how do cells make ATP

Chapter 8: How Cells Make ATP: Energy-Releasing Pathways

Food For Energy

• Every organism must extract energy from food molecules.

  • Organisms that manufacture their own food through photosynthesis are called autotrophs.

  • Organisms that obtain energy from the environment are called heterotrophs.

Metabolism

• Metabolism consists of two complementary components:

  • Catabolism: pathways that release energy by breaking down complex molecules into smaller components.

  • Anabolism: pathways that synthesize complex molecules from simpler building blocks.

• Endergonic reactions require ATP or another energy source to proceed.

Catabolic Pathways

• Most steps in catabolic pathways are energy-releasing (exergonic) and capture free energy for the cell.

  • Aerobic cellular respiration: process that requires oxygen.

  • Anaerobic cellular respiration: process that does not require oxygen.

  • Fermentation: also does not require oxygen and is less efficient than respiration.

Aerobic Respiration

• The general reaction for aerobic respiration is:
  $C<em>6H</em>{12}O<em>6 + 6 O</em>2 + 6 H<em>2O ightarrow 6 CO</em>2 + 12 H_2O + ext{energy}$

• Under aerobic conditions, nutrients are catabolized to carbon dioxide (CO2) and water (H2O).

• This process is a redox reaction:

  • Glucose is oxidized, while oxygen is reduced.

  • Electrons are transferred to oxygen in a series of steps.

Changes in Free Energy

• The equation detailing changes in free energy involved in metabolism is presented as:
  $E = e₁ + e₂ + e₃ + e₄ + e₅$

The Four Stages Of Aerobic Respiration

1. Glycolysis

   • Breakdown of glucose.

2. Formation of Acetyl-CoA

   • Conversion of pyruvate into acetyl-CoA.

3. Citric Acid Cycle (also known as the Krebs Cycle)

   • Process that further breaks down compounds.

4. Electron Transport and Chemiosmosis

   • Final stage where ATP is produced.

Reactions Involved in Aerobic Respiration

• Dehydrogenations: removal of two hydrogen atoms from a substrate, transferring them to NAD+ or FAD.

• Decarboxylations: removal of part of a carboxyl group (COOH) from the substrate, releasing CO2.

• Preparation reactions: rearrangement of molecules for further reactions like dehydrogenations or decarboxylations.

Glycolysis

• Glycolysis occurs in three phases:

  • Takes place in the cytosol

  • Net energy yield: results in 2 ATP and 2 NADH molecules.

  • Two major phases:

  • Energy Investment Phase: requires investments of ATP.

  • Energy Capture Phase: yields ATP and NADH.

Glycolysis Process (1 of 3)

• Phosphorylation Reactions: ATP transfers phosphate groups to glucose, forming phosphorylated sugar.

  • Enzymatically, the 6-carbon sugar (glucose) is split into two 3-carbon molecules (glyceraldehyde-3-phosphate, or G3P):
    $ext{glucose} + 2 ext{ ATP} <br>ightarrow 2 ext{ G3P} + 2 ext{ ADP}$

Glycolysis Process (2 of 3)

• Energy Capture Phase:

  • G3P converts to pyruvate through oxidation (electrons transfer to NAD+).

  • The reaction is represented as:
    $2 ext{ G3P} + 2 ext{ NAD}^+ + 4 ext{ ADP} <br>ightarrow 2 ext{ pyruvate} + 2 ext{ NADH} + 4 ext{ ATP}$

Formation of Acetyl-CoA

• Each pyruvate undergoes oxidative decarboxylation:

  • A carboxyl group is removed as CO2.

  • This process occurs in the mitochondria for eukaryotes.

  • The formation yields acetyl coenzyme A (acetyl CoA):
    $2 ext{ pyruvate} + 2 ext{ NAD}^+ + 2 ext{ CoA} <br>ightarrow 2 ext{ acetyl CoA} + 2 ext{ NADH} + 2 ext{ CO}_2$

Citric Acid Cycle

• Also known as the tricarboxylic acid (TCA) cycle or Szent-Györgyi–Krebs cycle.

• Takes place in the matrix of the mitochondria of eukaryotes.

• The cycle begins with acetyl CoA donating its two-carbon acetyl group to the four-carbon acceptor compound, oxaloacetate, forming citrate (a six-carbon compound):
  $ext{oxaloacetate} + ext{acetyl CoA} <br>ightarrow ext{citrate} + ext{CoA}$

Citric Acid Cycle Process (2 of 3)

• Citrate undergoes transformations, losing carboxyl groups as CO2.

• One ATP is synthesized per acetyl group through substrate-level phosphorylation; most oxidative energy is transferred to NAD+, yielding 3 NADH and FADH2.

Electron Transport and Chemiosmosis

• All electrons removed during the earlier stages (glycolysis, acetyl CoA formation, and the citric acid cycle) are transferred to NADH and FADH2.

• NADH and FADH2 enter the electron transport chain (ETC), where electrons are transferred from one acceptor to another, driving ATP synthesis through oxidative phosphorylation.

Electron Transport Chain (1 of 3)

• Also called the respiratory chain.

• The ETC is a series of electron carriers embedded in the inner mitochondrial membrane of eukaryotes.

• Electrons pass through the chain via redox reactions and undergo energy loss at each step.

• Key components of the ETC include:

  • Flavin mononucleotide

  • Ubiquinone

  • Iron-sulfur proteins

  • Cytochromes

Electron Transport Chain Structure (2 of 3)

• The ETC contains four large protein complexes:

  • Complex I: accepts electrons from NADH.

