USABO 9 : Cellular Respiration

Catabolic Pathways

  • fermentation is the partial degredatioon of sugars or otehr organiz fuel that occurs without the use of ocxygen

  • aerobic respiration is the catabolic pathway in which oxygen is consumed as a recatant along with the organic fuel

    • conducted by most eukaryotic and prokaryotic organisms

    • General process

      • organic compounds + oxygen → carbon dioxide + water + energy

  • anaerobic respiration is a similar catabolic pathway that uses substances otehr than oxygen as reactants

  • cellular respiration generally refers to both but most often only refers to aerobic respiration

Redox Reactions: Oxidations + Reduction

  • redox reactions, also kown as oxidation reduction reactions, are chemical reactants where one or more electrons is transfered from one reaction to another

    • oxidation is the loss of electrons from one subtsance in a redox reaction

    • reduction is the addition of electrons to another substance in a redox reaction

    • ex: Na+ Cl → Na+ + Cl-

      • Na becomes oxydized( loses electron

      • Cl becomes reduced

    • reducing agent is the subatnce that is donating electrons and “reducing” the other substance

    • oxidizing agent is the substance that is accepting electrons by removing the other substance’s electrons

  • some redox reactions only change the degree of electron sharing in covalent bonds

    • ex: CH4 + 2O2 → CO2 + Energy + 2H2O

      • methan is oxidized because oxygen is electronegative

    • O2 is the most powerful of all oxidizing agents because it is so electronegative

  • same process is applied to organic molecules such a glucose

    • organic molecules with loots of hydrogen are good as fuels because their bonds are a soruce of hill top electrons becayse their electrons will fall down an energy gradient during transfer to oxugen

    • this is why most fuels with c-h bonds are oxidized into rpoducts with c-o bonds

Stepwise Energy Harvest via NAD

  • NAD+, or nicotinamide adenine dinucleotide, is the oxidized form of the coenzyme electron carrier NAD functioning as an electron acceptor

  • NADH is the reduced from of NAD and represents stored energy in cellular respiration that can be tapped for ATP

  • enzymes removed a pair of hydrogen atoms from the sustrate and delivery 2 electrons + proton to it’s coenzyme NAD+ forming NADH and releasing a hydorgen ion

  • Dehydrogenases are the enzymes taht remove the pair of hydrogen atoms from the substaate and give them to coenzyem

  • Electron transport chain is a system of molecules, mostly proteins, built into the inner membrane of mitocondria that shuttle electrons removed by gluvose via NADH, down a series of redox reactions, to O2

    • e\electron transfer from NADH to oxygen is an exergonic reactions that generates energy, but the chain makes it so that a luttle energy is lost in a controlled way

    • electron forms O2

Glucose → NADH → electron transport chain → Oxygen

Cellular Respiration Stages

  • Glycolysis is the first catabolic pathway in cellular respiration that occurs in the cytosol and begins with breaking glucose into two pyruvate molecules

  • Pyruvate Oxidation is the oxidization of pyruvate into acetyl coA in eukarotes

  • Citric Acid Cyle is stage in cellular respiration where the breakdown of glucose into carbon dioxide is completed

  • Oxidative Phosphorylation is the usage of energy recieved from the electron transport chain to make ATP from ADP

  • subsrate level phosphorlyation is the mode of ATP synthesis caused by a few reactions in glyolysis and the citric acic cycle

    • transfers a phosphate group from

    • a substate molecule to ADP

Glycolysis

  • the energy investment phase of glycolysis is the phase of glycolysis where the cell spends ATP

  • the energy payoff phase of glycolysis the phase of glycolysis here ATP is produced from substrate level phosphorlyation and NAD+ is reduced to NADH

  • the net energy yield from glycolysis is 2 ATP + 2 NADH

Pyruvate Oxidation

  • occurs in mitochondria in eukaryotes and cytosol in prkaryotes

  • Acetyl CoA, or Acetyl coenzyme A, is converted from pyruvate in pyruvate oxidation

    1. pyruvates carboxyl group is fully oxidized and given off as a molecules of CO2

