Cellular Respiration and Fermentation: Study Guide

Chapter 7: Cellular Respiration and Fermentation

Overview

Cellular respiration is a series of metabolic pathways that convert organic fuels into energy in the form of ATP. This process involves glycolysis, the citric acid cycle, and oxidative phosphorylation, and can occur aerobically (with oxygen) or anaerobically (without oxygen, as in fermentation).

Key Concepts

• Catabolic pathways yield energy by oxidizing organic fuels.

• Glycolysis harvests chemical energy by oxidizing glucose to pyruvate.

• After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules.

• During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis.

• Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen.

• Glycolysis and the citric acid cycle connect to many other metabolic pathways.

Cellular Respiration: Pathways and Processes

Catabolic Pathways and Energy Yield

Catabolic pathways break down organic molecules to release energy. The main fuel for cellular respiration is glucose, but other carbohydrates, fats, and proteins can also be used.

• Fermentation: Occurs without oxygen; produces less ATP.

• Aerobic respiration: Requires oxygen; produces more ATP.

• Energy yield: The breakdown of glucose in cellular respiration yields energy, measured in kilocalories (Kcal).

Redox Reactions in Cellular Respiration

Cellular respiration involves many redox reactions, where electrons are transferred between molecules.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• General formula for a redox reaction: $Xe^- + Y \rightarrow X + Ye^-$

• Reducing agent: Donates electrons.

• Oxidizing agent: Accepts electrons.

Electron Carriers: NAD+ and NADH

Electron carriers are molecules that transport electrons during cellular respiration.

• NAD+: Oxidized form; accepts electrons.

• NADH: Reduced form; carries electrons and energy.

Carrier	Oxidized or Reduced?	Energy Level
NAD+	Oxidized	Lower Energy
NADH	Reduced	Higher Energy

Stages of Cellular Respiration

Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm. It breaks down glucose (a six-carbon sugar) into two molecules of pyruvate (three carbons each).

• Energy investment phase: ATP is used to phosphorylate glucose.

• Energy payoff phase: ATP and NADH are produced.

• Net products: 2 ATP, 2 NADH, 2 pyruvate per glucose.

• Glycolysis does not require oxygen.

Pyruvate Oxidation and Citric Acid Cycle

After glycolysis, pyruvate enters the mitochondria and is converted to acetyl CoA, which enters the citric acid cycle (Krebs cycle).

• Pyruvate oxidation: Produces acetyl CoA, CO2, and NADH.

• Citric acid cycle: Completes the breakdown of glucose, producing NADH, FADH2, ATP, and CO2.

• Each turn of the cycle produces: 3 NADH, 1 FADH2, 1 ATP, and 2 CO2 (per acetyl CoA).

• For each glucose, the cycle turns twice.

Electron Transport Chain and Oxidative Phosphorylation

The electron transport chain (ETC) is located in the inner mitochondrial membrane. Electrons from NADH and FADH2 are passed through a series of protein complexes, ultimately reducing oxygen to water.

• Electron carriers: Proteins and other molecules embedded in the membrane.

• Final electron acceptor: Oxygen (O2).

• Proton gradient: Electron flow pumps protons (H+) into the intermembrane space, creating a gradient.

• ATP synthesis: Protons flow back into the matrix through ATP synthase, driving the production of ATP (chemiosmosis).

Fermentation

Fermentation allows cells to produce ATP without oxygen by regenerating NAD+ from NADH.

• Lactic acid fermentation: Pyruvate is reduced to lactate.

• Alcohol fermentation: Pyruvate is converted to ethanol and CO2.

• Fermentation produces much less ATP than aerobic respiration.

Key Terms and Definitions

• ATP (Adenosine Triphosphate): The main energy currency of the cell.

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP to form ATP.

• Oxidative phosphorylation: ATP synthesis powered by the electron transport chain and chemiosmosis.

• Chemiosmosis: Movement of ions across a semipermeable membrane, generating ATP.

• Proton-motive force: The potential energy stored in the proton gradient across the mitochondrial membrane.

Summary Table: Major Steps and Products of Cellular Respiration

Stage	Main Location	Key Inputs	Key Outputs	ATP Yield
Glycolysis	Cytoplasm	Glucose, 2 ATP, 2 NAD+	2 Pyruvate, 4 ATP (2 net), 2 NADH	2
Pyruvate Oxidation	Mitochondrial Matrix	2 Pyruvate, 2 NAD+	2 Acetyl CoA, 2 NADH, 2 CO2	0
Citric Acid Cycle	Mitochondrial Matrix	2 Acetyl CoA, 6 NAD+, 2 FAD, 2 ADP	4 CO2, 6 NADH, 2 FADH2, 2 ATP	2
Oxidative Phosphorylation	Inner Mitochondrial Membrane	NADH, FADH2, O2	ATP, H2O	~26-28

Important Equations

• General equation for cellular respiration: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy (ATP + heat)}$

• Redox reaction formula: $\text{Xe}^- + \text{Y} \rightarrow \text{X} + \text{Ye}^-$

Connections to Other Metabolic Pathways

Glycolysis and the citric acid cycle are central hubs that connect to other metabolic pathways, including the breakdown and synthesis of carbohydrates, fats, and proteins.

Example: Energy Yield from Glucose

• Complete oxidation of one glucose molecule yields up to 30-32 ATP molecules.

• Most ATP is produced during oxidative phosphorylation.

Additional info:

• Some details, such as the exact number of ATP produced, may vary depending on cell type and conditions.

• Figures referenced in the original file (e.g., Figure 7.7, 7.8, 7.9, 7.10, 7.11) illustrate the steps and components of cellular respiration, including glycolysis, the citric acid cycle, and the electron transport chain.