Senior High School 11 General Biology 1 Quarter 2 Learning Module PDF

SENIOR HIGH SCHOOL 11 General Biology 1 Quarter 2 – Supplementary Module 4: Cellular Respiration CELLULAR RESPIRATION: OBTAINING ENERGY FROM FOOD Autotrophs like green plants and other chlorophyll-bearing organisms such as algae and certain bacteria produce their...

SENIOR HIGH SCHOOL 11 General Biology 1 Quarter 2 – Supplementary Module 4: Cellular Respiration CELLULAR RESPIRATION: OBTAINING ENERGY FROM FOOD Autotrophs like green plants and other chlorophyll-bearing organisms such as algae and certain bacteria produce their own food through the process of photosynthesis. However, animals and other heterotrophic organisms depend, directly or indirectly on plants and other photosynthetic organisms for food. Cellular respiration is a complex process by which energy, in the form of ATP, is released from food molecules ingested by organisms. Below is the summary equation of cellular respiration: Glucose + oxygen → carbon dioxide + water + energy (ATP) (C6H12O6) + 6 O2 → 6CO2 + H2O + energy In the past lesson, you have learned that photosynthesis happens in the chloroplast in plant cells. On the other hand, cellular respiration takes place in mitochondria (sing. mitochondrion). It refers to as the cell’s power plant - as a primary site of cellular respiration. It metabolizes glucose, the most common cell fuel, to generate ATP (Adenosine triphosphate). Mitochondria. Mitochondria (singular, mitochondrion)— Mitochondria are the sites of cellular respiration, the metabolic process that uses oxygen to drive the generation of ATP by extracting energy from sugars, fats, and other fuels. The mitochondria are oval-shaped organelles found in most eukaryotic cells. They are the ‘powerhouses’ of the cell. As the site of cellular respiration, mitochondria serve to transform molecules such as glucose into an energy molecule known as adenosine triphosphate (ATP). ATP fuels cellular Figure 1 Structure of Mitochondrion. Source: New Castle (2019) processes by breaking its high-energy chemical bonds. Mitochondria are most plentiful in cells that require significant amounts of energy to function, such as liver and muscle cells. The mitochondria have two membranes that are similar in composition to the cell membrane: Outer membrane is a selectively permeable membrane that surrounds the mitochondria. It is the site of attachment for the respiratory assembly of the electron transport chain and ATP Synthase. It has integral proteins and pores for transporting molecules just like the cell membrane fully surrounds the inner membrane, with a small intermembrane space in between has many protein-based pores that are big enough to allow the passage of ions and molecules as large as a small protein Inner membrane folds inward (called cristae) to increase surfaces for cellular metabolism. It contains ribosomes and the DNA of the mitochondria. The inner membrane creates two enclosed spaces within the mitochondria: intermembrane space between the outer membrane and the inner membrane; and matrix that is enclosed within the inner membrane. has restricted permeability like the plasma membrane is loaded with proteins involved in electron transport and ATP synthesis surrounds the mitochondrial matrix, where the citric acid cycle produces the electrons that travel from one protein complex to the next in the inner membrane. At the end of this electron transport chain, the final electron acceptor is oxygen, and this ultimately forms water (H20). At the same time, the electron transport chain produces ATP in a process called oxidative phosphorylation There are several processes under cellular respiration involving in the ATP formation.  1. Glycolysis. It occurs in the cytoplasm by which one glucose molecule (a six-carbon compound) is broken down into two pyruvic acid (or pyruvate) molecules (a three-carbon compound) generating two net ATPs and 2 NADH molecules (reduced NAD+ nicotinamide adenine dinucleotide molecule). There are two pathways in which ATP can be generated from pyruvic acid molecules - Aerobic and anaerobic. Three major stages: 1. Conversion of pyruvic acid to Acetyl-CoA. The three-carbon pyruvic acid is first converted into a two-carbon molecule or acetyl group called Acetyl- CoA. During the transformation, the pyruvic acid loses H2 and produces CO2 + NADH + H+ (as NAD+ accepts H2). 2. Krebs cycle (Citric acid cycle or Tricarboxylic acid cycle). Named after the British biochemist Sir Hans Adolf Krebs) refers to a series of enzyme-catalyzed reactions that break down acetyl-CoA completely into carbon dioxide and water. It takes place in the mitochondrial inner matrix. For every molecule of acetyl-CoA that entered the Krebs cycle, one molecule of ATP is produced with three molecules of NADH and one molecule of FADH2 (FAD – Flavin adenine dinucleotide). There are two turns of the cycle since 1 glucose = 2 pyruvic acid = 2 acetyl CoA, the number of molecules mentioned are doubled. 