Cellular Respiration: Glycolysis and the Citric Acid Cycle

Cellular Respiration: Glycolysis and the Citric Acid Cycle

Introduction to Cellular Respiration

Cellular respiration is the process by which cells extract energy from organic molecules, primarily glucose, to produce ATP, the universal energy currency of the cell. This process is essential for powering most cellular work and occurs in both plant and animal cells.

• ATP (Adenosine Triphosphate): The main molecule that stores and transfers energy in cells.

• Photosynthesis vs. Cellular Respiration: Photosynthesis captures light energy to produce organic molecules and oxygen, while cellular respiration breaks down these molecules to release energy, carbon dioxide, and water.

• Catabolic Pathways: Metabolic pathways that break down molecules into smaller units, releasing energy.

• Example: Glucose breakdown in cellular respiration.

Overview of Energy Production Pathways

Energy production in cells involves several interconnected pathways, each with specific roles in extracting and transferring energy.

• Glycolysis: The first step in energy production, occurring in the cytoplasm. It converts glucose into pyruvate and generates ATP and NADH.

• Citric Acid Cycle (Krebs Cycle): Takes place in the mitochondrial matrix, further oxidizing pyruvate to produce NADH, FADH2, ATP, and CO2.

• Oxidative Phosphorylation: Uses NADH and FADH2 to generate large amounts of ATP via the electron transport chain and chemiosmosis (covered in a later lecture).

Redox Reactions in Cellular Respiration

Definition and Importance

Redox (reduction-oxidation) reactions involve the transfer of electrons between molecules. These reactions are fundamental to energy extraction in biological systems.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Mnemonic: "LEO the lion says GER" — Loss of Electrons is Oxidation; Gain of Electrons is Reduction.

• Example Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy (ATP + heat)}$

• Electron Carriers: NAD+ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) are key molecules that accept electrons during redox reactions, becoming NADH and FADH2.

Glycolysis

Why Start with Glucose?

Glucose is the primary substrate for glycolysis due to its abundance and central role in metabolism. Most organisms, including plants, animals, and bacteria, utilize glucose as a key energy source.

• Ubiquity: Present in almost all living cells.

• Central Role: Many metabolic pathways converge on or diverge from glucose.

• Well-understood: The steps and regulation of glycolysis are extensively studied.

Phases of Glycolysis

Glycolysis consists of ten enzyme-catalyzed reactions, divided into two main phases:

• Energy Investment Phase (Steps 1-5): ATP is consumed to phosphorylate glucose and its intermediates.

• Energy Generation Phase (Steps 6-10): ATP and NADH are produced as glucose is split into two molecules of pyruvate.

• For anaerobic organisms: Glycolysis is the primary source of ATP, as they do not use oxygen as a final electron acceptor.

Summary Equation for Glycolysis

$\text{Glucose} + 2\text{ADP} + 2\text{P}_i + 2\text{NAD}^+ \rightarrow 2\text{Pyruvate} + 2\text{ATP} + 2\text{NADH} + 2\text{H}_2\text{O}$

Pyruvate Oxidation and the Citric Acid Cycle

Pyruvate Oxidation

Before entering the citric acid cycle, pyruvate produced from glycolysis is transported into the mitochondria and converted into acetyl-CoA.

• Transport: Pyruvate crosses the mitochondrial membranes into the matrix.

• Decarboxylation: One carbon is removed from pyruvate as CO2.

• Formation of Acetyl-CoA: The remaining two-carbon fragment is attached to coenzyme A.

The Citric Acid Cycle (Krebs Cycle)

The citric acid cycle is an eight-step metabolic pathway that completes the oxidation of glucose derivatives, producing energy-rich electron carriers and waste products.

• Main Goals:

  • Produce CO2 (oxidized waste product)

  • Generate NADH and FADH2 (reduced electron carriers)

  • Produce ATP (or GTP, depending on the cell type)

• Key Steps: Each turn of the cycle processes one acetyl-CoA, generating electron carriers for the electron transport chain.

Summary Equation for the Citric Acid Cycle

$2\text{Pyruvate} + 8\text{NAD}^+ + 2\text{FAD} + 2\text{ADP} + 2\text{P}_i \rightarrow 6\text{CO}_2 + 8\text{NADH} + 2\text{FADH}_2 + 2\text{ATP}$

Total Energy Yield from One Glucose Molecule

Process	ATP Produced	NADH Produced	FADH2 Produced	CO2 Produced
Glycolysis	2	2	0	0
Citric Acid Cycle (including pyruvate oxidation)	2	8	2	6
Total	4	10	2	6

Key Concepts and Review

• Redox reactions are central to energy extraction in cells.

• NADH and FADH2 are produced during glycolysis and the citric acid cycle and are used in the electron transport chain to generate more ATP.

• ATP is produced directly in glycolysis and the citric acid cycle, but most ATP is generated later via oxidative phosphorylation.

• CO2 is released as a waste product during pyruvate oxidation and the citric acid cycle.

Additional info: The electron transport chain and chemiosmosis, which use NADH and FADH2 to produce the majority of ATP, will be covered in a subsequent lecture.