Cellular Respiration

Introduction Cells break down (or metabolize) their organic nutrients by way of slow oxidation. A molecule, such as glucose, is acted on by a series of enzymes. The function of these enzymes is to catalyse a sequential series of reactions in which the covalent bonds are broken (oxidized) one at a time.

Introduction Each time a covalent bond is broken, a small amount of energy is released The ultimate goal of releasing energy in a controlled way is to trap the released energy in the form of ATP molecules.

Introduction Before food can be used to perform work, its energy must be released through the process of respiration. Two main types of respiration exist in living things. Both begin with glycolysis. Glycolysis: a process by which one glucose molecule is broken down into two pyruvic acid molecules. Fermentation (anaerobic respiration): pyruvic acid is broken down without the use of oxygen Oxidative Respiration (aerobic respiration): pyruvic acid is metabolized using oxygen

Fermentation (without oxygen) Aerobic Respiration Glucose Glycolysis Krebs cycle Electron transport Fermentation (without oxygen) Alcohol or lactic acid Anaerobic Respiration

Cells recycle the ATP ATP, adenosine triphosphate, is the pivotal molecule in cellular energetics. It is the chemical equivalent of a loaded spring. The close packing of three negatively charged phosphate groups is an unstable, energy-storing arrangement. Loss of the end phosphate group “relaxes” the “spring”. The price of most cellular work is the conversion of ATP to ADP and inorganic phosphate (Pi). An animal cell regenerates ATP from ADP and Pi by the catabolism of organic molecules. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The transfer of the terminal phosphate group from ATP to another molecule is phosphorylation. This changes the shape of the receiving molecule, performing work (transport, mechanical, or chemical). When the phosphate group leaves the molecule, the molecule returns to its alternate shape. Fig. 9.2 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Electron Carriers The “fall” of electrons during respiration is stepwise, via NAD+ and an electron transport chain Cellular respiration does not oxidize glucose in a single step that transfers all the hydrogen in the fuel to oxygen at one time. Rather, glucose and other fuels are broken down gradually in a series of steps, each catalyzed by a specific enzyme.

Electron Carriers At key steps, hydrogen atoms are stripped from glucose and passed first to a coenzyme, like NAD+ (nicotinamide adenine dinucleotide). Dehydrogenase enzymes strip two hydrogen atoms from the fuel (e.g., glucose), pass two electrons and one proton to NAD+ and release H+. H-C-OH + NAD+ -> C=O + NADH + H+ Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

This changes the oxidized form, NAD+, to the reduced form NADH. NAD + functions as the oxidizing agent in many of the redox steps during the catabolism of glucose. Fig. 9.4 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The electrons carried by NADH loose very little of their potential energy in this process. This energy is tapped to synthesize ATP as electrons “fall” from NADH to oxygen. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Oxidation and reduction Metabolism is the sum of all chemical reactions carried out by an organisms. These reactions involve: Catabolic pathways Anabolic pathways

Oxidation and reduction Catabolic pathways result in the breakdown of complex molecules to smaller molecules Anabolic pathways result in the synthesis of more complex molecules from simpler ones To understand these complex pathways, it is essential for understand two general types of chemical reactions: reduction and oxidation

Oxidation and reduction OIL RIG OIL=Oxidation is Loss RIG=Reduction is Gain Oxidation Reduction Loss of electrons Gain of electrons Gain of oxygen Loss of oxygen Loss of hydrogen Gain of hydrogen Results in many C-O bonds Results in many C-H bonds Results in a compound with lower potential energy Results in a compound with high potential energy

Oxidation and reduction The two reactions occur together during chemical reactions. Think of it in this way: one compound’s or elements loss is another compound’s or elements gain

Oxidation and reduction C6H12O6→ 2CH3CH2OH + 2CO2 + Energy In this equation, glucose is oxidized because electrons are transferred from it to oxygen. The protons follow the electrons to produce water The oxygen atoms that occur in the oxygen molecules on the reactant side of the equation are reduced

Oxidation and reduction Because oxidation and reduction always occur together, these chemical reactions are referred to as redox reactions. When redox reactions take place, the reduced form of a molecule always has more potential energy than the oxidized form of the molecule.

