Cellular Respiration and Fermentation

Overview of Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells extract energy from organic molecules, primarily glucose, to produce ATP, the main energy currency of the cell. This process involves a series of redox reactions and can occur with or without oxygen.

• Cellular respiration includes both aerobic (with oxygen) and anaerobic (without oxygen) pathways.

• Fermentation is a partial degradation of sugars that occurs without oxygen.

• The main stages of cellular respiration are glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.

Energy Flow and Chemical Recycling in Ecosystems

Energy enters ecosystems as sunlight and is converted by photosynthesis into chemical energy, which is then used by cellular respiration to generate ATP. The chemical elements involved are recycled between photosynthesis and respiration.

• Photosynthesis uses CO2 and H2O to produce organic molecules and O2.

• Cellular respiration uses organic molecules and O2 to produce ATP, CO2, and H2O.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Redox (reduction-oxidation) reactions are central to energy transfer in biological systems. In these reactions, electrons are transferred from one molecule to another, releasing energy that can be used to synthesize ATP.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance.

• Redox reactions always occur in pairs: one molecule is oxidized while another is reduced.

Examples of Redox Reactions

Redox reactions can be represented by the transfer of electrons between atoms or molecules. For example, in the reaction between sodium and chlorine:

• Sodium (Na) loses an electron (is oxidized) to become Na+.

• Chlorine (Cl) gains an electron (is reduced) to become Cl-.

Stages of Cellular Respiration

Overview of the Stages

Cellular respiration consists of three main stages, each occurring in specific regions of the eukaryotic cell:

1. Glycolysis (cytosol): Breaks down glucose into two molecules of pyruvate.

2. Pyruvate Oxidation and Citric Acid Cycle (mitochondrial matrix): Completes the breakdown of glucose to CO2.

3. Oxidative Phosphorylation (inner mitochondrial membrane): Electron transport chain and chemiosmosis produce most of the cell's ATP.

Substrate-Level and Oxidative Phosphorylation

ATP can be generated by two mechanisms:

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP by an enzyme.

• Oxidative phosphorylation: ATP synthesis powered by redox reactions in the electron transport chain.

Glycolysis

Phases and Net Yield

Glycolysis is the first stage of cellular respiration and occurs in the cytoplasm. It consists of two phases:

• Energy investment phase: 2 ATP are used to split glucose into two three-carbon sugars.

• Energy payoff phase: 4 ATP are produced (net gain of 2 ATP), 2 NAD+ are reduced to NADH, and 2 pyruvate molecules are formed.

Detailed Steps of Glycolysis

The process of glycolysis involves a series of enzyme-catalyzed reactions:

• Glucose is phosphorylated and rearranged to form fructose 1,6-bisphosphate.

• This molecule is split into two three-carbon sugars, which are further processed to generate ATP and NADH.

Pyruvate Oxidation and the Citric Acid Cycle

Oxidation of Pyruvate

After glycolysis, pyruvate is transported into the mitochondrion and converted to acetyl CoA. This step links glycolysis to the citric acid cycle.

• Pyruvate dehydrogenase catalyzes the conversion, producing CO2, NADH, and acetyl CoA.

The Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the oxidation of organic molecules, generating ATP, NADH, FADH2, and CO2 as a waste product. Each turn of the cycle processes one acetyl CoA.

• For each glucose molecule, the cycle runs twice (once per pyruvate).

• Products per turn: 1 ATP, 3 NADH, 1 FADH2, 2 CO2.

• The cycle regenerates oxaloacetate, making it a true cycle.

Oxidative Phosphorylation and Chemiosmosis

Electron Transport Chain

The electron transport chain (ETC) is a series of protein complexes embedded in the inner mitochondrial membrane. NADH and FADH2 donate electrons to the ETC, which are passed through a series of carriers to oxygen, the final electron acceptor.

• As electrons move through the chain, protons (H+) are pumped into the intermembrane space, creating a proton gradient.

• This gradient is known as the proton-motive force.

ATP Synthesis by Chemiosmosis

ATP synthase is an enzyme that uses the energy stored in the proton gradient to synthesize ATP from ADP and inorganic phosphate. This process is called chemiosmosis.

• Most ATP generated during cellular respiration is produced by oxidative phosphorylation via chemiosmosis.

Fermentation and Anaerobic Respiration

Fermentation Pathways

When oxygen is not available, cells can produce ATP by fermentation. Fermentation allows glycolysis to continue by regenerating NAD+ from NADH.

• Alcohol fermentation: Pyruvate is converted to ethanol and CO2 (used by yeast in brewing and baking).

• Lactic acid fermentation: Pyruvate is reduced to lactate (used by muscle cells and some bacteria).

Comparison of Fermentation and Cellular Respiration

Metabolic Integration and Regulation

Catabolism of Other Molecules

Other macromolecules such as fats and proteins can also be used as fuel for cellular respiration. They enter the pathway at different points after being broken down into their monomers.

• Fats are broken down into glycerol and fatty acids; fatty acids undergo beta-oxidation to form acetyl CoA.

• Proteins are deaminated and their carbon skeletons enter glycolysis or the citric acid cycle.

Regulation of Cellular Respiration

Cellular respiration is tightly regulated by feedback mechanisms. The enzyme phosphofructokinase is a key regulatory point, responding to levels of ATP and other metabolites to control the rate of glycolysis.

• High ATP levels inhibit phosphofructokinase, slowing glycolysis.

• Low ATP levels activate the enzyme, increasing glycolysis and ATP production.