Cellular Respiration and Fermentation

Overview

Cellular respiration and fermentation are essential metabolic pathways that allow cells to extract energy from high-potential energy compounds, such as glucose, to produce ATP. This chapter explores how cells generate ATP in the presence and absence of oxygen, the steps involved in these processes, and their regulation and significance in metabolism.

How Cells Produce Energy

ATP and Cellular Work

• ATP (Adenosine Triphosphate) is the primary energy currency of the cell, fueling various cellular processes.

• Cells harvest energy from high-energy molecules (e.g., glucose) through metabolic pathways.

• The energy released from these molecules is used to add a phosphate group to ADP, forming ATP.

Additional info: The process of adding a phosphate group to ADP is called phosphorylation.

Overview of Cellular Respiration

ATP Production and Glucose Utilization

• Cells contain only enough ATP for a few seconds to minutes of activity, so ATP must be continually produced.

• Plants produce glucose via photosynthesis; other organisms obtain glucose from food.

• Glucose can be stored as glycogen (in animals) or starch (in plants).

Oxidation of Glucose

• When glucose is oxidized to carbon dioxide, energy is released.

• In cells, this oxidation occurs through a series of controlled redox reactions, collectively known as cellular respiration.

• Much of the released energy is used to synthesize ATP.

Equation for complete glucose oxidation:

Cellular Respiration vs. Fermentation

• Cellular respiration fully oxidizes glucose, producing more ATP.

• Fermentation partially oxidizes glucose, producing less ATP and small, reduced organic molecules as waste.

Main Steps of Cellular Respiration

Four Key Processes

1. Glycolysis: Glucose (6C) is broken down into two pyruvate (3C) molecules.

2. Pyruvate Oxidation: Each pyruvate is oxidized to form acetyl CoA (2C).

3. Citric Acid Cycle: Each acetyl CoA is oxidized to CO2.

4. Electron Transport and Chemiosmosis: Electrons move through a transport chain, creating a proton gradient used to make ATP.

Metabolic Pathways and Intermediates

Catabolic and Anabolic Pathways

• Catabolic pathways break down molecules, releasing energy to produce ATP.

• Anabolic pathways synthesize larger molecules from smaller components, often using ATP.

Interconnection of Pathways

• Carbohydrates, fats, and proteins can all serve as substrates for cellular respiration.

• Fats are broken down into glycerol (enters glycolysis) and fatty acids (converted to acetyl CoA).

• Proteins are broken down into amino acids; amino groups are removed, and the remaining carbon skeletons enter glycolysis or the citric acid cycle.

Use of Intermediates

• Intermediates from glycolysis and the citric acid cycle are used to synthesize nucleotides, amino acids, fatty acids, and glucose (for storage as glycogen or starch).

• Metabolic regulation maintains homeostasis, allowing cells to adapt to changing conditions.

Glycolysis

Pathway and Regulation

• Glycolysis is a sequence of 10 enzyme-catalyzed reactions occurring in the cytosol.

• It consists of two phases:

  • Energy investment phase (steps 1-5): Consumes 2 ATP.

  • Energy payoff phase (steps 6-10): Produces 4 ATP (net gain 2 ATP) and 2 NADH.

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

• ATP is produced by substrate-level phosphorylation (direct transfer of a phosphate group to ADP).

Key Enzymes Consuming ATP in Glycolysis

• Hexokinase (step 1)

• Phosphofructokinase (step 3)

Regulation

• Glycolysis is regulated by feedback inhibition of phosphofructokinase.

• High ATP levels inhibit the enzyme by binding to a regulatory site, slowing glycolysis.

Pyruvate Oxidation

Conversion to Acetyl CoA

• In eukaryotes, pyruvate is transported into the mitochondrial matrix; in prokaryotes, it occurs in the cytosol.

• Pyruvate dehydrogenase catalyzes the conversion:

• Regulated by phosphorylation (inactivation) and dephosphorylation (activation) in response to energy needs.

Citric Acid Cycle (Krebs Cycle)

Oxidation of Acetyl CoA

• Occurs in the mitochondrial matrix (eukaryotes) or cytosol (prokaryotes).

• Each acetyl CoA is oxidized to 2 CO2.

• Cycle turns twice per glucose molecule.

• Produces per turn: 3 NADH, 1 FADH2, 1 ATP (or GTP), and 2 CO2.

• Regulated by feedback inhibition at multiple steps (e.g., high ATP or NADH slows the cycle).

Summary Table: Citric Acid Cycle Reactions

Electron Transport Chain (ETC) and Chemiosmosis

Electron Transport and ATP Synthesis

• ETC is a series of protein complexes (I-IV) and mobile carriers (ubiquinone/Q, cytochrome c) in the inner mitochondrial membrane.

• NADH and FADH2 donate electrons, which move through the chain, releasing energy.

• Energy is used to pump protons from the matrix to the intermembrane space, creating a proton gradient (proton-motive force).

• Oxygen is the final electron acceptor, forming water.

ATP Synthase and Chemiosmosis

• ATP synthase is a membrane-bound enzyme complex that synthesizes ATP as protons flow back into the matrix.

• This process is called oxidative phosphorylation.

• About 29 ATP are produced per glucose molecule (most via oxidative phosphorylation).

Summary Table: Electron Transport Chain Components

Aerobic vs. Anaerobic Respiration

Electron Acceptors

• Aerobic respiration: Oxygen is the final electron acceptor (most efficient, highest ATP yield).

• Anaerobic respiration: Other molecules (e.g., nitrate, sulfate) serve as electron acceptors in some prokaryotes (lower ATP yield).

Fermentation

Pathways and Importance

• Occurs when no electron acceptor (e.g., O2) is available; ETC stops, NADH accumulates, and NAD+ is depleted.

• Fermentation regenerates NAD+ from NADH, allowing glycolysis to continue and produce ATP by substrate-level phosphorylation.

• Yields only 2 ATP per glucose (much less than cellular respiration).

Types of Fermentation

• Lactic acid fermentation (e.g., in muscle cells): Pyruvate accepts electrons from NADH, forming lactate and regenerating NAD+.

• Alcohol fermentation (e.g., in yeast): Pyruvate is converted to acetaldehyde and CO2; acetaldehyde accepts electrons from NADH, forming ethanol and regenerating NAD+.

Summary Table: Fermentation Pathways

Facultative Anaerobes

• Some organisms can switch between fermentation and aerobic respiration depending on oxygen availability.

Key Concepts and Applications

• Cellular respiration is central to metabolism, providing energy and metabolic intermediates for biosynthesis.

• Regulation of these pathways ensures efficient energy production and homeostasis.

• Understanding these processes is crucial for fields such as medicine, biotechnology, and physiology.