4.11 The Citric Acid CycleNow that

4.11 The Citric Acid Cycle
Now that we have a grasp of how ATP is made in respiration, we
need to consider the important reactions in carbon metabolism
associated with formation of ATP. Our focus here is on the citric
acid cycle, also called the Krebs cycle, a key pathway in
virtually all cells.
Respiration of Glucose
The early biochemical steps in the respiration of glucose are the
same as those of glycolysis; all steps from glucose to pyruvate
(Figure 4.14) are the same. However, whereas in fermentation
pyruvate is reduced and converted into products that are
excreted, in respiration pyruvate is oxidized to CO2. The pathway
by which pyruvate is completely oxidized to CO2 is called the
citric acid cycle (CAC), summarized in Figure 4.21.
Pyruvate is first decarboxylated, leading to the production of
CO2, NADH, and the energy-rich substance acetyl-CoA (Figure
4.12). The acetyl group of acetyl-CoA then combines with the
four-carbon compound oxalacetate, forming the six-carbon compound citric acid. A series of reactions follow, and two additional
CO2 molecules, three more NADH, and one FADH are formed.
Ultimately, oxalacetate is regenerated to return as an acetyl
acceptor, thus completing the cycle (Figure 4.21).
CO
2 Release and Fuel for Electron Transport
The oxidation of pyruvate to CO2 requires the concerted activity
of the citric acid cycle and the electron transport chain. For each
pyruvate molecule oxidized through the citric acid cycle, three
CO2 molecules are released (Figure 4.21). Electrons released during the oxidation of intermediates in the citric acid cycle are
transferred to NAD1 to form NADH, or to FAD to form FADH2.
This is where respiration and fermentation differ in a major way.
Instead of being used in the reduction of pyruvate as in fermentation (Figure 4.14), in respiration, electrons from NADH and
FADH
2 are fuel for the electron transport chain, ultimately resulting in the reduction of an electron acceptor (O2) to H2O. This
allows for the complete oxidation of glucose to CO2 along with a
much greater yield of energy. Whereas only 2 ATP are produced
per glucose fermented in alcoholic or lactic acid fermentations
(Figure 4.14), a total of 38 ATP can be made by aerobically respiring the same glucose molecule to CO2 1 H2O (Figure 4.21b)
Biosynthesis and the Citric Acid Cycle
Besides playing a key role in catabolism, the citric acid cycle plays
another important role in the cell. The cycle generates several key
compounds, small amounts of which can be drawn off for biosynthetic purposes when needed. Particularly important in this
regard are -ketoglutarate and oxalacetate, which are precursors
of several amino acids (Section 4.14), and succinyl-CoA, needed to
form cytochromes, chlorophyll, and several other tetrapyrrole compounds (Figure 4.16). Oxalacetate is also important because it can
be converted to phosphoenolpyruvate, a precursor of glucose. In
addition, acetate provides the starting material for fatty acid biosynthesis (Section 4.15, and see Figure 4.27). The citric acid cycle thus
plays two major roles in the cell: bioenergetic and biosynthetic.
Much the same can be said about the glycolytic pathway, as certain
intermediates from this pathway are drawn off for various biosynthetic needs as well (Section 4.13
4.12 Catabolic Diversity
Thus far in this chapter we have dealt only with catabolism by
chemoorganotrophs. We now briefly consider catabolic diversity, some of the alternatives to the use of organic compounds as
electron donors, with emphases on both electron and carbon
flow. Figure 4.22 summarizes the mechanisms by which cells
generate energy other than by fermentation and aerobic respiration. These include anaerobic respiration, chemolithotrophy, and
phototrophy.
Anaerobic Respiration
Under anoxic conditions, electron acceptors other than oxygen
can be used to support respiration in certain prokaryotes. These
processes are called anaerobic respiration. Some of the electron
acceptors used in anaerobic respiration include nitrate (NO32,
reduced to nitrite, NO22, by Escherichia coli or to N2 by
Pseudomonas species), ferric iron (Fe31, reduced to Fe21 by
Geobacter species), sulfate (SO422, reduced to hydrogen sulfide,
H
2S, by Desulfovibrio species), carbonate (CO322, reduced to
methane, CH4, by methanogens or to acetate by acetogens), and
even certain organic compounds. Some of these acceptors, for
example Fe31, are often only available in the form of insolubleminerals, such as metal oxides. These common minerals, widely
distributed in nature, allow for anaerobic respiration in a wide
variety of microbial habitats.