3 molecules of NADH 2 molecules of CO2 1 molecule of FADH2 1 molecule of ATP 3. Electron transport chain or Oxidative phosphorylation. It transpires in the cristae of the mitochondrion. It refers to a series of oxidation processes where (H2+) carried by NADH2 and FADH2 are transferred to electron acceptors (Coenzyme Q and cytochrome b, c and a). ATPs are released and water as a by-product as electrons are transferred. The final electron acceptor is an oxygen molecule. In aerobic respiration, it involves the use of oxygen. It takes place in the mitochondrion’s inner matrix and in the cristae. Also, in aerobic respiration, the theoretical yield of ATP harvested from glucose is 38 molecules. This is reduced to 36 in eukaryotes because it takes 1 ATP to transport each NADH molecule that is generated by glycolysis inside the cytoplasm into the mitochondria. On the other hand, Anaerobic respiration. The term closely related to this is fermentation which is the breakdown of pyruvic acid without the use of oxygen. Fermentation comes in two forms: A. Alcohol or Ethanol fermentation. It occurs in some plant cells and some one-celled organisms like yeasts. During this process, pyruvic acid is converted to ethyl alcohol. NAD+ and ethanol (ethyl alcohol) are produced. The bread made with yeast release CO2 causes it to rise. Wine and beer are the products of this process carried out by some microorganisms. Pyruvic acid + NADH → alcohol + CO2 + NAD+ B. Lactic acid fermentation. It takes place when there is a short supply of oxygen in the cells. Lactic acid from glucose in the liver when pyruvic acid accepts hydrogen from NADH+. This contributes to muscle fatigue. Pyruvic acid + NADH → lactic acid+ NAD+  MAJOR FEATURES OF GLYCOLYSIS, KREBS CYCLE, ELECTRON TRANSPORT SYSTEM Series of energy-producing processes are involved in the production of energy molecules and other by-products. Moreover, molecules of ATP are also used in the initial process of cellular respiration. Each process in cellular respiration has distinct features that distinguish one from the other. Glycolysis Digestion results in the formation of glucose. Glucose is the source of energy for living cells. However, glucose must be converted to Adenosine Triphosphate (ATP) first so that it can fuel the metabolic processes in cells. ATP is the energy currency of cells. Glycolysis is the first step in cellular respiration that results in the breakdown of glucose to draw energy for cellular metabolism. Almost all organisms carry out glycolysis as part of their metabolism. Prokaryotic and eukaryotic cells undergo glycolysis. Glycolysis is anaerobic which means oxygen is not needed in the process. It takes place in the cytoplasm of the cell. The word glycolysis comes from Greek words meaning “sugar splitting,” which refers to the fact that the sugar glucose is metabolized. Glycolysis, which occurs in the cytosol, begins the degradation process by breaking six-carbon glucose molecule is into to two three-carbon molecules of pyruvate. The process begins with the six-carbon ring-shaped structure of a single glucose molecule and ends with two molecules of a three-carbon sugar called pyruvic acid or pyruvate. Glycolysis initially uses two ATP molecules as an energy source and produces four ATP molecules and two NADH (nicotinamide adenine dinucleotide) molecules in the latter stage. The process results to a net gain of two ATP molecules and two molecules of NADH. The following sequence of events take place in glycolysis. Enzymes catalyzed these reactions. Figure 11.3. Highlights of Glycolysis. Source: Khan Academy (2021). A. Energy-requiring phase. In this phase, the starting molecule of glucose gets rearranged, and two phosphate groups are attached to it. The phosphate groups make the modified sugar—now called fructose-1,6-bisphosphate— unstable, allowing it to split in half and form two phosphate-bearing three-carbon sugars. Because the phosphates used in these steps come from ATP two ATP molecules get used up. 1. The six-carbon sugar (glucose) is converted to glucose-6-phosphate with the enzyme hexokinase. One ATP is used in the process. 2. Glucose-6-phosphate is converted to fructose-6-phosphate with the enzyme isomerase. 3. Phosphorylation of fructose-6-phosphate to fructose-1,6- bisphosphate with the enzyme phosphofructokinase. The second ATP molecule is used. 4. Fructose-1,6-bisphosphate is converted to two three-carbon isomers: dihydroxyacetone-phosphate and glyceraldehyde-3- phosphate with the enzyme aldolase. 5. Transformation of the dihydroxyacetone-phosphate into its isomer, glyceraldehyde-3-phosphate with an isomerase enzyme. At this point in the pathway, there is a net investment of energy from two ATP molecules in the breakdown of one glucose molecule. B. Energy-releasing phase. In this phase, each three-carbon sugar is converted into another three-carbon molecule, pyruvate, through a series of reactions. In these reactions, two ATP molecules and one NADH molecule are made. Because this phase takes place twice, once for each of the two three-carbon sugars, it makes four 4 ATP and 2 NADH overall. 