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Chapter 7 Cellular Pathways that Harvest Chemical Energy. Biology 101 Tri-County Technical College Pendleton, SC. Principles of Metabolic Pathways. Complex chemical transformations in cell occur in number of small steps connected in a pathway Each reaction catalyzed by specific enzyme

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Chapter 7 Cellular Pathways that Harvest Chemical Energy

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Chapter 7 cellular pathways that harvest chemical energy

Chapter 7 Cellular Pathways that Harvest Chemical Energy

Biology 101

Tri-County Technical College

Pendleton, SC

Principles of metabolic pathways

Principles of Metabolic Pathways

  • Complex chemical transformations in cell occur in number of small steps connected in a pathway

  • Each reaction catalyzed by specific enzyme

  • Pathways similar in all organisms

  • Some pathways compartmentalized with certain steps occurring inside organelle

  • Pathways regulated by activities of a few enzymes

Pass the glucose please

Pass the glucose…please?

  • Glucose MOST common fuel for living cells

  • Many other compounds serve as food but almost all converted to glucose or intermediate compounds

  • Cells obtain energy from glucose by process called oxidation

  • If glucose burned in flame, it forms carbon dioxide and water and lots of energy

Glucose cont

Glucose, cont.

  • Happens ONLY if oxygen gas is present

  • C6H12O6 + 6O2 6CO2 + 6H2O + energy

    • Energy in the form of heat and light

  • Same equations applies to metabolism of glucose in cells except is multi-step controlled series of reactions

    • Captures about 40% of energy in form of ATP

  • Complete oxidation of glucose = -686 kcal/mol

  • Some cells, unable to obtain or use oxygen, metabolize glucose incompletely and obtain LESS ATP per glucose

Metabolic pathways

Metabolic Pathways

  • Glycolysis = splitting of sugar and begins metabolism of glucose in ALL cells

  • Universal pathway

  • Produces 2 three-carbon compounds called pyruvate (pyruvic acid)

  • Small amount of ATP and NADH produced

  • NADH is electron carrier (NAD+)

  • Cellular respiration occurs when environment is aerobic (contains oxygen gas)

Pathways cont

Pathways, cont.

  • Converts pyruvate into carbon dioxide

  • Includes “preparatory Krebs,” Krebs, and ETC

  • Great deal of energy stored in pyruvate is released and trapped in ATP

  • Fermentation occurs when environment is anaerboic (lacking oxygen gas)

  • Instead of energy poor CO2 relatively energy rich molecules (lactic acid/ethanol) are produced

  • Energy harvest MUCH less

Pathways iii

Pathways III

  • True fermenter MUST exist on only 2 ATP per glucose

  • Even see a 160 lb. yeast cell?

  • Estes will make a distinction here between aerobic and anaerobic respiration

  • We will leave the fermenters alone…or will we?

Redox reactions

Redox Reactions

  • Reaction in which one substance transfers one or more electrons to another substance is called oxidation-reduction reaction

    • Redox for short

  • Gain or 1 or more electrons by atom, ion, or molecule is called reduction

  • Loss of 1 or more electrons by atom, ion, or molecule is called oxidation

Redox cont

Redox, cont.

  • Defined in terms of electrons, but can be thought of as gain/loss of hydrogen atoms

  • Reducing agents and oxidizing agents

  • You will not get this by osmosis…give it some thought

  • In redox reactions, energy is transferred

  • Some key reactions in glycolysis and cellular respiration are highly exergonic redox reactions

Redox visual

Redox Visual

Ones to remember

Ones to Remember

  • Main pair of oxidizing and reducing agents in cells is based on NAD (nicotinamide adenine dinucleotide)

  • NAD+ is oxidized form and NADH is reduced form

  • FAD is another important electron carrier

  • Redox reactions important in glycolysis, PreKrebs, Krebs, and ETC



  • Defined as nonprotein molecule that plays role in catalysis by an enzyme

  • May be part of enzyme molecule or free in solution

  • Some coenzymes are oxidizing and reducing agents


Metabolic pathways1

Metabolic Pathways

  • When O2 available as FEA, four pathways operate

  • Glycolysis, Pyruvate oxidation (preKrebs), Krebs (citric acid cycle), and respiratory chain (ETC)

  • When O2 unavailable, pyruvate oxidation, citric acid cycle, and respiratory chain do NOT function, and fermentation is added to the pathway


Pathways visual

Pathways Visual

Pathways visual ii

Pathways Visual II



  • glucose2 pyruvate + 2 ATP (net) + 2 NADH

  • Known as Universal Pathway

  • Occurs in cytoplasm of cell and does NOT require the presence of O2

  • Has energy investment stage (2 ATP) and energy payback stage

  • Chalk talk time on glycolysis; we will begin with concept of 3 kinds of phosphorylation

Oxidation of pyruvate

Oxidation of Pyruvate

  • 2 pyruvic acid produced by glycolysis are in cytoplasm of cell

  • In eukaryotes, they must be moved into the mitochondrion

  • Enzyme complex for pyruvate oxidation attached to inner mitochondrion membrane (cristae)

  • Oxidized to acetyl group and CO2 released

  • Part of energy from this step saved by reduction of NAD+ to NADH + H+

Pyruvate oxidation cont

Pyruvate Oxidation, cont.

