Cellular Respiration: Mechanisms and Key Concepts

Cellular Respiration

Introduction to Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. This process is essential for the survival of most organisms, as ATP serves as the primary energy currency of the cell.

• Purpose: To transform organic molecules, such as glucose, into usable cellular energy (ATP).

• Efficiency: Approximately 34% of the energy from glucose is captured as ATP; the remainder is released as heat.

Balanced Equation for Cellular Respiration

The overall chemical reaction for aerobic cellular respiration is:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Chemical Energy (ATP)} + \text{Heat}$

• Reactants: Glucose and oxygen

• Products: Carbon dioxide, water, ATP, and heat

Key Concepts and Terms

Cofactors and Electron Carriers

Cofactors are non-protein chemical compounds that assist enzymes in catalyzing reactions, often by transferring electrons, ions, or functional groups.

• NAD+ (Nicotinamide adenine dinucleotide): Accepts electrons and a hydrogen ion to become NADH.

• FAD (Flavin adenine dinucleotide): Accepts electrons and hydrogen ions to become FADH2.

• NADH, NADPH, and FADH2: Function as electron shuttles, transporting high-energy electrons to the electron transport chain.

How NAD+ Functions as an Electron Carrier

• NAD+ receives one hydrogen atom (1 proton + 1 electron) and one additional electron from an organic molecule.

• This reduction forms NADH, which is electrically neutral.

• NADH carries electrons to molecules with a higher electron affinity, such as oxygen, in the electron transport chain.

Electron Transport Chain (ETC)

The electron transport chain is a series of protein complexes located in the inner mitochondrial membrane (or thylakoid membrane in chloroplasts) that transfer electrons from electron carriers to oxygen, generating ATP in the process.

• After NADH donates its electrons, it is free to accept more electrons and hydrogen ions.

• The ETC extracts energy from electrons and uses it to pump protons, creating a gradient used for ATP synthesis.

Oxidation-Reduction (Redox) Reactions

Redox reactions are chemical processes in which electrons are transferred between molecules, playing a central role in cellular respiration.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance.

• These processes always occur together; when one molecule is oxidized, another is reduced.

• Example: $\text{NAD}^+ + e^- \rightarrow \text{NADH}$

Exergonic and Endergonic Reactions

Cellular respiration and photosynthesis are examples of exergonic and endergonic reactions, respectively.

• Exergonic reactions: Release energy to the environment (e.g., cellular respiration).

• Endergonic reactions: Require an input of energy; products have more energy than reactants (e.g., photosynthesis).

Important Terms to Know

• Glycolysis: The breakdown of glucose into pyruvate.

• Krebs Cycle (Citric Acid Cycle): Series of reactions that generate electron carriers and CO2.

• Oxidative & Substrate Level Phosphorylation: Mechanisms for ATP production.

• Electron Transport Chain: Final stage of aerobic respiration, producing most ATP.

• Pyruvate: End product of glycolysis, substrate for the Krebs cycle.

• Acetyl Co-A: Entry molecule for the Krebs cycle.

• Oxaloacetate: Molecule that combines with Acetyl Co-A to begin the Krebs cycle.

• NADH, FADH2: Reduced electron carriers.

Stages of Cellular Respiration

Overview of the Process

Cellular respiration consists of several stages, each contributing to the overall production of ATP from glucose.

• Location: Most steps occur in the mitochondria; glycolysis occurs in the cytoplasm.

• Major Steps:

  1. Glycolysis

  2. Intermediate Reaction (Pyruvate to Acetyl Co-A)

  3. Krebs Cycle (Citric Acid Cycle)

  4. Oxidative Phosphorylation (Electron Transport Chain and Chemiosmosis)

Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm. It involves the oxidation of glucose to produce pyruvate, ATP, and NADH.

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

• Payoff Phase: 4 ATP are produced (net gain: 2 ATP) via substrate-level phosphorylation.

• Products: 2 pyruvate, 2 NADH, 2 ATP (net)

• Type of Reaction: Catabolic (breakdown of glucose)

Intermediate Step (Pyruvate Oxidation)

This step bridges glycolysis and the Krebs cycle, converting pyruvate into Acetyl Co-A in the mitochondrial matrix.

• Location: Across the outer and inner mitochondrial membranes into the matrix.

• Process: Each pyruvate loses a carbon (released as CO2), and NAD+ is reduced to NADH.

• Products (per glucose): 2 Acetyl Co-A, 2 NADH, 2 CO2

• ATP: None produced or consumed in this step.

Krebs Cycle (Citric Acid Cycle)

The Krebs cycle completes the oxidation of glucose derivatives, generating electron carriers and ATP.

• Location: Mitochondrial matrix

• For each Acetyl Co-A (per turn):

  • 1 ATP (via substrate-level phosphorylation)

  • 3 NADH

  • 1 FADH2

  • 2 CO2

• Cycle regenerates oxaloacetate to continue the process.

Oxidative Phosphorylation and Chemiosmosis

This final stage uses the energy stored in NADH and FADH2 to generate ATP through the electron transport chain and chemiosmosis.

• Location: Begins in the mitochondrial matrix, involves the inner mitochondrial membrane, and returns to the matrix.

• Process: Electrons from NADH and FADH2 are transferred through the ETC to oxygen (the final electron acceptor), forming water.

• Proton Gradient: Energy from electron transfer pumps H+ ions into the intermembrane space, creating a gradient.

• ATP Synthase: H+ ions flow back into the matrix through ATP synthase, driving the synthesis of ATP (chemiosmosis).

• ATP Yield: Up to 34 ATP per glucose (3 ATP per NADH, 2 ATP per FADH2).

Anaerobic ATP Synthesis (Fermentation)

When oxygen is unavailable, cells can generate ATP through fermentation, which allows glycolysis to continue.

• Lactic Acid Fermentation: Occurs in muscle cells and some bacteria; pyruvate is reduced to lactic acid.

• Alcohol Fermentation: Occurs in yeast; pyruvate is converted to ethanol and CO2.

• ATP Yield: Limited to the 2 ATP produced during glycolysis.

Summary Table: ATP and Electron Carrier Yield per Glucose

Stage	ATP (net)	NADH	FADH2	CO2
Glycolysis	2	2	0	0
Pyruvate Oxidation	0	2	0	2
Krebs Cycle	2	6	2	4
Oxidative Phosphorylation	~34	-	-	0
Total	~38	10	2	6

Additional info: Actual ATP yield may vary (typically 30-32 ATP per glucose in eukaryotes due to transport and membrane leakages).

Key Questions for Review

• In which steps are cofactors reduced and oxidized?

• What is the final product of cellular respiration, and what is its significance?

• How much ATP and how many energized cofactors are produced from 3 glucose molecules?

• Is cellular respiration anabolic or catabolic?

• Why is it called aerobic cellular respiration? Can it continue without O2? How?

• List all products of oxidative phosphorylation.

• How much ATP does each NADH and FADH2 generate?