6. Oxidation of the sugar glyceraldehyde-3-phosphate to extract high-energy electrons, which are picked up by the electron carrier NAD+, producing NADH.  7. Phosphorylation of glyceraldehyde-3-phosphate by the addition of a second phosphate group, producing 1, 3- bisphosphoglycerate. 8. Remaining phosphate group in 3-phosphoglycerate moves from the third carbon to the second carbon, producing 2-phosphoglycerate. A mutase (isomerase) catalyzes this step. 9. 2-phosphoglycerate loses water from its structure; this is a dehydration reaction, resulting in the formation of a double bond that increases the potential energy in the remaining phosphate bond and produces phosphoenolpyruvate (PEP). Enolase catalyzes this step. 10. The last step in glycolysis is catalyzed by the enzyme pyruvate kinase (the enzyme in this case is named for the reverse reaction of pyruvate’s conversion into PEP) and results in the production of a second ATP molecule by substrate level phosphorylation and the compound pyruvic acid (or its salt form, pyruvate). Figure 11. 5. Detailed Steps of Glycolysis. (1-5) Energy Requiring Phase. (6-10) Energy Releasing Phase. Source: Khan Academy (2021) Each reaction in glycolysis is catalyzed by its own enzyme. The most important enzyme for regulation of glycolysis is phosphofructokinase, which catalyzes formation of the unstable, two-phosphate sugar molecule, fructose-1,6-bisphosphate. Phosphofructokinase speeds up or slows down glycolysis in response to the energy needs of the cell. Overall, glycolysis converts one six-carbon molecule of glucose into two three-carbon molecules of pyruvate. The net products of this process are two molecules of ATP (4 ATP produced- 2 ATP used up) and two molecules of NADH. Oxidation of Pyruvate If oxygen is sufficient, aerobic respiration takes place. In eukaryotes, the pyruvate molecules formed in glycolysis enter the mitochondria, where they are converted to acetyl coenzyme A (acetyl CoA). These reactions occur in the cytosol of aerobic prokaryotes. In this series of reactions, pyruvate undergoes a process known as oxidative decarboxylation yielding acetyl. (During electron transport) to form additional ATP molecules. Pyruvate is converted into an acetyl group within the mitochondrial matrix, which is picked up and activated by a carrier compound called coenzyme A (CoA). The compound resulting from this is called acetyl CoA. The cell can use Acetyl CoA in several ways, but its main function is to deliver the pyruvate-derived acetyl group to the next stage of the glucose catabolism pathway which is the Krebs cycle. This process also generates carbon dioxide and NADH. Note that the original glucose molecule has now been partially oxidized, yielding two acetyl groups and two CO2 molecules. The electrons removed have reduced NAD+ to NADH. Figure 11.4. Conversion of pyruvate to acetyl CoA. The junction between glycolysis and the citric acid cycle. Source: Campbell Biology (2010) At this point in aerobic respiration, four NADH molecules have been formed because of the catabolism of a single glucose molecule: two during glycolysis and two during the formation of acetyl CoA from pyruvate. Keep in mind that these NADH molecules will be used later (during electron transport) to form additional ATP molecules. Krebs Cycle/Citric Acid Cycle /Tricarboxylic Acid Cycle (TAC) The citric acid cycle is also called the tricarboxylic acid cycle or the Krebs cycle, the latter honoring Hans Krebs, the German- British scientist who was largely responsible for working out the pathway in the 1930. In eukaryotes, the citric acid cycle takes place in the matrix of the mitochondria, just like the conversion of pyruvate to acetyl CoA. In prokaryotes, these steps both take place in the cytoplasm. The citric acid cycle is a closed loop; the last part of the pathway reforms the molecule used in the first step. The cycle includes eight major steps. 1. Condensation of Acetyl CoA (two- Figure 11.6. Highlights of Krebs Cycle. Source: Khan Academy (2021). carbon acetyl group) by combining with oxaloacetate to form a six-carbon molecule of citrate or citric acid. 2. Citrate loses one water molecule and gains another to form its isomer, isocitrate. 3. Oxidation of isocitrate to produce a five-carbon molecule, α- ketoglutarate. A molecule of CO2 and NADH are formed. 4. CoA binds with the succinyl group to form succinyl CoA. In the next step, a phosphate group is substituted for coenzyme A, and a high-energy bond is formed. This energy is used in substrate-level phosphorylation (during the conversion of the succinyl group to succinate) to form either guanine triphosphate (GTP) or ATP. 5. Succinate is converted to fumarate. Two hydrogen atoms are transferred to FAD, producing FADH 2 (flavin adenine dinucleotide). 6. Water is added to fumarate to form malate. 7. Malate is oxidized to regenerate oxaloacetate. Another molecule of NADH is produced in the process. The Citric Acid Cycle is a metabolic pathway involving biochemical reactions which produces NADH, carbon dioxide, ATP and FADH2. NADH and FADH2 are electron carriers that will be used in the next step of cellular respiration. The cycle takes place in the inner matrix of mitochondria.  