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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.
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.
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4.11 The Citric Acid CycleNow that we have a grasp of how ATP is made in respiration, weneed to consider the important reactions in carbon metabolismassociated with formation of ATP. Our focus here is on the citricacid cycle, also called the Krebs cycle, a key pathway invirtually all cells.Respiration of GlucoseThe early biochemical steps in the respiration of glucose are thesame as those of glycolysis; all steps from glucose to pyruvate(Figure 4.14) are the same. However, whereas in fermentationpyruvate is reduced and converted into products that areexcreted, in respiration pyruvate is oxidized to CO2. The pathwayby which pyruvate is completely oxidized to CO2 is called thecitric acid cycle (CAC), summarized in Figure 4.21.Pyruvate is first decarboxylated, leading to the production ofCO2, NADH, and the energy-rich substance acetyl-CoA (Figure4.12). The acetyl group of acetyl-CoA then combines with thefour-carbon compound oxalacetate, forming the six-carbon compound citric acid. A series of reactions follow, and two additionalCO2 molecules, three more NADH, and one FADH are formed.Ultimately, oxalacetate is regenerated to return as an acetylacceptor, thus completing the cycle (Figure 4.21).CO2 Release and Fuel for Electron TransportThe oxidation of pyruvate to CO2 requires the concerted activityof the citric acid cycle and the electron transport chain. For eachpyruvate molecule oxidized through the citric acid cycle, threeCO2 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. Inaddition, acetate provides the starting material for fatty acid biosynthesis (Section 4.15, and see Figure 4.27). The citric acid cycle thusplays two major roles in the cell: bioenergetic and biosynthetic.Much the same can be said about the glycolytic pathway, as certainintermediates from this pathway are drawn off for various biosynthetic needs as well (Section 4.134.12 Catabolic DiversityThus far in this chapter we have dealt only with catabolism bychemoorganotrophs. We now briefly consider catabolic diversity, some of the alternatives to the use of organic compounds aselectron donors, with emphases on both electron and carbonflow. Figure 4.22 summarizes the mechanisms by which cellsgenerate energy other than by fermentation and aerobic respiration. These include anaerobic respiration, chemolithotrophy, andphototrophy.Anaerobic RespirationUnder anoxic conditions, electron acceptors other than oxygencan be used to support respiration in certain prokaryotes. Theseprocesses are called anaerobic respiration. Some of the electronacceptors used in anaerobic respiration include nitrate (NO32,reduced to nitrite, NO22, by Escherichia coli or to N2 byPseudomonas species), ferric iron (Fe31, reduced to Fe21 byGeobacter species), sulfate (SO422, reduced to hydrogen sulfide,H2S, by Desulfovibrio species), carbonate (CO322, reduced tomethane, CH4, by methanogens or to acetate by acetogens), andeven 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 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.
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.
