Cellular Respiration and Fermentation: Key Concepts and Processes

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Chapter 7: Cellular Respiration and Fermentation

Overview

This chapter explores how cells extract energy from organic molecules, primarily glucose, to generate ATP through cellular respiration and fermentation. These processes are central to metabolism in both plant and animal cells.

Key Terms

Acetyl CoA: A molecule that conveys carbon atoms into the citric acid cycle.
Aerobic respiration: Respiration using oxygen as the final electron acceptor.
Alcohol fermentation: Anaerobic process producing ethanol and CO2.
ATP synthase: Enzyme that synthesizes ATP using a proton gradient.
Citric acid cycle (Krebs cycle): Series of reactions that complete the breakdown of glucose.
Electron transport chain: Sequence of electron carrier molecules that transfer electrons and release energy for ATP synthesis.
FAD: Electron carrier reduced to FADH2 during the citric acid cycle.
Fermentation: Anaerobic process that allows glycolysis to continue by regenerating NAD+.
Glycolysis: The breakdown of glucose into pyruvate, producing ATP and NADH.
Lactic acid fermentation: Anaerobic process producing lactate.
Mitochondria matrix: The site of the citric acid cycle in eukaryotes.
NAD+: Electron carrier reduced to NADH during glycolysis and the citric acid cycle.
Oxidation: Loss of electrons from a substance.
Oxidative phosphorylation: ATP synthesis powered by the electron transport chain.
Pyruvate: End product of glycolysis, substrate for the citric acid cycle.
Redox reaction: Chemical reaction involving the transfer of electrons.
Reduction: Gain of electrons by a substance.

How is the Chemical Energy Stored in Food Used to Generate ATP?

Plant and animal cells break down organic molecules using cellular respiration in mitochondria.
Chemical energy in organic molecules is transformed into chemical energy in ATP.
Some energy is released as heat.
Photosynthesis and cellular respiration are complementary processes in the global carbon and energy cycles.

Catabolic Pathways Yield Energy by Oxidizing Organic Fuels

Energy Flow and Recycling

Energy enters ecosystems as light and exits as heat.
Chemical elements essential to life are recycled.
Photosynthesis uses CO2 and H2O to make organic molecules and produces O2 as a waste product.
Cellular respiration uses O2 and organic molecules to make ATP, producing CO2 and H2O as waste products.

Types of Catabolic Pathways

Fermentation: Partial degradation of sugars without O2.
Aerobic respiration: Consumes organic molecules and O2; most efficient ATP production.
Anaerobic respiration: Similar to aerobic but uses electron acceptors other than O2.

General Equation for Cellular Respiration

The overall process can be summarized as:

Carbohydrates, fats, and proteins can all serve as fuel, but glucose is the primary molecule traced in most studies.
ATP functions as a chemical drive shaft, linking catabolic and cellular work.

Redox Reactions in Cellular Respiration

Definitions and Principles

Oxidation: Loss of electrons from a substance.
Reduction: Gain of electrons by a substance (reduces positive charge).
Redox reactions involve the transfer of electrons, releasing energy stored in organic molecules.
The electron donor is the reducing agent; the electron acceptor is the oxidizing agent.
In biological systems, redox reactions often change electron sharing in covalent bonds rather than complete electron transfer.

Example: Methane Combustion

Methane (CH4) is oxidized; oxygen is reduced.
Electrons move from less electronegative atoms (C, H) to more electronegative atoms (O), releasing energy.

Stepwise Energy Harvest via NAD+ and the Electron Transport Chain

Glucose and other organic molecules are broken down in a series of steps.
Electrons are first transferred to NAD+, a coenzyme and oxidizing agent, forming NADH.
Dehydrogenase enzymes facilitate this transfer, releasing a hydrogen ion (H+).
NADH stores energy and donates electrons to the electron transport chain (ETC).
Electrons are passed through a series of increasingly electronegative carriers, ultimately reducing O2 to H2O.
This stepwise transfer allows energy to be released in manageable amounts for ATP synthesis.

Stages of Cellular Respiration

Overview of the Three Main Stages

Glycolysis (in cytosol): Breaks down glucose (6C) into two pyruvate (3C) molecules.
Pyruvate Oxidation and Citric Acid Cycle (in mitochondrial matrix): Pyruvate is oxidized to acetyl CoA, which enters the citric acid cycle, completing glucose breakdown.
Oxidative Phosphorylation (in inner mitochondrial membrane): Electron transport and chemiosmosis produce most ATP.

NAD+ and FAD are reduced to NADH and FADH2 during glycolysis and the citric acid cycle, carrying electrons to the ETC.
Most ATP is produced by oxidative phosphorylation; a smaller amount is produced by substrate-level phosphorylation in glycolysis and the citric acid cycle.
Each glucose molecule can yield up to 32 ATP molecules.