Overall, one turn of the citric acid cycle releases two carbon dioxide molecules and produces three NADH, one FADH2, and one ATP or GTP. The citric acid cycle goes around twice for each molecule of glucose that enters cellular respiration because there are two pyruvates— and thus, two acetyl CoA—made per glucose. Why do we need oxygen? As it turns out, the reason you need oxygen is so your cells can use this molecule during oxidative phosphorylation, the final stage of cellular respiration. Oxygen sits at the end of the electron transport chain, where it accepts electrons and picks up protons to form water. If oxygen isn’t there to accept electrons (for instance, because a person is not breathing in enough oxygen), the electron transport chain will stop running, and ATP will no longer be produced by chemiosmosis. Without enough ATP, cells can’t carry out the reactions they need to function, and, after a long enough period, may even die. Oxidative phosphorylation is made up of two closely connected components: the electron transport chain and chemiosmosis. Figure 11. 7. Detailed Steps of Krebs Cycle. Source: OpenStax College, Biology In the electron transport chain, electrons are passed from one molecule to another, and energy released in these electron transfers is used to form an electrochemical gradient. In chemiosmosis, the energy stored in the gradient is used to make ATP. The electron transport chain is a series of proteins and organic molecules found in the inner membrane of the mitochondria. Electrons are passed from one member of the transport chain to another in a series of redox reactions. Energy released in these reactions is captured as a proton gradient, which is then used to make ATP in a process called chemiosmosis. Together, the electron transport chain and chemiosmosis make up oxidative phosphorylation. The key steps of this process, shown in simplified form in the diagram left, include: Figure 11.8. Highlights of Oxidative Phosphorylation. Source: Khan Academy (2021). Delivery of electrons by NADH and FADH2. Reduced electron carriers (NADH and FADH2) from other steps of cellular respiration transfer their electrons to molecules near the beginning of the transport chain. In the process, they turn back into NAD+ and FAD, which can be reused in other steps of cellular respiration. Electron transfer and proton pumping. As electrons are passed down the chain, they move from a higher to a lower energy level, releasing energy. Some of the energy is used to pump H + ions, moving them out of the matrix and into the intermembrane space. This pumping establishes an electrochemical gradient. Splitting of oxygen to form water. At the end of the electron transport chain, electrons are transferred to molecular oxygen, which splits in half and takes up H+ to form water. Gradient-driven synthesis of ATP. As H+ ions flow down their gradient and back into the matrix, they pass through an enzyme called ATP synthase, which harnesses the flow of protons to synthesize ATP. ELECTRON TRANSPORT CHAIN The electron transport chain is a collection of membrane-embedded proteins and organic molecules, most of them organized into four large complexes labeled I to IV. In eukaryotes, many copies of these molecules are found in the inner mitochondrial membrane. In prokaryotes, the electron transport chain components are found in the plasma membrane. As the electrons travel through the chain, they go from a higher to a lower energy level, moving from less electron- hungry to more electron-hungry molecules. Energy is released in these “downhill” electron transfers, and several of the protein complexes use the released energy to pump protons from the mitochondrial matrix to the intermembrane space, forming a proton gradient. All the electrons that enter the transport chain come from NADH and FADH2 molecules produced during earlier stages of cellular respiration: glycolysis, pyruvate oxidation, and the citric acid cycle. NADH is very good at donating electrons in redox reactions (that is, its Figure 11. 9. Electron Transport Chain. Source: DifferenceBetween.com (2017) electrons are at a high energy level), so it can transfer its electrons directly to complex I, turning back into NAD+. As electrons move through complex I in a series of redox reactions, energy is released, and the complex uses this energy to pump protons from the matrix into the intermembrane space. FADH2 is not as good at donating electrons as NADH (that is, its electrons are at a lower energy level), so it cannot transfer its electrons to complex I. Instead, it feeds them into the transport chain through complex II, which does not pump protons across the membrane. Because of this "bypass," each FADH2 molecule causes fewer protons to be pumped (and contributes less to the proton gradient) than an NADH. Beyond the first two complexes, electrons from NADH and FADH2 travel exactly the same route. Both complex I and complex II pass their electrons to a small, mobile electron carrier