Cellular Respiration and Fermentation

An overview of cellular respiration

"During glycolysis, each glucose molecule is broken down into two molecules of the compound pyruvate. In eukaryotic cells, as shown here, the pyruvate enters the mitochondrion. There it is oxidized to acetyl CoA, which is further oxidized to CO2 in the citric acid cycle. NADH and a similar electron carrier, a coenzyme called FADH2, transfer electrons derived from glucose to electron transport chains, which are built into the inner mitochondrial membrane. (In prokaryotes, the electron transport chains are located in the plasma membrane.) During oxidative phosphorylation, electron transport chains convert the chemical energy to a form used for ATP synthesis in the process called chemiosmosis."

Citric Acid Cycle

"The citric acid cycle functions as a metabolic furnace that oxidizes organic fuel derived from pyruvate." 8 steps, each catalyzed by a different enzyme: 1. Acetyl CoA (from oxidation of pyruvate) adds its two-carbon acetyl group to oxaloacetate, producing citrate. 2. Citrate is converted to its isomer, isocitrate, by removal of one water molecule and addition of another. 3. Isocitrate is oxidized, reducing NAD+ to NADH. Then the resulting compound loses a CO2 molecule. 4. Another CO2 is lost, and the resulting compound is oxidized, reducing NAD+ to NADH. The remaining molecule is then attached to coenzyme A by an unstable bond. 5. CoA is displaced by a phosphate group, which is transferred to GDP, forming GTP, a molecule with functions similar to ATP. GTP can also be used, as shown, to generate ATP. 6. Two hydrogens are transferred to FAD, forming FADH2 and oxidizing succinate. 7. Addition of a water molecule rearranges bonds in the substrate 8. Addition of a water molecule rearranges bonds in the substrate 2 NET ATP

Glycolysis Steps

1. Hexokinase transfers a phosphate group from ATP to glucose, making it more chemically reactive. The charge on the phosphate also traps the sugar in the cell. 2. Glucose 6-phosphate is converted to fructose 6-phosphate 3. Phosphofructokinase transfers a phosphate group from ATP to the opposite end of the sugar, investing a second molecule of ATP. This is a key step for regulation of glycolysis. 4. Aldolase cleaves the sugar molecule into two different three-carbon sugars. 5. Conversion between DHAP and G3P: This reaction never reaches equilibrium; G3P is used in the next step as fast as it forms. 6. Two sequential reactions: (1) The sugar is oxidized by the transfer of electrons to NAD+, forming NADH. (2) Using energy from this exergonic redox reaction, a phosphate group is attached to the oxidized substrate, making a high-energy product. 7. The phosphate group is transferred to ADP (substrate-level phosphorylation) in an exergonic reaction. The carbonyl group of G3P has been oxidized to the carboxyl group (—COO-) of an organic acid (3-phosphoglycerate). 8. This enzyme relocates the remaining phosphate group. 9. Enolase causes a double bond to form in the substrate by extracting a water molecule, yielding phosphoenolpyruvate (PEP), a compound with a very high potential energy. 10. The phosphate group is transferred from PEP to ADP (a second example of substrate-level phosphorylation), forming pyruvate.

2 phases of glycolysis

1. energy investment phase: spends ATP 2. energy payoff phase: investment repaid NET: 2 Pyruvate, 2 H2O, 2 ATP, 2 NADH, 2 H+ All of the carbon originally present in glucose is accounted for in the two molecules of pyruvate; no carbon is released as CO2 during glycolysis.

Cellular Respiration Net ATP Yeild

30-32 Majority of ATP released in Oxidative phosphorylation

Anaerobic Respiration

A process that harvests chemical energy without oxygen

Chapter 9 Section 3

After pyruvate is oxidized, the citirc acid cycle completes the energy-tielding oxidation of organix molecules

Cellular Respiration

C6H12O6 + 6 O2 S 6 CO2 + 6 H2O + Energy (ATP + heat)

The catabolism of various molecules from food.

Carbohydrates, fats, and proteins can all be used as fuel for cellular respiration. Monomers of these molecules enter glycolysis or the citric acid cycle at various points. Glycolysis and the citric acid cycle are catabolic funnels through which electrons from all kinds of organic molecules flow on their exergonic fall to oxygen.

