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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

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

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



  • 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)

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?