Ch 8

Harvesting Energy: Glycolysis and Cellular Respiration

Photosynthesis

• Definition: Photosynthetic organisms capture the energy of sunlight and store it in the form of glucose.

• Overall Equation: The overall equation for photosynthesis is:
  $6 ext{CO}<em>2 + 6 ext{H}</em>2 ext{O} <br>ightarrow ext{C}<em>6 ext{H}</em>{12} ext{O}<em>6 + 6 ext{O}</em>2$

Glucose

• Key Role: Glucose is a critical energy-storing molecule.

  • Nearly all cells metabolize glucose for energy.

  • Glucose metabolism is straightforward and efficient.

  • Other organic molecules can be converted to glucose for energy harvesting.

• Energy Release: During glucose breakdown, all cells release solar energy that was originally captured by plants through photosynthesis, utilizing it to produce ATP (adenosine triphosphate).

 Overview of Glucose Breakdown

• Overall Equation: The complete breakdown of glucose can be represented as:
  $ext{C}<em>6 ext{H}</em>{12} ext{O}<em>6 + 6 ext{O}</em>2 <br>ightarrow 6 ext{CO}<em>2 + 6 ext{H}</em>2 ext{O} + ext{ATP}$

• Main Stages of Glucose Metabolism:

  1. Glycolysis

  2. Cellular Respiration

Glycolysis

• Location: Glycolysis occurs in the cytosol of the cell.

• Oxygen Requirement: Glycolysis does not require oxygen (anaerobic process).

• Process: It breaks glucose into pyruvate, yielding a net of two molecules of ATP per molecule of glucose.

Fermentation (in Absence of Oxygen)

• Process: If oxygen is absent, fermentation occurs where pyruvate is converted into either lactate or ethanol and CO2.

Cellular Respiration

• Location: Cellular respiration occurs in mitochondria (only in eukaryotic cells).

• Oxygen Requirement: Requires oxygen (aerobic process).

• Process: Breaks down pyruvate into carbon dioxide and water, producing an additional 32-34 ATP molecules, depending on cell type.

Detailed Phases of Glycolysis

• Phases:

  1. Glucose Activation Phase

  2. Energy Harvesting Phase

• Summary: Each molecule of glucose is broken down into two molecules of pyruvate, and a net of two ATP molecules and two NADH (high-energy electron carriers) are formed.

Aerobic vs Anaerobic Conditions

• Under aerobic conditions, NADH produced in glycolysis is transported to ATP-generating reactions in the mitochondria, allowing NAD+ to recycle back in glycolysis.

• Under anaerobic conditions, pyruvate is converted into lactate or ethanol through fermentation; this does not produce additional ATP but is crucial for regenerating NAD+, enabling glycolysis to continue.

Fermentation in Human Cells

• Human Muscle Cells: Can perform fermentation under anaerobic conditions, particularly when oxygen is consumed faster than it can be supplied (e.g., during sprinting), resulting in lactic acid (lactate) production.

Fermentation in Microbes

• Some microbes ferment pyruvate to form acids like lactate (e.g., in cheese, yogurt, and sour cream production).

• Other microbes may ferement exclusively instead of performing aerobic respiration.

• Yeast Cells: Perform alcoholic fermentation where glucose is fermented to ethanol and CO2.

  • Examples:

  • Champagne production involves yeast with sugars in fruits where CO2 creates fizz.

  • Bread making involves yeast, sugar, and flour, causing dough to rise from CO2 bubbles.

Cellular Respiration Process Overview

1. Glycolysis: Glucose is first broken down into pyruvate in the cytoplasm.

2. Transport: Pyruvate is transported into the mitochondrion and split into CO2 and a 2-carbon acetyl group.

Krebs Cycle (Part of Cellular Respiration)

3. The acetyl group is further broken down into CO2 in the Krebs Cycle (within the matrix space) while electron carriers are loaded.

Electron Transport Chain (ETC)

4. Electron carriers NADH and FADH2 donate electrons to the electron transport chain located along the inner mitochondrial membrane.

5. Hydrogen Ion Gradient: A hydrogen ion gradient created by the ETC is utilized to synthesize ATP through the process of chemiosmosis.

6. ATP is then transported out of the mitochondrion to power cellular activities.

Chemiosmosis Mechanism

1. Electron Transfer: Energy released from electrons as they pass through the electron transport chain.

2. Ion Pumping: Released energy is used to pump hydrogen ions across the inner membrane, creating a concentration gradient in the intermembrane space.

3. Stored Energy: Hydrogen ions flow back into the matrix through an ATP synthesizing enzyme (ATP synthase), generating ATP.

4. ATP Production: Flow of hydrogen ions allows for the phosphorylation of ADP forming 32-34 ATP.

5. ATP diffuses out of the mitochondrion for use in energy-requiring cellular activities.

Influence on Organism Function

• Metabolic processes highly depend on ATP generation. Notably, cyanide is lethal as it inhibits ATP production.

• Muscle cells exhibit metabolic flexibility, switching between fermentation and aerobic respiration based on O2 availability.

Alternative Fuel Sources

• Other carbohydrates, proteins, and fats can also serve as energy sources for the cells.

Evolutionary Aspects

• Fat storage has evolved as a survival strategy during times of abundance, allowing organisms to have energy reserves.