Cellular Respiration Biology Lecture PDF

Cellular Respiration Cells require energy in order to grow, reproduce, manufacture, transport materials, move and maintain cell structure All of this energy comes from the sun Light energy is converted into chemical energy via the process of photosynthesis...

Cellular Respiration Cells require energy in order to grow, reproduce, manufacture, transport materials, move and maintain cell structure All of this energy comes from the sun Light energy is converted into chemical energy via the process of photosynthesis Some bacteria, all plants and algae do photosynthesis Photosynthesis uses the energy provided by the sun to rearrange bonds between CO2 and H2O in order to form glucose and O2 Cellular Respiration breaks down glucose and O2 produced by photosynthesis, producing CO2 and H2O It also stores the chemical energy that is released in the bonds of ATP Some of the energy is lost as heat during the various conversions  Breathing Provides the Oxygen Needed for Cellular Respiration Respiration is defined as an exchange of gases Organisms obtain O2 from their environment and release CO2 as waste Respiration may also be described as the aerobic harvesting of energy from food molecules by cells Distinguishes cellular respiration from breathing Breathing and Cellular Respiration are tightly connected When the runner breathes in air, O2 that is taken up enters into the blood stream traveling to all body cells The muscles cells actively involved in running take up the O2 which is used in the mitochondria of the muscle cells In the mitochondria energy from glucose and other organic molecules in conjunction with the O2 is harvested in order to generate ATP via cellular respiration This provides the energy for the muscular contraction CO2 is expelled from the lungs as a waste product  Cellular Respiration uses ATP for Energy Storage Cellular respiration ultimately function to generate ATP for cellular work Glucose is used as an example however there are a number of other organic molecules that can be used to generate ATP Bonds between atoms in glucose and oxygen break and reform as CO2 and H2O releasing energy that is then stored in the bonds of ATP Cellular respiration may produce as many as 32 ATP molecules per one glucose molecule This 32 ATP only represents ~40% of the total energy available per glucose molecule The remaining energy is lost as heat  Bodily Activity that Uses ATP In order to breathe, maintain a heart beat and maintain body temperature at 37oC the body requires a constant supply of energy The most energy required by the body each day is for the brain Brain cells burn ~120 grams of glucose each day and use 15% of the bodies total oxygen consumption In order to maintain life and carry out necessary bodily functions, approximately 75% of the total energy taken in each day is utilized by the body Voluntary activities are also powered by cellular respiration How Exactly is Energy Harvested from Glucose Chemical bonds that hold a molecule together result from shared electrons These shared electrons contain the energy that is available to the cell These electrons are transferred to O2 as C-H bonds in glucose are broken and the H-O bonds in water are formed during cellular respiration O2 attracts these electrons very strongly and as the electrons fall down a ‘staircase’ from the bonds of glucose to O2 lots of energy is released which is then stored in the chemical bonds of ATP Glucose looses hydrogen atoms as it is converted to CO2 O2 gains hydrogen atoms as it is converted into water Hydrogen transfers represent electron transfers because each hydrogen consists of 1 H+ and 1 e-  How Exactly is Energy Harvested from Glucose When electrons are moved from one molecule to another the reaction is called a redox reaction or an oxidation-reduction reaction The loss of electrons from one molecule is called oxidation An atom that has lost electrons is referred to as having been oxidized The gain of electrons by another molecule is called reduction An atom that has gained electrons is referred to as having been reduced A transfer of electrons always requires a donor and a recipient, therefore oxidation and reduction reactions always occur together Overall result of cellular respiration: glucose looses electrons as H atoms and becomes oxidized and O2 gains electrons in the form of H atoms and is reduced During H transfers, electrons loose potential energy which is then released How Exactly is Energy Harvested from Glucose During glucose oxidation there are two key players: 1. An enzyme called Dehydrogenase 2. A co-enzyme called NAD+ NAD+ (Nicotinamide Adenine Dinucleotide) is an organic molecule that is used to shuttle electrons during redox reactions Dehydrogenase strips two electrons from glucose These two electrons (as H) are picked up by NAD + which becomes NADH + H + How Exactly is Energy Harvested from Glucose NADH + H+ delivers the electrons to the electron transport chain where they are then transferred to O2 forming water and