Cellular Respiration and Fermentation: Processes, Mechanisms, and Energy Yield (chapter 7)

Cellular Respiration and Fermentation

Overview and Learning Objectives

Cellular respiration and fermentation are essential metabolic processes that enable cells to extract energy from organic molecules. These notes cover the relationship between photosynthesis and cellular respiration, redox reactions, ATP structure and function, and the mechanisms of substrate-level and oxidative phosphorylation.

• Relationship between Photosynthesis and Cellular Respiration: Photosynthesis converts light energy into chemical energy, while cellular respiration (including fermentation) converts chemical energy into ATP to power cellular work.

• Redox Reactions: Understanding oxidation and reduction, and identifying oxidizing and reducing agents.

• Phosphorylation Mechanisms: Differentiating substrate-level phosphorylation and oxidative phosphorylation.

• Electron Carriers: Roles of NAD+/NADH and FAD/FADH2 in metabolism.

• Key Steps in Cellular Respiration: Glycolysis, pyruvate oxidation, citric acid cycle, and oxidative phosphorylation.

Energy Flow in Ecosystems

Photosynthesis and Cellular Respiration

Energy transformation in ecosystems involves the conversion of light energy to chemical energy and then to ATP, which is used for cellular work.

• Photosynthesis: Occurs in chloroplasts; uses CO2 and H2O to produce organic molecules and O2.

• Cellular Respiration: Occurs in mitochondria; breaks down organic molecules to generate ATP, releasing CO2 and H2O.

• Energy Loss: Some energy is lost as heat during these processes.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Redox reactions are central to energy transfer in cells. They involve the movement of electrons between molecules.

• Oxidation: Loss of electrons; the molecule becomes more positive.

• Reduction: Gain of electrons; the molecule becomes more negative.

• Reducing Agent: Donates electrons and becomes oxidized.

• Oxidizing Agent: Accepts electrons and becomes reduced.

Example:

In the reaction: A (reducing agent) + B (oxidizing agent) → A+ + B- A loses electrons (oxidized), B gains electrons (reduced).

ATP Structure and Function

Adenosine Triphosphate (ATP)

ATP is the primary energy currency of the cell, composed of adenosine (adenine + ribose) and three phosphate groups.

• Hydrolysis of ATP: Releases energy by breaking the terminal phosphate bond. $ATP + H_2O ightarrow ADP + P_i + ext{energy}$

• Energy Coupling: ATP hydrolysis is coupled to endergonic (energy-requiring) reactions in the cell.

• ATP Cycle: ATP is recycled; synthesized from ADP and inorganic phosphate during catabolic reactions.

Phosphorylation Mechanisms

Substrate-Level Phosphorylation

Direct transfer of a phosphate group from a substrate to ADP, forming ATP. This occurs during glycolysis and the citric acid cycle.

• Enzyme-Catalyzed: An enzyme facilitates the transfer of the phosphate group.

• Example Reaction: $ext{Substrate-P} + ADP ightarrow ext{Product} + ATP$

Oxidative Phosphorylation

ATP is produced using energy derived from the electron transport chain and chemiosmosis in mitochondria.

• Electron Transport Chain (ETC): Electrons are transferred through a series of protein complexes, ultimately reducing oxygen to water.

• ATP Synthase: Uses the proton gradient generated by the ETC to synthesize ATP from ADP and inorganic phosphate.

• Overall Reaction: $NADH + H^+ + 1/2 O_2 ightarrow NAD^+ + H_2O$ $ADP + P_i ightarrow ATP$

Electron Carriers in Cellular Metabolism

NAD+/NADH and FAD/FADH2

Electron carriers are molecules that transport electrons during cellular respiration.

• NAD+ (Nicotinamide Adenine Dinucleotide): Accepts electrons to become NADH.

• FAD (Flavin Adenine Dinucleotide): Accepts electrons to become FADH2.

• Role: Carry electrons from glycolysis, pyruvate oxidation, and the citric acid cycle to the electron transport chain.

Summary Table: Key Processes in Cellular Respiration

Process	Main Location	Inputs	Outputs	ATP Yield
Glycolysis	Cytosol	Glucose, 2 NAD+, 2 ADP	2 Pyruvate, 2 NADH, 2 ATP (net)	2
Pyruvate Oxidation	Mitochondrial Matrix	2 Pyruvate, 2 NAD+, 2 CoA	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, ADP	NAD+, FAD, H2O, ATP	~26-28

Additional info:

• These notes are based on lecture slides and class notes for a General Biology college course, focusing on cellular respiration and fermentation.

• Further details on glycolysis, pyruvate oxidation, citric acid cycle, and fermentation are covered in subsequent sections of the course.