Cellular Respiration and Fermentation: ATP Production in Cells

Cellular Respiration and Fermentation

Overview of Cellular Respiration

Cellular respiration is a series of metabolic processes by which cells harvest energy from organic molecules, primarily glucose, to produce adenosine triphosphate (ATP). This process is essential for providing the energy required for cellular activities. Cellular respiration can occur in the presence (aerobic) or absence (anaerobic) of oxygen, with the latter leading to fermentation.

• ATP (Adenosine Triphosphate): The main energy currency of the cell, used to power various cellular functions.

• Key Steps: Glycolysis, Pyruvate Processing, Citric Acid Cycle, Electron Transport Chain & Oxidative Phosphorylation.

• Fermentation: An alternative pathway when oxygen is not available, allowing glycolysis to continue by regenerating NAD+.

Major Steps of Cellular Respiration

1. Glycolysis: Occurs in the cytosol; glucose (6C) is split into two pyruvate (3C) molecules, producing a net gain of 2 ATP and 2 NADH.

2. Pyruvate Processing: Each pyruvate is converted into acetyl CoA (2C), releasing CO2 and generating NADH.

3. Citric Acid Cycle (Krebs Cycle): Acetyl CoA is oxidized to CO2 in the mitochondrial matrix, producing NADH, FADH2, and ATP (or GTP).

4. Electron Transport Chain (ETC) and Oxidative Phosphorylation: NADH and FADH2 donate electrons to the ETC, creating a proton gradient that drives ATP synthesis via chemiosmosis.

Glycolysis

Pathway and Energy Yield

Glycolysis is the first step in glucose catabolism, occurring in the cytosol. It consists of 10 enzyme-catalyzed reactions divided into two phases: the energy investment phase and the energy payoff phase.

• Energy Investment Phase: 2 ATP are consumed to phosphorylate glucose and its intermediates.

• Energy Payoff Phase: 4 ATP and 2 NADH are produced, resulting in a net gain of 2 ATP per glucose molecule.

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

Pyruvate Processing

Conversion to Acetyl CoA

Pyruvate produced in glycolysis is transported into the mitochondrial matrix (in eukaryotes), where it is converted to acetyl CoA by the pyruvate dehydrogenase complex. This step links glycolysis to the citric acid cycle.

• Reactants: 2 pyruvate, 2 NAD+, 2 Coenzyme A

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

Citric Acid Cycle (Krebs Cycle)

Oxidation of Acetyl CoA

The citric acid cycle completes the oxidation of glucose derivatives. Each acetyl CoA enters the cycle, which consists of eight enzyme-catalyzed steps in the mitochondrial matrix. The cycle runs twice per glucose molecule.

• Reactants per turn: 1 acetyl CoA, 3 NAD+, 1 FAD, 1 GDP (or ADP), 2 H2O

• Products per turn: 2 CO2, 3 NADH, 1 FADH2, 1 GTP (or ATP)

• Total per glucose: 4 CO2, 6 NADH, 2 FADH2, 2 ATP (or GTP)

Electron Transport Chain and Oxidative Phosphorylation

ATP Synthesis via Chemiosmosis

The electron transport chain (ETC) is located in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through a series of protein complexes, ultimately reducing oxygen to water. The energy released pumps protons into the intermembrane space, creating a proton gradient.

• Proton Motive Force: The electrochemical gradient drives protons back into the matrix through ATP synthase, synthesizing ATP from ADP and inorganic phosphate.

• Maximum ATP Yield: Up to 29 ATP per glucose molecule (theoretical maximum).

Fermentation

ATP Production in the Absence of Oxygen

When oxygen is not available as the final electron acceptor, cells use fermentation to regenerate NAD+ so glycolysis can continue. Fermentation produces far less ATP than aerobic respiration.

• Lactic Acid Fermentation: Occurs in animals; pyruvate is reduced to lactate.

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

• Purpose: Regenerate NAD+ for glycolysis.

Type	Organism	End Products
Lactic Acid Fermentation	Animals (e.g., muscle cells)	Lactate
Alcohol Fermentation	Yeast	Ethanol, CO2

Summary Table: Cellular Respiration Pathways

Stage	Location	Inputs	Outputs	ATP Produced
Glycolysis	Cytosol	Glucose, 2 NAD+, 2 ATP	2 Pyruvate, 2 NADH, 4 ATP (2 net)	2 (net)
Pyruvate Processing	Mitochondrial Matrix	2 Pyruvate, 2 NAD+, 2 CoA	2 Acetyl CoA, 2 NADH, 2 CO2	0
Citric Acid Cycle	Mitochondrial Matrix	2 Acetyl CoA, 6 NAD+, 2 FAD, 2 GDP/ADP	4 CO2, 6 NADH, 2 FADH2, 2 ATP/GTP	2
ETC & Oxidative Phosphorylation	Inner Mitochondrial Membrane	10 NADH, 2 FADH2, O2	H2O, ~25 ATP	~25

Key Equations

• Overall Cellular Respiration: $\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP + heat)}}$

• ATP Yield (Theoretical Maximum): $\mathrm{\text{Glycolysis:}\ 2\ ATP,\ \text{Citric\ Acid\ Cycle:}\ 2\ ATP,\ \text{ETC:}\ 25\ ATP}$

Additional info: The actual ATP yield in cells is often lower than the theoretical maximum due to leaky membranes and the use of the proton gradient for other processes.