creating ATP The electron transport chain consists of a series of electron transfers (redox reactions) that end with O2 Each of these electron transfers releases a little bit of energy all of which will be used to produce ATP  Cellular Respiration: 4 Stages There are four key stages that collectively make up cellular respiration: 1. Glycolysis 2. The Intermediate Step 3. The Citric Acid Cycleà also called the TCA (Tricarboxylic Acid) Cycle and the Kreb’s Cycle 4. Oxidative Phosphorylation All three stages occur in prokaryotes and eukaryotes but the location differs 1 occurs in the cytoplasm of prokaryotes and eukaryotes 2 and 3 occur in the cytoplasm of prokaryotes and in the mitochondrial matrix of eukaryotes 4 occurs in the plasma membrane of prokaryotes and in the inner mitochondrial membrane of eukaryotes  Cellular Respiration: Summary 1. Glycolysis functions to break glucose into two three carbon sugars called pyruvate 2. The Intermediate step converts pyruvate into acetyl coA 3. The Citric Acid Cycle converts acetyl coA into CO2 A very small amount of ATP is also produced during stages 1 and 3 Glycolysis, the intermediate step and the citric acid cycle function to supply stage 4 (oxidative phosphorylation) with a supply of electrons 4. Oxidative Phosphorylation: makes use of the electron transport chain and a process called chemiosmosis NADH and FADH2 bring the electrons generated during stages 1, 2 and 3 to the electron transport chain embedded within the inner mitochondrial membrane The bulk of ATP produced during cellular respiration occurs during this step as electrons fall from NADH and FADH2 to O2 releasing lots of energy that is then used to phosphorylate ATP  Cellular Respiration: Summary The electron transport chain must somehow be coupled to ATP synthesis As the electron transport chain moves electrons down the staircase from carrier to carrier energy is released and protons are pumped across the inner mitochondrial membrane into the intermembrane space using this released energy This creates a concentration gradient of H+ across the membrane with a higher concentration of H+ between the two membranes than in the cytoplasm of the mitochondria Chemiosmosis uses this H+ gradient to generate ATP driving the diffusion of H+ through ATP synthases which are protein complexes built into the inner mitochondrial membrane that are used to synthesize ATP  Glycolysis This is the first stage of cellular respiration Glycolysis literally means the splitting of sugar which is exactly what happens Glycolysis begins with a single molecule of glucose and ends with 2 molecules of pyruvate 1 6C molecule à 2 3C molecules The breakdown of glucose into two pyruvate molecules occurs over 10 different chemical reactions Each reaction is catalyzed by a particular enzyme Glycolysis from start to finish generates: 2 NADH + H+ from 2 NAD+ 2 ATP The ATP is generated by substrate-level phosphorylation  Glycolysis Substrate-level phosphorylation: The direct transfer of a phosphate group from a substrate molecule to ADP forming ATP This process is responsible for the production of a small amount of ATP during stages 1 and stage 3 Following the breakdown of glucose that occurs during glycolysis all of the energy that is released is stored in the form of ATP and NADH The ATP is available for immediate use but the NADH that is produced must first enter the electron transport chain located in the inner mitochondrial membrane The bulk of the energy that is available from glucose is still stored in the bonds of the 2 pyruvate molecules and goes on to be harvested in the citric acid cycle  Pyruvate is Groomed for the Citric Acid Cycle Once the 2 molecules of pyruvate are formed at the end of glycolysis they move from the cytoplasm of the cell into a mitochondrion Pyruvate as it is formed from glycolysis enters the intermediate step 1. A COOH group is removed as a CO2 molecule leaving behind a 2 carbon molecule 2. The remaining 2 carbon compound is oxidized producing one NADH from NAD+ 3. A molecule called coenzyme A (derived from vitamin B) joins to the two carbon group forming a molecule called acetyl coenzyme A (acetyl CoA) Acetyl CoA is a high energy molecule that enters into the citric acid cycle For every one molecule of glucose that enters glycolysis, 2 molecules of acetyl CoA are produced and enter the citric acid cycle  The Citric Acid Cycle The two carbon portion of acetyl CoA participates in the citric acid cycle The coenzyme A portion helps the acetyl group enter the citric acid cycle and will break off and be recycled This stage occurs in the mitochondrial matrix and each individual step is catalyzed by a different enzyme specific for reaction This stage releases 2 CO2 molecules as well as 1 ATP via substrate-level phosphorylation The ATP is actually produced as an energy equivalent called GTP It also produces 3 NADH from NAD+ and 1 FADH2 from FAD Since 2 molecules of acetyl-CoA enter this cycle the total yield is 2 GTP, 6 NADH and 2 FADH2 Considerably more energy released than