Harvesting Cellular Energy: Cellular Respiration

Cellular Respiration

Pp 69 73 & 217 - 2371Define cell respirationCell respiration is the controlled release of energy from organic compounds in cells to form ATPGlucose is the major substrate for respirationAdenosine triphosphates (ATP) is the molecule which directly fuels the majority of biological reactions.

Why cell respiration?Cells require a constant source of energy to perform various tasks e.g.Movement Transport Division

Types of Respiration:(i) Anaerobic Respiration(ii) Aerobic RespirationOccurs in the absence of OxygenOccurs in the cells cytoplasm

Yields small amount of ATP (2 molecules) per molecule of glucose

Involves fermentation of pyruvate to lactate in muscles/CO2 & ethanol in plant & yeast

Occurs in presence of Oxygen

Occurs in the cells mitochondria Yields large amount of ATP (38 molecules) per molecule of glucoseDoes not involve fermentationTypes of RespirationOccurs in the absence of Oxygen(ii) Aerobic RespirationOccurs in presence of OxygenOccurs in the cells cytoplasmOccurs in the cells mitochondria Yields small amount of ATP (2 molecules) per molecule of glucoseYields large amount of ATP (38 molecules) per molecule of glucoseDoes not involve fermentationInvolves fermentation of pyruvate to lactate in muscles/CO2 & ethanol in plant & yeast(i) Anaerobic RespirationComparison between Aerobic & Anaerobic Respiration

Adenosine triphosphate (ATP):ATP the molecule which directly fuels the majority of biological reactionsAbout 1025 ATP molecules are hydrolysed to ADP and Pi dailyADP is reduced back to ATP using the free energy from the oxidation of organic molecules

ATP Cycle

in cell respiration, glucose in the cytoplasm is broken down by glycolysis into pyruvate, with a small yield of ATP

Glycolysis and Cell Respiration:

Glycolysis occurs in the cytoplasm of the cell1 glucose molecules is broken down into 2 pyruvate moleculesThere is a net production of 2 ATP moleculesGlycolysis does not require oxygenFate of pyruvate depends on presence or absence of oxygen Anaerobic Cell Respiration:Summary EquationThe summary equation for cellular respiration is: C6H12O6 + O2 CO2 + H2O +

Glucose + oxygen carbon dioxide + water + ATPATP12Key players in this processGlucose: source of fuelNAD+: electron carrier Enzymes: mediate entire processMitochondria: site of aerobic respirationATP: principal end productProtons/Electrons: sources of potential energyOxygen: final electron acceptor

13Redox reactions Reduction: reducing overall positive charge by gaining electronsOxidation: loss of electrons

14NAD+: an electron carrierIn order for electrons to be passed from one compound to another, an electron carrier is neededNAD+ is reduced to NADH when picking up electronsIt is oxidized back to NAD+ when losing them

15Where do the electrons come from?Remember all those hydrogen atoms that make up glucose?Hydrogens are a part of fats, too.Hydrogen = 1e-, so here, H = e-

16Respiration is a controlled release of energyIts a highly exergonic, but well-controlled processMediated by enzymes, electron carriersOtherwise, it would be like an explosionNot compatible with life!

17PhosphorylationAddition of a phosphate group to a molecule; in this case, to ADP, forming ATPSubstrate level phosphorylation vs. oxidative phosphorylation 18Substrate-level phosphorylationAn enzyme transfers a phosphate group from a substrate to ADPIneffective in generating large amounts of ATP

19Oxidative phosphorylationRefers to phosphorylation that occurs due to redox reactions transferring electrons from food to oxygen Happens on electron transport chains

20Mitochondrion Structure

Outer membraneInner membraneCristae (folds)Matrix (liquid)DNA (mtDNA)Intermembrane space21Three stages of respirationStage 1: Glycolysis (energy investment) Some ATP is made, some is usedStage 2: Krebs Cycle (oxidation of pyruvate) Generation of CO2Stage 3: Oxidative Phosphorylation Generation of most ATP 22Three stages of respiration

23Stage 1: GlycolysisWhereCells cytoplasmWhyTo break glucose down into pyruvate, which feeds into the Krebs CycleTo regenerate NAD, an electron carrier