called ubiquinone (Q), which is reduced to form QH2 and travels through the membrane, delivering the electrons to complex III. As electrons move through complex III, more H+ ions are pumped across the membrane, and the electrons are ultimately delivered to another mobile carrier called cytochrome C (Cyt C). Cyt C carries the electrons to complex IV, where a final batch of H+ ions is pumped across the membrane. Complex IV passes the electrons to O2, which splits into two oxygen atoms and accepts protons from the matrix to form water. Four electrons are required to reduce each molecule of O2 and two water molecules are formed in the process. Overall, electron transport chain has two important functions: Regenerates electron carriers. NADH and FADH2 pass their electrons to the electron transport chain, turning back into NAD+ and FAD. This is important because the oxidized forms of these electron carriers are used in glycolysis and the citric acid cycle and must be available to keep these processes running. Makes a proton gradient. The transport chain builds a proton gradient across the inner mitochondrial membrane, with a higher concentration of H+ in the intermembrane space and a lower concentration in the matrix. This gradient represents a stored form of energy, and, as we’ll see, it can be used to make ATP. Chemiosmosis Complexes I, III, and IV of the electron transport chain are proton pumps. As electrons move energetically downhill, the complexes capture the released energy and use it to pump H+ ions from the matrix to the intermembrane space. This pumping forms an electrochemical gradient across the inner mitochondrial membrane. The gradient is sometimes called the proton-motive force, and you can think of it as a form of stored energy, kind of like a battery. Like many other ions, protons can't pass directly through the phospholipid bilayer of the membrane because its core is too hydrophobic. Instead, H+ can move down their concentration gradient only with the help of channel proteins that form hydrophilic tunnels across the membrane. In the inner mitochondrial membrane, H+ ions have just one channel available: a membrane-spanning protein known as ATP synthase. Conceptually, ATP synthase is a lot like a turbine in a hydroelectric power plant. Instead of being turned by water, it’s turned by the flow of Figure 11.10. ATP synthase, a molecular mill. H+ ions moving down their electrochemical gradient. As ATP synthase The ATP synthase protein complex functions as a turns, it catalyzes the addition of a phosphate to ADP, capturing energy mill. powered by the flow of hydrogen ions. Source: from the proton gradient as ATP. Pearson Education Inc. (2010) This process, in which energy from a proton gradient is used to make ATP, is called chemiosmosis. More broadly, chemiosmosis can refer to any process in which energy stored in a proton gradient is used to do work. Although chemiosmosis accounts for over 80% of ATP made during glucose breakdown in cellular respiration, it’s not unique to cellular respiration. For instance, chemiosmosis is also involved in the light reactions of photosynthesis. What would happen to the energy stored in the proton gradient if it weren't used to synthesize ATP or do other cellular work? It would be released as heat, and some types of cells deliberately use the proton gradient for heat generation rather than ATP synthesis. This might seem wasteful, but it's an important strategy for animals that need to keep warm. For instance, hibernating mammals (such as bears) have specialized cells known as brown fat cells. In the brown fat cells, uncoupling proteins are produced and inserted into the inner mitochondrial membrane. These proteins are simply channels that allow protons to pass from the intermembrane space to the matrix without traveling through ATP synthase. By providing an alternate route for protons to flow back into the matrix, the uncoupling proteins allow the energy of the gradient to be dissipated as heat Figure 11.11. ATP yield per molecule of glucose at each stage of cellular respiration. Source: Campbell Biology (2010).  ATP YIELD TABLE 11.1. INVENTORY FOR THE BREAKDOWN OF ONE MOLECULE OF GLUCOSE: STAGE DIRECT PRODUCTS (NET) ULTIMATE ATP YIELD (NET) Glycolysis 2 ATP 2 ATP 2 NADH 3-5 ATP Pyruvate oxidation 2 NADH 5 ATP Citric acid cycle 2 ATP/GTP 2 ATP 6 NADH 15 ATP 2 FADH2 3 ATP Total 30-32 ATP SUMMARY OF CELLULAR RESPIRATION Stage Summary Starting Materials End Products Series of reactions in which glucose is 1. Glycolysis (in degraded to pyruvate; net profit of 2 ATPs; Glucose, ATP, NAD+, Pi Pyruvate, ATP, NADH cytosol) hydrogen atoms are transferred to carriers; can proceed anaerobically 2. Formation of Pyruvate is degraded and combined with acetyl CoA coenzyme A to form acetyl CoA; hydrogen Pyruvate, coenzyme A, Acetyl CoA, CO2, (In mitochondria) atoms are transferred to carriers; CO2 is NAD+ NADH released 3. Citric acid cycle Series of reactions in which the acetyl (in portion of acetyl CoA is degraded to CO2; Acetyl CoA, H2O, CO2, NADH, mitochondria) hydrogen atoms