glycolysis 2ATP and 2 NADH  The Citric Acid Cycle At this point the breakdown of glucose has generated 4 ATP equivalents (2 ATP and 2 GTP), 10 NADH and 2 FADH2 The FADH2 and the NADH must now enter the electron transport chain so that the high energy electrons may be transferred to O2 producing water and ATP via oxidative phosphorylation  The Electron Transport Chain The final stage of cellular respiration is oxidative phosphorylation This stage involves the electron transport chain and chemiosmosis The electron transport chain is built into the inner mitochondrial membrane This membrane is highly folded à large amount of space to house many electron transport chains and ATP synthase complexes The electrons move from NADH and FADH2 through the electron transport chain to O2 which acts as the terminal electron receptor Each oxygen atom (1/2 O2) picks up two electrons in the form of hydrogen ions, resulting in 2 H20 molecules à products of cellular respiration  The Electron Transport Chain There are four main protein complexes that make up the electron transport chain (these are embedded within the membrane) There are also two mobile electron carriers that shuttle electrons between the stationary complexes All of these carriers bind and then release electrons in redox reactions This functions to move the electrons down the energy staircase Three of the stationary protein complexes use the energy from the movement of the electrons to transport H+ from the matrix into the inter-membranous space where it is less concentrated Establishes an H+ gradient with a [higher] in the inter-membranous space than in the matrix, this gradient can be used to provide the energy needed to form ATP  The Electron Transport Chain Because the membrane is not permeable to H+ the ion travels down the newly established concentration gradient through a membrane protein called ATP synthase This movement causes the ATP synthase to spin which actively catalyzes the attachment of a phosphate group onto ADP generating ATP This is called oxidative phosphorylation because energy from oxidation reactions is used to phosphorylate ADP The H+ gradient created during electron transport is used to drive ATP synthesis via chemiosmosis  The Effect of Chemicals on Cellular Respiration There are three different categories of poison that effect cellular respiration: 1. Poisons that Block the Electron Transport Chain Retinone is a poison that tightly binds to the first electron carrier in the chain This prevents electrons from being transferred further down the electron transport chain Used to kill insects and fish Inhibits ATP synthesis and starves an organism’s cells of energy Cyanide and carbon monoxide bind to the fourth electron carrier which blocks the flow of electrons to O2 No H+ gradient is generated therefore no ATP is made  The Effect of Chemicals on Cellular Respiration 2. Poisons that Block ATP Synthase Ex: oligomycin blocks the flow of protons through the H+ channel in ATP synthase This antibiotic is used topically to combat fungal infections by inhibiting the production of ATP 3. Poisons that are Uncouplers The mitochondrial membrane becomes leaky to H+ H+ leaks through the membrane depleting the hydrogen ion gradients Electron transport continues but ATP is not synthesized O2 is still consumed and often at an accelerated rate Ex: DNP (dinitrophenol)  Review of Cellular Respiration Per one glucose molecule: Glycolysis: occurs in cellular cytoplasm and produces 2 ATP The Citric Acid Cycle: occurs in the mitochondrial matrix and produces 2 GTP **Above ATP equivalents are produced by substrate-level phosphorylation The Electron Transport Chain and Chemiosmosis Harvests energy that is stored in NADH and FADH2 produced during glycolysis, the intermediate step and the citric acid cycle ~28 ATP are formed via oxidative-phosphorylation  Review of Cellular Respiration It is assumed that each NADH that transfers a pair of high energy electrons from glucose to the electron transport chain contributes enough H+ to the gradient to produce 2.5 ATP It is also assumed that each FADH2 generated contributes enough to the H + gradient to produce only 1.5 ATP Because FADH2 enters the electron transport chain at a later spot than NADH **The above estimates for NADH and FADH2 are maximums and actual numbers may be a little bit lower depending on the circumstances Ex: some energy that is stored in the H+ gradient may be used for transport purposes rather than for ATP generation  The Effect of Oxygen Concentration on ATP Generation Most of the ATP generation from glucose occurs during oxidative phosphorylation (~28 ATP) This requires an adequate O2 supply O2 must be present to act as an electron acceptor during the electron transport chain If O2 is not present, chemiosmosis cannot occur and the cell will die from energy starvation Muscle cells as an example are able to continue on for some period of time without O2, generating ATP via fermentation  Fermentation The ONLY ATP generated during fermentation is that generated during