Cellular Respiration and Fermentation: Study Notes (Campbell Biology, Chapter 9)

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Cellular Respiration and Fermentation

Introduction

Cellular respiration and fermentation are essential metabolic processes by which cells extract energy from organic molecules. These processes allow both plant and animal cells to generate ATP, the primary energy currency of the cell, by breaking down carbohydrates, fats, and proteins.

Cellular respiration occurs mainly in the mitochondria and involves a series of redox reactions to convert chemical energy in food into ATP.
Fermentation is an alternative pathway that enables ATP production in the absence of oxygen.

Catabolic Pathways Yield Energy by Oxidizing Organic Fuels

Overview of Energy Flow in Ecosystems

Energy enters ecosystems as light and exits as heat, while essential elements are recycled. Photosynthesis and cellular respiration are interconnected processes in the energy cycle.

Photosynthesis uses CO2 and H2O to make organic molecules and O2.
Cellular respiration uses O2 and organic molecules to make ATP; CO2 and H2O are produced as waste.

Catabolic Pathways and ATP Production

Exergonic reactions release stored energy by breaking down complex molecules.
Fermentation is a partial degradation of sugars that occurs without oxygen.
Aerobic respiration consumes organic molecules and oxygen, yielding ATP.
Anaerobic respiration is similar to aerobic respiration but uses compounds other than oxygen as the final electron acceptor.

General equation for cellular respiration:

Redox Reactions: Oxidation and Reduction

Definitions and Mechanisms

Redox reactions involve the transfer of electrons between reactants, releasing energy stored in organic molecules. This energy is ultimately used to synthesize ATP.

Oxidation: Loss of electrons from a substance.
Reduction: Gain of electrons by a substance (reduces positive charge).
Reducing agent: Electron donor (gets oxidized).
Oxidizing agent: Electron acceptor (gets reduced).

Example redox reaction:

Electrons move from less electronegative atoms to more electronegative atoms (such as oxygen), releasing energy.

Stages of Cellular Respiration

Overview

Cellular respiration consists of three main stages, each occurring in specific locations within the cell:

Glycolysis (cytosol): Breaks down glucose into two molecules of pyruvate.
Pyruvate oxidation and the citric acid cycle (mitochondrial matrix): Completes the breakdown of glucose to CO2.
Oxidative phosphorylation (mitochondrial inner membrane): Electron transport chain and chemiosmosis synthesize most of the cell's ATP.

Substrate-Level vs. Oxidative Phosphorylation

Substrate-level phosphorylation: Enzyme transfers a phosphate group directly from a substrate to ADP, forming ATP.
Oxidative phosphorylation: ATP is generated by redox reactions powered by the electron transport chain.

Up to 32 molecules of ATP are produced per molecule of glucose degraded to CO2 and H2O.

Glycolysis

Phases and Products

Glycolysis occurs in the cytoplasm and consists of two major phases:

Energy investment phase: 2 ATP are used to split glucose into two three-carbon sugars.
Energy payoff phase: 4 ATP are synthesized, 2 NAD+ are reduced to NADH, and the small sugars are oxidized to form 2 pyruvate and 2 H2O.

Net products per glucose:

2 pyruvate
2 ATP (net gain)
2 NADH

Glycolysis does not release CO2 and occurs whether or not O2 is present.

Pyruvate Oxidation and the Citric Acid Cycle (Krebs Cycle)

Pyruvate Oxidation

Before entering the citric acid cycle, pyruvate is converted to acetyl coenzyme A (acetyl CoA) in the mitochondria.

Oxidation of pyruvate's carboxyl group releases CO2.
NAD+ is reduced to NADH.
Remaining two-carbon fragment combines with coenzyme A to form acetyl-CoA.

Citric Acid Cycle

The citric acid cycle completes the breakdown of glucose by oxidizing acetyl CoA to CO2. Each turn of the cycle generates:

1 ATP
3 NADH
1 FADH2
2 CO2 (waste)

Since two pyruvate are produced per glucose, the cycle runs twice per glucose molecule.

