Cellular Respiration PDF

CHAPTER 6: CELLULAR RESPIRATION Objectives: o Explain the major features and sequence the chemical events of cellular respiration. o Distinguish major features of glycolysis, Krebs cycle, electron transport system, and chemiosmosis o Describe reactions that produce and consume ATP...

CHAPTER 6: CELLULAR RESPIRATION Objectives: o Explain the major features and sequence the chemical events of cellular respiration. o Distinguish major features of glycolysis, Krebs cycle, electron transport system, and chemiosmosis o Describe reactions that produce and consume ATP o Describe the role of oxygen in respiration and describe pathways of electron flow in the absence of oxygen o Explain the advantages and disadvantages of fermentation and aerobic respiration o Differentiate aerobic from anaerobic respiration Autotrophs and heterotrophs share the same energy molecule to drive their cellular processes. Interestingly, autotrophs have the capacity to release the energy from the same molecule that they have created to power up the non-photosynthetic parts of its system. Trapped light from the sun is processed by photosynthesis which produces energy. This usable energy is stored as a chemical bond in a molecule known as adenosine triphosphate (ATP). All organisms use this fuel molecule during cellular respiration as a chemical energy source. Metabolism of large molecules, that is, carbohydrates by glycolysis and fats by B-oxidation produces substrates for cellular respiration. Structure and Function of ATP and the ATP-ADP Cycle Adenosine triphosphate (ATP) is the energy currency used throughout the cell. ATP provides energy for the cell to do work, such as mechanical work (to move cilia and vesicles), transport substances across the membrane, and perform various chemical reactions. ATP is composed of phosphate groups, ribose, and adenine. In the structure of ATP, there are three phosphate groups attached to adenosine. The last two bonds on the phosphate groups contain especially high energy and are therefore very useful for doing work within living cells. The bonds that hold phosphate groups are easily broken by hydrolysis which results in the release of energy. This reaction is exergonic and releases 7.3 kcal of energy per mole of ATP hydrolyzed. Figure 6.1 is summarized in the equation: ATP + 𝑯𝟐 𝑶 → ADP + 𝑷𝑶𝟒− The phosphate bonds of ATP are sometimes called high-energy phosphate bonds because of the release of energy by hydrolysis. However, the "high energy" term is misleading because the release of energy during hydrolysis of ATP comes from the chemical change to a step of low free energy, not from the phosphate bonds themselves. The usefulness of ATP as an energy currency of the cells is further exemplified by the presence of three phosphate groups that are negatively charged. The similarity in the charges results in repulsion that contributes to the instability of this region in the ATP molecule. This triphosphate tail is comparable to a compressed spring. Heat is generated when ATP is released as free energy. This is beneficial to an organism. The process of shivering by muscle contraction utilizes the process of ATP hydrolysis to generate heat and warm the body. However, this process is energy intensive, so the cells require specific enzymes to couple the energy of the ATP hydrolysis to endergonic processes by transferring a phosphate group from ATP to some other molecule. The recipient of the phosphate group is phosphorylated in this process. The phosphorylated intermediate molecule is the key to coupling an exergonic and endergonic reaction because these intermediate molecules are more reactive than its original unphosphorylated counterpart. Since hydrolysis releases energy, ATP is manufactured during various processes such as fermentation, cellular respiration, and photosynthesis. The cells utilize adenosine diphosphate (ADP) as a starting molecule, then add phosphorus to it. This process occurs in the cytoplasm or mitochondria. When the mitochondrion undergoes chemiosmotic phosphorylation, the ADP is charged to form ATP. A unique feature of the mitochondrion is its ability to produce an electrical chemical gradient from the accumulated H+ ions in the space between the inner and outer membrane. As the charge builds up, an electrical potential is generated to release the energy and causes the flow of hydrogen ions. This also leads to the attachment of an enzyme to the ADP that catalyzes the addition of phosphorus to form ATP. The mitochondria in plants are also capable of performing this process, but plants primarily produce ATP by using sunlight. For the eukaryotic cells of animals, the conversion of fuel molecule to pyruvate then acetyl coenzyme A (CoA) leads to the release of energy when acetyl CoA enters the Krebs cycle, thus reverting ADP back to ATP (Bergman, 1999). In the process of ATP utilization, ATP usually gives the phosphate to another molecule so that the stored high energy is released with the aid of ATPase. The last phosphate will be cut off from the bond, turning the molecule into ADP. The release of this energy renders the cell capable of performing various functions such as the transfer of solutes across membrane channels. When large molecules are consumed by an organism (e.g., carbohydrates and lipids), these molecules are broken down into simple derivatives. In this process, energy is produced and used to reattach the phosphate to AD. The new molecule that received the phosphate group becomes phosphorylated. The phosphorylated molecule, being unstable, releases the energy again, repeating the bond-breaking and bond-making cycle. Cellular Respiration A candy, which is made up of glucose, is broken down by a series of reactions in the cytoplasm of a cell in a process called glycolysis. Glucose is converted to pyruvic acid which can enter the cycle either through aerobic respiration or anaerobic respiration. In aerobic respiration, pyruvic acid molecules enter the mitochondria through a series of chemical reactions known as the citric acid cycle (Krebs cycle) via electron transport chain. In the Krebs cycle, the pyruvic acid is converted to carbon dioxide. The electron transport chain accepts the electron from the breakdown products of the Krebs cycle and glycolysis via the NADH and FADH2. At the end of the chain, the electrons are combined with hydrogen ions and molecular oxygen to form water. This process can produce ATP. During this process, the glucose molecule is broken down and the carbon atoms released from glucose are combined with oxygen to produce carbon dioxide. In anaerobic respiration, pyruvic acid is converted to lactic acid. There is a production of two ATP molecules for each glucose molecule. Process of Cell Respiration 1. Glycolysis - the initial stage of processing large molecules such as starch and glycogen into glucose. ✓ This is processed in the cytosol of eukaryotic or prokaryotic cells. ✓ This energy-releasing pathway involves both the consumption and release of energy. However, the net energy released is greater than what is needed to complete glycolysis.  Steps: A. Initially, two molecules of ATP are used to phosphorylate and change glucose to 3-C molecule glyceraldehyde 3-phosphate (G3P). B. Then, G3P will be oxidized to pyruvate. This process is coupled with ATP synthesis where four ATPs are formed and two are used. In addition, four electrons and two hydrogen atoms are received by two molecules of NAD+, an electron acceptor, to produce NADH. ✓ The glycolytic pathway is said to be a primitive form of producing energy as this does not involve the utilization of oxygen in the process. However, as autotrophic organisms evolve, carbohydrates are directly produced from carbon dioxide and water, which results in the production and accumulation of oxygen in the biosphere. ✓ This oxygen-rich environment paved the way for the introduction of oxidative respiration as a pathway to allow extraction of up to 34 more molecules of ATP, resulting in a maximum of 36 ATP molecules when coupled with glycolysis. Despite this high-energy production by oxidative phosphorylation, glycolysis still remains as the major pathway when a short burst of energy is needed. ▪ Intermediate pathway - The pyruvate produced during glycolysis is converted to 2- carbon acetyl CoA. This releases CO2 and two electrons, and one proton accepted by NAD+ to form NADH. 2. Citric Acid Cycle/Krebs Cycle 1. The Acetyl CoA combines with a 4-carbon molecule oxaloacetate which releases coenzyme A and forms the 6-carbon molecule citrate. 2. Citrate transforms into isocitrate, which is the usable form of molecules for a series of oxidation processes. 3. Isocitrate then releases CO2 plus two electrons and one proton, which is received by NAD+. 4. A 5-carbon molecule will be formed from isocitrate to alpha-ketoglutarate, undergoing a similar process and producing isocitrate. 5. Succinyl CoA, a 4-carbon molecule, is converted via a series of reactions to form oxaloacetate, thus completing the citric acid cycle or Krebs cycle. When this series of reactions returns to oxaloacetate, it produces electrons and protons that form FADH, and NADH and one ATP molecule. Note that NADH oxidation is equivalent to 3ATP, which requires 1ATP to transport the molecule across the mitochondrial membrane, resulting in a net yield of 2ATP. FADH2 oxidation is equivalent to 2ATP. Let’s Try This! I. Compute for the number of ATPs needed or gained in cellular respiration. Goal Number of ATP needed or gained Break 2 glucose molecules down to form pyruvates Take the pyruvate and convert it to acetyl CoA Put the acetyl CoA into the Krebs Cycle to ETC and produce NADH and FADH2 Guide Questions: 1. How many ATP are usually produced from 4 NADH? ________________________________ 2. How many NADH molecules are formed upon the complete breakdown of 5 molecules of glucose? ___________ II. Compute for the number of ATPs needed or gained and ATP lost in cellular respiration. Input Number of ATP Gained Number of ATP lost 6 glucose molecules entered glycolysis Pyruvate decarboxylation Krebs cycle and ETC III. Compute for the number of ATPs gained and lost, include the NADH and FADH2 produced. Number of ATPs NADH and/or FADH2 Input Number of ATPs Lost Gained Produced 9 glucose molecules entered glycolysis Pyruvate decarboxylation Krebs cycle and ETC 3. Electron Transport Chain (ETC) The number of electron acceptors in the cell is very few compared to the demand to process the incoming carbon molecule. Thus, the ability of the cell to recycle NAD+ and FAD