Metabolic Pathways: Catabolic Pathways and Cellular Respiration (Chapter 9: Concepts 9.1–9.6)

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Metabolic Pathways: Catabolic Pathways

Introduction to Catabolic Pathways

Catabolic pathways are essential metabolic routes that break down complex molecules into simpler ones, releasing energy that the cell can use to perform work. Cellular respiration is a central catabolic pathway in most organisms, converting biochemical energy from nutrients into adenosine triphosphate (ATP).

Catabolism: The breakdown of complex molecules into simpler ones, releasing energy.
Anabolism: The synthesis of complex molecules from simpler ones, requiring energy input.
Redox Reactions: Chemical reactions involving the transfer of electrons between molecules; central to energy extraction in catabolism.

Redox Reactions in Metabolism

Definitions and Examples

Oxidation: Loss of electrons from a molecule.
Reduction: Gain of electrons by a molecule.
Reducing Agent: Donates electrons and becomes oxidized.
Oxidizing Agent: Accepts electrons and becomes reduced.
Example: In cellular respiration, glucose is oxidized and oxygen is reduced.

Glycolysis

Overview and Phases

Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. It occurs in the cytoplasm and does not require oxygen.

Energy Investment Phase: 2 ATP are used to phosphorylate glucose and its intermediates.
Energy Payoff Phase: 4 ATP and 2 NADH are produced per glucose molecule.
Net Yield: 2 ATP, 2 NADH, and 2 pyruvate per glucose.

Pyruvate Oxidation and the Citric Acid Cycle

Pyruvate Oxidation

Pyruvate (from glycolysis) is transported into the mitochondrion and converted to acetyl-CoA.
This process produces NADH and releases CO2.

Citric Acid Cycle (Krebs Cycle)

Acetyl-CoA enters the cycle, which completes the oxidation of glucose derivatives.
Each turn of the cycle produces 3 NADH, 1 FADH2, 1 ATP (via substrate-level phosphorylation), and 2 CO2.

Electron Transport Chain (ETC) and Chemiosmosis

Pathway of Electron Transport

Electrons from NADH and FADH2 are transferred through a series of protein complexes in the inner mitochondrial membrane.
FADH2 enters the ETC at complex II, resulting in less ATP production compared to NADH.
Oxygen is the final electron acceptor, forming water.

Chemiosmosis and ATP Synthase

ATP Synthase: Enzyme complex that synthesizes ATP from ADP and inorganic phosphate using the energy from the proton gradient.
Protons (H+) flow down their gradient through ATP synthase, causing it to rotate and catalyze ATP formation.
This process is called oxidative phosphorylation.

Proton-Motive Force

The ETC pumps protons from the mitochondrial matrix to the intermembrane space, creating an electrochemical gradient (proton-motive force).
Energy stored in this gradient is used to drive ATP synthesis.

Accounting of ATP Production by Cellular Respiration

ATP Yield per Glucose Molecule

Glycolysis: 2 ATP (substrate-level phosphorylation), 2 NADH
Pyruvate Oxidation: 2 NADH
Citric Acid Cycle: 2 ATP (substrate-level phosphorylation), 6 NADH, 2 FADH2
ETC and Oxidative Phosphorylation: ~28 ATP
Total Maximum Yield: ~32 ATP per glucose

Note: The actual ATP yield varies due to differences in shuttle mechanisms and the use of the proton-motive force for other cellular work.

What Happens if There is No Oxygen?

Anaerobic Respiration

Uses an electron transport chain with a final electron acceptor other than oxygen (e.g., SO42- in some bacteria).
Produces less ATP than aerobic respiration.

Fermentation

Does not use an electron transport chain or oxygen.
Regenerates NAD+ from NADH, allowing glycolysis to continue.
Yields only 2 ATP per glucose (from glycolysis).

Types of Fermentation

Alcohol Fermentation

Pyruvate is converted to ethanol in two steps:
1. CO2 is released from pyruvate, forming acetaldehyde.
2. Acetaldehyde is reduced by NADH to ethanol, regenerating NAD+.
Occurs in yeast and some bacteria.

Lactic Acid Fermentation

Pyruvate is reduced directly by NADH to form lactate (lactic acid).
No CO2 is released.
Occurs in some bacteria, fungi, and animal muscle cells during intense exercise.
Lactate can be transported to the liver and converted back to pyruvate when oxygen is available.

Summary of Catabolism

All three methods (aerobic respiration, anaerobic respiration, fermentation) use glycolysis to oxidize glucose to pyruvate.
All use NAD+ as the oxidizing agent.
Fermentation: Uses an organic molecule as the final electron acceptor (e.g., pyruvate or acetaldehyde).
Cellular Respiration: Uses an electron transport chain to regenerate NAD+ and produces more ATP.

Process	Final Electron Acceptor	ATP Yield per Glucose
Aerobic Respiration	O2	~32
Anaerobic Respiration	Other inorganic molecules (e.g., SO42-)	Varies (<32)
Fermentation	Organic molecule (e.g., pyruvate, acetaldehyde)	2

Obligate Anaerobes: Organisms that only carry out fermentation or anaerobic respiration.
Facultative Anaerobes: Organisms that can switch between fermentation and aerobic respiration depending on oxygen availability.

Connections to Other Metabolic Pathways

Integration of Catabolism

Proteins are broken down into amino acids, which can enter glycolysis or the citric acid cycle after deamination.
Carbohydrates are hydrolyzed to sugars, which enter glycolysis.
Fats are broken down by beta oxidation into glycerol (enters glycolysis) and fatty acids (converted to acetyl-CoA for the citric acid cycle).

Key Equations

Overall equation for aerobic cellular respiration:

ATP synthesis by substrate-level phosphorylation (SLP):

ATP synthesis by oxidative phosphorylation (OP):

Summary Table: ATP Yield from Cellular Respiration

Stage	ATP Produced (per glucose)	NADH Produced	FADH2 Produced
Glycolysis	2	2	0
Pyruvate Oxidation	0	2	0
Citric Acid Cycle	2	6	2
Oxidative Phosphorylation	~28	-	-
Total	~32	10	2

Additional info: The actual ATP yield can vary depending on the cell type and the shuttle systems used to transport electrons into the mitochondria.