Cellular Respiration & Fermentation: Study Notes for General Biology

Cellular Respiration & Fermentation

Introduction

Cellular respiration and fermentation are essential metabolic processes that allow living cells to extract energy from organic molecules. These processes are fundamental to the survival of all organisms, providing the ATP required for cellular work.

Metabolic Pathways Produce ATP

Overview of Energy Transfer

• Living cells require energy from external sources to perform life's tasks.

• The sun provides energy to organic molecules in food, which in turn supply energy to organisms.

• Catabolic pathways break down complex molecules, releasing stored energy.

• The arrangement of electrons in chemical bonds gives organic compounds their potential energy.

• Catabolic pathways are linked to cellular work by the production of ATP (adenosine triphosphate).

Aerobic Respiration

Definition and Efficiency

• A catabolic pathway for organic molecules that uses oxygen as the final electron acceptor in the electron transport chain (ETC), ultimately producing ATP.

• This is the most efficient catabolic pathway.

• Carried out by most eukaryotic and many prokaryotic organisms.

• Anaerobic respiration is used by some prokaryotes and uses substances other than oxygen as reactants.

Glucose Breakdown

Energy Release from Glucose

• The major source of carbohydrates is starch, which is broken down into glucose.

• The breakdown of glucose is exergonic (releases energy) with $\Delta G = -686$ kcal/mol.

• The overall reaction for cellular respiration is:

$\mathrm{C_6H_{12}O_6} + 6\mathrm{O_2} \rightarrow 6\mathrm{CO_2} + 6\mathrm{H_2O}$

• Energy released during this process is used to synthesize ATP.

Oxidation and Reduction Reactions

Redox Reactions in Metabolism

• Catabolic pathways break down fuel for energy through the transfer of electrons.

• These electron transfers are called oxidation-reduction (redox) reactions.

• Reduction: Electrons are added to a molecule (adding a negative charge, reducing the positive charge).

• Oxidation: Electrons are lost from a molecule.

The Principle of Redox

Agents and Electron Transfer

• Oxidized = loses an electron.

• Reduced = gains an electron.

• The electron donor is the reducing agent and becomes oxidized.

• The electron acceptor is the oxidizing agent and becomes reduced.

• Oxidation and reduction always occur together.

Example: $\mathrm{Na} + \mathrm{Cl} \rightarrow \mathrm{Na}^+ + \mathrm{Cl}^-$ Sodium is oxidized (loses electron), chlorine is reduced (gains electron).

Redox Reactions

Types and Importance

• Not all redox reactions involve complete electron transfer; some change the degree of electron sharing in covalent bonds.

• Oxygen is one of the most powerful oxidizing agents.

Example: Methane ($\mathrm{CH_4}$) reacts with oxygen ($\mathrm{O_2}$) to form carbon dioxide ($\mathrm{CO_2}$) and water, with electrons shifting from methane to oxygen.

Redox Reactions Require Energy

Electronegativity and Energy Transfer

• Energy is required to pull an electron away from an atom.

• The more electronegative an atom, the more energy needed to take an electron.

• Electrons lose potential energy when transferred from a less electronegative atom to a more electronegative one.

• Molecules with many hydrogens are good fuel sources because their electrons can be released and fall down an energy gradient toward oxygen.

Stepwise Energy Harvest

Controlled Release of Energy

• If fuel was broken down in one explosive step, energy could not be efficiently harvested.

• Cellular respiration breaks down glucose in a stepwise manner, each step catalyzed by an enzyme.

• At key steps, electrons are stripped from glucose.

Cellular Respiration

Pathways and Electron Transport Chain

• Catabolic pathways of aerobic and anaerobic respiration break down organic molecules and use an electron transport chain (ETC) for ATP production.

• Aerobic respiration uses oxygen as the final electron acceptor in the ETC and produces ATP (most eukaryotes and many prokaryotes).

• Anaerobic respiration completes the process without oxygen (some prokaryotes).

Stages of Cellular Respiration

Three Main Stages

• Glycolysis

• Pyruvate Oxidation & the Citric Acid Cycle

• Oxidative Phosphorylation

Example: Glycolysis occurs in the cytosol, pyruvate oxidation and the citric acid cycle occur in the mitochondria, and oxidative phosphorylation involves the electron transport chain and ATP synthesis.

Glycolysis

Process and Yield

• "Sugar splitting" process that occurs in the cytosol.

• Consists of a series of reactions that break down glucose.

• Serves as the starting point for fermentation or cellular respiration.

• Two phases: energy investment and energy payoff.

• One glucose molecule yields 2 ATP and 2 NADH.

Example: During glycolysis, glucose ($\mathrm{C_6H_{12}O_6}$) is converted to two molecules of pyruvate, producing ATP and NADH.