are transferred to carriers; NAD+, FAD, ADP, FADH2, ATP ATP is synthesized Pi Chain of several electron transport 4. Electron molecules; electrons are passed along transport and chain; released energy is used to form a NADH, FADH2, O2, ATP, H2O, NAD+, FAD chemiosmosis (in proton gradient; ATP is synthesized as ADP, Pi mitochondria) protons diffuse down the gradient; oxygen is final electron acceptor  SENIOR HIGH SCHOOL 11 General Biology 1 Quarter 2 – Supplementary Module 5: Aerobic and Anaerobic Respiration  ROLE OF OXYGEN IN RESPIRATION Oxygen is vital to survival of almost all organisms. Without it, generating enough energy to fuel cellular processes is impossible. However, in some cases, the absence of oxygen may still be beneficial to a number of organisms as this leads to the formation of useful products The Role of Oxygen in Respiration In aerobic cellular respiration, an oxygen molecule, O2, is the final electron acceptor for the electron transport chain. If aerobic respiration takes place, then, during the electron transport chain and chemiosmosis, molecules of ATP will be formed using energy from high-energy electrons brought to the electron transport chain by NADH or FADH2. NAD+ and FAD are regenerated when NADH or FADH2 send their high energy electrons to the electron transport chain. These low-energy molecules cycle back to glycolysis and/or citric acid, where they capture more electrons of high energy and allow the process to begin. If there is no NAD+ present to pick up electrons as the reactions continue, glycolysis and the citric acid cycle do not occur. This is not a problem when oxygen is present, all of the NADH and FADH2 that were created during glycolysis and the citric acid cycle is transformed into NAD+ and FAD after the electron transport chain. The electron transport chain is a series of electron transporters or complexes embedded in the inner mitochondrial membrane which shuttle electrons from NADH and FADH2 to molecular oxygen. In the process, protons are pumped to the intermembrane space from the mitochondrial matrix, and oxygen is reduced to form water. Figure 1. Oxidative Phosphorylation Source: IStock (2020) There are four electron complexes to which electron move through before ending with oxygen as the final acceptor. Complex I, Complex II, Complex III, and Complex IV. Complex V is better known as ATP synthase, that which generates ATP. There are two other molecules associated with the chain which are the coenzyme Ubiquinone (CoU) and cytochrome C (CytC).  The process starts when NADH approaches complex I and gives up its proton and electron and becomes NAD. NADH donates its electrons to complex I and make it supercharged which causes the pumping of H + ions from the mitochondrial matrix to the intermembrane space. Then electrons from complex I is picked up by CoU. FADH2 approaches complex II gives up and donates its electron. Unlike Complex I, complex II is not supercharged so it cannot pump H + ions. The electrons in complex II is passed to CoU. Then, electrons from CoU are passed to Complex III and become supercharged thus, allowing protons to be pumped out to the intermembrane space. The buildup of H+ (protons) creates a proton gradient. Electrons from Complex III moves to CytC and then passed to complex IV then becomes supercharged. Complex IV can pumped more H+ (protons) to the intermembrane space. Electrons from Complex IV are passed to oxygen as the final acceptor. Oxygen molecule splits into two. Two H 2O molecules are formed. NAD+ and FAD become available in the cell for cellular respiration to begin. Pathways of Electron Flow in the Absence of Oxygen If there is no NAD+ present to pick up electrons, glycolysis, and the citric acid cycle do not occur. This is not a problem when oxygen is present, all the NADH and FADH2 that were created during glycolysis and the citric acid cycle is transformed after the electron transport chain back into NAD+ and FAD. In aerobic respiration, the electron transport chain does not occur when no oxygen is present since nothing will serve as the final acceptor of electrons. This means that NADH's electrons will not be recognized by the ETC as its power source, so NAD+ will not be regenerated. For cells to continue to produce ATP, NADH must be converted back to NAD+ for use as an electron carrier. Anaerobic processes use different mechanisms, but all function to convert NADH back to NAD+ Figure 2). There are two ways in which this is done. First is the use of an organic molecule to regenerate NAD+ from NADH. This is collectively called fermentation. Second is the use of inorganic molecule (such as nitrate or sulfur) to regenerate NAD+. Both methods are referred to as anaerobic cellular respiration. They do not require oxygen to regenerate NAD+ and enable organisms and convert energy for use in the absence of oxygen. Figure 2. A. Lactic acid fermentation. NAD+ is regenerated as In anaerobic respiration, glycolysis occurs. The 2 molecules of pyruvate is reduced to lactate. B. Alcohol fermentation. NAD+ NADH that are generated in the process