Because of the positions of these alternative electron acceptors
on the redox tower (none has an as positive as the O2/H2O
couple; Figure 4.9), less energy is released when they are reduced
instead of oxygen (recall that DG09 is proportional to ;
Section 4.6). Nevertheless, because O2 is often limiting or absent
in many microbial habitats, anaerobic respirations can be very
important means of energy generation. As in aerobic respiration,
anaerobic respirations involve electron transport, generation of a
proton motive force, and the activity of ATPase.
Chemolithotrophy
Organisms able to use inorganic chemicals as electron donors are
called chemolithotrophs. Examples of relevant inorganic electron donors include H2S, hydrogen gas (H2), Fe21, and NH3.
Chemolithotrophic metabolism is typically aerobic and
begins with the oxidation of the inorganic electron donor
(Figure 4.22). Electrons from the inorganic donor enter an electron transport chain and a proton motive force is formed in exactly the same way as for chemoorganotrophs (Figure 4.19).
However, one important distinction between chemolithotrophs
and chemoorganotrophs, besides their electron donors, is their
source of carbon for biosynthesis. Chemoorganotrophs use
organic compounds (glucose, acetate, and the like) as carbon
sources. By contrast, chemolithotrophs use carbon dioxide
(CO2) as a carbon source and are therefore autotrophs (organisms capable of biosynthesizing all cell material from CO2 as
the sole carbon source). We consider many examples of
chemolithotrophy in Chapter 13.
Phototrophy
Many microorganisms are phototrophs, using light as an energy
source in the process of photosynthesis. The mechanisms by
which light is used as an energy source are complex, but the end
result is the same as in respiration: generation of a proton motive
force that is used to drive ATP synthesis. Light-mediated ATP
synthesis is called photophosphorylation. Most phototrophs
use energy conserved in ATP for the assimilation of CO2 as the
carbon source for biosynthesis; they are called photoautotrophs.
However, some phototrophs use organic compounds as carbon sources with light as the energy source; these are the photoheterotrophs (Figure 4.22).
As we discussed in Chapter 2, there are two types of photosynthesis: oxygenic and anoxygenic. Oxygenic photosynthesis, carried out by cyanobacteria and their relatives and also by green
plants, results in O2 evolution. Anoxygenic photosynthesis is a
simpler process used by purple and green bacteria that does not
evolve O2. The reactions leading to proton motive force formation in both forms of photosynthesis have strong parallels, as we
see in Chapter 13.
The Proton Motive Force
and Catabolic Diversity
Microorganisms show an amazing diversity of bioenergetic
strategies. Thousands of organic compounds, many inorganic
compounds, and light can be used by one or another microorganism as an energy source. With the exception of fermentations,
in which substrate-level phosphorylation occurs (Section 4.8),
energy conservation in respiration and photosynthesis is driven
by the proton motive force.
Whether electrons come from the oxidation of organic or
inorganic chemicals or from phototrophic processes, in all
forms of respiration and photosynthesis, energy conservation is
linked to the pmf through ATPase (Figure 4.20). Considered in
this way, respiration and anaerobic respiration are simply metabolic variations employing different electron acceptors. Likewise,
chemoorganotrophy, chemolithotrophy, and photosynthesis are
simply metabolic variations upon a theme of different electron
donors. Electron transport and the pmf link all of these
processes, bringing these seemingly quite different forms of
metabolism into a common focus. We pick up on this theme in
Chapters 13 and 14.