should be efficient to maintain the metabolic rate. This is achieved via the ETC. The energy conserved in FADH, and NADH is converted to ATP in this chain of reaction. There is no definite number of energies produced in this process since some energy is used by the mitochondria. NADH and FADH, transfer electrons to carrier proteins found in the plasma membrane (prokaryotes) or inner membrane of mitochondria (eukaryote). Electrons are removed and passed through several cytochrome electron carrier protein complexes. Translocation of protons also occurs. Cytochrome c oxidase, the final protein complex, uses four electrons and four protons to reduce O2 to H2O. Oxygen is thus considered as the final electron acceptor, producing water as the end product. This returns the NAD+ and FAD back to the cycle. Checkpoint: What are the major features of glycolysis, pyruvate decarboxylation, Krebs cycle, and ETC? Summarize them in the table. Pyruvate Glycolysis Krebs Cycle ETC Decarboxylation Occurs in Initial Source End product (other than ATP) ATP yield Anaerobic Respiration The fate of the pyruvate molecule depends on the presence and absence of oxygen in the system. If oxygen is present, then the pyruvate enters the glycolytic pathway of cellular respiration. In the absence of oxygen, the process shifts to fermentation. There are some bacteria known as obligate anaerobes that immediately die if there is oxygen. Some others can selectively utilize oxygen depending on their presence in the environment. These are called facultative anaerobes. Many prokaryotes are known to be facultative anaerobes, (switching between aerobic respiration, fermentation, or anaerobic respiration) depending on the availability of oxygen. This is advantageous in such a way that they can generate more ATPs per glucose when needed or lesser when not needed by the system. Our muscle cells behave like facultative anaerobes. They use aerobic respiration when oxygen is present but change to lactic acid fermentation when there is a lack of oxygen. In the absence of oxygen, these organisms can extract energy from fuel molecules, such as glucose, by anaerobic respiration and fermentation. In both aerobic and anaerobic respiration, electrons are derived from a fuel molecule (often in the form of glucose) and are passed energetically "downhill" through an ETC. The energy in the ETC is released and used to form ATP. Fermentation: An Anaerobic Form of Respiration In the event that the cell lacks enough oxygen to perform the high ATP-yielding aerobic reaction, the cell can still produce ATP by fermentation. This generally involves glycolysis joining with an additional reaction to consume NADH and produce pyruvate. This reaction also ensures that NAD+ is recycled back for glycolysis. In this process, there is no ETC and no oxidative phosphorylation. Instead, the sole energy extraction pathway is glycolysis in addition to one or two extra steps. These extra steps at the end of fermentation are needed to regenerate NAD+ from the NADH. In order to do this, the electrons are transferred from (1) NADH to pyruvate, (2) the end product of glycolysis, or to (3) one of its derivatives. This is an important step to recycle the limited NAD+ pools of the system. In the absence of this NAD+ regeneration, all the NAD+ will be converted to NADH, and glycolysis will not progress. 2 Common Types of Fermentation: 1. Lactic Acid Fermentation In lactic acid fermentation, NADH transfers its electrons directly to pyruvate to form lactate as a by-product. The bacterium that produces yogurt and red blood cells in our body performs these processes. Anaerobic fermentation that uses lactate produces products such as cheese, yogurt, and sauerkraut, whereas bacterial fermentation produces important industrial chemicals such as butyric acid, propionic acid, and acetic acid. 2. Alcohol Fermentation In the case of alcohol fermentation, the process is almost the same as lactic acid fermentation; however, the NADH donates its electrons to a derivative of pyruvate to produce ethanol as an end product. This is a two-step process: (1) a carboxyl group is removed from pyruvate and released as CO2 to produce a 2-C molecule, acetaldehyde, then (2) NADH passes its electrons to acetaldehyde, regenerating NAD+ forming ethanol. An example of an excellent alcohol fermenter is yeast which is used to produce beverages like beer and wine. When yeast ferments, they produce both ethyl alcohol and carbon dioxide. In other fermentation products, the alcohol evaporates. However, in some cases, the yeast is eventually killed by the alcohol that they produce. Other than ethanol formation, another pathway of anaerobic reaction occurs in plants, microorganisms, and humans. We are aware of cramps (or pulikat) experienced during strenuous exercises such as sprints or 100 m dashes. The rapid burst of ATP during lactic acid fermentation sustains muscle contraction. Lactate accumulates in the cells, with some carried by blood, away from the muscles. Eventually, lactate builds up, resulting in a change in the pH and causing muscles to fatigue and cramp. This is often a result of oxygen debt and is needed to rid the body of lactate. The body recovers by transporting the lactate to the liver where it is converted back to pyruvate.