Cellular Respiration: Key Pathways and Processes

Cellular Respiration

Overview of 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 both prokaryotic and eukaryotic organisms.

• Purpose: To generate ATP, the energy currency of the cell, from organic molecules such as glucose.

• Main Stages: Glycolysis, Krebs Cycle (Citric Acid Cycle), and Electron Transport Chain (ETC).

• Key Molecules: ATP, NADH, FADH2, CO2, H2O

Glycolysis

Pathway and Products

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm. It breaks down one molecule of glucose (6C) into two molecules of pyruvate (3C each).

• Location: Cytoplasm

• Inputs: Glucose, 2 NAD+, 2 ADP, 2 Pi

• Outputs: 2 Pyruvate, 2 NADH, 2 ATP (net gain)

• ATP Yield: 2 ATP (net)

Equation:

$\text{Glucose} + 2\ \text{NAD}^+ + 2\ \text{ADP} + 2\ \text{P}_i \rightarrow 2\ \text{Pyruvate} + 2\ \text{NADH} + 2\ \text{ATP} + 2\ \text{H}_2\text{O}$

Krebs Cycle (Citric Acid Cycle)

Pathway and Products

The Krebs cycle occurs in the mitochondrial matrix (or cytoplasm in prokaryotes). It processes pyruvate (converted to Acetyl-CoA) to produce electron carriers and CO2.

• Location: Mitochondrial matrix (eukaryotes), cytoplasm (prokaryotes)

• Inputs: 2 Acetyl-CoA, 6 NAD+, 2 FAD, 2 ADP, 2 Pi

• Outputs: 4 CO2, 6 NADH, 2 FADH2, 2 ATP

• ATP Yield: 2 ATP per glucose

Equation:

$2\ \text{Acetyl-CoA} + 6\ \text{NAD}^+ + 2\ \text{FAD} + 2\ \text{ADP} + 2\ \text{P}_i \rightarrow 4\ \text{CO}_2 + 6\ \text{NADH} + 2\ \text{FADH}_2 + 2\ \text{ATP}$

Electron Transport Chain (ETC) and Chemiosmosis

Pathway and Products

The electron transport chain is the final stage of cellular respiration, located in the inner mitochondrial membrane (or plasma membrane in prokaryotes). Electrons from NADH and FADH2 are transferred through protein complexes, driving the production of ATP via oxidative phosphorylation.

• Location: Inner mitochondrial membrane (eukaryotes), plasma membrane (prokaryotes)

• Inputs: NADH, FADH2, O2, ADP, Pi

• Outputs: H2O, ATP

• ATP Yield: About 34 ATP per glucose

Equation:

$10\ \text{NADH} + 2\ \text{FADH}_2 + 6\ \text{O}_2 + 34\ \text{ADP} + 34\ \text{P}_i \rightarrow 10\ \text{NAD}^+ + 2\ \text{FAD} + 12\ \text{H}_2\text{O} + 34\ \text{ATP}$

Summary Table: ATP Yield from Cellular Respiration

Stage	ATP Produced (per glucose)
Glycolysis	2
Krebs Cycle	2
Electron Transport Chain	~34
Total	~38

Key Electron Carriers

NADH and FADH2

During glycolysis and the Krebs cycle, electrons are transferred to the carriers NAD+ and FAD, forming NADH and FADH2. These molecules donate electrons to the ETC, driving ATP synthesis.

• NADH: Nicotinamide adenine dinucleotide (reduced form)

• FADH2: Flavin adenine dinucleotide (reduced form)

Fermentation (Anaerobic Respiration)

Alternative Pathway

When oxygen is not available, cells may undergo fermentation to regenerate NAD+ from NADH, allowing glycolysis to continue. Fermentation produces less ATP and results in end products such as lactic acid or ethanol.

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

• End Products: Lactic acid (in animals), ethanol and CO2 (in yeast)

Example: Muscle cells produce lactic acid during intense exercise when oxygen is limited.

Additional info: The maximum theoretical yield of ATP per glucose is about 38 in prokaryotes and 36 in eukaryotes due to differences in mitochondrial transport efficiency.