BIO 121 Chapter 9

studied byStudied by 36 people
get a hint

Metabolic pathways

1 / 50

Tags and Description


51 Terms


Metabolic pathways

  • harvest energy from high-energy molecules such as glucose

  • cellular respiration is critical and often interacts with other pathways

  • comprises thousands of different chemical reactions that may be organized and regulated

New cards

Cellular respiration

  • occurs through a long series of carefully controlled redox reactions (conserves energy/prevents mini explosions) that use the electrons of high-energy molecules to make ATP

  • oxygen atoms are reduced to form water

  • glucose + 6 oxygen gas + ADP + inorganic Phosphate —> 6 carbon dioxide + 6 water + ATP

  • consists of 4 processes: glycolysis, pyruvate processing, Krebs cycle, and electron transport and oxidative phosphorylation

New cards

Glycolysis overview

  • a series of 10 reactions that occurs in the cytosol of eukaryotes and prokaryotes

  • net yield of 2 NADH, 2 ATP, 2 H2O, and 2 pyruvate for every glucose

  • glucose + 2 ATP —> 2 (NADH + H+) + 4 ATP + 2 Pyruvate + 2 H2O

New cards

Phosphofructokinase (PFK)

  • uses ATP to phosphorylate the end of fructose-6-phosphate to form fructose-1,6-bisphosphate

  • increases potential energy

New cards

Pyruvate processing

  • occurs in the matrix of the mitochondria or the cytosol of prokaryotes

  • for eukaryotes, pyruvate is transported from the cytosol to the mitochondrial matrix

  • catalyzed by pyruvate dehydrogenase, an enormous enzyme complex which is regulated by a negative feedback loop involving ATP

  • 2 pyruvate + 2 NAD+ + 2 Coenzyme A —> 2 acetyl CoA + 2 CO2 + 2 NADH

  • Decarboxylation: the carboxyl group on pyruvate (the 3rd carbon) is released as CO2

New cards


  • enzyme that catalyzes the transfer of a phosphate group from ATP to another molecule

New cards


  • enzyme that removes phosphate group

New cards

Krebs cycle (citric acid cycle)

  • also occurs in the matrix of the mitochondria or the cytosol of prokaryotes

  • 2 turns of the citric acid cycle for each glucose molecule

  • Potential energy is released to reduce coenzymes

  • The acetyl group (2C) from acetyl CoA is transferred to oxaloacetate (4C) to form citrate (6C); oxaloacetate is regenerated at the end (cycle)

  • 8 reactions

  • 2 acetyl coA —> 6 NADH + 6H+ + 2 FADH2 + 2 ATP + 4CO2

New cards

Electron transport and oxidative phosphorylation

  • electron transport chain consisting of 4 main protein complexes establishes a proton gradient that is used to produce ATP

  • uses NADH and FADH2 produced in previous steps to generate the protein gradient, which contributes to the phosphorylation of ADP

  • uses O2 (oxygen gas) and produces ATP and water

  • occurs across the inner membrane of the mitochondria or the plasma membrane + the periplasm of prokaryotes

  • a small amount of energy is released in each reaction; each successive bond/molecule in the ETC holds less potential energy ; after the ETC, most of the chemical energy from glucose is accounted for by a proton electrochemical gradient

  • primary goal: make ATP

  • secondary goal: regenerate NAD+

New cards


  • space between the cell wall and the plasma membrane

New cards

2 fundamental requirements of cellular life

  • energy to generate ATP

  • a source of carbon to use as raw materials for synthesizing macromolecules

New cards

Catabolic pathways

  • involve the breakdown of molecules

  • often harvest stored chemical energy to produce ATP

New cards

Anabolic pathways

  • result in the synthesis of larger molecules from smaller components

  • often use energy in the form of ATP

New cards


  • maintenance of a stable internal environment under different environmental conditions

New cards

Energy investment phase (glycolysis)

  • reactions 1 through 5

  • uses 2 ATP molecules

  • regulation of the metabolic pathway occurs during this phase (reaction 3, regulation of phosphofructokinase)

New cards

Energy payoff phase (glycolysis)

  • reactions 6 through 10

  • NADH is made and ATP is produced by substrate-level phosphorylation

New cards

Substrate-level phosphorylation

  • 1 way to make ATP

  • the ONLY way to produce ATP through glycolysis

  • enzyme facilitates the transfer of a phosphate group from a substrate to ADP

New cards

Glycolysis regulation

  • regulated by feedback inhibition

  • high levels of ATP inhibit the third enzyme/step of glycolysis (phosphofructokinase), which have two binding sites for ATP

  • when ATP binds to the regulatory site of phosphofructokinase, the reaction rate slows dramatically

New cards

Regulation of pyruvate processing

  • when products of glycolysis and pyruvate processing are abundant, pyruvate dehydrogenase is phosphorylated, inducing a conformational change in the enzyme and inhibiting its activity