Metabolic pathways

Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration. Many carbohydrates can enter glycolysis, most often after conversion to glucose. Amino acids of proteins must be deaminated before being oxidized. The fatty acids of fats undergo beta oxidation to 2-carbon fragments and then enter the citric acid cycle as acetyl CoA. Anabolic pathways can use small molecules from food directly or build other substances using intermediates of glycolysis or the citric acid cycle.

Chapter 9 Section 1

Catabolic pathways yield energy by oxidizing organic fuels

Cellular Respiration from Concept Map

Cellular respiration is a redox reaction. glucose gets oxidized to CO2, O2 gets reduced to H2O. Redox reactions involve a transfer of electrons. Cellular respiration is a catabolic pathway that releases stored energy as ATP (30-32 net ATP). Catabolic pathways can either be fermentation (anaerobic respiration) or aerobic respiration. 2 types of fermentation are lactic acid fermentation and ethanol fermentation (both used in food production). The 3 steps of cellular respiration are 1. GLYCOLYSIS, 2. PYRUVATE OXIDATION and the CITRIC ACID CYCLE and 3. OXIDATIVE PHOSPHORYLATION. Glycolysis takes place in the cytosol and breaks down glucose to 2 pyruvate molecules. The pyruvate enters the mitochondrion where is it oxidized by AcetylCoA, which is further oxidized to CO2. Pyruvate Oxidation / Citric Acid Cycle takes place in the mitochondrion. Electron carriers NADH and FADH2 transfer electrons that were originally from glucose to electron transport chains in the mitochondrial membrane. Oxidative Phosphorylation involves ETCs converting chemical energy to ATP by performing chemiosmosis.

allosteric enzymes

Cellular respiration is controlled by allosteric enzymes at key points in glycolysis and the citric acid cycle.

Oxidative Phosphorylation STEPS

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.

Chapter 9 Section 4

During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis

Chapter 9 Section 5

Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen

Comparing Fermentation with Anaerobic and Aerobic Respiration

Fermentation, anaerobic respiration, and aerobic respiration are three alternative cellular pathways for producing ATP by harvesting the chemical energy of food. All three use glycolysis to oxidize glucose and other organic fuels to pyruvate, with a net production of 2 ATP by substrate-level phosphorylation. And in all three pathways, NAD+ is the oxidizing agent that accepts electrons from food during glycolysis. A key difference is the contrasting mechanisms for oxidizing NADH back to NAD+, which is required to sustain glycolysis. In fermentation, the final electron acceptor is an organic molecule such as pyruvate (lactic acid fermentation) or acetaldehyde (alcohol fermentation). In cellular respiration, by contrast, electrons carried by NADH are transferred to an electron transport chain, which regenerates the NAD+ required for glycolysis. Another major difference is the amount of ATP produced. Fermentation yields 2 molecules of ATP, produced by substrate-level phosphorylation. Cellular respiration harvests much more energy from each sugar molecule than fermentation can. Aerobic respiration yields up to 32 molecules of ATP per glucose molecule—up to 16 times as much as does fermentation.

The Stages of Cellular Respiration

GLYCOLYSIS PYRUVATE OXIDATION and the CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION: Electron transport and chemiosmosis

Glycolysis

Glucose, a six carbon sugar, is split into two three-carbon sugars. These smaller sugars are then oxidized and their remaining atoms rearranged to form two molecules of pyruvate.

Chapter 9 Section 6

Glycolysis and the citric acid cycle connect to many other metabolic pathways

Chapter 9 Section 2

Glycolysis harvests chemical energy by oxidizing glucose to pyruvate

Alcohol Fermentation

In alcohol fermentation, pyruvate is converted to ethanol (ethyl alcohol) in 2 steps. 1st step: CO2 released from pyruvate, which is converted to the 2-carbon compound acetaldehyde. 2nd step: acetaldehyde reduced by NADH to ethanol. This regenerates the supply of NAD+ needed for the continuation of glycolysis. Many bacteria carry out alcohol fermentation under anaerobic conditions. Yeast (a fungus) also carries out alcohol fermentation. Used in brewing, winemaking, and baking. CO2 bubbles generated by baker's yeast during alcohol fermentation allow bread to rise.

Catabolic pathways

Metabolic pathways that release stored energy by breaking down complex molecules

Lactic Acid Fermentation

Pyruvate is reduced directly by NADH to form lactate as an end product, with no release of CO2. Used in the dairy industry to make cheese and yogurt. BIO EX. Human muscle cells make ATP by lactic acid fermentation when oxygen is scarce. This occurs during strenuous exercise, when sugar catabolism for ATP production outpaces the muscle's supply of oxygen from the blood. Under these conditions, the cells switch from aerobic respiration to fermentation. Excess lactate is gradually carried away by the blood to the liver, where it is converted back to pyruvate by liver cells. Because oxygen is available, this pyruvate can then enter the mitochondria in liver cells and complete cellular respiration.