    2. two carbon fragment is oxidized and electrons are transfered to NAD+ storing energy in NADH

    3. CoA, a sulfer containing comoound derived from a B vitamin is attached via sukfur atom to the two carbon intermediate forming acetyl coA and transfered to a moleule in the citric acid cycle

The Citric Acid Cycle

  • functions as a metabolic furnace that further oxidizes organic fuel derived from pyruvate

  • generates one ATP per turn from substrate level phosphorlyation but most of chem energy is transferred to NAD+ and FAD

  • eight steps

    1. acetycl group of acetyl CoA joins the cycle by combining with the compoun oxaloacetate gorming citrate

    2. next 7 steps decompose citrate back to oxaloacetate

      1. step 5 sometimes forms an ATP molecules

      2. step 5 often prodcues guanosine triphosphate

  • 3 NAD+ are reduced to NADH for each acetyl grou0

Oxidative Phosphorlyation

  • already produce 4 ATP molecules per glucose molecule from substrate level phosphorlyation

  • NADH accounts for most energy, however

Electron Transport

  • prosthetic groups are nonprotein componenets such as cofactors or coenzymes essecntial for the catalytic function of certain enzymes

  • electron carrues alternate between reduced and. oxidized shapes as they accept and donate electrons

  • each “downhill neighbor” has a higher affinity for electrons, starting from NADH with the lowest and oxygen with the highest

  • uses the exergonic flow of electrons to pump H+ across the membrane creating a gradien

  1. electrons aquired from glucose by NAD+ during glycolysis and the citric acid cycle are transferred from NADH to the first molecules of the electron transport chain in complex 1

    1. flavoproteins are the first molecule of the electron complex chains and have the prosthetic group flavin mononucleotide

  2. From flavoproteins are passed to an iron sulfur protein

  3. from iron sulfer protein to a compound called ubiquinone

    1. only member of transport chain that is not a protein

  4. cytochromes are the proteins that make up most of the remaining electron carries between ubiquinone and oxygen and has a heme prosthetic group with iron

  • FADH2 is the other reduced product of the citric acid cycle and adds its electrons from within complex 2

Chemiosmosis

  • ATP synthase is a protein complex/enzyme that populates the inner membrane of mitochondria and makes ATP from ADP and inorganicphosphate

    • functions like a waterwheel where protons move one way by binding sites on the rotor and catalyzing ATP production from ADP and Pi

  • chemiosmos is the process in which energy stored in the form of a hydrogen ion gradient across a membrane is used to drive cellular work such as ATP synthesis

  • proton motive force is the H+ gradient that results from transport electron carriers

Fermentation/Anaerobic Respiration

  • in anaerobic respiration, instead of oxygen at the bottom of the electron transport chain, it is instead a sulfate ion that pulls the electrons down

Fermentation

  • fermentation consist of glycolysis plus reactions that generate NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate

  • alcohol fermentation is fermentation where pyruvate is converted to ethanol in two steps

    • first step releases co2, and then the pyruvate is converted to acethaldehyde

    • acetaldehyde is then reduced by the NADH to become alcohok

  • lactic acid fermentation is fermenatation where pyruvate is reduced to form lactate

    • reduced directly by NADH

    • lactate production in humans occurs when 02 is in short supply and not referred to as fermentation

  • obligate anaerobes are organisms that carry out only fermenattion or anaerobic respiration

  • facuktative anaerobes are organisms that can make enough ATP from either fermentation or respration

Connections

  • deanimation is the process of removing amino groups from amino acids before they can be fed into glycolysis or the citric acid cyle

    • prodcues ammonia

  • beta oxidation is a metabolic sequence that breaks down fatty acids into two carbon fragments which enter the citric acid cycle as acetyl CoA

  • compounds can be formed from the intermediates in the citric acid cycle being diverted

If ATP concentration drops, cellualr respiration speeds up. If it increases, cellualr respiration slows down

  • phosphofructokinase is an enzyme in step 3 of glycolysis and controls the rate of cellular respiratopn

    • inhibited by AMP from ADP