are converted back into is regenerated as pyruvate is broken down to CO2 and ethanol. Source: Taylor, Martha et.al. (2020) NAD+ so that glycolysis can continue. Since glycolysis generates only two net ATP molecules, anaerobic respiration is much less efficient than aerobic respiration. However, 2 ATP molecules is so essential as the cell dies if it does not generate any ATP at all. In anaerobic respiration, the cell must continue performing glycolysis to produce a minimal energy molecule (2 ATP). Anaerobic respiration uses molecules other than oxygen as the terminal electron acceptors in the electron transport chain. The electron transport chain is still included in this form, but without using oxygen as the terminal electron acceptor. Instead, as electron acceptors, molecules like sulfate (SO 42-), nitrate (NO3- ), or sulfur (S) are used. These molecules have a lower reduction potential than oxygen, so in anaerobic versus aerobic environments, less energy is released per glucose molecule. For fermentation, pyruvate serves as an electron acceptor to recycle NAD+. Some bacteria use sulphate ion and produce hydrogen sulfide instead of water which produces an odor like a rotten egg in some environment. Anaerobic respiration uses several different types of electron acceptors. The use of nitrate (NO3-) as the terminal electron acceptor is denitrification. Nitrate has a high reduction potential, like oxygen. This process is widespread and is used by many proteobacteria members.  Ferric iron (Fe2+) and various organic electron acceptors can also be used by several denitrifying bacteria. Sulfate reduction uses sulfate (SO2−4) as the acceptor of electrons, generating as a metabolic result hydrogen sulfide (H2S). A relatively energetically weak method is sulfate reduction, which is used by many Gram-negative bacteria found within the δ-Proteobacteria. Acetogenesis is a form of microbial metabolism that uses hydrogen (H2) for the development of acetate as an electron donor and carbon dioxide (CO2) as an electron acceptor, the same electron donors and methanogenesis acceptors. A widespread anaerobic terminal electron acceptor used by both autotrophic and heterotrophic species is ferric iron (Fe3+). The flow of electrons in these species is similar to the flow of electrons, resulting in oxygen or nitrate, except that the final enzyme in this mechanism is ferric iron reductase in ferric iron-reducing species. A. Lactic Acid Fermentation In lactic acid fermentation (Figure 2), glycolysis occurs the way it happens in aerobic respiration. A glucose molecule breaks to form two pyruvate molecules. The pyruvate molecules in this process does not undergo decarboxylation but will be directly reduced by NADH to produce lactate. This allows the regeneration of NAD+ for use during glycolysis. It is involving in no release of CO2. The fermentation method used by animals and some bacteria like those in yogurt is lactic acid fermentation. This occurs routinely in mammalian red blood cells and in skeletal muscle that does not have enough oxygen to allow aerobic respiration to continue (such as in muscles after hard exercise). The chemical reaction of lactic acid fermentation is the following: PYRUVIC ACID + NADH ↔ LACTIC ACID + NAD+ B. Alcohol Fermentation In alcohol fermentation (Figure 3) a glucose molecule breaks to form two pyruvate molecules two ATP molecules using NAD+ in the process. Each pyruvate undergoes decarboxylation to produce acetaldehyde. And finally, acetaldehyde is reduced to ethanol using 2 molecules of NADH in the process. NADH is converted back to NAD+. The chemical reaction of alcohol fermentation is the following: PYRUVIC ACID + NADH ↔ ETHYL ALCOHOL + CO2 + NAD+ The fermentation of pyruvic acid by yeast produces the ethanol found in alcoholic beverages. If the carbon dioxide produced by the reaction is not released from the fermentation chamber, for example in beer and sparkling wines, it remains dissolved in the medium until the pressure is released. Ethanol above 12 percent is toxic to yeast, so natural levels of alcohol in wine occur at a maximum of 12 percent. Figure 3. Alcohol fermentation in yeast and Lactic acid fermentation in muscle cells  ADVANTAGES AND DISADVANTAGES OF FERMENTATION AND AEROBIC RESPIRATION A series of metabolic pathways known as cellular respiration extract the energy from the bonds in glucose and convert it into a form that all living things can use- both producers and consumers. The two metabolic pathways of cellular respiration are called aerobic and anaerobic respiration Difference Between Aerobic and Anaerobic Respiration. AEROBIC RESPIRATION ANAEROBIC RESPIRATION HOW ALIKE? Both undergo glycolysis in the cytoplasm of the cell Both undergo substrate-level phosphorylation and oxidative phosphorylation and chemiosmosis in producing ATP molecules Both split the 6-carbon glucose into two molecules of pyruvate, the three-carbon molecule Both involve a series of enzyme-controlled reactions that take place in the cytoplasm Both use NAD+ (nicotinamide adenine dinucleotide), a redox