Oxidative Phosphorylation: Electron Transport Chain and Chemiosmosis

Electron Transport Chain (ETC)

The electron transport chain is a series of protein complexes embedded in the inner mitochondrial membrane. NADH and FADH2 donate electrons to the chain, which are passed through carrier molecules (including cytochromes) and ultimately to oxygen, forming water.

Electrons drop in free energy as they move down the chain.
The ETC breaks the large energy drop from glucose to O2 into manageable steps.

Chemiosmosis

Energy released by the ETC is used to pump protons (H+) from the mitochondrial matrix to the intermembrane space, creating a proton gradient (proton-motive force).

Protons flow back into the matrix through ATP synthase, driving the phosphorylation of ADP to ATP.
This process is called chemiosmosis.

ATP Yield

Most energy flows in the sequence: glucose → NADH → electron transport chain → proton-motive force → ATP. About 34% of the energy in glucose is transferred to ATP (about 32 ATP per glucose); the rest is lost as heat.

Fermentation and Anaerobic Respiration

Fermentation

Fermentation allows cells to produce ATP without oxygen by extending glycolysis. NADH is oxidized by transferring electrons to pyruvate or its derivatives.

Alcohol fermentation: Pyruvate is converted to ethanol and CO2; NAD+ is regenerated.
Lactic acid fermentation: Pyruvate is reduced directly to lactate by NADH; NAD+ is regenerated (no CO2 released).

Anaerobic Respiration

Anaerobic respiration uses an electron transport chain with a final electron acceptor other than oxygen (e.g., sulfate ion SO42-), producing products like hydrogen sulfide (H2S).

Comparison Table: Fermentation vs. Anaerobic and Aerobic Respiration

Process	Final Electron Acceptor	ATP Yield (per glucose)	Location
Fermentation	Organic molecule (e.g., pyruvate, acetaldehyde)	2 (substrate-level phosphorylation)	Cytosol
Anaerobic Respiration	Inorganic molecule (e.g., SO42-)	Varies (more than fermentation, less than aerobic)	Cytosol & membrane
Aerobic Respiration	O2	Up to 32 (oxidative phosphorylation)	Cytosol & mitochondria

Regulation and Integration of Metabolism

Regulation of Cellular Respiration

Cellular respiration is regulated by feedback mechanisms, primarily feedback inhibition. This prevents wasteful production of ATP.

If ATP concentration drops, respiration speeds up.
If there is plenty of ATP, respiration slows down.
Regulation occurs by controlling the activity of enzymes at strategic points in the pathway.

Integration with Other Metabolic Pathways

Glycolysis and the citric acid cycle are major intersections for various metabolic and anabolic pathways.

Carbohydrates, fats, and proteins can all be used as fuel for cellular respiration.
Proteins must be digested to amino acids and deaminated; nitrogenous waste is excreted as ammonia (NH3), urea, or other products.
Fats are digested to glycerol (used in glycolysis) and fatty acids (broken down by beta oxidation to yield acetyl CoA, NADH, and FADH2).
An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate.

Organisms use small molecules from food to build macromolecules, such as proteins from amino acids. These small molecules may come directly from food, glycolysis, or the citric acid cycle.

Types of Organisms Based on Oxygen Requirement

Obligate anaerobes: Carry out fermentation or anaerobic respiration and cannot survive in the presence of O2.
Facultative anaerobes: Can survive using either fermentation or cellular respiration (e.g., yeast, many bacteria).

Summary Table: Key Steps and Locations in Cellular Respiration

Step	Main Reactants	Main Products	Location
Glycolysis	Glucose, 2 NAD+, 2 ATP	2 Pyruvate, 2 NADH, 4 ATP (2 net)	Cytosol
Pyruvate Oxidation	2 Pyruvate, 2 NAD+, 2 CoA	2 Acetyl CoA, 2 NADH, 2 CO2	Mitochondrial matrix
Citric Acid Cycle	2 Acetyl CoA, 6 NAD+, 2 FAD, 2 ADP	4 CO2, 6 NADH, 2 FADH2, 2 ATP	Mitochondrial matrix
Oxidative Phosphorylation	NADH, FADH2, O2, ADP	ATP (about 26-28), H2O	Inner mitochondrial membrane

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