  • Some of remaining energy stored temporarily by combining acetyl group with CoA

  • Oxidation of pyruvate to acetate is link between glycolysis and cellular respiration

  • Pyruvate cannot enter Krebs cycle

  • Chalk talk time on preparatory Krebs (oxidation of pyruvate)

Krebs cycle

Krebs Cycle

  • Krebs occurs in mitochondrial matrix

  • Starts with acetyl-CoA

  • Chalk talk time on Krebs (citric acid cycle)

  • Inputs are acetate, water, and oxidized electron carriers (NAD+ and FAD)

  • Outputs are carbon dixoide, reduced electrons carriers, and ATP (yield of 2 per substrate level phosphorylation)

Krebs visual

Krebs Visual

Respiratory chain etc

Respiratory Chain (ETC)

  • ETC located in inner mitochondrial membrane (cristae)

  • Series of electron carriers located there

  • Series of redox reactions provide energy to actively pump H+s across inner membrane

  • Critical thinking time on the three “things” that result from ETC

  • Eventually, H+s diffuse back across membrane through ATP synthase (ATPase)

Respiratory chain cont

Respiratory Chain, cont.

  • Diffusion of H+s back across membrane through ATPase coupled with synthesis of ATP (oxidative phosphorylation)

  • Several Key Questions:

  • What does redox reactions of ETC accomplish?

  • What actually synthesizes the ATP?

  • Process actually called “chemiosmosis”

Chemiosmosis and atp

Chemiosmosis and ATP

  • Each NADH (on average) traded for 3 ATP

  • Each FADH2 traded for 2 ATP

  • Chalk talk time on chemiosmosis and critical thinking on why the “trades”

  • Mitochondrion has 2 membranes (outer and inner)

  • This creates intramembranous space essential for establishment of both [ ] for H+s and for increasing membane potential across inner

  • Called Proton Motive Force

Chemiosmosis cont

Chemiosmosis, cont.

  • Inner membrane highly folded to > surface area and provide more room for ETC

  • Each contains three large protein complexes with carrier molecules and associated enzymes

  • Have small protein called cytochrome c and nonprotein component called ubiquinone (Q)

  • Why are preparatory Krebs and Krebs located within the matrix

Jethroe and ciphering

Jethroe and Ciphering

  • Glycolysis: 2 ATP (net) and 2 NADH

  • Preparatory Krebs: 2 NADH

  • Krebs Cycle: 2 ATP, 6 NADH, and 2 FADH2

  • Totals (net): 4 ATP, 10 NADH, and 2 FADH2

  • Swapping ECs yields 32-34 ATP so total is 36-38….why?

To ferment or not

To ferment…or not!!

  • If O2 supply to aerobic respiring cell is cut off and cell has no other metabolic pathway available to itauf Wiedersehen

  • Many cells can switch to fermentation

  • With fermentation, ATP comes solely from glycolysis (2 ATP net per glucose)

  • Review: glycolysis produces 2 pyruvate, 2 ATP (net) and 2 NADH

Fermentation cont

Fermentation, cont.

  • NAD+ is required for glycolysis and is reduced to NADH

  • Fermenting cell has to regenerate NAD+ or glycolysis will SHUT down

  • Catch 22 to be sure—what to do with all that pyruvate that is essentially “worthless”

  • What a plan—use the pyruvate to regenerate NAD+

Fermentation iii

Fermentation III

  • Using pyruvate as oxidizing agent for NADH results in changing it into something else

  • Some organisms choose to ferment even in presence of oxygen

  • Some do anaerobic respiration by using inorganic molecule other than oxygen as FEA

    • nitrate to nitrite; sulfate to hydrogen sulfide; carbonate to methane

Why fermentation

Why Fermentation?

  • Allows glycolysis to produce small but sustained amount of ATP

  • Glycolysis rate >s 10X to produce needed ATP

  • Allows some cells lacking oxygen to perform needed functions

  • “Aerobic” cells cannot ferment indefinitely

Products of fermentation

Products of Fermentation

  • Lactic acid fermentation reduces pyruvate to lactate (lactic acid)

  • Human muscle cells can ferment but NOT neurons or cardiac muscle cells

  • Is what makes one’s muscles sore

  • Eventually, lactate converted back to pyruvate and “burned” in process of paying “oxygen debt”

Products cont

Products, cont.

  • Certain yeasts and plant cells perform alcoholic fermentation

  • Requires two enzymesone removes a carbon dioxide from pyruvate leaving acetaldehyde

  • Acetaldehyde reduced by NADH producing NAD+ and ethyl alcohol

  • Where would be be without beer and bread?

  • Note that some microorganism produce wide variety of acids and end products other than two above

In for a penny in for a pound

In for a penny…in for a pound

  • Aerobic respiration can produce 36-38 ATP per glucose

  • Anaerobic respiration can produce an equivalent amount

  • Fermentation produces only 2 ATP (net) per glucose

  • How does one “burn” lipids, proteins, and nucleic acids?

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