Cellular Respiration and Fermentation: Mechanisms of Energy Harvest in Cells

Cellular Respiration and Fermentation

Introduction to Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells extract energy from organic molecules, primarily glucose, to produce adenosine triphosphate (ATP). This process is essential for all living organisms, providing the energy required for cellular activities. Cellular respiration can occur in the presence (aerobic) or absence (anaerobic) of oxygen, with different pathways and efficiencies.

Energy Flow in the Biosphere

The energy cycle in the biosphere involves the transformation of light energy into chemical energy via photosynthesis, and the subsequent release of that energy through cellular respiration. Organic molecules such as glucose are synthesized during photosynthesis and later oxidized during cellular respiration to produce ATP, carbon dioxide, and water.

Overview of Cellular Respiration

Major Steps of Cellular Respiration

Cellular respiration consists of four main stages:

• Glycolysis: Breakdown of glucose (6C) into two molecules of pyruvate (3C) in the cytosol.

• Pyruvate Processing: Conversion of pyruvate to acetyl CoA (2C) with the release of CO2.

• Citric Acid Cycle (Krebs Cycle): Complete oxidation of acetyl CoA to CO2 in the mitochondrial matrix.

• Electron Transport Chain (ETC) and Oxidative Phosphorylation: Transfer of electrons from NADH and FADH2 to oxygen, generating a proton gradient used to synthesize ATP.

Redox Reactions in Cellular Respiration

Cellular respiration is driven by redox reactions, where glucose is oxidized and oxygen is reduced. The overall equation is:

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

Glycolysis

Pathway and Regulation

Glycolysis is a sequence of 10 enzyme-catalyzed reactions occurring in the cytosol. It consists of two phases:

• Energy Investment Phase (steps 1–5): Consumes 2 ATP to phosphorylate glucose and its intermediates.

• Energy Payoff Phase (steps 6–10): Produces 4 ATP (net gain of 2 ATP), 2 NADH, and 2 pyruvate molecules.

Glycolysis is regulated by feedback inhibition, primarily at the enzyme phosphofructokinase, which is inhibited by high levels of ATP.

Summary Table: The Reactions of Glycolysis

Step	Enzyme	Reaction
1	Hexokinase	Uses ATP to phosphorylate glucose, increasing its potential energy.
2	Phosphoglucose isomerase	Converts glucose-6-phosphate to fructose-6-phosphate.
3	Phosphofructokinase	Uses ATP to phosphorylate fructose-6-phosphate to fructose-1,6-bisphosphate.
4	Aldolase	Cleaves fructose-1,6-bisphosphate into two 3-carbon sugars.
5	Triose phosphate isomerase	Converts dihydroxyacetone phosphate to glyceraldehyde-3-phosphate.
6	Glyceraldehyde-3-phosphate dehydrogenase	Oxidizes and phosphorylates glyceraldehyde-3-phosphate, producing NADH.
7	Phosphoglycerate kinase	Transfers a phosphate to ADP, forming ATP.
8	Phosphoglycerate mutase	Rearranges 3-phosphoglycerate to 2-phosphoglycerate.
9	Enolase	Removes water from 2-phosphoglycerate, forming phosphoenolpyruvate.
10	Pyruvate kinase	Transfers a phosphate to ADP, forming ATP and pyruvate.

Pyruvate Processing

Conversion to Acetyl CoA

Pyruvate produced in glycolysis is transported into the mitochondrial matrix (or cytosol in prokaryotes) and converted to acetyl CoA by the enzyme complex pyruvate dehydrogenase. This process releases CO2 and generates NADH. Pyruvate processing is regulated by feedback inhibition.

Citric Acid Cycle (Krebs Cycle)

Oxidation of Acetyl CoA

The citric acid cycle completes the oxidation of acetyl CoA to CO2 in the mitochondrial matrix. Each turn of the cycle produces:

• 3 NADH

• 1 FADH2

• 1 ATP (or GTP)

• 2 CO2

Since two acetyl CoA molecules are produced per glucose, the cycle turns twice for each glucose molecule.

Summary Table: The Reactions of the Citric Acid Cycle

Step	Enzyme	Reaction
1	Citrate synthase	Combines acetyl CoA and oxaloacetate to form citrate.
2	Aconitase	Converts citrate to isocitrate.
3	Isocitrate dehydrogenase	Oxidizes isocitrate, producing NADH and CO2.
4	α-Ketoglutarate dehydrogenase	Oxidizes α-ketoglutarate, producing NADH and CO2.
5	Succinyl-CoA synthetase	Converts succinyl-CoA to succinate, producing ATP (or GTP).
6	Succinate dehydrogenase	Oxidizes succinate, producing FADH2.
7	Fumarase	Converts fumarate to malate.
8	Malate dehydrogenase	Oxidizes malate to regenerate oxaloacetate, producing NADH.

Regulation of the Citric Acid Cycle

The citric acid cycle is regulated by feedback inhibition. High levels of ATP and NADH inhibit key enzymes, slowing the cycle when energy is abundant.

Electron Transport Chain and Oxidative Phosphorylation

Electron Transport Chain (ETC)

The ETC is a series of protein complexes located in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through these complexes to oxygen, the terminal electron acceptor, forming water. The energy released is used to pump protons across the membrane, creating an electrochemical gradient.

ATP Synthase and Chemiosmosis

ATP synthase is an enzyme that synthesizes ATP as protons flow back into the mitochondrial matrix through its channel, driven by the proton gradient. This process is called chemiosmosis and is responsible for most of the ATP generated during cellular respiration.

ATP Yield from Cellular Respiration

Cellular respiration can yield up to 29 ATP molecules per glucose molecule under optimal conditions, with the majority produced during oxidative phosphorylation.

Aerobic vs. Anaerobic Respiration

Aerobic Respiration

In aerobic respiration, oxygen serves as the terminal electron acceptor in the ETC. This process is highly efficient and is used by all eukaryotes and many prokaryotes.

Anaerobic Respiration

Some prokaryotes use alternative electron acceptors such as nitrate, sulfate, or carbon dioxide in the absence of oxygen. Anaerobic respiration is less efficient than aerobic respiration.

Fermentation

Fermentation Pathways

When no terminal electron acceptor is available, cells switch to fermentation to regenerate NAD+ from NADH, allowing glycolysis to continue. Fermentation produces only 2 ATP per glucose molecule.

• Lactic Acid Fermentation: Pyruvate is reduced to lactate (in muscle cells).

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2 (in yeast).

Fermentation is less efficient than cellular respiration but allows ATP production in the absence of oxygen.

Catabolic Pathways and Metabolic Integration

Other Substrates for Cellular Respiration

Besides glucose, cells can metabolize proteins, fats, and other carbohydrates. These molecules enter the cellular respiration pathway at various points, allowing flexibility in energy production.

Summary Table: Comparison of Aerobic Respiration, Anaerobic Respiration, and Fermentation

Process	Terminal Electron Acceptor	ATP Yield (per glucose)	Location
Aerobic Respiration	O2	~29	Mitochondria (eukaryotes)
Anaerobic Respiration	Nitrate, sulfate, CO2	Less than aerobic	Cytoplasm or plasma membrane (prokaryotes)
Fermentation	Organic molecules (e.g., pyruvate)	2	Cytoplasm

Key Terms

• ATP (Adenosine Triphosphate): The main energy currency of the cell.

• NADH/FADH2: Electron carriers that store energy used to make ATP.

• Substrate-level phosphorylation: Direct synthesis of ATP during glycolysis and the citric acid cycle.

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

• Feedback inhibition: Regulatory mechanism where the end product inhibits an earlier step in the pathway.