Glycolysis

Glycolysis All cells begin the of cell respiration in the same way. Glucose enters a cell through the plasma membrane and floats in the cytoplasm An enzyme modifies the glucose slightly, than a second enzyme modifies this molecule even more. This is followed by an entire series of reactions which ultimately cleave the 6-carbon glucose into two 3-carbon molecules Each of these three molecules is called Pyruvate

Glycolysis Some of the covalent bonds in the glucose were broken during this series of reactions. Some of the energy released from the breaking of these bonds was used to form a small number of ATP molecules

Glycolysis Look at Picture!

Glycolysis Two ATP’s are used to start the process A total of four ATP’s are produced—a net gain of two ATP’s Two molecules of NADH are produced Involves substrate phosphorylation, lysis, oxidation, and ATP formation The metabolic pathway is controlled by enzymes. Whenever ATP levels in the cell are high, feedback inhibition will block the first enzyme of the pathway. This will slow or stop the process. Glycolysis occurs in the cytoplasm. Two pyruvate molecules are present at the end of the pathway

Glycolysis

Cell Respiration The term ‘cell respiration’ refers to a variety of biochemical pathways that can be used to metabolize glucose. Glycolysis is a metabolic pathway that is common to all organisms on Earth. Some organisms derive their ATP completely without the use of oxygen and are referred to an anaerobic (fermentation) Two main types of fermantation: alcoholic fermentation and lactic acid fermentation

Fermentation (Anaerobic Respiration) Fermentation is the breakdown of pyruvic acid without the use of oxygen. Glycolysis + Fermentation = Anaerobic Respiration The metabolism of pyruvic acid during fermentation does not produce any ATP. Instead, the function of fermentation is to break down pyruvic acid and regenerate NAD+ for reuse in glycolysis.

Alcoholic Fermentation Yeast is a common single-celled fungus that uses alcoholic fermentation for ATP generation. Yeast cells take in glucose from their environment and generate a net gain of two ATP’s by way of glycolysis (products are two ATP’s and 2 pyruvate molecules) The pyruvate are then converted into molecules of ethanol Ethanol is a 2-carbon molecule, so a carbon atoms is “lost” in this conversion

Alcoholic Fermentation The ‘lost’ carbon atom is given off in a carbon dioxide molecule. Baking Production of drinking alchol

Lactic Acid Fermentation Organisms that use an aerobic cell respiration sometimes find themselves in a metabolic situation where they cannot supply enough oxygen to their cells Exercising Remember—that glycolysis is used by all cells to begin the cell respiration sequence In a low-oxygen situation, excess pyruvate molecules are converted into lactic acid

Lactic Acid Fermentation Like pyruvate, lactic acid is a 3 carbon molecule so there is not production of carbon dioxide. So what benefit does this serve? It allows the glycolysis to continue with a small gain of ATP generated in addition to the ATP which is already being generated through the aerobic pathway

Fermentation Lactic Acid Fermentation Alcoholic Fermentation To Glycolysis Fermentation Lactic Acid Fermentation NADH + H+ NAD+ + 2 H+ + 2 e- Pyruvic Acid Lactic Acid Alcoholic Fermentation NADH + H+ NAD+ + 2 H+ + 2 e- Pyruvic Acid Ethyl Alcohol (Ethanol) CO2

Oxidative Respiration Aerobic Respiration Oxidative Respiration

Aerobic Respiration Cells that have mitochondria use an aerobic pathway for cell respiration This pathway also begins with glycolysis and thus has a net gain of ATP and two pyruvate molecules The two pyruvate molecules now enter a mitochondrion and are further metabolized Each pyruvate first loses a carbon dioxide (decarboxylated) molecule, than the acetyl group is then oxidized with the formation of a reduced NAD+. Finally, the acetyl group combines with coenzyme A (CoA) to form molecule known as acetyl-CoA

