Oceanic algae contribute enormous quantities of food and oxygen to global food chains. Plants are also photoautotrophs , a type of autotroph that uses sunlight and carbon from carbon dioxide to synthesize chemical energy in the form of carbohydrates. Heterotrophs are organisms incapable of photosynthesis that must therefore obtain energy and carbon from food by consuming other organisms.
Even if the food organism is another animal, this food traces its origins back to autotrophs and the process of photosynthesis. Humans are heterotrophs, as are all animals.
Heterotrophs depend on autotrophs, either directly or indirectly. Cellular respiration is the process by which individual cells break down food molecules, such as glucose and release energy.
This is because cellular respiration releases the energy in glucose slowly, in many small steps. It uses the energy that is released to form molecules of ATP, the energy-carrying molecules that cells use to power biochemical processes. Cellular respiration involves many chemical reactions, but they can all be summed up with this chemical equation:. Because oxygen is required for cellular respiration, it is an aerobic process.
Cellular respiration occurs in the cells of all living things, both autotrophs and heterotrophs. All of them catabolize glucose to form ATP. The reactions of cellular respiration can be grouped into three main stages and an intermediate stage: glycolysis , Transformation of pyruvate , the Krebs cycle also called the citric acid cycle , and Oxidative Phosphorylation.
The first stage of cellular respiration is glycolysis. ATP is produced in this process which takes place in the cytosol of the cytoplasm. Enzymes split a molecule of glucose into two molecules of pyruvate also known as pyruvic acid.
Glucose is first split into glyceraldehyde 3-phosphate a molecule containing 3 carbons and a phosphate group. This process uses 2 ATP. Next, each glyceraldehyde 3-phosphate is converted into pyruvate a 3-carbon molecule. Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules. These two molecules go on to stage II of cellular respiration.
The energy to split glucose is provided by two molecules of ATP. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP. As a result, there is a net gain of two ATP molecules during glycolysis. In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into mitochondria, which are sites of cellular respiration. If oxygen is available, aerobic respiration will go forward.
In mitochondria, pyruvate will be transformed into a two-carbon acetyl group by removing a molecule of carbon dioxide that will be picked up by a carrier compound called coenzyme A CoA , which is made from vitamin B 5.
Acetyl CoA can be used in a variety of ways by the cell, but its major function is to deliver the acetyl group derived from pyruvate to the next pathway step, the Citric Acid Cycle.
Before you read about the last two stages of cellular respiration, you need to review the structure of the mitochondrion, where these two stages take place. The space between the inner and outer membrane is called the intermembrane space. The space enclosed by the inner membrane is called the matrix. The second stage of cellular respiration, the Krebs cycle, takes place in the matrix. The third stage, electron transport, takes place on the inner membrane. Recall that glycolysis produces two molecules of pyruvate pyruvic acid.
Pyruvate, which has three carbon atoms, is split apart and combined with CoA, which stands for coenzyme A. The product of this reaction is acetyl-CoA. These molecules enter the matrix of a mitochondrion, where they start the Citric Acid Cycle. The third carbon from pyruvate combines with oxygen to form carbon dioxide, which is released as a waste product.
High-energy electrons are also released and captured in NADH. This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle. After citric acid forms, it goes through a series of reactions that release energy. This energy is captured in molecules of ATP and electron carriers. Carbon dioxide is also released as a waste product of these reactions. 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. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.
Use this quiz to check your understanding and decide whether to 1 study the previous section further or 2 move on to the next section.
Skip to main content. Module 6: Metabolic Pathways. Search for:. Learning Objectives Describe the process of glycolysis and identify its reactants and products Describe the process of pyruvate oxidation and identify its reactants and products Describe the process of the citric acid cycle Krebs cycle and identify its reactants and products Describe the respiratory chain electron transport chain and its role in cellular respiration.
Figure 1. Reactants and products of glycolysis. In Summary: Glycolysis Glycolysis is the first pathway used in the breakdown of glucose to extract energy. Figure 4 shows the entire process of glycolysis in one image: Figure 4. In Summary: Pyruvate Oxidation In the presence of oxygen, pyruvate is transformed into an acetyl group attached to a carrier molecule of coenzyme A.
You can click through each step of the citric acid cycle here. In Summary: Citric Acid Cycle The citric acid cycle is a series of redox and decarboxylation reactions that remove high-energy electrons and carbon dioxide. Practice Question Figure 8. DNP is an effective diet drug because it uncouples ATP synthesis; in other words, after taking it, a person obtains less energy out of the food he or she eats.
Interestingly, one of the worst side effects of this drug is hyperthermia, or overheating of the body. Since ATP cannot be formed, the energy from electron transport is lost as heat. Practice Question Figure 9. 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.
Also during the Krebs cycle, the two carbon atoms of acetyl-CoA are released and each forms a carbon dioxide molecule. Thus, for each acetyl-CoA entering the cycle, two carbon dioxide molecules are formed. Since two acetyl-CoA molecules enter the cycle, and each has two carbon atoms, four carbon dioxide molecules will form.
Add these four molecules to the two carbon dioxide molecules formed in the conversion of pyruvic acid to acetyl-CoA, and the total is six carbon dioxide molecules. These six CO 2 molecules are given off as waste gas in the Krebs cycle. They represent the six carbons of glucose that originally entered the process of glycolysis. At the end of the Krebs cycle, the final product formed is oxalo-acetic acid , identical to the oxaloacetic acid which begins the cycle.
The molecule is now ready to accept another acetyl-CoA molecule to begin another turn of the cycle. The electron transport system. The electron transport system occurs at the bacterial cell membrane and in the cristae of the mitochondria in eukaryotic cells. Here, a series of cytochromes cell pigments and coenzymes exist. These cytochromes and coenzymes act as carrier molecules and transfer molecules.
They accept high-energy electrons and pass the electrons to the next molecule in the system. At key proton-pumping sites, the energy of the electrons is used to transport protons across the cell membrane or into the outer compartment of the mitochondrion.
Each NADH molecule is highly energetic. It accounts for the transfer of six protons across the membrane. Each FADH 2 molecule accounts for the transfer of four protons. The final electron acceptor is an oxygen atom. The electron-oxygen combination then takes on two protons to form a molecule of water H 2 O. As a final electron receptor, oxygen is responsible for removing electrons from the system. If oxygen were not available, electrons could not be passed among the coenzymes, the energy in electrons could not be released, the proton pump could not be established, and ATP could not be produced.
The actual production of ATP in cellular respiration takes place during chemiosmosis. As previously noted, chemiosmosis involves the pumping of protons through special channels in the membranes of mitochondria from the inner to the outer compartment.
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