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4.11 chu trình axit chanh.Bây giờ, chúng ta đã nắm được cách ở trong chúng ta thở,Cần cân nhắc quan trọng trong phản ứng chuyển hóa carbonVới đội hình.Chúng ta có vấn đề là chanh.Chu trình axit, cũng được gọi là chu kỳ, một con đường quan trọngGần như tất cả các tế bào.Hơi thở của glucoseTrong hơi thở của glucose trong bước đầu là sinh học.Cùng đường men pyruvate giải; từ glucose đến tất cả các bước(đồ 4.14) là như nhau.Tuy nhiên, và đang trong quá trình lên menPyruvate giảm, và biến nó thành một sản phẩm, làXả trong ống nghiệm, thở pyruvate bị oxy hóa thành carbon dioxide.Con đườngBy pyruvate hoàn toàn bị oxy hóa thành carbon dioxide được gọi làChu trình axit chanh (CAC), tổng kết ở Đồ 4.21.Pyruvate decarboxylation là đầu tiên, dẫn đến sản xuất.CO2, NADH, và năng lượng vật chất acetyl coenzyme A (Fig.4.12).Acetyl coenzyme A của acetyl, sau đó kết hợpBốn hợp chất carbon sáu carbon, hình thành hợp chất axit chanh.Một loạt các phản ứng theo, và thêm haiBa phân tử CO2, NADH và FADH được hình thành.Cuối cùng, tái sinh trở lại với một acetylThụ, để hoàn thành chu kỳ (đồ 4.21).Carbon monoxide2 điện tử được thả và thua vận chuyển nhiên liệuPyruvate oxit cần phù hợp với hoạt động của khí carbon dioxideChu trình axit chanh và chuỗi chuyển điện tử.Đối với mỗiQua vòng của phân tử oxy hóa pyruvate axit chanh, ba2 giải phóng phân tử (đồ 4.21).Trong chu kỳ của các oxit axit chanh trong quá trình giải phóng điện làQuay nad1 tạo NADH, hoặc hình thành FADH2 thời trang.Đó là hơi thở và lên men một cách chính ở nơi khác.Thay vì được dùng để phục hồi cho lên men pyruvate (đồ 4.14), thở động từ NADH và điện tử.FADHChuỗi chuyển điện tử 2 là nhiên liệu, cuối cùng dẫn đến giảm thụ điện tử (O), lấy nước.Cho phép hoàn chỉnh cho carbon dioxide và oxy hóa glucose - 1Năng lượng lớn hơn.Và chỉ có 2 người tạo raMỗi khi uống rượu hay lên men axit lactic lên men glucose(đồ 4.14), tổng cộng 38 ATP có thể thở oxy qua có cùng một phân tử CO2 glucose - 1 H2O (đồ 4.21b) với chu trình axit chanh.Ngoài việc chơi ở vai trò quan trọng của phân hủy chất chuyển hóa trong chu trình axit chanhTrong một tế bào khác có tác dụng quan trọng.Chu kỳ tạo ra vài phím.Hợp chất, một ít, có thể trong khi mục đích của cần chia sẻ.Đặc biệt quan trọng làÊ là α - oxoglutarate và, đó là điềm báoMột vài axit amin (4.14 hải lý), và Amber coenzym A, cầnHình thành sắc tố tế bào, diệp lục, và một vài hợp chất khác (hình 4 pyrrole 4.16). cũng rất quan trọng, vì nó có thểCó thể chuyển hóa thành phosphate ethanol thức pyruvate, tiền thân của glucose.ỞBên cạnh đó, axit muối của axit béo, bắt đầu từ cung cấp nguyên liệu (số 4.15 Festival, 4.27).Do đó, chu trình axit chanh.Tế bào trong 2 tác dụng: năng lượng và.Cũng có thể nói, đường men cách giải, như một sốTừ con đường này của trung cấp cũng bị miễn cho các nhu cầu của (ngày thứ 4.134.12 trao đổi chất đa dạng.Cho đến nay, trong cuốn "Chúng tôi chỉ đối phó với các quá trình trao đổi chấtChemoorganotrophs.Chúng ta bây giờ chủ yếu về trao đổi chất diversity, một hợp chất hữu cơ sử dụng phương ánNhà tài trợ tập trung vào điện, điện tử và carbonChảy.Đồ 4.22 của tế bào cơ chế tổng hợp.Ngoài sự lên men và tạo ra năng lượng có oxy để thở.Chúng bao gồm thở, dinh dưỡng, vàTự dưỡng quang hợp. thởTrong điều kiện thiếu oxy, thụ điện tử ngoài oxy.Ở một số sinh vật nhân sơ có thể được dùng để hỗ trợ thở.NhữngQuá trình này được gọi là thở.Một vài điện tửKỵ khí thở cho thụ bao gồm nitrat (lấy nitrat,Trở lại thành nitrit, 22, by E. coli hoặc N2 bào vi khuẩn), sắt (fe31, giảm FE21.Geobacter trong họ Cerambycidae), sulfat (so422, giảm hydro sulfua,H2S, Lưu Huỳnh Lưu Huỳnh nói), cacbonat (co322, giảmMetan, metan, by metan hay axit acetic), vàThậm chí một số hợp chất hữu cơ.Thụ thể này và choVí dụ fe31, thường chỉ có thể ở dạng insolubleminerals, như oxit kim loại.Những khoáng vật phổ biến, rộng lớn.Trong tự nhiên, phân phối, cho phép trong một thở rất rộng.Môi trường sống vi sinh vật đa dạng.Vì những thay thế vị trí của thụ thể điện tử.Trong quá trình oxy hóa (không có một tháp là tích cực, vì không khí / nướcCặp đôi; đồ 4.9), ít hơn. Năng lượng được giải phóng khi họ giảmChứ không phải tỷ lệ thuận với dg09 oxy (nhớ?Đường dài 4, 6 hải lý).Tuy nhiên, vì không khí thường là hạn chế hoặc không tồn tại.Trong nhiều môi trường sống của vi sinh vật, thở có thể
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