Kết quả (Việt) 1:

4.11 chu trình KrebsBây giờ mà chúng tôi có một nắm bắt cách ATP được thực hiện trong hô hấp, chúng tôicần phải xem xét các phản ứng quan trọng trong chuyển hóa cacbongắn liền với sự hình thành của ATP. Chúng tôi tập trung ở đây là trên các citricchu trình axit, còn được gọi là chu trình Krebs, một con đường quan trọng tronghầu như tất cả các tế bào.Sự hô hấp của GlucoseSinh hóa bước đầu trong việc hô hấp của glucose cácgiống như những người glycolysis; Tất cả các bước từ glucose để pyruvat(Hình 4,14) đều giống nhau. Tuy nhiên, trong khi trong quá trình lên menPyruvat là giảm và chuyển đổi thành sản phẩmbài tiết, hô hấp pyruvat là mầu để CO2. Con đườngbởi pyruvat mà hoàn toàn bị ôxi hóa để CO2 được gọi là cácchu trình Krebs (CAC), được tóm tắt trong hình 4.21.Pyruvat là decarboxylated đầu tiên, dẫn đến việc sản xuấtCO2, NADH, và năng lượng phong phú chất acetyl-CoA (hình4.12). Nhóm axetyl acetyl-CoA sau đó kết hợp với các4-Bon hợp chất oxalacetate, tạo thành axít citric hợp chất của sáu-cacbon. Một loạt các phản ứng sau, và hai bổ sungCác phân tử khí CO2, ba thêm NADH và một FADH được thành lập.Cuối cùng, oxalacetate tái sinh để trở lại như một axetylTìm, do đó hoàn thành chu kỳ (hình 4.21).CO2 phát hành và nhiên liệu cho giao thông vận tải điện tửQuá trình oxy hóa của pyruvat để CO2 yêu cầu hoạt động phối hợpchu trình axit citric và chuỗi vận tải điện tử. Đối với mỗiPyruvat phân tử bị ôxi hóa thông qua chu trình axit citric, baCO2 molecules are released (Figure 4.21). Electrons released during the oxidation of intermediates in the citric acid cycle aretransferred to NAD1 to form NADH, or to FAD to form FADH2.This is where respiration and fermentation differ in a major way.Instead of being used in the reduction of pyruvate as in fermentation (Figure 4.14), in respiration, electrons from NADH andFADH2 are fuel for the electron transport chain, ultimately resulting in the reduction of an electron acceptor (O2) to H2O. Thisallows for the complete oxidation of glucose to CO2 along with amuch greater yield of energy. Whereas only 2 ATP are producedper glucose fermented in alcoholic or lactic acid fermentations(Figure 4.14), a total of 38 ATP can be made by aerobically respiring the same glucose molecule to CO2 1 H2O (Figure 4.21b)Biosynthesis and the Citric Acid CycleBesides playing a key role in catabolism, the citric acid cycle playsanother important role in the cell. The cycle generates several keycompounds, small amounts of which can be drawn off for biosynthetic purposes when needed. Particularly important in thisregard are -ketoglutarate and oxalacetate, which are precursorsof several amino acids (Section 4.14), and succinyl-CoA, needed toform cytochromes, chlorophyll, and several other tetrapyrrole compounds (Figure 4.16). Oxalacetate is also important because it canbe converted to phosphoenolpyruvate, a precursor of glucose. InNgoài ra, axetat cung cấp các tài liệu khởi đầu cho sinh tổng hợp axit béo (phần 4.15, và xem hình 4,27). Axít citric chu kỳ như vậyđóng hai vai trò chính trong các tế bào: bioenergetic và viêm.Nhiều như vậy có thể nói về con đường glycolytic, như một sốTrung gian từ con đường này được rút ra các viêm nhu cầu cũng như (phần 4,134.12 catabolic đa dạngVậy, đến nay trong chương này chúng tôi đã xử lý chỉ với catabolism bởichemoorganotrophs. Chúng tôi bây giờ một thời gian ngắn xem xét catabolic đa dạng, một số lựa chọn thay thế để sử dụng các hợp chất hữu cơ nhưCác nhà tài trợ điện tử, với emphases trên cả hai electron và carbondòng chảy. Con số 4,22 tóm tắt cơ chế bởi các tế bào đótạo ra năng lượng khác hơn bằng cách lên men và sự hô hấp hiếu khí. Chúng bao gồm kỵ khí hô hấp, chemolithotrophy, vàphototrophy.Kỵ khí hô hấpTrong điều kiện thiếu ôxy, chất nhận khác ở điện tử khác với oxycó thể được sử dụng để hỗ trợ hô hấp trong sinh nhất định. Nhữngquá trình được gọi là kỵ khí hô hấp. Một số các electronchất nhận khác ở được sử dụng trong kỵ khí hô hấp bao gồm nitrat (NO32,giảm nitrit, NO22, bởi Escherichia coli hoặc N2 bởiPseudomonas loài), sắt sắt (Fe31, giảm đến Fe21 bằngGeobacter loài), sulfat (SO422, giảm đến sulfua hiđrô,H2S, bởi