Cellular Respiration and Fermentation: Study Notes

Chapter 9: Cellular Respiration and Fermentation

Introduction

Cellular respiration and fermentation are essential metabolic pathways that allow cells to extract energy from organic molecules. These processes are fundamental to life, providing the ATP required for most cellular work. This chapter explores the steps, mechanisms, and regulation of cellular respiration and fermentation, as well as their interconnections with other metabolic pathways.

Catabolic Pathways and Energy Harvest

Catabolic Pathways Yield Energy by Oxidizing Organic Fuels

Catabolic pathways break down complex molecules into simpler ones, releasing stored energy. This energy is used to synthesize ATP, the main energy currency of the cell.

• 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.

• Energy enters ecosystems as light and exits as heat, while essential elements are recycled.

Types of Catabolic Pathways

• Fermentation: Partial degradation of sugars without oxygen.

• Aerobic respiration: Consumes organic molecules and oxygen, yielding ATP.

• Anaerobic respiration: Similar to aerobic respiration but uses compounds other than oxygen as final electron acceptors.

Overall Equation for Cellular Respiration

The process is often summarized using glucose as the fuel:

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

• Cells must constantly regenerate ATP from ADP and phosphate to sustain life.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Redox reactions involve the transfer of electrons between reactants, releasing energy stored in organic molecules.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance (reduces positive charge).

• Reducing agent: Electron donor (becomes oxidized).

• Oxidizing agent: Electron acceptor (becomes reduced).

Example redox reaction:

$CH_4 + 2 O_2 \rightarrow CO_2 + 2 H_2O$

• Methane (CH4) is oxidized; oxygen (O2) is reduced.

Electron Carriers: NAD+ and NADH

• NAD+ (Nicotinamide adenine dinucleotide): Functions as an electron carrier and oxidizing agent.

• NADH: The reduced form, stores energy used to synthesize ATP.

• Enzymes called dehydrogenases remove hydrogen atoms from substrates, transferring electrons to NAD+ to form NADH.

Stages of Cellular Respiration

Overview of Stages

Cellular respiration consists of three main stages:

1. Glycolysis: Breaks down glucose into two molecules of pyruvate (occurs in cytosol).

2. Pyruvate oxidation and the Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose to CO2 (occurs in mitochondria).

3. Oxidative phosphorylation: Includes the electron transport chain and chemiosmosis, producing most ATP (occurs in mitochondria).

Substrate-Level vs. Oxidative Phosphorylation

• Substrate-level phosphorylation: Enzyme transfers a phosphate group directly from a substrate to ADP, forming ATP (occurs in glycolysis and citric acid cycle).

• Oxidative phosphorylation: Accounts for about 90% of ATP production, powered by redox reactions in the electron transport chain.

ATP Yield

• Up to 32 ATP molecules are produced per molecule of glucose during cellular respiration.

• Exact yield varies due to differences in shuttle mechanisms and use of proton-motive force for other work.

Glycolysis

Process and Phases

Glycolysis occurs in the cytoplasm and consists of two phases:

• Energy investment phase: 2 ATP are used to split glucose into two three-carbon sugars.

• Energy payoff phase: 4 ATP are produced (net gain of 2 ATP), 2 NAD+ are reduced to 2 NADH, and 2 pyruvate and 2 H2O are formed.

Key facts:

• All carbon from glucose is accounted for in pyruvate.

• No CO2 is released during glycolysis.

• Glycolysis occurs with or without oxygen.

Pyruvate Oxidation and the Citric Acid Cycle

Pyruvate Oxidation

• In eukaryotes, pyruvate enters mitochondria if O2 is present.

• Pyruvate is converted to acetyl coenzyme A (acetyl CoA) by pyruvate dehydrogenase.

• Three steps: oxidation (releases CO2), reduction of NAD+ to NADH, and combination with coenzyme A.

Citric Acid Cycle (Krebs Cycle)

• Completes the breakdown of glucose by oxidizing acetyl CoA to CO2.

• Each turn generates: 1 ATP, 3 NADH, 1 FADH2, and 2 CO2 (per acetyl CoA).

• Cycle runs twice per glucose molecule.

• NADH and FADH2 carry electrons to the electron transport chain.

Oxidative Phosphorylation

Electron Transport Chain (ETC)

• Located in the inner mitochondrial membrane (cristae) in eukaryotes; plasma membrane in prokaryotes.

• Consists of multiprotein complexes, including cytochromes (proteins with heme groups).

• NADH and FADH2 donate electrons, which are passed through the chain to O2, forming H2O.

• Electron transfer releases energy used to pump H+ into the intermembrane space, creating a proton gradient.

Chemiosmosis and ATP Synthase

• H+ flows back into the mitochondrial matrix through ATP synthase, driving ATP production.

• This process is called chemiosmosis.

• The H+ gradient is known as the proton-motive force.

Fermentation and Anaerobic Respiration

Fermentation

• Allows ATP production without oxygen by extending glycolysis.

• Regenerates NAD+ by transferring electrons from NADH to pyruvate or its derivatives.

• Two common types:

  • Alcohol fermentation: Pyruvate is converted to ethanol; CO2 is released.

  • Lactic acid fermentation: Pyruvate is reduced to lactate; no CO2 is released.

Anaerobic Respiration

• Uses an electron transport chain with a final electron acceptor other than oxygen (e.g., sulfate ion SO42-).

• Produces less ATP than aerobic respiration.

Comparison Table: Fermentation vs. Anaerobic and Aerobic Respiration

Process	Final Electron Acceptor	ATP Yield (per glucose)	Oxygen Required?
Fermentation	Organic molecule (e.g., pyruvate, acetaldehyde)	2	No
Anaerobic Respiration	Inorganic molecule (not O2, e.g., SO42-)	Varies (less than aerobic)	No
Aerobic Respiration	O2	~32	Yes

Metabolic Integration and Regulation

Connections to Other Metabolic 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, urea, or other products.

• Fats are broken down by beta oxidation to yield acetyl CoA, NADH, and FADH2.

• Fats yield more than twice as much ATP per gram as carbohydrates.

Regulation of Cellular Respiration

• Feedback inhibition is the most common mechanism of metabolic control.

• If ATP concentration drops, respiration speeds up; if ATP is abundant, respiration slows down.

• Regulation occurs at key enzymatic steps, such as phosphofructokinase in glycolysis.

Key Terms and Concepts

• ATP (Adenosine triphosphate): Main energy currency of the cell.

• Glycolysis: First step in glucose breakdown, occurs in cytosol.

• Citric Acid Cycle (Krebs Cycle): Completes glucose oxidation, occurs in mitochondria.

• Electron Transport Chain: Series of proteins that transfer electrons and pump protons to generate ATP.

• Chemiosmosis: Process by which ATP is produced using a proton gradient.

• Fermentation: Anaerobic process that allows glycolysis to continue by regenerating NAD+.

• Obligate anaerobes: Organisms that cannot survive in the presence of oxygen.

• Facultative anaerobes: Organisms that can survive using either fermentation or respiration.