Cellular Metabolism

Introduction to Metabolism

Cellular metabolism refers to the chemical processes that occur within living cells to maintain life. These processes include the breakdown of nutrients to produce energy and the synthesis of molecules needed for cellular function.

• Metabolism is divided into catabolism (breaking down molecules to release energy) and anabolism (building up molecules using energy).

• Energy produced is primarily stored and transferred in the form of ATP (adenosine triphosphate).

ATP Synthesis

Mechanisms of ATP Production

ATP is synthesized in cells through several mechanisms, each involving different biochemical pathways and cellular locations.

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from a phosphorylated intermediate. Occurs in glycolysis and the Krebs cycle.

• Oxidative phosphorylation: ATP is produced using energy derived from the transfer of electrons through the electron transport chain to oxygen. Occurs in mitochondria.

General equations:

• Substrate-level phosphorylation:

• Oxidative phosphorylation:

Glucose Oxidation

Overview of Glucose Oxidation

Glucose oxidation is the primary pathway for energy production in most cells. It involves a series of reactions that convert glucose into carbon dioxide, water, and ATP.

• Overall reaction:

• Energy change: kcal/mole (energy released per mole of glucose oxidized)

Efficiency of Glucose Oxidation

Not all energy released from glucose oxidation is captured as ATP; a significant portion is lost as heat.

• 1 mole of glucose produces 32 ATP molecules.

• Energy in 32 ATP: kcal

• Efficiency:

• Remaining 67% of energy is released as body heat.

Stages of Glucose Oxidation

Major Steps

Glucose oxidation occurs in several distinct stages, each contributing to ATP production.

1. Glycolysis: Occurs in the cytosol; converts glucose to pyruvate, producing ATP and NADH.

2. Link Reaction (Pyruvate to Acetyl CoA): Pyruvate is converted to acetyl CoA in mitochondria, producing NADH and CO2.

3. Krebs Cycle (Citric Acid Cycle): Acetyl CoA enters the cycle, generating NADH, FADH2, ATP, and CO2.

4. Electron Transport Chain & Oxidative Phosphorylation: NADH and FADH2 donate electrons, driving ATP synthesis and producing water.

Summary Table: Glucose Oxidation Steps and Products

Glycolysis

Process and Products

Glycolysis is the first step in glucose metabolism, occurring in the cytosol. It breaks down one molecule of glucose into two molecules of pyruvate.

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

• Equation:

• Substrate-level phosphorylation is responsible for ATP production in glycolysis.

Link Reaction: Pyruvate to Acetyl CoA

Conversion and Significance

Pyruvate produced in glycolysis is transported into mitochondria and converted to acetyl CoA, which enters the Krebs cycle.

• Each pyruvate yields 1 acetyl CoA, 1 NADH, and 1 CO2.

• Equation:

• This step is unidirectional and links glycolysis to the Krebs cycle.

Krebs Cycle (Citric Acid Cycle)

Cycle Overview

The Krebs cycle is a series of reactions in the mitochondrial matrix that further oxidizes acetyl CoA, producing high-energy electron carriers and ATP.

• Each turn of the cycle (per acetyl CoA) produces: 3 NADH, 1 FADH2, 1 ATP, and 2 CO2.

• Since two acetyl CoA are produced per glucose, the cycle turns twice per glucose molecule.

• Equation (per glucose):

Electron Transport Chain & Oxidative Phosphorylation

ATP Generation and Final Products

NADH and FADH2 produced in earlier steps donate electrons to the electron transport chain, driving the synthesis of ATP and producing water.

• Oxygen acts as the final electron acceptor, forming water.

• Most ATP is generated in this stage (~26-28 ATP per glucose).

• Equation:

Summary of Aerobic Glucose Metabolism

Key Points

• One molecule of glucose yields up to 32 ATP via aerobic metabolism.

• Major products: ATP, CO2, H2O, NADH, FADH2.

• Efficiency of energy capture is about 33%, with the remainder lost as heat.

Summary Table: Products of Aerobic Glucose Metabolism

Example: During intense exercise, muscle cells rely on these metabolic pathways to rapidly generate ATP for contraction.

Additional info: Some details, such as the exact ATP yield and the efficiency calculation, were inferred and expanded for academic completeness.