coenzyme that accepts two electrons plus a hydrogen (H+) that becomes NADH Both performed by eukaryotic and prokaryotic cells HOW DIFFERENT? Maximum yield of 36 to 38 ATP molecules per glucose Maximum yield of 2 ATP molecules per glucose for obligate anaerobes Complete breakdown of glucose to carbon dioxide and Partial degradation of glucose without the use of oxygen water with the use of oxygen (Obligate anaerobes) Multiple metabolic pathways Single metabolic pathway (in fermentation) Pyruvate proceeds to acetyl formation in the Pyruvate is broken down to ethanol and carbon dioxide or mitochondrion lactate (in fermentation) The presence of enough oxygen in the cell makes the cell Cause burning sensation in the muscle during strenuous perform its job smoothly without burning sensation exercise (in fermentation) More efficient in harvesting energy from glucose with estimated Less efficient in harvesting energy from glucose with 2% energy 39% energy efficiency (36-38 ATP) in eukaryotic organisms but efficiency (for obligate anaerobes) much higher ATP production (38 to 40 ATP) in prokaryotic organisms Outputs are carbon dioxide, water, and ATP Outputs are lactate, alcohol, and carbon dioxide (in fermentation); but reduced inorganic compound in anaerobic respiration Products produce are for biochemical cycling and for the Produce numerous products with economic and industrial cellular processes that require energy importance through fermentation Slow glucose breakdown Rapid breakdown of glucose Electrons in NADH are transferred to electron transport chain Electrons in NADH are transferred to electron transport chain; but in fermentation electrons in NADH are transferred to organic molecule Mechanism of ATP synthesis is by substrate-level and oxidative Mechanism of ATP synthesis is by substrate-level and phosphorylation/chemiosmosis oxidative phosphorylation/chemiosmosis; but in fermentation substrate-level phosphorylation only during glycolysis O2 is the final electron acceptor of the electron transport system In anaerobic respiration, inorganic substances like NO3- or SO4 2- are the final acceptor of the electron transport system; but in fermentation, there is no electron acceptor because it has no electron transport system. Brain cells in the human body can only live aerobically. They die if Some organisms like yeasts (eukaryotic), many bacteria molecular oxygen is absent. (prokaryotic) and the human muscle cells (eukaryotic) can make enough ATP to survive in facultative anaerobes (can live in the absence or presence of oxygen). But under anaerobic conditions lactic acid fermentation occurs. A facultative anaerobe needs to consume the nutrient at a much faster rate when doing the fermentation or anaerobic process. ADVANTAGES OF AEROBIC RESPIRATION All available energy extracted from glucose is 36 to 38 ATP 39% energy transferred from glucose to ATP Slow breakdown of glucose in ATP Organisms can do more work for a longer time with the slow and efficient breakdown of ATP. Animals and the human muscle cells can adapt and perform lactic acid fermentation for a rapid burst of energy. Can breathe heavily to refill the cells with oxygen so that lactate is removed from the muscle cells. Lactate is returned to the liver to become pyruvate or glucose again. Complete breakdown of glucose. DISADVANTAGES OF AEROBIC RESPIRATION 61% of glucose metabolism becomes heat and enters the environment. Human brain cells cannot perform lactic acid fermentation. Human muscle cells feel the burning sensations and pain when lactate accumulates in the cell and experience oxygen debt. ADVANTAGES OF ANAEROBIC RESPIRATION/ FERMENTATION All available energy extracted from glucose is 2 ATP. Certain bacteria produce chemicals of industrial importance such as isopropanol, butyric acid, acetic acid when bacteria ferment- breakdown of sugars in the absence of oxygen. Foods that are fermented last longer because these fermenting organisms have removed many of the nutrients that would attract other microorganisms. Yeast ferment fruits and wine is produced. Grain is also fermented to produce beer. They also caused the bread to rise due to carbon dioxide, a by-product, and alcohol is lost in the bread. Yeast and Lactobacillus together produce sour taste in wheat beer. Yeasts and Acetobacter aceti spoil wine to become vinegar. Bacterial fermentation produces yogurt (due to Streptococcus thermophilus and Lactobacillus bulgaris), sour cream, cheese, brine cucumber pickles, and kimchi. Clostridium bacteria can produce nail polish remover and rubbing alcohol from the acetone and isopropanol they make. Soy sauce is produced by adding mold (Aspergillus), yeasts and fermenting bacteria. DISADVANTAGES OF ANAEROBIC RESPIRATION/FERMENTATION Consumption of 2 ATP is fast Ethanol and lactate, the by-products of fermentation, have a lot of energy reserves- prokaryotes and eukaryotes cannot extract the energy in lactate and ethanol using anaerobic method. Needs a large supply of glucose to perform the same work as in aerobic respiration. Glucose is partially oxidized.