New cards

Citric acid cycle regulation

  • can be turned off at multiple points via several different mechanisms of feedback inhibitions

  • regulated at steps 1, 3, and 4 by ATP and NADH

  • reaction rates are high when ATP and NADH are scarce; rates are low when ATP or NAHD are abundant

New cards

Oxidation of NADH and FADH2

  • oxidized by membrane complexes

  • NADH is oxidized when combined with the inner membrane of the mitochondria; in prokaryotes, it is oxidized by the plasma membrane

  • molecules in the inner mitochondrial membrane can cycle between oxidized and reduced states

New cards

ETC Protein complexes

  • most are composed of easily-oxidized proteins

  • some accept only electrons, while others accept electrons plus protons; each complex has differing redox potentials

New cards

Ubiquinone (coenzyme Q, or simply Q)

  • lipid-soluble, non-protein

  • critical component of the ETC

  • reduced by complexes I and II; moves throughout the hydrophobic interior of the electron transport chain membrane, where it is oxidized by complex III

New cards

Redox potential

  • ability to accept electrons

  • High positive value = more potential to GAIN electrons

  • strong negative value = more potential to LOSE electrons

New cards

Complex I (ETC)

  • NADH dehydrogenase oxidizes NADH

  • transfers 2 electrons through proteins containing FMN prosthetic groups and Fe-S cofactors to reduce an oxidized form of Q

  • 4 protons pumped out of the matrix to the intermembrane space per pair of electrons

New cards

Complex II (ETC)

  • Succinate dehydrogenase oxidizes FADH2

  • transfers the two electrons through proteins containing Fe-S cofactors to reduce an oxidized form of Q

  • this complex is also used in step 6 of the Krebs cycle

  • does not produce sufficient energy to pump protons

New cards

Complex III

  • cytochrome c reductase oxidizes Q

  • transfers 1 electron at a time through proteins containing heme prosthetic groups and Fe-S cofactors to reduce an oxidized form of cytochrome c

  • 4 protons for each pair of electrons is transported from the matrix to the intermembrane space

New cards

Cyt c (cyctochrome c)

  • reduced by accepting a single electron from complex III

  • moves along the surface of the ETC membrane, where it is oxidized by complex IV

New cards

Complex IV

  • cytochrome c oxidase oxidizes cyt c

  • transfers each electron through proteins containing heme prosthetic groups to reduce oxygen gas, which picks up two protons from the matrix to produce water

  • 2 additional protons are pumped out of the matrix of the intermembrane space

New cards

ATP Synthesis (ETC)

  • fueled by chemiosmosis; uses the established proton gradient to create ATP using ATP synthase

New cards

ATP synthase

  • located in the inner mitochondrial membrane in eukaryotes, or the plasma membrane in prokaryotes

  • creates energy from the proton motive force of the proton gradient to chemical bond energy in ATP

  • is a rotary machine that makes ATP as it spins

  • consists of 2 components—an ATPase "knob"/F1 unit, and a membrane-bound, proton-transporting base/F0 unit, which is a rotor that turns as protons flow through it—that are connected by a shaft and held in place by a stator

  • the spinning F0 unit changes the conformation of the F1 unit so that it phosphorylates ADP to form ATP

New cards

Oxidative phosphorylation

  • oxidative = FADH2 and NADH are being oxidized

  • phosphorylation = ADP —> ATP

  • different from substrate-level phosphorylation because instead of potential energy activating the enzyme, kinetic energy activates the enzyme (movement of protons down their gradient)

  • yields ~24-28 ATP per glucose

New cards

Chemiosmotic hypothesis

  • the linkage between electron transport and ATP production by ATP synthase is indirect

  • the synthesis of ATP only requires a proton gradient

New cards

Aerobic respiration

  • O2, which has a very high redox potential, is the final electron acceptor

  • most efficient—CO2 (single-carbon compound) is the byproduct

New cards

Anaerobic respiration

  • some other compound is the final electron acceptor

  • has a lower energy yield compared to aerobic respiration because oxygen is super electronegative and has a high redox potential

  • less efficient—some other carbon-containing (organic) molecule is the byproduct (ethanol, lactic acid, etc)

  • seen in some prokaryotes

New cards


  • a metabolic pathway that regenerates NAD+ from NADH

  • the electron in NADH is transferred to pyruvate

  • serves as an emergency backup for aerobic respiration when there is not enough oxygen

  • incomplete oxidation of glucose; much less efficient than cellular respiration

  • produces 2 ATP per glucose, compared with about 29 ATP per glucose in cellular respiration

New cards

Lactic acid fermentation

  • fermentation in which the product is lactic acid

  • occurs in humans in the absence of oxygen

  • muscle cramps = the accumulation of lactic acid

  • in humans, lactic acid fermentation results in the production of yogurt, cheese, etc

