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AP Biology Notes Outline
Chapter 9: Cellular Respiration
 Cellular Respiration is the process that releases
energy by breaking down food molecules in the
presence of oxygen
o Organisms that respire: animals, fungi,
plants (during night hours)
 Cellular respiration requires a food molecule (such as
glucose), oxygen, and it gives off carbon dioxide
 Because is occurs in the PRESENCE of OXYGEN,
cellular respiration is known as AEROBIC
respiration
 The chemical formula for cellular respiration is:
o 6O2 + C6H12O6 → 6 CO2 + 6 H2O + Energy
o oxygen + glucose → carbon dioxide + water + energy
 The reactants of cellular respiration are:
o oxygen (O2) & glucose (C6H12O6)
 The products of cellular respiration are:
o carbon dioxide (CO2) and water (H2O)
 The 3 main stages of cellular respiration are:
o Glycolysis
o Krebs Cycle (Citric Acid Cycle)
o Electron Transport
 USEFUL ANIMATION:
o http://www.sumanasinc.com/webcontent/animations/content/cellularrespiration.html
Glycolysis takes place in cytoplasm of cell. The Krebs Cycle and Electron Transport chain take place inside the mitochondria.
AP Biology Notes Outline
Chapter 9: Cellular Respiration
 Cellular respiration and fermentation are energy-yielding pathways (catabolic).
 To understand what happens when a catabolic pathway decomposes glucose, you must first understand:
o Redox Reactions: a chemical reaction involving the transfer of one or more electrons from one reactant to another.
 OXIDATION: loss of electrons from a substance
 REDUCTION: addition of electrons to a substance
 As electrons from redox reactions are transferred from one substance to another, energy is transferred as well.
o RECALL THAT ELECTRONS HAVE ENERGY!
o Oxidized substances LOSE energy
o Reduced substances GAIN energy
 The fate of glucose in the body can take one of two
catabolic pathways:
o CELLULAR RESPIRATION
 a.k.a. aerobic respiration
 absolutely requires oxygen
 occurs in mitochondria
o FERMENTATION
 occurs without oxygen
 less efficient than cell respiration (makes less ATP)
 SUBSTRATE LEVEL PHOSPHORYLATION:
 Some ATP created during cell respiration is made by direct
enzymatic transfer of a phosphate group from a substrate to ADP.
 OXIDATIVE PHOSPHORYLATION:
 ATP synthesis can be powered by the flow of H+ back across mitochondrial
membrane (chemiosmosis).
AP Biology Notes Outline
Chapter 9: Cellular Respiration
 The first set of reactions in cellular respiration is glycolysis:
 Glycolysis is the process in which 1 molecule of glucose is broken in half, producing 2
molecules of pyruvic acid.
 Glycolysis occurs in the cytoplasm of the cell.
 Glycolysis produces a total of 4 ATP, but requires 2 ATP in the beginning to get the pyruvic
acid through the membrane of the mitochondria to enter the next phase.
 Therefore, the NET ATP YIELD of glycolysis is 2 ATP!
 Glycolysis means “splitting of sugar”.
 During glycolysis, a six-carbon sugar is split into 2 three-carbon sugars.
 These smaller sugars are then oxidized (lose electrons) and their remaining atoms rearranged
to form 2 molecules of pyruvate.
 This is a 10-step process, each catalyzed by a specific enzyme…with an energy input and
energy payoff phase (Figure 9.8 page 161).
 The conversion of pyruvate to acetyl CoA is the junction between glycolysis (step 1) and the Krebs cycle
(step 2).
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If oxygen is present, Pyruvate (3 C each) from glycolysis enters the mitochondrion.
 Using Coenzyme A, each pyruvate is converted into a molecule of Acetyl CoA (2 C each).
What happened to the other carbon from each molecule of pyruvate?
 CO2 released!
NAD+ is reduced to form NADH
AP Biology Notes Outline
Chapter 9: Cellular Respiration
 In the presence of oxygen, the pyruvic acid produced during glycolysis passes to the second stage of cellular respiration: the
Krebs Cycle.
o During the Krebs cycle, pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions.
Every time you exhale, you expel the CO2 produced by the Krebs cycle.
 For each turn of the 8-step cycle, 5 pairs of high energy electrons are captured by the carrier molecules NADH and
FADH2.
