Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is a series of metabolic processes that convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. It involves the oxidation of organic molecules, primarily glucose, to produce ATP, which cells use for energy-requiring activities.

• Purpose: To extract energy from glucose and other organic molecules to produce ATP.

• Organisms: Occurs in nearly all eukaryotic and many prokaryotic cells.

• Summary Reaction:

• Final Electron Acceptor: Oxygen (O2) in aerobic respiration.

Redox Reactions in Catabolic Pathways

Redox (reduction-oxidation) reactions are central to cellular respiration, transferring energy by moving electrons from one molecule to another.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance.

• Electron Carriers: NAD+ and FAD accept electrons (are reduced) to become NADH and FADH2, transporting high-energy electrons to the electron transport chain.

• Energy Transfer: As glucose is oxidized, electrons are transferred to NAD+/FAD, then to the electron transport chain, ultimately reducing O2 to H2O.

Stages of Cellular Respiration

Cellular respiration consists of four main stages, each occurring in specific cellular locations:

1. Glycolysis (cytosol)

2. Pyruvate Oxidation (mitochondrial matrix)

3. Citric Acid Cycle (Krebs Cycle) (mitochondrial matrix)

4. Oxidative Phosphorylation (inner mitochondrial membrane)

Glycolysis

Glycolysis is the first step in cellular respiration, breaking down glucose into two molecules of pyruvate.

• Location: Cytosol

• Oxygen Required? No (anaerobic process)

• Phases:

  • Energy Investment Phase: 2 ATP are used to phosphorylate glucose intermediates.

  • Energy Payoff Phase: 4 ATP and 2 NADH are produced.

• Substrate: Glucose (C6H12O6)

• Products (per glucose): 2 pyruvate, 2 ATP (net), 2 NADH, 2 H2O

• ATP Production: Substrate-level phosphorylation

• CO2 Produced? No

Pyruvate Oxidation

Pyruvate produced in glycolysis is transported into the mitochondria and converted to acetyl CoA.

• Location: Mitochondrial matrix

• Oxygen Required? Indirectly (process does not occur without O2 present for the electron transport chain)

• Substrates: 2 pyruvate (per glucose)

• Products: 2 acetyl CoA, 2 CO2, 2 NADH

• ATP Production: None

• Oxidized: Pyruvate

• Reduced: NAD+ to NADH

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the oxidation of organic molecules, generating electron carriers and some ATP.

• Location: Mitochondrial matrix

• Oxygen Required? Indirectly (cycle halts without O2 for electron transport chain)

• Substrates: 2 acetyl CoA (per glucose)

• Products (per glucose): 4 CO2, 6 NADH, 2 FADH2, 2 ATP

• ATP Production: Substrate-level phosphorylation

• CO2 Produced? Yes

• Oxidized: Acetyl group of acetyl CoA

• Reduced: NAD+ and FAD

Oxidative Phosphorylation

Oxidative phosphorylation includes the electron transport chain and chemiosmosis, producing the majority of ATP in cellular respiration.

• Location: Inner mitochondrial membrane

• Oxygen Required? Yes (O2 is the final electron acceptor)

• Substrates: NADH, FADH2, O2

• Products: ATP, H2O, NAD+, FAD

• ATP Production: Oxidative phosphorylation (via chemiosmosis)

• ATP Yield: ~26-28 ATP per glucose

Electron Transport Chain (ETC)

• Function: Transfers electrons from NADH and FADH2 to O2 through a series of protein complexes, releasing energy.

• Energy Flow: Electrons move from high to low energy, pumping protons (H+) across the membrane, creating a proton gradient.

• Final Electron Acceptor: O2, forming H2O

Chemiosmosis

• Definition: The process by which energy stored in a proton gradient is used to drive ATP synthesis via ATP synthase.

• Energy Source: Proton-motive force (H+ gradient)

• Product: ATP

ATP Yield and Phosphorylation Methods

ATP is produced by two main mechanisms during cellular respiration:

• Substrate-level Phosphorylation: Direct transfer of a phosphate group to ADP from a substrate (occurs in glycolysis and citric acid cycle).

• Oxidative Phosphorylation: ATP synthesis powered by the electron transport chain and chemiosmosis.

Total ATP Yield: Approximately 30-32 ATP per glucose molecule (varies by cell type and conditions).

Fermentation

Fermentation is an anaerobic process that allows glycolysis to continue in the absence of oxygen by regenerating NAD+.

• Purpose: To regenerate NAD+ so glycolysis can continue producing ATP when O2 is unavailable.

• Occurs When: Oxygen is absent or limited.

• First Step: Glycolysis always occurs first.

• Types:

  • Alcohol Fermentation: Pyruvate is converted to ethanol; final electron acceptor is acetaldehyde (in yeast).

  • Lactic Acid Fermentation: Pyruvate is reduced to lactate; final electron acceptor is pyruvate (in animals, e.g., muscle cells).

• ATP Yield: 2 ATP per glucose (from glycolysis only)

Regulation of Cellular Respiration

Cellular respiration is tightly regulated to meet the cell's energy needs. The enzyme phosphofructokinase (PFK) is a key regulatory point in glycolysis.

• Phosphofructokinase (PFK): Catalyzes an early step in glycolysis; acts as a metabolic "valve" for glucose breakdown.

• Regulation: Allosterically inhibited by ATP and citrate (negative feedback); activated by AMP (positive feedback).

• Importance: Essential for life; without PFK, glycolysis and thus ATP production would halt.

Efficiency of Cellular Respiration

• Energy Efficiency: Cellular respiration captures about 34% of the energy in glucose as ATP; the rest is lost as heat.

• ATP Yield: 30-32 ATP per glucose (theoretical maximum).

Comparison: Cellular Respiration vs. Fermentation

• ATP Yield: Respiration (30-32 ATP/glucose) vs. Fermentation (2 ATP/glucose)

• Oxygen Requirement: Respiration requires O2; fermentation does not.

• End Products: Respiration produces CO2 and H2O; fermentation produces ethanol or lactate.

• Electron Acceptors: Respiration uses O2; fermentation uses organic molecules (acetaldehyde or pyruvate).

Example: During intense exercise, muscle cells switch from aerobic respiration to lactic acid fermentation when oxygen is scarce, allowing ATP production to continue temporarily.

Additional info: The actual ATP yield per glucose can vary due to differences in shuttle mechanisms for NADH, proton leak, and other cellular conditions.