken bonds, is transferred to 2 ATP and 2 NADH molecules.
::这个过程的第一阶段,即水解过程的第一阶段,是古老的、普遍的和厌氧的。 在大部分的细胞图层中,每6碳的甘蔗糖分子的玻璃解析断裂成两种3碳的蒸汽分子。 化学能源已经储存在现在的破碎的债券中,被转移到2个ATP和2个NADH分子中。

The fate of pyruvate depends on the and the presence or absence of oxygen. If oxygen is present to drive subsequent reactions, pyruvate enters the mitochondrion, where the Krebs Cycle (Stage 2) and electron transport chain (Stage 3) break it down and oxidize it completely to CO ₂ and H ₂ O. The energy released builds many more ATP molecules, though of course some is lost as heat. Let's explore the details of how mitochondria use oxygen to make more ATP from glucose by aerobic respiration .
::蒸发剂的命运取决于氧气的存在和缺乏。如果氧气的存在能够推动随后的反应,蒸发剂进入了米托昆德里翁,在那里,Krebs循环(Stage 2)和电子运输链(Stage 3)将其分解并完全氧化为CO2和H2O。释放的能量造就了更多的ATP分子,当然有些分子作为热量而消失。让我们探讨Mitochondria如何利用氧气通过有氧呼吸从甘蔗中制造更多ATP的细节。

The Krebs Cycle: Capturing Energy from Pyruvate
::Krebs 循环:从火速中获取能源

Aerobic respiration begins with the entry of the product of glycolysis, pyruvate, into the mitochondria. For each initial glucose molecule, two pyruvate molecules will enter the mitochondria. Pyruvate, however, is not the molecule that enters the Krebs cycle. Before entry into this cycle, pyruvate must be converted into a 2-carbon acetyl-CoenzymeA (acetyl-CoA) unit. The conversion of pyruvate into acetyl-CoA is referred to as the pyruvate dehydrogenase reaction. It is catalyzed by the pyruvate dehydrogenase complex. This process produces one NADH electron carrier while releasing a CO ₂ molecule. This step is also known as the link reaction or transition step, as it links glycolysis and the Krebs cycle. Of course, as two pyruvates result from glycolysis, two acetyl-CoAs are produced as are 2 NADH molecules.
::腐蚀性呼吸从凝胶解产物( 浮化物) 进入 mitochondria 开始。 对于每个初始的浮化物分子, 两个浮化物分子将进入 mitochondria 。 但是, 浮化物不是进入 Krebs 循环的分子。 在进入此循环之前, 浮化物必须转换成 2 碳 乙酰- CoenzymeA ( acytyl- CoA) 单位。 将浮化物转化为 乙酰- CoA ( ectyl- CoA) , 被称为 丙酰胺脱氢酶反应。 由 浮化物脱氢酶复合体催化。 这个过程产生一个NADH 电子载体, 释放一个 CO2 分子。 这个步骤也被称为连接反应或过渡步骤, 因为它与 石解法和 Krebs 循环联系。 当然, 由 精化产生两个硫化物, 两个乙基- CoA 产生为 2 NADH 分子 。

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1. Within the mitochondria, each pyruvate is broken apart and combined with a coenzyme known as CoA to form a 2-carbon molecule, Acetyl-CoA, which can enter the Krebs Cycle. A single atom of carbon (per pyruvate) is “lost” as carbon dioxide. The energy released in this breakdown is captured in two NADH molecules. See the Figure  . Fatty acids can also break down into acetyl-CoA. By this means, , like fats, can be “burned” to make ATP using the Krebs Cycle.
   ::在米托乔因德里亚内部,每个回旋剂都分解出来,并且与一种被称为CoA的可溶酶结合,形成一种2-碳分子,即Acetyl-CoA,可以进入Krebs循环。一个碳原子(Per pyruvate)“丢失”为二氧化碳。在这种分解中释放的能量用两个NADH分子捕捉。见图。脂肪酸也可以分解成乙酰-CoA。通过这个方法,像脂肪一样,可以用Krebs循环“燃烧”来制造ATP。
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2. The Krebs Cycle ( Figure  ) begins by combining each acetyl-CoA with a four-carbon carrier molecule to make a 6-carbon molecule of citric acid (or citrate, its ionized form). For this reason, the Krebs Cycle, named after a scientist who worked out its details, is also called the Citric Acid Cycle .
   ::Krebs 循环( 图表 ) , 开始于将每种乙酰CoA与四碳载体分子相结合, 形成一个6碳的柠檬酸分子( 或其离子化形态 ) 。 因此, Krebs 循环以一位研究其细节的科学家命名,也被称为 Citric酸循环。
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3. The cycle carries citric acid through a series of chemical reactions which gradually release energy and capture it in several carrier molecules. For each Acetyl-CoA that enters the cycle, 3 NAD ⁺ are reduced to NADH, one molecule of FAD (another temporary energy carrier) is reduced to FADH ₂ , and one molecule of ATP (actually a precursor, GTP, guanine triphosphate) is produced. Study the Figure  to locate each of these energy-capturing events.
   ::通过一系列化学反应逐渐释放出能量并用若干载体分子捕捉到这种能量,循环中含有柠檬酸的循环。对于每个进入周期的乙基-可溶胶分子,3个NAD+将减至NADH,一个FAD(其他临时能量载体)的分子将减至FADH2,并产生一个ATP分子(实际上是一种前体、GTP、guanine 三磷酸)。研究这些能量捕捉事件的每一个位置。
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4. Note what happens to carbon atoms (black dots in the Figure  ). For each 2-carbon Acetyl-CoA that enters the cycle, two molecules of carbon dioxide are released, completing the breakdown of the original 6-carbon glucose molecule. The final step regenerates the original 4-carbon molecule which began the cycle so that another Acetyl-CoA can enter the cycle.
   ::注意碳原子( 图中的黑点) 发生的情况。 对于每个进入该循环周期的2-碳乙酰- 可乐分子,释放了2个二氧化碳分子,完成原6-碳葡萄糖分子的分解。最后一步是原4-碳分子的再生,该分子开始循环,以便另一个乙酰-可乐可以进入循环。