 NADH and FADH2 are energy carriers! They carry electrons captured during the Krebs Cycle (AND THEIR
HYDROGENS) to the Electron Transport Chain – so we can make ATP!!!
AP Biology Notes Outline
Chapter 9: Cellular Respiration
 Following the Krebs cycle, the electrons captured by NADH and FADH 2 are passed to the electron transport chain.
NADH and FADH2 also dump their hydrogen atoms here!
o NADH and FADH2 shuttle high-energy electrons extracted from food during glycolysis and the Krebs cycle to an
electron transport chain built into the inner mitochondrial membrane.
 As each complex of the chain accepts and then donates electrons, it pumps hydrogen ions (protons) from
the mitochondrial matrix into the inner membrane space – creating a gradient of H+ across the
membrane.
o The hydrogen ions flow back, down their gradient, through a channel in an ATP synthase (a protien) – causing it
to rotate.
 Each time it rotates, the enzyme ATP synthase grabs a low energy ADP and attaches a phosphate, forming highenergy ATP
o The ATP synthase harnesses the proton-motive force to phosphorylate ADP, forming ATP.
 NOTE: this is oxidative phosphorylation because it is driven by the loss of electrons from food!
 THE USE OF AN H+ GRADIENT TO
TRANSFER ENERGY FROM REDOX
REACTIONS TO CELLULAR WORK
(ATP) is called CHEMIOSMOSIS.
The complete breakdown of 1 glucose molecule through cellular respiration results in the production of 36 molecules of ATP.
AP Biology Notes Outline
Chapter 9: Cellular Respiration
 When oxygen is NOT present, glycolysis is followed by a different pathway called fermentation.
o Fermentation releases energy from food molecules in the absence of oxygen
o Because fermentation does not require oxygen, it is said to be anaerobic
 The 2 main types of fermentation are:
o alcoholic fermentation
o lactic acid fermentation
 Fermentation – aside from the original 2 ATP’s made
during glycolysis, the only energy produced is that
which is in the bonds of…
o ethyl alcohol -- C2H6O
o lactic acid – C3H5O3
 In the absence of oxygen, a cell will use fermentation to
produce ATP by substrate-level phosphorylation.
 Lactic Acid Fermentation converts glucose into lactic acid. This type of fermentation occurs in human muscle cells during
strenuous exercise when breathing cannot supply the cells with enough oxygen.
AP Biology Notes Outline
Chapter 9: Cellular Respiration
Pyruvate is a key juncture in catabolism:
o Glycolysis is common to fermentation AND respiration.
o The end product of glycolysis is pyruvate…represents a fork in the
catabolic pathways of glucose oxidation.
o In a cell capable of both respiration and fermentation, pyruvate is
committed to one of those two pathways, usually depending on the
presence of oxygen.
During glycolysis, each glucose
molecule is broken down into
two molecules of the compound
pyruvate. The pyruvate
crosses the double membrane
of the mitochondrion to enter
the matrix.
The Krebs cycle decomposes pyruvate into Carbon Dioxide. Don’t forget conversion to Acetyl CoA FIRST!!!
Pyruvate is broken down into 3 molecules of CO2…including the molecule of CO2 released during pre-Krebs cycle conversion
of pyruvate to acetyl CoA.
AP Biology Notes Outline
Chapter 9: Cellular Respiration
NADH or FADH2 transfers electrons from molecules undergoing glycolysis and the Krebs cycle to the electron transport
chains…which are built into the inner mitochondrial membrane.
The ETC converts the chemical energy to a form used to drive oxidative phosphorylation. Cycle generates 1 ATP per turn by
substrate phosphorylation…but most of the chemical energy is transferred during the redox reactions to NAD+ and FAD.
The reduced coenzymes, NADH and FADH2, shuttle their cargo of high-energy electrons to the electron transport chain,
which uses the energy to synthesize ATP by oxidative phosphorylation.
Electron transport and
pumping of protons (H+),
which create an H+
gradient across the
membrane.
The ATP synthase protein complex functions as a mill, powered by the flow of
hydrogen ions. This complex resides in the mitochondrial and chloroplast membranes
of eukaryotes and in the plasma membranes of prokaryotes. Each of the four parts of
ATP synthase consists of a number of polypeptide subunits.
CHEMIOSMOSIS IS AN ENERGY-COUPLING MECHANISM!!!
ATP synthesis powered by
the flow of H+ back across
them membrane
(CHEMIOSMOSIS).