Glycolysis: Harvesting Chemical Energy

Phases of Glycolysis

Energy Investment Phase: 2 ATP are used to phosphorylate glucose and its intermediates.
Energy Payoff Phase: 4 ATP and 2 NADH are produced as G3P is oxidized to pyruvate.

Net products per glucose:

2 ATP (net gain)
2 NADH + 2 H+
2 Pyruvate + 2 H2O

Glycolysis can occur with or without oxygen. If O2 is present, pyruvate enters the mitochondria for further oxidation.

Pyruvate Oxidation and the Citric Acid Cycle

Pyruvate Oxidation

In the presence of O2, pyruvate is transported into the mitochondrial matrix (eukaryotes) or remains in the cytosol (prokaryotes).
Each pyruvate loses a CO2 and is oxidized by NAD+ to form NADH and an acetyl group.
The acetyl group combines with coenzyme A to form acetyl CoA.

Citric Acid Cycle (Krebs Cycle)

Acetyl CoA combines with oxaloacetate to form citrate.
Through a series of eight enzyme-catalyzed steps, citrate is oxidized, releasing CO2, NADH, FADH2, and ATP (or GTP).
Each turn of the cycle (per pyruvate) yields: 1 ATP, 3 NADH, 1 FADH2, and 2 CO2.
The cycle turns twice per glucose molecule.

Oxidative Phosphorylation: Electron Transport and Chemiosmosis

Electron Transport Chain (ETC)

Located in the inner mitochondrial membrane (eukaryotes) or plasma membrane (prokaryotes).
Composed of multiprotein complexes (I-IV) and mobile carriers (e.g., cytochromes).
Electrons from NADH and FADH2 are transferred through the chain, dropping in free energy.
O2 is the final electron acceptor, forming H2O.
The ETC does not directly produce ATP; it creates a proton gradient for chemiosmosis.

Chemiosmosis and ATP Synthase

Energy from electron transfer is used to pump H+ into the intermembrane space, creating a proton gradient (proton-motive force).
H+ diffuses back into the matrix through ATP synthase, driving ATP production.
This process is called chemiosmosis.

ATP Yield

Most energy flows: glucose → NADH → ETC → proton-motive force → ATP.
About 34% of glucose's energy is transferred to ATP (about 32 ATP per glucose).
Exact ATP yield varies due to differences in shuttle mechanisms and use of the proton gradient for other work.

Stage	ATP Produced (per glucose)	NADH Produced	FADH2 Produced
Glycolysis	2 (net)	2	0
Pyruvate Oxidation	0	2	0
Citric Acid Cycle	2	6	2
Oxidative Phosphorylation	~28	-	-
Total	~32	-	-

Fermentation and Anaerobic Respiration

ATP Production without Oxygen

Without O2, the ETC stops and oxidative phosphorylation ceases.
Cells can generate ATP via glycolysis, coupled to fermentation or anaerobic respiration.
Anaerobic respiration uses an ETC with a final electron acceptor other than O2 (e.g., sulfate).
Fermentation regenerates NAD+ by transferring electrons from NADH to pyruvate or its derivatives.

Types of Fermentation

Alcohol fermentation: Pyruvate is converted to ethanol and CO2 (e.g., yeast).
Lactic acid fermentation: Pyruvate is reduced to lactate (e.g., muscle cells, some bacteria).
Fermentation yields 2 ATP per glucose (from glycolysis).

Comparison Table: Fermentation vs. Anaerobic and Aerobic Respiration

Process	Final Electron Acceptor	ATP Yield (per glucose)	ETC Used?
Fermentation	Organic molecule (e.g., pyruvate, acetaldehyde)	2	No
Anaerobic Respiration	Inorganic molecule (e.g., SO42-)	Varies (<32)	Yes
Aerobic Respiration	O2	~32	Yes

Facultative and Obligate Anaerobes

Obligate anaerobes: Only use fermentation or anaerobic respiration; cannot survive in O2.
Facultative anaerobes: Can use either fermentation or aerobic respiration (e.g., yeast, many bacteria).

Connections to Other Metabolic Pathways

Glycolysis and the citric acid cycle are central to both catabolic and anabolic pathways.
Carbohydrates, fats, and proteins can all be funneled into cellular respiration at various points.
Fats are broken down by beta oxidation to acetyl CoA, yielding more ATP per gram than carbohydrates.
Proteins are deaminated and their carbon skeletons enter glycolysis or the citric acid cycle.
Some intermediates serve as precursors for biosynthesis (anabolic pathways).

Evolutionary Significance of Glycolysis

Glycolysis is a universal pathway, indicating it evolved early in the history of life.
It does not require oxygen and occurs in the cytosol, suggesting it predates mitochondria and atmospheric O2.