24Glycolysis: How

Glucose is phosphorylated.-1 ATPA series of enzymes produces intermediate products.An intermediate is phosphorylated.-1 ATPThis diphosphate compound is unstable and breaks into 2 PGAL.The PGAL molecules generate ATP through substrate-level phosphorylation.+4 ATPNAD is reduced to NADH, 1 each per PGAL+2 NADH + H+Summary: 2 pyruvate produced2 NADH + H+ producedNet 2 ATP produced

25Stage 2: Krebs cycle Where:Matrix of mitochondria, but only if O2 presentWhy:To oxidize pyruvate to CO2To build up a H+ ion gradient used in electron transport

26Krebs Cycle: How

Pyruvate is decarboxylated.-1 CO2Resulting acetic acid (2C) is oxidized by NAD.+1 NADH + H+Acetyl group (2C) has Coenzyme A added1 Acetyl co-A: link reactionAcetyl co-A (2C) is added to a 4C base molecule, forming a 6C intermediate NAD oxidizes these intermediatesCO2 is given off as a byproduct+2 NADH + H+-1 CO2 To regenerate the original 4C base, ATP is generated and FAD oxidizes an intermediate+1 ATP +1 FADH2NAD oxidizes a 4C to form original 4C molecule: +1 NADH + H+ 27Krebs cycle summaryPer pyruvate that enters: 1 ATP made3 CO2 given off4 NADH produced1 FADH2 producedThink: how many pyruvates entered the cycle?How many times must this cycle happen to break down ONE glucose?28Stage 3: Oxidative phosphorylationWhere:Inner membrane of mitochondria (on cristae)Why:To produce ATP from H+ ion gradient generated during Krebs cycleRequires oxygen!

29Oxidative Phosphorylation: How

matrixIntermembrane spaceH+H+H+H+H+H+H+H+H+H+H+H+H+H+H+H+ ions accumulate in the matrix as a result of NADH picking them up during the Krebs cycle.cytochromes30Oxidative Phosphorylation: How

matrixIntermembrane spaceH+The H+ ions diffuse out of the matrix through protein channels into the intermembrane space where they split into H protons and electrons. H+H+H+H+H+H+e-e-e-e-e-e-cytochromes31Oxidative Phosphorylation: How

matrixIntermembrane spaceH+H+H+H+H+H+The accumulated H+ ions then move through a pump called ATP synthase to produce ATP. What powers the pump is the electrochemical gradient produced.cytochromese-e-e-e-e-e-ADPATP32Oxidative Phosphorylation: How

matrixIntermembrane spaceH+H+H+H+H+H+While the H+ protons move through ATP synthase, electrons carried by NADH are passed along electron transport chains composed of cytochromes.cytochromese-e-e-e-e-e-ADPATP33Oxidative Phosphorylation: How

matrixH+H+H+H+H+H+Now the H+ atoms are in the matrix. The hydrogen atoms and electrons combine with oxygen, the final electron acceptor of oxidative phosphorylation, to form water.cytochromese-e-e-e-e-e-ADP34 ATPOH+H+34Summary: Oxidative Phosphorylation34 ATP madeH2O generated NADH oxidized back to NADVery efficient process! Produces a lot of energy.35

36But what if theres no oxygen?Remember, the first living organisms lived in an anaerobic environmentWithout oxygenNADH cannot be oxidized back to NAD+In order for aerobic respiration to occur, NADH must be oxidized and some intermediate compound must be reduced Fermentation Includes glycolysisAlso side reactions that allow for NADH to be oxidized back to NAD+ by shuttling electrons to intermediate products such as ethanol and lactate

Alcoholic FermentationGlycolysis happensPyruvate is then converted to acetaldehyde, CO2 is releasedAcetaldehyde is reduced by NADH to ethanol No additional ATP is madeOccurs in yeasts, some bacteria

Lactic acid fermentationGlycolysis happensPyruvate is reduced by NADH and forms lactate (lactic acid)No CO2 is releasedNo additional ATP is formedDone by bacteria, muscle cells

Other molecules can be used in respirationProteins: must be deaminated, then converted to pyruvateFats: undergo beta-oxidationCells prefer carbs