  • produces only 2 ATP (by substrate-level phosphorylation)

New cards

Ethanol fermentation

  • some yeast cells can perform alcohol fermentation

  • pyruvate is converted to acetaldehyde and CO2

  • acetaldehyde accepts electrons from NADH

  • ethanol and NAD+ are produced

New cards

Faculative anaerobes

  • organisms that can switch between fermentation and aerobic respiration

  • only use fermentation if an electron acceptor is not available

  • E.coli, yeast, etc

New cards

Glycolysis step 1

  • hexokinase uses ATP to phosphorylate glucose, increasing its potential energy

  • forms glucose-6-phosphate and ADP

New cards

Glycolysis Step 2

  • phosphoglucose isomerase converts glucose-6-phosphate to fructose-6-phosphate (an isomer)

New cards

Glycolysis Step 3

  • Phosphofructokinase uses ATP to phosphorylate the opposite end of fructose-6-phosphate, increasing its potential energy

  • forms fructose-1,6-bisphosphate

New cards

Glycolysis Step 4

  • fructose-bis-phosphate aldolase cleaves fructose-1,6-bisphosphate into 2 different 3-carbon sugars (DAP and G3P)

New cards

Glycolysis Step 5

  • triose phosphate isomerase converts dihydroxyacetone phosphate (DAP) to glyceraldehyde-3-phosphate (G3P)

  • reaction is fully reversible, but DAP-to-G3P reaction is favored because G3P can be immediately used as a substrate for step 6

New cards

Glycolysis Step 6

  • glyceraldehyde-3-phosphate (G3P) dehydrogenase catalyzes a 2-step reaction

  • first oxidizes G3P using the NAD+ coenzyme to produce NADH

  • Energy from this reaction is used to attach an inorganic phosphate to the oxidized product to form 1,3-bisphosphoglycerate

New cards

Glycolysis Step 7

  • phosphoglycerate kinase transfers a phosphate from 1,3-bisphosphoglycerate to ADP to make 3-phosphoglycerate and ATP (PRODUCES ATP—1 for each 3-carbon intermediate)

New cards

Glycolysis Step 8

  • phosphoglycerate mutase rearranges the phosphate in 3-phosphoglycerate to form 2-phosphoglycerate

New cards

Glycolysis Step 9

  • enolase removes a water molecule from 2-phosphoglycerate to form a C=C double bond and produce phosphoenolpyruvate

New cards

Glycolysis Step 10

  • remaining phosphate groups are added to 2 ADP molecules to form 2 ATP and pyruvate

  • pyruvate kinase transfers a phosphate fro phosphoenolpyruvate to ADP to make pyruvate and ATP

New cards

Electron Transport Chain Theoretical Yield

  • 1 NADH = 3 ATP

  • 1 FADH2 = 2 ATP (lower because complex II, where FADH2 is oxidized, has a lower redox potential than complex I, where NADH is oxidized)

New cards

ETC Actual Yield

  • 1 NADH = ~2.25 ATP

  • 1 FADH2 = ~1.25 ATP

New cards

Explore top notes

note Note
studied byStudied by 17 people
Updated ... ago
4.0 Stars(2)
note Note
studied byStudied by 6 people
Updated ... ago
5.0 Stars(1)
note Note
studied byStudied by 7 people
Updated ... ago
4.0 Stars(1)
note Note
studied byStudied by 17 people
Updated ... ago
5.0 Stars(1)
note Note
studied byStudied by 6 people
Updated ... ago
5.0 Stars(1)
note Note
studied byStudied by 102 people
Updated ... ago
5.0 Stars(1)
note Note
studied byStudied by 23 people
Updated ... ago
5.0 Stars(2)
note Note
studied byStudied by 12551 people
Updated ... ago
4.8 Stars(51)

Explore top flashcards

flashcards Flashcard66 terms
studied byStudied by 91 people
Updated ... ago
5.0 Stars(3)
flashcards Flashcard46 terms
studied byStudied by 23 people
Updated ... ago
5.0 Stars(1)
flashcards Flashcard100 terms
studied byStudied by 4 people
Updated ... ago
5.0 Stars(2)
flashcards Flashcard37 terms
studied byStudied by 2 people
Updated ... ago
5.0 Stars(1)
flashcards Flashcard31 terms
studied byStudied by 6 people
Updated ... ago
5.0 Stars(2)
flashcards Flashcard61 terms
studied byStudied by 2 people
Updated ... ago
4.0 Stars(1)
flashcards Flashcard42 terms
studied byStudied by 3 people
Updated ... ago
5.0 Stars(1)
flashcards Flashcard464 terms
studied byStudied by 33766 people
Updated ... ago
4.1 Stars(393)