Aerobic Respiration Each acetyl-CoA enters into a series of reactions called the Krebs cycle (Tricarboxylic Acid cycle). During this series of reactions, two more carbon dioxide molecules are produced from each original pyruvate that entered. The Krebs cycle series of reactions is a cycle because each time it returns to a molecule that once again reacts with another incoming acetyl-CoA molecule

Aerobic Respiration Step One: Acetyl CoA from the link reaction combines with a 4-carbon compound called oxaloacetate. The result is a 6-carbon compound called citrate

Aerobic Respiration Step Two: Citrate (6-carbon compound) is oxidized to form a 5-carbon compound. In this process, the carbon is released from the cell as carbon dioxide. While the 6-carbon compound is oxidized, NAD+ is reduced to form NADH

Aerobic Respiration Step Three The 5-carbon is oxidized and decarboxylated to form a 4-carbon compound. Again, the removed carbon combines with oxygen and released as carbon dioxide. Another NAD+ is reduced for form NADH

Aerobic Respiration Step Four The 4 carbon compound undergoes various changes resulting in several products. One Product is another NADH. The coenzyme FAD is reduced to form FADH2. There is also a reduction of an ADP to form ATP. The 4-carbon compound is changed during these steps to re-form the starting compound of the cycle, oxaloactetate. The oxaloactetate may than begin the cycle again.

Aerobic Respiration Some ATP is directly generated during the Krebs cycle and some is indirectly generated through a later series of reactions directly involving oxygen Aerobic cell respiration breaks down (or completely oxidizes) a glucose molecule and the end-products are carbon dioxide and water.

Aerobic Respiration The reason aerobic cell respiration is so much more efficient than anaerobic cell respiration is that anaerobic cell respiration is that anaerobic pathways do not completely oxidize the glucose molecule. Aerobic cell respiration does leaves no by products such as lactic acid or ethanol and results in a yield of ATP per glucose.

Aerobic Respiration It is important to remember that the Krebs cycle will run twice for each glucose molecule entering cellular respiration. This is because a glucose molecule forms two pyruvate molecules. Each pyruvate produces one acetyl CoA which enters the cycle.

Aerobic Respiration Products of Kreb cycle for one glucose molecule (two turns of the cycle) Two ATP molecules Six molecules of NADH Two molecules of FADH2 Four molecules of carbon dioxide So far, only four ATP’s have been gained: six are generated (four from glycolysis and two from the Krebs cycle) but two are used to start the process f glycolysis

Aerobic Respiration The result of glycolysis and aerobic respiration is shown by the reaction: C6H12O6 + 6 O2 → 6 H2O + 6 CO2 + 38 ATP Aerobic respiration occurs in the mitochondria outer and inner membrane matrix: dense solution enclosed by inner membrane cristae: the folds of the inner membrane that house the electron transport chain and ATP synthase Steps: Conversion of Pyruvic Acid Kreb’s Cycle Electron Transport Chain

Structure of Mitochondrion

Conversion of PA Kreb’s Cycle

Conversion of Pyruvic Acid NAD+ NADH + H+ Pyruvic Acid C Acetyl-CoA C CO2 C CoA CoA Kreb's Cycle Citric Acid C Oxaloacetic Acid C NAD+ NADH + H+ NADH + H+ CO2 C NAD+ Ketoglutaric Acid C Malic Acid C NAD+ NADH + H+ Succinic Acid C CO2 C FADH2 ATP ADP + P FAD

Electron Transport Chain Glycolysis, the conversion of PA to acetyl-CoA, and the Krebs Cycle complete the breakdown of glucose. Up to this point: 4 ATP (2 from glycolysis, 2 from Krebs) 10 NADH + H+ (2 from glycolysis, 2 from the conversion of PA, 6 from Krebs) 2 FADH2 (from Krebs)

Electron Transport Chain The electron transport chain is where most of the ATPs from glucose catabolism are produced It is the first stage of cellular respiration where oxygen is actually needed and it occurs in the mitochondria Krebs cycle-matrix of mitochondria ETC-on the inner mitochondrial membrane of the cristae

