Cellular Respiration: Oxidation-Reduction, Glycolysis, Krebs Cycle, and Oxidative Phosphorylation

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Cellular Respiration

Oxidation-Reduction (Redox) Reactions

Oxidation-reduction reactions are fundamental to cellular respiration, involving the transfer of electrons and hydrogen atoms between molecules. These reactions help cells extract energy from nutrients.

Oxidation: The process in which a substance loses electrons and hydrogen, becoming more positively charged.
Reduction: The process in which a substance gains electrons and hydrogen, becoming more negatively charged.

Process	Electron Movement	Resulting Charge
Oxidation	Loses electrons/hydrogen	More positive
Reduction	Gains electrons/hydrogen	More negative

Reducing agent: Donates electrons to another reactant (is itself oxidized).
Oxidizing agent: Accepts electrons from another reactant (is itself reduced).

The agent does what to the other reactant?	The agent is	The agent undergoes what process?
Gives electrons	Electron donor	Oxidation
Takes electrons	Electron acceptor	Reduction

General Formula of Cellular Respiration

Cellular respiration is the process by which cells convert the chemical energy of glucose into ATP, with oxygen as the final electron acceptor.

Reactants: Glucose (C6H12O6) and oxygen (O2)
Products: Carbon dioxide (CO2), water (H2O), and ATP

Reactants	Products
Glucose, Oxygen	Carbon dioxide, Water, ATP

Glucose loses electrons (is oxidized); oxygen gains electrons (is reduced).
Carbon dioxide results from the oxidation of glucose.
Water results from the reduction of oxygen.
ATP is produced from the breaking of covalent bonds in organic molecules.

Equation:

Electron Carriers and NAD+/NADH

Electron carriers such as NAD+ and FAD play a crucial role in transferring electrons during cellular respiration.

NAD+ is the oxidized form; NADH is the reduced form.
Movement of electrons is facilitated by these carriers.
NAD+ is derived from nucleotides (DNA/RNA).

Substrate-Level Phosphorylation vs. Oxidative Phosphorylation

ATP can be produced by two main mechanisms during cellular respiration: substrate-level phosphorylation and oxidative phosphorylation.

Type	Visualization	Definition
Substrate-Level Phosphorylation		An enzyme makes ATP by transferring a phosphate group from a substrate to ADP.
Oxidative Phosphorylation		Redox reactions of the Electron Transport Chain (ETC) create a hydrogen ion gradient that powers chemiosmosis (production of ATP by ATP synthase).

Glycolysis

Overview and Steps

Glycolysis is the first step of cellular respiration, occurring in the cytoplasm. It breaks down glucose into two molecules of pyruvate, producing ATP and NADH.

Main goal: Convert chemical energy of covalent bonds in glucose into ATP.
Glucose is split into two 3-carbon molecules.

Step	Description	Visualization
1-5	Energy investment phase: ATP is consumed; glucose is phosphorylated to form G3P and DHAP.
6-10	Energy payoff phase: ATP is produced; G3P molecules are oxidized to pyruvate.

Oxygen is not required for glycolysis.
Enzymes involved include kinases, isomerases, and dehydrogenases.
Fructose 6-phosphate is the intermediate with the most potential energy.

Inputs	Outputs	Net
Glucose, ADP, NAD+	Pyruvate, ATP, NADH	Pyruvate, ATP, NADH

Pyruvate Oxidation and Acetyl CoA Formation

Conversion of Pyruvate to Acetyl CoA

Pyruvate produced from glycolysis is transported into the mitochondria, where it is converted to acetyl CoA, releasing CO2 and producing NADH.

The carboxyl group of pyruvate is released as CO2.
Coenzyme A attaches to the remaining 2-carbon fragment, forming acetyl CoA.
NAD+ is reduced to NADH.

Inputs (per glucose)	Outputs (per glucose)
Pyruvate, Coenzyme A, NAD+	Acetyl CoA, CO2, NADH

Krebs Cycle (Citric Acid Cycle)

Overview and Steps

The Krebs cycle occurs in the mitochondrial matrix and completes the oxidation of glucose derivatives, producing CO2, NADH, FADH2, and ATP.

Acetyl CoA enters the cycle; coenzyme A is released.
Each turn of the cycle oxidizes the acetyl group, releasing two molecules of CO2.
NAD+ and FAD are reduced to NADH and FADH2.
ATP is produced by substrate-level phosphorylation.

Inputs (per glucose)	Outputs (per glucose)
Acetyl CoA, NAD+, FAD, ADP	CO2, NADH, FADH2, ATP

Oxidative Phosphorylation

Electron Transport Chain (ETC) and Chemiosmosis

Oxidative phosphorylation occurs in the inner mitochondrial membrane, where electrons from NADH and FADH2 are transferred through protein complexes, creating a proton gradient that drives ATP synthesis.

NADH transfers electrons to Complex I (is oxidized).
FADH2 transfers electrons to Complex II (is oxidized).
Electrons move through the chain; the final electron acceptor is oxygen (is reduced to water).
Electron transfer causes protein complexes to pump H+ ions into the intermembrane space.
H+ diffuses back into the matrix through ATP synthase, phosphorylating ADP to ATP.

Step	Description
Electron Transfer	NADH and FADH2 donate electrons; oxygen is the final acceptor.
Proton Pumping	Protein complexes pump H+ into the intermembrane space.
ATP Synthesis	H+ flows back through ATP synthase, generating ATP.

Movement of electrons through the ETC is aided by increasing electronegativity of protein complexes.
ATP synthase uses the proton gradient to synthesize ATP from ADP and inorganic phosphate.

Equation for ATP synthesis:

Summary Table: Inputs and Outputs of Cellular Respiration

Stage	Inputs	Outputs
Glycolysis	Glucose, NAD+, ADP	Pyruvate, NADH, ATP
Pyruvate Oxidation	Pyruvate, NAD+, CoA	Acetyl CoA, NADH, CO2
Krebs Cycle	Acetyl CoA, NAD+, FAD, ADP	CO2, NADH, FADH2, ATP
Oxidative Phosphorylation	NADH, FADH2, O2, ADP	ATP, H2O, NAD+, FAD

Key Terms and Concepts

ATP synthase: Enzyme that synthesizes ATP using the proton gradient.
Electron Transport Chain (ETC): Series of protein complexes that transfer electrons and pump protons.
Substrate-level phosphorylation: Direct transfer of phosphate to ADP.
Oxidative phosphorylation: ATP production powered by redox reactions and chemiosmosis.

Example: During intense exercise, muscle cells rely on glycolysis for rapid ATP production, even in the absence of oxygen.