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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?