The 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 of 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. 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.

Oxidative Phosphorylation

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.

Chemiosmosis

The movement of ions across a selectively permeable membrane, down their electrochemical gradient. During chemiosmosis, electron carriers like NADH and FADH donate electrons to the electron transport chain. The electrons cause conformation changes in the shapes of the proteins to pump H+ across a selectively permeable cell membrane. The uneven distribution of H+ ions across the membrane establishes both concentration and electrical gradients (thus, an electrochemical gradient) owing to the hydrogen ions' positive charge and their aggregation on one side of the membrane. If the membrane were open to diffusion by the hydrogen ions, the ions would tend to spontaneously diffuse back across into the matrix, driven by their electrochemical gradient. However, 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 a membrane protein called ATP synthase. This protein acts as a tiny generator turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient. The turning of this molecular machine harnesses the potential energy stored in the hydrogen ion gradient to add a phosphate to ADP, forming ATP. Chemiosmosis is used to generate 90 percent of the ATP made during aerobic glucose catabolism. The production of ATP using the process of chemiosmosis in mitochondria is called oxidative phosphorylation. It is also the method used in the light reactions of photosynthesis to harness the energy of sunlight in the process of photophosphorylation. 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.

NO OXYGEN

Without the electronegative oxygen to pull electrons down the transport chain, oxidative phosphorylation eventually ceases However, fermentation and anaerobic respiration can still occur

electron transport chain

a number of molecules, mostly proteins, built into the inner membrane of the mitochondria of eukaryotic cells most electrons travel the following "downhill" route: glucose --> NADH --> electron transport chain --> oxygen

Redox Reactions: Oxidation and Reduction

a transfer of one or more electrons (e-) from one reactant to another oxidation: the loss of electrons from one substance reduction: the addition of electrons to another substance LEO says GER reducing agent: electron donor, allows a substance to be reduced oxidizing agent: electron acceptor, allows a substance to be oxidized Because an electron transfer requires both an electron donor and an acceptor, oxidation and reduction always go hand in hand. BIO EX. NAD+ reduces to NADH, NADH oxidizes to NAD+

obligate anaerobes

carry out only fermentation or anaerobic respiration cannot survive in the presence of oxygen

Fermentation

catabolic pathway a partial degradation of sugars or other organic fuel that occurs without the use of oxygen

The Evolutionary Significance of Glycolysis

early prokaryotes may have generated ATP exclusively from glycolysis The fact that glycolysis is today the most widespread metabolic pathway among Earth's organisms suggests that it evolved very early in the history of life.

2 types of fermentation

lactic acid fermentation alcohol fermentation

Pyruvate Oxidation

most of the energy is still stockpiled in pyruvate the pyruvate in eukaryotic cells enters a mitochondrion, where the oxidation of glucose is completed. carried out by multienzyme process 1. Pyruvate's carboxyl group (—COO-), which is already fully oxidized and thus has little chemical energy, is removed and given off as a molecule of CO2. This is the first step in which CO2 is released during respiration 2. The remaining two-carbon fragment is oxidized, forming acetate (CH3COO-, which is the ionized form of acetic acid). The extracted electrons are transferred to NAD+, storing energy in the form of NADH. 3. Finally, coenzyme A (CoA), a sulfur-containing compound derived from a B vitamin, is attached via its sulfur atom to the acetate, forming acetyl CoA, which has a high potential energy; in other words, the reaction of acetyl CoA to yield lower-energy products is highly exergonic. This molecule will now feed its acetyl group into the citric acid cycle for further oxidation.

facultative anaerobes

organisms, including yeasts and many bacteria, that can make enough ATP to survive using either fermentation or respiration Under aerobic conditions, pyruvate can be converted to acetyl CoA, and oxidation continues in the citric acid cycle via aerobic respiration. Since it occurs in the cytosol, it has been occurring 1 billion years before the evolution of the first prokaryotic cell

Aerobic Respiration

the most efficient catabolic pathway oxygen is consumed as a reactant along with the organic fuel The cells of most eukaryotic and many prokaryotic organisms can carry out aerobic respiration.