Electron Transport Chain Embedded in the involved membrane are molecules that are easily reduced and oxidized. These carriers of electrons are close together and pass the electrons from one to another due to an energy gradient. Each carrier molecule has a slightly different electronegativity and, therefore, a different attraction for electrons Most carriers are proteins called cytochromes One is one a protein called coenzyme Q

Electron Transport Chain NADH + H+ and FADH2 carry electrons to an electron transport chain, where additional ATP is produced. 10 NADH 30 ATP 2 FADH2 4 ATP

Electron Transport Chain In this chain, electrons pass from one carrier to another because the receiving molecule has a higher electronegativity and, therefore, a stronger attraction for electrons In the process of electron transport, small amounts of energy are released The source of the electrons that move down the electron transport chain are the coenzymes NADH and FADH2 from the previous stages of cellular respiration

Electron Transport Chain Oxygen is the final electron acceptor because it has a very high electronegativity and therefore a strong attraction for electrons When the electrons combine with the oxygen so do two hydrogen ions from the aqueous surroundings=results in water

Electron Transport Chain So energy is now available as a result of the electron transport chain. This is the energy that allows the addition of phosphate and energy to ADP to form ATP. The process by which this occurs is called chemiosmosis

Chemiosmosis Chemiosmosis involves the movement of protons (hydrogen ions) to provide energy to the phosphorylation can occur Because this phosphorylation uses an ETC, it is called oxidative phosphorylation not substrate level phosphorylation

Chemiosmosis The membrane in which this occurs has enzymes and the other necessary compounds embedded in it. The inner membrane of the mitochondrion have numerous copies of an enzyme called ATP synthase. This enzymes uses the energy of an ion gradient to allow the phosphorylation of ADP The ion gradient is created by a hydrogen ion concentration difference that occurs across the cristae membrane

Chemiosmosis Look at the picture! Note the three labeled areas on the left: intermembrane space, inner mitochondrial membrane, and mitochondrial matrix Also note the hydrogen ions are being pumped out of the matrix into the intermembrane space. The energy for this pumping action is provided by the electrons as they are de-energized moving through the ETC. This creates the different hydrogen ion concentration on the two sides of the cristae membranes

Chemiosmosis With the higher hydrogen ion concentration in the intermembrane space, these ions begin to passively move through a channel in ATP synthase back into the mitochondrial matrix. As the hydrogen ions move through the ATP synthase channel, the enzyme harnesses the available energy thus allowing the phosphorylation of ADP

Electron Transport Chain

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix NADH + H+ FADH2 NAD+ FAD

Electron Transport Chain Intermembrane Space Inner Membrane Matrix ADP + P ~ e ATP NADH + H+ FADH2 NAD+ FAD

Electron Transport Hydrogen Ion Movement ATP Production ATP synthase Channel Inner Membrane Matrix Intermembrane Space

Energy Yield

Energy Yield Reasons why ATP yield can be less than 38: Process ATP used ATP produced Net ATP gain Glycolysis 2 4 Krebs cycle ETC/ chemiosmosis 32 Total 38 36 Reasons why ATP yield can be less than 38: Sometimes energy is required to transport NADH + H+ formed by glycolysis from the cytoplasm through the inner mitochondrial membrane. Some H+ in chemiosmosis may leak through the membrane.

Energy Yield

Energy Yield Aerobic Respiration is generally 19 times more efficient than anaerobic respiration. The ATP produced during aerobic respiration represents about 1/2 of the energy stored in a molecule of glucose.

Final Look at respiration and the mitochondrion Outer mitochondrial membrane Separates the contents of the mitochondrion form the rest of the cell Matrix Internal cytosol-like area that contains the enzymes for the link reaction and the Krebs cycle Cristae Tubular regions surrounded by membranes increasing surface area for oxidative phosphorylation Inner mitochondrial membrane Contains the carriers for the electron transport chain and ATP synthase for chemiosmosis Space between inner and outer membranes Reservoir for hydrogen ions (protons), the high concentration for hydrogen ions is necessary for chemiosmosis