Desulfovibrio loài), cacbonat (CO322, giảm xuốngmêtan, CH4, loài sinh metan hoặc acetate bởi acetogens), vàeven certain organic compounds. Some of these acceptors, forexample Fe31, are often only available in the form of insolubleminerals, such as metal oxides. These common minerals, widelydistributed in nature, allow for anaerobic respiration in a widevariety of microbial habitats.Because of the positions of these alternative electron acceptorson the redox tower (none has an as positive as the O2/H2Ocouple; Figure 4.9), less energy is released when they are reducedinstead of oxygen (recall that DG09 is proportional to ;Section 4.6). Nevertheless, because O2 is often limiting or absentin many microbial habitats, anaerobic respirations can be veryimportant means of energy generation. As in aerobic respiration,anaerobic respirations involve electron transport, generation of aproton motive force, and the activity of ATPase.ChemolithotrophyOrganisms able to use inorganic chemicals as electron donors arecalled chemolithotrophs. Examples of relevant inorganic electron donors include H2S, hydrogen gas (H2), Fe21, and NH3.Chemolithotrophic metabolism is typically aerobic andbegins with the oxidation of the inorganic electron donor(Figure 4.22). Electrons from the inorganic donor enter an electron transport chain and a proton motive force is formed in exactly the same way as for chemoorganotrophs (Figure 4.19).However, one important distinction between chemolithotrophsand chemoorganotrophs, besides their electron donors, is theirsource of carbon for biosynthesis. Chemoorganotrophs useorganic compounds (glucose, acetate, and the like) as carbonsources. By contrast, chemolithotrophs use carbon dioxide(CO2) as a carbon source and are therefore autotrophs (organisms capable of biosynthesizing all cell material from CO2 asthe sole carbon source). We consider many examples ofchemolithotrophy in Chapter 13.PhototrophyMany microorganisms are phototrophs, using light as an energysource in the process of photosynthesis. The mechanisms bywhich light is used as an energy source are complex, but the endresult is the same as in respiration: generation of a proton motiveforce that is used to drive ATP synthesis. Light-mediated ATPsynthesis is called photophosphorylation. Most phototrophsuse energy conserved in ATP for the assimilation of CO2 as thecarbon source for biosynthesis; they are called photoautotrophs.However, some phototrophs use organic compounds as carbon sources with light as the energy source; these are the photoheterotrophs (Figure 4.22).As we discussed in Chapter 2, there are two types of photosynthesis: oxygenic and anoxygenic. Oxygenic photosynthesis, carried out by cyanobacteria and their relatives and also by greenplants, results in O2 evolution. Anoxygenic photosynthesis is asimpler process used by purple and green bacteria that does notevolve O2. The reactions leading to proton motive force formation in both forms of photosynthesis have strong parallels, as wesee in Chapter 13.The Proton Motive Forceand Catabolic DiversityMicroorganisms show an amazing diversity of bioenergeticstrategies. Thousands of organic compounds, many inorganiccompounds, and light can be used by one or another microorganism as an energy source. With the exception of fermentations,in which substrate-level phosphorylation occurs (Section 4.8),energy conservation in respiration and photosynthesis is drivenby the proton motive force.Whether electrons come from the oxidation of organic orinorganic chemicals or from phototrophic processes, in allforms of respiration and photosynthesis, energy conservation islinked to the pmf through ATPase (Figure 4.20). Considered inthis way, respiration and anaerobic respiration are simply metabolic variations employing different electron acceptors. Likewise,chemoorganotrophy, chemolithotrophy, and photosynthesis aresimply metabolic variations upon a theme of different electrondonors. Electron transport and the pmf link all of theseprocesses, bringing these seemingly quite different forms ofmetabolism into a common focus. We pick up on this theme inChapters 13 and 14.