What are brassinosteroids? What is the immediate result of a plant's egg and sperm combining? Both cytokinins and gibberellins promote plant growth. What are the ways in which these hormones What are the roles of plastids in the life of a plant? The overall mechanism of cellular respiration involves four subdivisions: glycolysis , in which glucose molecules are broken down to form pyruvic acid molecules; the Krebs cycle , in which pyruvic acid is further broken down and the energy in its molecule is used to form high-energy compounds such as NADH; the electron transport system , in which electrons are transported along a series of coenzymes and cytochromes and the energy in the electrons is released; and chemiosmosis , in which the energy given off by electrons is used to pump protons across a membrane and provide the energy for ATP synthesis.
The process of glycolysis is a multistep metabolic pathway that occurs in the cytoplasm of microbial cells and the cells of other organisms. At least six enzymes operate in the metabolic pathway. In the first and third steps of the pathway, ATP is used to energize the molecules. Thus, two molecules of ATP must be expended in the process. Further along in the process, the six-carbon glucose molecule is converted into intermediary compounds and then is split into two three-carbon compounds.
The latter undergo additional conversions and eventually form pyruvic acid at the conclusion of the process. During the latter stages of glycolysis, four ATP molecules are synthesized using the energy given off during the chemical reactions. The reduced coenzyme NADH will later be used in the electron transport system, and its energy will be released.
During glycolysis, two NADH molecules are produced. As glycolysis does not use oxygen, the process is considered to be anaerobic. For certain anaerobic organisms, such as certain bacteria and fermentation yeasts, glycolysis is the sole source of energy. It is a somewhat inefficient process because much of the cellular energy remains in the two molecules of pyruvic acid. The Krebs cycle. Following glycolysis, the mechanism of cellular respiration then involves another multistep process called the Krebs cycle , also called the citric acid cycle and the tricarboxylic acid cycle.
An overview of the processes of cellular respiration showing the major pathways and the places where ATP is synthesized. The Krebs cycle occurs at the cell membrane of bacterial cells and in the mitochondria of eukaryotic cells. Each of these sausage-shaped organelles of eukaryotic microorganisms possesses inner and outer membranes, and therefore an inner and outer compartment.
The inner membrane is folded over itself many times; the folds are called cristae. Along the cristae are the important enzymes necessary for the proton pump and for ATP production. Prior to entering the Krebs cycle, the pyruvic acid molecules are processed.
Each three-carbon molecule of pyruvic acid undergoes conversion to a substance called acetyl-coenzyme A, or acetyl-CoA. In the process, the pyruvic acid molecule is broken down by an enzyme, one carbon atom is released in the form of carbon dioxide, and the remaining two carbon atoms are combined with a coenzyme called coenzyme A. This combination forms acetyl-CoA. Acetyl-CoA now enters the Krebs cycle by combining with a four-carbon acid called oxaloacetic acid.
The combination forms the six-carbon acid called citric acid. Citric acid undergoes a series of enzyme-catalyzed conversions. The conversions, which involve up to 10 chemical reactions, are all brought about by enzymes. In many of the steps, high-energy electrons are released to NAD. Also, in one of the reactions, enough energy is released to synthesize a molecule of ATP.
Since there are two pyruvic acid molecules entering the system, two ATP molecules are formed. A prosthetic group is a non-protein molecule required for the activity of a protein. Prosthetic groups are organic or inorganic, non-peptide molecules bound to a protein that facilitate its function; prosthetic groups include co-enzymes, which are the prosthetic groups of enzymes. The enzyme in complex I is NADH dehydrogenase and is a very large protein, containing 45 amino acid chains. Complex I can pump four hydrogen ions across the membrane from the matrix into the intermembrane space, and it is in this way that the hydrogen ion gradient is established and maintained between the two compartments separated by the inner mitochondrial membrane.
The compound connecting the first and second complexes to the third is ubiquinone Q. The Q molecule is lipid soluble and freely moves through the hydrophobic core of the membrane.
Once it is reduced, QH 2 , ubiquinone delivers its electrons to the next complex in the electron transport chain. This enzyme and FADH 2 form a small complex that delivers electrons directly to the electron transport chain, bypassing the first complex.
Since these electrons bypass and thus do not energize the proton pump in the first complex, fewer ATP molecules are made from the FADH 2 electrons. The number of ATP molecules ultimately obtained is directly proportional to the number of protons pumped across the inner mitochondrial membrane. The third complex is composed of cytochrome b, another Fe-S protein, Rieske center 2Fe-2S center , and cytochrome c proteins; this complex is also called cytochrome oxidoreductase.
Cytochrome proteins have a prosthetic group of heme. The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen. The heme molecules in the cytochromes have slightly different characteristics due to the effects of the different proteins binding them, giving slightly different characteristics to each complex.
Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes cytochrome c is the acceptor of electrons from Q; however, whereas Q carries pairs of electrons, cytochrome c can accept only one at a time. The fourth complex is composed of cytochrome proteins c, a, and a 3. This complex contains two heme groups one in each of the two cytochromes, a, and a 3 and three copper ions a pair of Cu A and one Cu B in cytochrome a 3.
The cytochromes hold an oxygen molecule very tightly between the iron and copper ions until the oxygen is completely reduced. The reduced oxygen then picks up two hydrogen ions from the surrounding medium to make water H 2 O.
The removal of the hydrogen ions from the system contributes to the ion gradient used in the process of chemiosmosis. In chemiosmosis, the free energy from the series of redox reactions just described is used to pump hydrogen ions protons across the membrane. If the membrane were open to diffusion by the hydrogen ions, the ions would tend to diffuse back across into the matrix, driven by their electrochemical gradient. Recall that many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels.
Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane through an integral membrane protein called ATP synthase Figure 8. This complex protein acts as a tiny generator, turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient. The turning of parts of this molecular machine facilitates the addition of a phosphate to ADP, forming ATP, using the potential energy of the hydrogen ion gradient.
Figure 8. Credit: modification of work by Klaus Hoffmeier. Dinitrophenol DNP is an uncoupler that makes the inner mitochondrial membrane leaky to protons. It was used until as a weight-loss drug. What effect would you expect DNP to have on the change in pH across the inner mitochondrial membrane? Why do you think this might be an effective weight-loss drug? Chemiosmosis Figure 9 is used to generate 90 percent of the ATP made during aerobic glucose catabolism; it is also the method used in the light reactions of photosynthesis to harness the energy of sunlight in the process of photophosphorylation.
Recall that the production of ATP using the process of chemiosmosis in mitochondria is called oxidative phosphorylation. The overall result of these reactions is the production of ATP from the energy of the electrons removed from hydrogen atoms. These atoms were originally part of a glucose molecule. At the end of the pathway, the electrons are used to reduce an oxygen molecule to oxygen ions. The extra electrons on the oxygen attract hydrogen ions protons from the surrounding medium, and water is formed.
Figure 9. Cyanide inhibits cytochrome c oxidase, a component of the electron transport chain. If cyanide poisoning occurs, would you expect the pH of the intermembrane space to increase or decrease? What effect would cyanide have on ATP synthesis?
The number of ATP molecules generated from the catabolism of glucose varies. For example, the number of hydrogen ions that the electron transport chain complexes can pump through the membrane varies between species. Another source of variance stems from the shuttle of electrons across the membranes of the mitochondria.
The NADH generated from glycolysis cannot easily enter mitochondria. Another factor that affects the yield of ATP molecules generated from glucose is the fact that intermediate compounds in these pathways are used for other purposes.
Glucose catabolism connects with the pathways that build or break down all other biochemical compounds in cells, and the result is somewhat messier than the ideal situations described thus far.
For example, sugars other than glucose are fed into the glycolytic pathway for energy extraction. Moreover, the five-carbon sugars that form nucleic acids are made from intermediates in glycolysis. Certain nonessential amino acids can be made from intermediates of both glycolysis and the citric acid cycle. Lipids, such as cholesterol and triglycerides, are also made from intermediates in these pathways, and both amino acids and triglycerides are broken down for energy through these pathways.
Overall, in living systems, these pathways of glucose catabolism extract about 34 percent of the energy contained in glucose. The electron transport chain is the portion of aerobic respiration that uses free oxygen as the final electron acceptor of the electrons removed from the intermediate compounds in glucose catabolism. The electron transport chain is composed of four large, multiprotein complexes embedded in the inner mitochondrial membrane and two small diffusible electron carriers shuttling electrons between them.
The electrons are passed through a series of redox reactions, with a small amount of free energy used at three points to transport hydrogen ions across a membrane.
This process contributes to the gradient used in chemiosmosis. The electrons passing through the electron transport chain gradually lose energy, High-energy electrons donated to the chain by either NADH or FADH 2 complete the chain, as low-energy electrons reduce oxygen molecules and form water.
The end products of the electron transport chain are water and ATP. A number of intermediate compounds of the citric acid cycle can be diverted into the anabolism of other biochemical molecules, such as nonessential amino acids, sugars, and lipids.
These same molecules can serve as energy sources for the glucose pathways. Cellular respiration is a collection of three unique metabolic pathways: glycolysis, the citric acid cycle, and the electron transport chain. Glycolysis is an anaerobic process, while the other two pathways are aerobic. In order to move from glycolysis to the citric acid cycle, pyruvate molecules the output of glycolysis must be oxidized in a process called pyruvate oxidation. Glycolysis is the first pathway in cellular respiration.
This pathway is anaerobic and takes place in the cytoplasm of the cell. This pathway breaks down 1 glucose molecule and produces 2 pyruvate molecules. There are two halves of glycolysis, with five steps in each half. This half splits glucose, and uses up 2 ATP. If the concentration of pyruvate kinase is high enough, the second half of glycolysis can proceed. Some cells e.
However, most cells undergo pyruvate oxidation and continue to the other pathways of cellular respiration. In eukaryotes, pyruvate oxidation takes place in the mitochondria. Pyruvate oxidation can only happen if oxygen is available.
In this process, the pyruvate created by glycolysis is oxidized. In this oxidation process, a carboxyl group is removed from pyruvate, creating acetyl groups, which compound with coenzyme A CoA to form acetyl CoA. This process also releases CO 2. The citric acid cycle also known as the Krebs cycle is the second pathway in cellular respiration, and it also takes place in the mitochondria. The rate of the cycle is controlled by ATP concentration.
This pathway is a closed loop: the final step produces the compound needed for the first step. The citric acid cycle is considered an aerobic pathway because the NADH and FADH 2 it produces act as temporary electron storage compounds, transferring their electrons to the next pathway electron transport chain , which uses atmospheric oxygen. Most ATP from glucose is generated in the electron transport chain.
It is the only part of cellular respiration that directly consumes oxygen; however, in some prokaryotes, this is an anaerobic pathway. In eukaryotes, this pathway takes place in the inner mitochondrial membrane. In prokaryotes it occurs in the plasma membrane. The electron transport chain is made up of 4 proteins along the membrane and a proton pump.
A cofactor shuttles electrons between proteins I—III. Click here for a text-only version of the activity. Answer the question s below to see how well you understand the topics covered in the previous section.
0コメント