• living cells require enery from outside sources to perform tasks that are essential to life
  • the energy stored in organic food molecules ultimately comes from the sun (energy flows into an ecosystem as sunlight and leaves as heat)
  • chemical elements are recycled through the ecosystem
  • Photosynthesis generates oxygen and organic molecules as fuel for cellular respiration
  • respiration breaks down this fuel, generating ATP
  • the waste products from respiration (carbon dioxide and water) are the raw materials for photosynthesis
  • the three key pathways of respiration are glycolsis, the citric acid cycle, and oxidative phosphorylation
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Catabolic Pathways
  • organic compounds possess potential energy as a result of their arrangement of atoms
  • compounds that can participate in exergonic reactions can act as fuels
  • enzymes help a cell systematically degrade complex organic molecules rich in potential energy to simpler waste products that have less energy
  • one catabolic process is fermentationwhich is a partial degradation of sugars without oxygen present
  • most efficient catabolic pathway is aerobic respiration which occurs in the presence of oxygen
  • cells of most eukaryotic and many prokaryotic organisms can carry out aerobic respiration
  • some prokaryotes use a substance other than oxygen as the reactant in similar process called anaerobic respiration that harvests chemical energy without oxygen present
  • cellular respiration includes both aerobic and anaerobic processes (but refers to the aerobic process)
  • food provides the fuel for respiration -- the exhaust is carbon dioxide and water
  • formula for aerobic respiration: organic compounds + oxygen ---> carbon dioxide + water + energy
  • we will track the degradation of glucose through celular respiration, but carbs, fats, and proteins can also be used as fuel
    • C6H12O6 + 6O2 -------> 6CO2 + 6H2O + energy ( ATP + heat)

    • glucose is the fuel that cells most often use
    • breakdown of glucose is exergonic (free energy exchange of -686 kcal per mole of glucose decomposed)
  • catabolic pathways are linked to work by ATP (cell must regenerate ATP to keep working)

Redox Reactions
  • chemical reaction in which one or more electrons are transferred from one reactant to another
  • called oxidation-reduction reactions
  • loss of electrons from one substance is called oxidation
  • gain of electrons to another substance is called reduction
    • example Na + Cl ----> Na+ + Cl-

  • electron donor is the reducing agent
  • electron acceptor is the oxidizing agent
  • not all redox reactions involve the complete transfer of electrons from one substance to the other - some change the degree of electron sharing in covalent bonds
  • redox reaction of greatest interest to biologists is the oxidation of glucose and other molecules in food
    • C6H12O6 becomes oxidized ---> 6CO2
    • 6O2 becomes reduced---> 6H2O

  • organic molecules with an abundance of hydrogen are excellent fuels due to their bonds that are a source of "hilltop" electrons whose energy is released as they fall down the energy gradient when they are transferred to oxygen
  • main energy foods such as carbs and fats are reservoirs of electrons associated with hydrogen
  • the barrier of activation energy is the only thing holding back the electrons from going to a lower energy state
  • energy cannot be harnessed if it is all relesed at once
  • so glucose and organic fuels are broken down in a series of steps, each one catalyzed by an enzyme
    • in oxidation reactions, each electron travels with a proton (thus as a hydrogen atom)
    • the hyrdogen atoms are tansferred to the electron carrier NAD+
      • enzymes called dehydrogenases remove a pair of hydrogen atoms from the substrate (oxidizing it)
      • enzyme delievers 2 electrons with 1 proton to NAD+
      • the other proton is released into the surrounding solution
      • H--C--OH + NAD+ --------> C=O + NADH + H+

      • receiving 2 electrons and 1 proton neutralizes NAD+ when it is reduced to NADH
  • NAD+ is the most versatile electron acceptor
  • each NADH molecule fromed during respiration represents stored energy that can be used to make ATP when the electrons complete their fall down the energy gradient from NADH to oxygen
  • cellular respiration also brings hydrogen and oxygen together to form water
    • the hydrogen that reacts with the oxygen is derived from organic molecules
    • respiration uses an electron transport chain to break the fall of electrons to oxygen in energy releasing steps
  • electron transport chain consists ofmolecules that are mostly proteins that are built into the inner membrane of mitochondria of eukaryotic cells and the plasma membrane of prokaryotic cells
    • electrons removed from glucose are taken by NADH to the "top" end of the chain
    • at the "bottom" of the chain O2 captures the electrons along with hydrogen nuclei (H+) forming water
  • in respiration, most electrons folow the "downhill" route - glucose ----> NADH ----> eletron transport chain ---->oxygen

Stages of Cellular Respiration

The three metabolic stages that make up Cellular Respiration are:
  1. Glycolysis
  2. The Citric Acid Cycle (Krebs Cycle)
  3. Oxidative Phosphorylation: electron transport and chemiosis

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  • glycolysis and the citric acid cycle are catabolic pathways that break down glucose and other organic fuels
  • glycolysis breaks glucose down into two molecules of pyruvate
  • citric acid cycle completes the breakdown of glucose by oxidizing a derivative of pyruvate to carbon dioxide
  • some of the steps in these two processes are redox reactions that transfer electrons
  • in the third phase of cellular respiration, the electron transport chain accepts electrons and passes them from one molecule to another
  • the energy released at each step of the chain is stored so that the mitochondria can make ATP
    • this is called oxidative phosphorylation because it is powered by the redox reactions of the electron transport chain
    • in eukaryotes the electron transport chain along with chemiosmosis is found in the inner membrane of the mitochondria - these are the two steps that make up oxidative phosphorylation
    • in prokaryotes these processes are in the plasma membrane
  • a smaller amount of ATP is formed DIRECTLY in some of the reactions of glycolysis and the citric acid cycle through substrate-level phosphorylation (shown below)

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Next: Glycolysis