Lecture-6-Cellular respirationFermentation

Cellular Respiration and Fermentation Overview

Lecture Date: February 26, 2025This overview focuses on the intricate processes of cellular respiration and fermentation, crucial for energy generation in living organisms.

Energy Sources

Origin of Energy:The energy present in food molecules ultimately originates from the sun, captured through the process of photosynthesis in plants. Photosynthesis results in the production of oxygen and organic molecules like glucose, which are essential for cellular processes.Mitochondria, the powerhouses of the cell, utilize these organic molecules for cellular respiration, leading to the generation of ATP (adenosine triphosphate), the energy currency of the cell. The waste products produced during this process, primarily carbon dioxide (CO2) and water (H2O), serve as raw materials for subsequent photosynthesis.

ATP Generating Processes

There are several key methods for ATP production in cells, which include:

• Glycolysis with Fermentation: A series of reactions that breaks down glucose to produce ATP without oxygen.

• Aerobic Cellular Respiration: Requires oxygen, where glucose is fully oxidized to CO2 and water.

• Anaerobic Cellular Respiration: Takes place in the absence of oxygen, allowing certain organisms to produce energy through alternative pathways. Some organisms may rely on one process, while others can switch between these methods based on environmental conditions and availability of oxygen.

Glucose as Energy Source

Glucose (C6H12O6):Glucose is the primary energy molecule utilized by cells. It undergoes oxidation in a multi-step process through various metabolic pathways to release energy. The primary purpose of cellular respiration is to generate ATP, which powers numerous cellular functions crucial for life.

Aerobic Cellular Respiration

Key Fuels:Aerobic respiration primarily utilizes carbohydrates, fats, and proteins for energy, with glucose being the most commonly used substrate.The breakdown of glucose is an exergonic reaction, defined by a negative Gibbs free energy change (ΔG = -2870 kJ/mol), indicating that the process releases energy that can be harnessed by the cell.

General Formula:

Organic Compounds + O2 + ADP + Pi → CO2 + H2O + energy (ATP & Heat)

Steps of Aerobic Respiration

1. Glycolysis:

   • Location: Cytoplasm

   • Anaerobic process that converts glucose into pyruvate, yielding a net production of 2 ATP and 2 NADH.

2. Pyruvate Oxidation:

   • Location: Mitochondrial matrix

   • Converts pyruvate into acetyl CoA, releasing NADH and CO2 in the process.

3. Citric Acid Cycle (Krebs Cycle):

   • Continues the breakdown of acetyl CoA into CO2, generating 2 ATP, 6 NADH, and 2 FADH2 per cycle.

4. Electron Transport Chain:

   • Utilizes NADH and FADH2, transferring electrons through a series of protein complexes, ultimately producing ATP via oxidative phosphorylation. This results in the majority of ATP generation during respiration.

Glycolysis Breakdown

Stages:

1. Energy Investment:

   • Initial investment of 2 ATP molecules to modify glucose, facilitating later breakdown.

2. Energy Harvesting:

   • Produces 4 ATP and 2 NADH through substrate-level phosphorylation.

Total ATP Production:In total, glycolysis yields a net gain of 2 ATP, 2 NADH, and 2 pyruvate molecules for every glucose molecule processed.

Pyruvate Oxidation

Location:

• Plasma membrane in prokaryotes; mitochondrial matrix in eukaryotes.Conversion of pyruvate to acetyl CoA is crucial as it also produces NADH (which will be used in the electron transport chain) and CO2 as a byproduct.

Citric Acid Cycle Summary

The cycle processes one acetyl CoA at a time, yielding:

• 2 ATP

• 6 NADH

• 2 FADH2

• 4 CO2 per complete round of the cycle.

Oxidative Phosphorylation

Two Processes Involved:

1. Electron Transport Chain:

   • Oxidizes NADH and FADH2, leading to the pumping of H+ ions across the mitochondrial membrane, creating an electrochemical gradient.

2. Chemiosmosis:

   • The H+ ion gradient powers ATP synthase, which synthesizes ATP from ADP and inorganic phosphate (Pi).Total ATP Yield:Overall, aerobic respiration can yield approximately 32-38 ATP molecules from one glucose molecule, significantly enhancing the cell's energy efficiency.

Fermentation

Anaerobic Conditions:Occurs in environments devoid of oxygen as a means to regenerate NAD+ for glycolysis to continue producing ATP.

• Lactic Acid Fermentation:

  • Converts pyruvate into lactic acid, typically occurring in muscle cells during intense exercise.

• Alcohol Fermentation:

  • Converts pyruvate into ethanol and CO2, commonly used by yeast in brewing and baking.Net ATP Production from Fermentation:Both types of fermentation yield only 2 ATP per glucose molecule, which is significantly less than that obtained from aerobic respiration.

Energy Yields Comparison

In summary, aerobic respiration yields significantly more energy per glucose molecule (up to 32-38 ATP) compared to fermentation (2 ATP). This illustrates why most aerobic organisms prefer oxygen-based pathways for energy generation when available.

Interconnecting Metabolic Pathways

Metabolic pathways not only generate energy but also provide essential materials for the biosynthesis of vital macromolecules, including carbohydrates, proteins, and lipids.These pathways are intricately regulated by feedback mechanisms to maintain metabolic homeostasis within the cell.

Regulation of Pathways

Metabolic Regulation:In some cases, metabolic regulation is achieved through allosteric enzymes and feedback loops that help maintain the balance between catabolism (breaking down molecules for energy) and anabolism (building up molecules for storage and growth).Changes in cellular energy levels or nutrient availability can significantly influence these pathways and enzyme activities, ensuring that the cell operates efficiently and responds to its energetic demands.