Chapter 7: Cellular Respiration and Fermentation – Study Guide

Cellular Respiration and Fermentation

Overview of Cellular Respiration

Cellular respiration is a series of metabolic processes by which cells harvest energy from organic molecules, primarily glucose. This energy is used to produce ATP, the main energy currency of the cell. Cellular respiration can be aerobic (requiring oxygen) or anaerobic (not requiring oxygen), and includes glycolysis, the citric acid cycle, and oxidative phosphorylation.

• Catabolic pathways break down organic fuels to yield energy.

• Fermentation is an anaerobic process that allows ATP production without oxygen.

• Aerobic respiration uses oxygen as the final electron acceptor in the electron transport chain.

Key Concepts and Pathways

Catabolic Pathways Yield Energy by Oxidizing Organic Fuels

• Fermentation vs. Aerobic Respiration: Fermentation is the partial degradation of sugars without oxygen, while aerobic respiration completely breaks down organic molecules using oxygen.

• Redox Reactions: Cellular respiration involves oxidation-reduction (redox) reactions, where electrons are transferred from one molecule to another.

• General Redox Formula:

$Xe^- + Y \rightarrow X + Ye^-$

• The molecule losing electrons is oxidized; the molecule gaining electrons is reduced.

• When compounds lose electrons, they lose energy; when they gain electrons, they gain energy.

• NAD+ is a key electron carrier (coenzyme) in cellular respiration, reduced to NADH during glycolysis and the citric acid cycle.

Electron Transport and Oxidative Phosphorylation

• Electrons from NADH and FADH2 are transferred to the electron transport chain (ETC), a series of protein complexes located in the inner mitochondrial membrane (eukaryotes) or plasma membrane (prokaryotes).

• Oxygen is the final electron acceptor, forming water.

• The ETC creates a proton (H+) gradient across the membrane, driving ATP synthesis via chemiosmosis and ATP synthase.

Glycolysis: Harvesting Chemical Energy by Oxidizing Glucose to Pyruvate

Glycolysis is the first step of cellular respiration, occurring in the cytosol and not requiring oxygen. It splits one glucose (6C) into two pyruvate (3C) molecules, producing a net gain of 2 ATP and 2 NADH per glucose.

• Energy Investment Phase: 2 ATP are used to phosphorylate glucose and its intermediates.

• Energy Payoff Phase: 4 ATP and 2 NADH are produced.

• Net gain: 2 ATP and 2 NADH per glucose.

Citric Acid Cycle (Krebs Cycle)

Pyruvate is transported into the mitochondrion and converted to acetyl CoA, which enters the citric acid cycle. Each turn of the cycle oxidizes acetyl CoA, producing NADH, FADH2, ATP (or GTP), and CO2.

• For each glucose (2 turns):

  • 6 NADH

  • 2 FADH2

  • 2 ATP (or GTP)

  • 4 CO2

Oxidative Phosphorylation

Oxidative phosphorylation consists of the electron transport chain and chemiosmosis. The ETC transfers electrons from NADH and FADH2 to oxygen, pumping protons to create a gradient. ATP synthase uses this gradient to produce ATP.

• ATP Yield: About 30–32 ATP per glucose, though the exact number varies.

• Key Terms: Chemiosmosis (movement of H+ across a membrane) and proton-motive force (potential energy stored as a proton gradient).

Fermentation and Anaerobic Respiration

Fermentation allows cells to produce ATP without oxygen by regenerating NAD+ from NADH. Common types include lactic acid fermentation (producing lactate) and alcohol fermentation (producing ethanol and CO2).

• Lactic Acid Fermentation: Occurs in muscle cells and some bacteria; pyruvate is reduced to lactate.

• Alcohol Fermentation: Occurs in yeast; pyruvate is converted to ethanol and CO2.

• Fermentation yields much less ATP than aerobic respiration.

Connections to Other Metabolic Pathways

Glycolysis and the citric acid cycle are central hubs for metabolism, connecting to the breakdown of other macromolecules:

• Starch and Glycogen: Broken down to glucose, entering glycolysis.

• Proteins: Deaminated and their carbon skeletons enter glycolysis or the citric acid cycle.

• Fats: Glycerol enters glycolysis; fatty acids are broken down by beta-oxidation to acetyl CoA.

Summary Table: Electron Carriers in Cellular Respiration

Molecule	Oxidized or Reduced?	Higher Energy/Lower Energy
NAD+	Oxidized	Lower Energy
NADH	Reduced	Higher Energy

Key Equations

• Overall Equation for Aerobic Cellular Respiration:

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

• ATP Synthesis by Chemiosmosis:

$\text{ADP} + P_i \rightarrow \text{ATP}$

Additional info:

• Some questions in the file prompt students to label diagrams and trace the flow of electrons and energy through cellular respiration, reinforcing the importance of understanding the integration of metabolic pathways.

• Students are encouraged to compare fermentation and aerobic respiration, understand the role of electron carriers, and relate the processes to the evolution of metabolism.