Cellular Respiration and Fermentation: Mechanisms of Energy Harvest in Cells

Cellular Respiration and Fermentation

Overview of Energy Flow in Ecosystems

Energy enters ecosystems as sunlight and is converted by photosynthesis into chemical energy stored in organic molecules. Cellular respiration in mitochondria then extracts this energy to produce ATP, which powers most cellular work. The process releases heat energy as a byproduct.

• Photosynthesis: Converts CO2 and H2O into organic molecules and O2.

• Cellular Respiration: Breaks down organic molecules, using O2, to generate ATP and release CO2 and H2O.

• ATP: The main energy currency of the cell, used for cellular work.

Pathways of Cellular Respiration

Cells extract energy from organic molecules through a series of metabolic pathways. The breakdown of these molecules is exergonic, releasing energy that is captured in ATP. There are three main types of respiration:

• Aerobic Respiration: Uses O2 as the final electron acceptor; yields the most ATP.

• Anaerobic Respiration: Uses inorganic molecules other than O2 as electron acceptors.

• Fermentation: Partial degradation of sugars without O2; yields less ATP and produces organic end products.

Redox Reactions in Cellular Respiration

Cellular respiration involves a series of redox (reduction-oxidation) reactions, where electrons are transferred from fuel molecules (like glucose) to electron carriers, releasing energy used to synthesize ATP.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Electron Carriers: NAD+ and FAD are key coenzymes that shuttle electrons during respiration.

Energy Harvest via NAD+

During cellular respiration, electrons from organic molecules are first transferred to NAD+, forming NADH. NADH stores energy and donates electrons to the electron transport chain (ETC), which ultimately drives ATP synthesis.

• NAD+: Oxidized form; accepts electrons.

• NADH: Reduced form; carries electrons to the ETC.

Stages of Cellular Respiration

Overview of the Three Main Stages

Cellular respiration consists of three main stages, each occurring in specific cellular locations and catalyzed by distinct enzymes:

• Glycolysis: Occurs in the cytoplasm; splits glucose into two pyruvate molecules.

• Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose in the mitochondrial matrix.

• Oxidative Phosphorylation: Includes the electron transport chain and chemiosmosis; produces most ATP.

Glycolysis

Glycolysis is the first step in the breakdown of glucose, occurring in the cytoplasm. It consists of two phases:

• Energy Investment Phase: 2 ATP are used 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.

Glycolysis does not require oxygen and occurs in both aerobic and anaerobic conditions.

Oxidation of Pyruvate to Acetyl CoA

Before entering the citric acid cycle, pyruvate is transported into the mitochondrion and converted to acetyl CoA. This step links glycolysis to the citric acid cycle and involves decarboxylation, reduction of NAD+ to NADH, and attachment of coenzyme A.

The Citric Acid Cycle (Krebs Cycle)

The citric acid cycle is a series of eight enzyme-catalyzed reactions that complete the oxidation of acetyl CoA to CO2. It generates NADH and FADH2 for the electron transport chain and produces a small amount of ATP directly.

• Key Steps: Acetyl CoA combines with oxaloacetate to form citrate; subsequent reactions regenerate oxaloacetate.

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

Oxidative Phosphorylation: Electron Transport Chain and Chemiosmosis

Oxidative phosphorylation is the final stage of cellular respiration, where most ATP is produced. NADH and FADH2 donate electrons to the electron transport chain (ETC), a series of protein complexes in the inner mitochondrial membrane. The energy released pumps protons (H+) across the membrane, creating a proton gradient.

• Electron Transport Chain: Transfers electrons to O2, forming H2O.

• Chemiosmosis: H+ flows back through ATP synthase, driving ATP production.

• ATP Yield: About 32–36 ATP per glucose molecule.

ATP Yield from Cellular Respiration

Most energy from glucose is transferred to ATP through the sequence: glucose → NADH → ETC → proton-motive force → ATP. Approximately 34% of the energy in glucose is captured as ATP, with the remainder lost as heat.

Fermentation

Alcohol Fermentation vs. Lactic Acid Fermentation

Fermentation allows glycolysis to continue in the absence of oxygen by regenerating NAD+. There are two main types:

Ethanol Fermentation	Both	Lactic Acid Fermentation
Generates ethanol	Regenerates NAD+ that can be used in glycolysis	Produces lactic acid (lactate)
Used by yeast cells	Considered to be an anaerobic process	Used by animal cells

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2; used by yeast and some bacteria.

• Lactic Acid Fermentation: Pyruvate is reduced to lactate; used by animal muscle cells and some bacteria.

Example: Muscle cells use lactic acid fermentation during intense exercise when oxygen is scarce.