  • Complex II: accepts electrons from FADH2.

  • Complex III: accepts electrons from reduced coenzyme Q (ubiquinone) and transfers them to cytochrome c.

  • Complex IV: reduces O2 to form H2O from electrons passed from cytochrome c.

Electron Transport Chain Function (3 of 3)

• Oxygen acts as the final electron acceptor in the ETC.

  • Absence of oxygen halts the entire ETC and ceases ATP production.

  • Some poisons, such as cyanide, inhibit cytochrome activity by binding to iron, blocking ATP production.

Electron Transport And Heat

• In some cells, mitochondria can uncouple the ETC from ATP production to generate heat instead of ATP.

Chemiosmosis

• This process connects electron transport with ATP synthesis through the formation of a proton gradient across the inner mitochondrial membrane.

• Protons diffuse from the intermembrane space into the matrix via complex V (ATP synthase).

  • ATP synthase catalyzes the phosphorylation of ADP to form ATP.

Key Processes in Chemiosmosis (2 of 2)

• By utilizing exergonic redox reactions, chemiosmosis facilitates ATP production via oxidative phosphorylation.

Energy Yield of Substrate-Level Phosphorylation

1. Glycolysis:

   • $ext{glucose} + 2 ext{ ATP} <br>ightarrow 2 ext{ pyruvates} + 2 ext{ NADH} + 4 ext{ ATPs}$

2. Pyruvate Conversion:

   • $2 ext{ pyruvates} <br>ightarrow 2 ext{ acetyl CoA} + 2 ext{ CO}_2 + 2 ext{ NADH}$

3. Citric Acid Cycle:

   • $2 ext{ acetyl CoA} <br>ightarrow 4 ext{ CO}<em>2 + 6 ext{ NADH} + 2 ext{ FADH}</em>2 + 2 ext{ ATPs}$

4. Total Yield:

   • Total ATP: 4 ATP + 10 NADH + 2 FADH2

Energy Yield of Oxidative Phosphorylation

• The oxidation of NADH in the ETC yields up to 3 ATPs per molecule.

• The oxidation of FADH2 yields 2 ATPs per molecule.

• The maximum number of ATPs from NADH varies from 28 to 30 due to energy expenditure for shuttling across the mitochondrial membrane.

Net Energy Yield by Aerobic Respiration Per Glucose

• Substrate-Level Phosphorylation:

  • From glycolysis: 2 ATP

  • From pyruvate acetyl CoA conversion: 2 NADH (4-6 ATP)

  • From citric acid cycle: 6 NADH (18 ATP) + 2 ATP + 2 FADH2 (4 ATP)

  • Total ATP from substrate-level phosphorylation: 32-34 ATP

Catabolism Of Nutrients Other Than Glucose

• Other nutrients are transformed into metabolic intermediates that enter glycolysis or the citric acid cycle.

  • Amino Acids: Amino group (NH2) is removed; carbon chain is utilized in aerobic respiration.

  • Lipids:

  • Glycerol is converted into a compound that enters glycolysis.

  • Fatty acids are converted via $\beta$-oxidation to acetyl CoA entering the citric acid cycle.

Anaerobic Respiration and Fermentation

• Anaerobic respiration does not utilize oxygen as the final electron acceptor.

• It is predominantly used by prokaryotes in environments like waterlogged soil and stagnant ponds.

• Electrons from glucose pass from NADH down an ETC coupled to ATP synthesis via chemiosmosis, with an inorganic substance like nitrate or sulfate as the final acceptor.

Products of Anaerobic Respiration

• The end products include:

  • CO2, one or more reduced inorganic substances, and ATP.

  • Example:
    $C<em>6H</em>{12}O<em>6 + 12 KNO</em>3 <br>ightarrow 6 CO<em>2 + 6 H</em>2O + 12 KNO_2 + ext{energy}$

Fermentation

• Fermentation is an anaerobic pathway that lacks an ETC.

• Generates only 2 ATPs per glucose via substrate-level phosphorylation during glycolysis.

• NADH donates H atoms to organic molecules, regenerating NAD+ for glycolysis.

Types of Fermentation (1 of 2)

• Alcohol Fermentation:

  • Carried out by yeasts, which can switch to alcohol fermentation when deprived of oxygen.

  • Enzymes decarboxylate pyruvate, resulting in acetaldehyde, which is reduced to ethyl alcohol by NADH.

Types of Fermentation (2 of 2)

• Lactate Fermentation:

  • Certain fungi and bacteria perform this type.

  • NADH reduces pyruvate to lactate.

  • Vertebrate muscle cells generate lactate under oxygen depletion during strenuous exercise.

Comparison of Aerobic and Anaerobic Respiration, and Fermentation

1. Aerobic Respiration:

   • Transfers electrons to the ETC.

   • Terminal electron acceptor is O2, producing water.

   • ATP synthesized via oxidative phosphorylation, chemiosmosis, substrate-level phosphorylation.

2. Anaerobic Respiration:

   • Transfers electrons to the ETC, with inorganic substances as electron acceptors.

   • Produces reduced inorganic substances.

   • ATP synthesized by oxidative phosphorylation and substrate-level phosphorylation.

3. Fermentation:

   • Transfers electrons to organic molecules without the ETC.

   • Produces alcohol or lactate.

   • ATP generated only by substrate-level phosphorylation during glycolysis.

References

• Solomon, E. P., Martin, C. E., Martin, D. W., Berg, L. R. (Eleventh Edition). Biology. ISBN: 978-1-337-39293-8.

• Cengage Learning, Inc. https://www.cengage.com/ (unless otherwise stated).