Metabolism and Cellular Respiration: Study Notes for General Biology

Metabolism

Overview of Metabolic Pathways

Metabolism refers to the chemical processes that occur within living organisms to maintain life. In cellular respiration, organic molecules such as glucose are catabolized to release energy through a series of interconnected pathways.

• Glycolysis (occurs in the cytosol)

• Krebs Cycle (also known as the Citric Acid Cycle; occurs in the mitochondria)

• Electron Transport Chain and Oxidative Phosphorylation (occurs in the mitochondria and requires oxygen)

These pathways are essential for the production of ATP, the energy currency of the cell.

Redox Reactions in Metabolism

Oxidation-Reduction (Redox) Reactions

Energy release in metabolism is dependent on the transfer of electrons through redox reactions. These reactions involve the movement of electrons from one molecule to another.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Mnemonic: "OIL RIG" – Oxidation Is Loss, Reduction Is Gain (of electrons).

Redox reactions are fundamental to cellular respiration, allowing cells to extract energy from nutrients.

Cellular Respiration

Overall Reaction

Cellular respiration is the process by which cells extract energy from glucose. The overall chemical equation is:

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

• Glucose is oxidized to carbon dioxide.

• Oxygen is reduced to water.

• This process occurs in multiple steps to ensure controlled energy release.

Glycolysis

Conversion of Glucose to Pyruvic Acid

Glycolysis is the first step in cellular respiration, occurring in the cytosol. It converts one molecule of glucose (6 carbons) into two molecules of pyruvic acid (3 carbons each).

• Initial Investment: 2 ATP molecules are used to phosphorylate glucose, making it more reactive.

• Key Intermediates: Glucose-6-phosphate, Fructose-6-phosphate, and Glyceraldehyde-3-phosphate.

• Net Gain: 2 ATP molecules (4 produced, 2 consumed) and 2 NADH molecules per glucose.

Phosphorylation of glucose is essential for trapping it in the cell and preparing it for further breakdown.

ATP Production in Glycolysis

Substrate-Level Phosphorylation

ATP is produced directly in glycolysis by substrate-level phosphorylation, where a phosphate group is transferred from a substrate molecule to ADP, forming ATP.

• Enzyme Example: Pyruvate kinase catalyzes the final step, producing pyruvate and ATP.

• Net ATP Yield: 2 ATP per glucose molecule.

Krebs Cycle (Citric Acid Cycle)

Overview and Steps

The Krebs cycle occurs in the mitochondria and requires oxygen. It completes the oxidation of glucose derivatives, producing electron carriers for the electron transport chain.

• Pyruvic acid is converted to Acetyl-CoA, which enters the cycle.

• Each glucose yields two Acetyl-CoA molecules, so the cycle turns twice per glucose.

• Products per glucose:

  • 2 ATP (via substrate-level phosphorylation)

  • 6 NADH

  • 2 FADH2

  • 4 CO2 (released as waste)

Most ATP is not produced directly in the Krebs cycle but via the electron transport chain using NADH and FADH2.

Electron Transport Chain and Oxidative Phosphorylation

Mechanism and ATP Yield

The electron transport chain (ETC) is located in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through protein complexes, ultimately reducing oxygen to water.

• Electron transfer drives the pumping of protons (H+) into the intermembrane space, creating a proton gradient.

• ATP synthase uses this gradient to synthesize ATP from ADP and inorganic phosphate.

• ATP Yield:

  • Each NADH: ~2.3 ATP

  • Each FADH2: ~1.4 ATP

  • Total ATP per glucose (theoretical maximum): ~30-32 ATP

Some protons may leak back into the matrix without passing through ATP synthase, reducing the actual ATP yield and producing heat.

Proton Leak and Heat Production

Uncoupling Proteins and Thermogenesis

Proton leak occurs when protons diffuse back into the mitochondrial matrix through channels other than ATP synthase, such as uncoupling proteins. This process is exergonic and releases heat, contributing to thermogenesis.

• Uncoupling proteins are important in brown adipose tissue for heat generation.

• Proton leak decreases the efficiency of ATP production.

Reactive Oxygen Species (ROS) Production

Byproducts of Electron Transport

The electron transport chain can produce reactive oxygen species (ROS) when electrons escape and react with oxygen. ROS include superoxide (O2-), hydrogen peroxide (H2O2), and hydroxyl radicals (OH•).

• ROS can damage lipids, proteins, and DNA.

• Oxidative damage is linked to aging and diseases such as cancer.

Anaerobic Metabolism and Fermentation

Metabolism Without Oxygen

When oxygen is unavailable, cells cannot use the electron transport chain. Instead, they rely on fermentation to regenerate NAD+ and allow glycolysis to continue.

• Lactic Acid Fermentation: Pyruvate is reduced to lactic acid (common in muscle cells).

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2 (common in yeast).

• Fermentation yields only 2 ATP per glucose, much less than aerobic respiration.

Fermentation is less efficient and can lead to the accumulation of acidic or toxic byproducts.

Summary Table: ATP Yield from Glucose Metabolism

Pathway	ATP Produced (per glucose)	NADH Produced	FADH2 Produced
Glycolysis	2	2	0
Krebs Cycle	2	6	2
Electron Transport Chain	~26-28	-	-
Total (Aerobic)	~30-32	-	-
Fermentation (Anaerobic)	2	0	0

Additional info: The actual ATP yield may vary due to proton leak and other cellular inefficiencies.