In summary, the Krebs Cycle completes the breakdown of glucose which begins with glycolysis. Its chemical reactions oxidize all six of the original carbon atoms to CO ₂  and capture the energy released in 2 ATP, 6 NADH, and 2 FADH ₂ . These energy carriers join the 2 ATP and 2 NADH produced in glycolysis and the 2 NADH produced in the conversion of 2 pyruvates to 2 Acetyl-CoA molecules.
::简言之,Krebs循环系统完成了以水解为开端的葡萄糖分解,其化学反应将所有6个原碳原子氧化为CO2,并捕捉了2个ATP、6个NADH和2个FADH2中释放的能量。 这些能量载体加入了2个ATP和2个NADH,这些ATP和2个NADH都是以水解制成的,2个NADH是把2个火化物转化为2个Acetyl-CoA分子产生的。

At the conclusion of the Krebs Cycle, glucose is completely broken down, yet only four ATP have been produced. Moreover, although oxygen is required to drive the Krebs Cycle, the cycle's chemical reactions do not themselves consume O ₂ . The conclusion of cellular respiration, stage 3, produces the majority of the ATP.
::Krebs循环结束时,葡萄糖完全分解,但只生产了4个ATP。 此外,尽管驱动Krebs循环需要氧气,但循环的化学反应本身并不消耗氧。 细胞呼吸,第3阶段的结束产生大部分ATP。

Summary
::摘要

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• Oxygen produced by the first photosynthetic organisms was probably toxic to early anaerobic life forms, but later organisms evolved a way to harness the power of oxygen to make ATP.
  ::第一批光合生物产生的氧可能对早期厌氧生物体有毒,但后来的生物体演变成一种利用氧力制造ATP的方法。
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• In eukaryotic cells, if oxygen is present, the pyruvate molecules produced by glycolysis in the cytoplasm enter the mitochondria for further breakdown and energy release. The Krebs Cycle harnesses the energy that remains in pyruvate after glycolysis.
  ::在eukaryodelle中,如果存在氧气,则通过在cytopolam中进行凝解生成的微积分分子进入mitochondria,以便进一步分解和释放能量。Krebs循环利用在血清解后残留的微积分能量。
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• The Krebs Cycle removes energy from citric acid in small steps, storing it in diverse energy carrier molecules: ATP, NADH, and FADH ₂ .
  ::Krebs循环以小步从柠檬酸中去除能量,储存在不同的能量载体分子中:ATP、NADH和FADH2。
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• The Krebs Cycle produces two molecules of CO ₂ per acetyl-CoA, completing the breakdown of glucose.
  ::Krebs循环生成了两种分子,即每乙酰-CoA的CO2,完成葡萄糖的分解。

Review
::回顾

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1. Explain why the appearance of oxygen in the atmosphere between two and three billion years ago was both “good news and bad news” for life on Earth.
   ::解释为什么20亿至30亿年前氧气在大气层中的出现, 对地球上的生命来说, 既是好消息,也是坏消息。
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2. In eukaryotic cells when oxygen is present, what is the fate of the pyruvate produced in glycolysis?
   ::当氧存在时,在水晶细胞中,以液解法产生的水晶石流的命运是什么?
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3. Define the Krebs cycle.
   ::定义 Krebs 周期 。
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4. Trace the six carbon atoms originally from acetyl-CoA through the Krebs Cycle. Trace the flow of energy from the pyruvates produced in glycolysis through the Krebs Cycle.
   ::追踪六种碳原子,这些碳原子最初来自乙基-碳原子,后来又来自Krebs循环系统。追踪通过Krebs循环系统以液解方式产生的冶金体的能量流动。
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5. How many energy carriers are produced during the Krebs cycle per acetyl-CoA?
   ::每个丙基-辛醇-辛醇-空气分配系数的Krebs周期生产了多少个能源载体?