glycolysis The total ATP generated during glycolysis is 2 ATP Glycolysis requires no O2 and generates 2 ATP by oxidizing Glucose to 2 molecules of pyruvate Simultaneously reduces 2 NAD+ to 2 NADH The 2 ATP produced by fermentation is considerably less than the 32 ATP produced by oxidative phosphorylation Enough to allow muscle contraction when the need for ATP is not met by the rate of O2 delivery Many microorganisms that are incapable of oxidative phosphorylation and generate all of their energy via fermentation There are two types of fermentation: 1. Lactic Acid Fermentation 2. Alcohol Fermentation  Lactic Acid Fermentation Lactic acid fermentation is used by muscle cells and some bacteria A few other cell types are also capable of lactic acid fermentation NADH is oxidized to NAD+ while the pyruvate generated during glycolysis is reduced to lactate During periods of prolonged exercise, muscle cells switch to lactic acid fermentation Lactic acid that builds up is then carried via the blood to the liver where it is converted back into pyruvate Lactic acid fermentation is used in industry to generate yogurt and cheese  Alcohol Fermentation This is used to produce wine, beer, and baking Yeast accomplish the production of the above products Yeast usually use aerobic respiration but switch to alcohol fermentation when O2 is absent Alcohol fermentation generates NAD+ from NADH while reducing pyruvate to CO2 and ethanol The CO2 produced provides the bubbles in champagne and beer It also is what causes dough to rise when yeast is added Ethanol produced is toxic to the fermentative organism When wine alcohol concentration reaches 14% the yeast within the container will die Obligate anaerobes cannot survive in the presence of O2 Facultative anaerobes use oxidative phosphorylation when O2 is present but switch to fermentation when O2 is absent  Glycolysis Evolved Early On Glycolysis takes place in every kind of living cell that exists: Bacterial cells, liver cells, skin cells, plants cells, fungal cells etc. Ancient prokaryotic cells likely made use of glycolysis to generate all ATP long before the atmosphere contained O2 Fossils are present from over 3.5 billion years ago O2 did not accumulate in the atmosphere until 2.7 billion years ago For the billion years that life existed in an O2-free atmosphere glycolysis was the only means of energy generation Glycolysis also does not require any membrane bound organelles Membrane bound organelles evolved greater than one billion years after the prokaryotic cell  Various Organic Molecules are Used for Cell Respiration Although glucose is the starting point for glycolysis, free glucose is not common in our diet Lipids, proteins, starch and disaccharides such as glucose are common in our diet All of the above molecules can also be used to generate ATP because they can be pushed into glycolysis following some modifications Starch and glycogen can be broken down into glucose via hydrolytic enzymes Proteins are broken down into their constituent amino acids Most of these amino acids are used to synthesize cellular proteins Unused amino acids are converted into glycolysis or citric acid cycle intermediates so that their energy can be harvested by cellular respiration Amino groups that are removed during this process are disposed of in urine as urea  Various Organic Molecules are Used for Cell Respiration Fats contain a great deal of energy that can be harvested by glycolysis There are a number of hydrogen atoms on a fat molecule and each of these hydrogen atoms are energy-rich electrons Fats are first broken down into the fatty acid and glycerol components Glycerol is made into a glycolysis intermediate The fatty acid tails are made into acetyl coA for the citric acid cycle Broken down like this a gram of fat will provide twice as much energy as a gram of starch  Food Molecules and Biosynthesis Not all of the food that we eat is digested in order to generate ATP Food is also responsible for providing the raw materials needed to synthesize organic molecules Cells must be capable of synthesizing raw materials needed to perform life functions and generate and maintain cellular structure Amino acids present from protein digestion can be used directly to synthesize cellular proteins Glycolysis intermediates must first be converted but may also be used in biosynthesis  Food Molecules and Biosynthesis Cellular Respiration and biosynthesis are closely connected to one another Feedback inhibition is used in order to prevent excess production of certain building blocks Ex: if there is an abundance of a particular amino acid, the pathway that is responsible for its production is turned off An end product inhibits an enzyme that works early on in the amino acid synthesis pathway When the amino acid is depleted, the inhibition is terminated and production of the amino acid is turned on once again The ability to generate sugars from CO2 and H2O is not universal Animals cells are incapable of this conversion while plant cells are able to photosynthesize