Harvesting Energy: Glycolysis and Cellular Respiration

Introduction to Energy Flow in Ecosystems

Energy flows through ecosystems while chemicals within an ecosystem are recycled. The process of energy harvesting involves the conversion of light energy into organic molecules, which are then used to produce ATP and heat.

• Energy Flow: Light → organic molecules → ATP + heat

• Photosynthesis: Converts light energy into organic molecules.

• Cellular Respiration: Converts organic molecules into ATP and heat.

Overall Reaction of Cellular Respiration:

Glycolysis

Glycolysis is a catabolic pathway that breaks down glucose into pyruvic acid, harvesting chemical energy. It is a multi-step process that occurs in the cytoplasm and can proceed with or without oxygen.

• Definition: The breakdown of a six-carbon glucose molecule into two three-carbon pyruvate molecules.

• Location: Cytoplasm

• Oxygen Requirement: Can occur aerobically or anaerobically.

Phases of Glycolysis

• 1. Glucose Activation Phase:

  • Uses ATP to phosphorylate glycolysis intermediates.

  • Costs two ATP molecules per glucose.

• 2. Energy Harvest Phase:

  • Produces ATP.

  • Yields 4 ATP molecules per glucose.

  • 2 molecules of NAD+ are reduced to NADH per glucose.

Steps in Glycolysis

1. Phosphorylation of Glucose: Makes glucose more reactive and traps it in the cytoplasm.

2. Rearrangement: Shuffles functional groups.

3. Second Phosphorylation: Regulatory enzyme controls the pathway; substrate becomes fructose-6-phosphate.

4. Splitting: 6-carbon sugar splits into two 3-carbon sugars (glycolysis).

5. Rearrangement of 3-carbon sugars: Only one form proceeds through the remainder of the pathway.

6. Reduction and Phosphorylation:

   • 2 NADH molecules produced per glucose.

   • High-energy phosphate bond created.

7. Substrate-Level Phosphorylation: Phosphate transferred to ADP to produce ATP (2 ATP per glucose).

8. Further Rearrangement and Phosphorylation: Additional ATP produced, ending with 2 pyruvic acid molecules.

Cellular Respiration

Cellular respiration is the process by which cells harvest energy from organic molecules, primarily glucose, in the presence of oxygen. It consists of multiple stages, including glycolysis, the Krebs cycle, and electron transport.

• Overall Reaction: Organic compounds + Oxygen → Carbon dioxide + Water + Energy (ATP)

• ATP Production: Results in the complete degradation of sugars.

Stages of Cellular Respiration

1. Krebs Cycle (Citric Acid Cycle):

   • Occurs in the mitochondrial matrix.

   • Completes the breakdown of glucose started by glycolysis.

   • Pathway discovered by Hans Krebs.

2. Electron Transport Chain and Oxidative Phosphorylation:

   • Occurs in the inner mitochondrial membrane and between the intermembrane space and the matrix.

   • Couples electron transport to ATP synthesis.

Bridge Reaction

Connects glycolysis to the Krebs cycle by converting pyruvic acid to acetyl CoA.

• Removal of CO2

• Production of NADH from NAD+

• Attachment of coenzyme A to form acetyl CoA

Krebs Cycle Details

• Occurs in the mitochondrial matrix.

• Completes the oxidation of organic molecules.

• Produces NADH, FADH2, and a small amount of ATP via substrate-level phosphorylation.

• Two turns of the cycle are needed to break down one glucose molecule.

Summary Equation for Glycolysis and Krebs Cycle:

Electron Transport System and Chemiosmosis

The electron transport system is located in the inner mitochondrial membrane and is responsible for the majority of ATP production during cellular respiration.

• Accepts electrons from NADH and FADH2.

• Uses energy from electron transfers to make ATP via oxidative phosphorylation.

• Produces most (90%) of the ATP of cellular respiration.

Chemiosmosis

Chemiosmosis is the energy-coupling mechanism that uses the proton gradient generated by the electron transport chain to drive ATP synthesis.

• Proton gradient forms across the inner mitochondrial membrane.

• ATP synthase uses the gradient to produce ATP as protons diffuse back into the matrix.

• Multiple copies of ATP synthase increase ATP production.

• Infoldings of the inner membrane increase surface area for chemiosmosis.

ATP Yield:

• For every NADH, 3 protons are moved, resulting in 1 ATP synthesized.

• For every FADH2, 2 protons are moved, resulting in 1 ATP synthesized.

Overall Summary Equation:

ATP Yield Table

Other Fuels in Respiration

Cellular respiration can accept components from most major types of macromolecules found in food.

• Carbohydrates: Most polysaccharide breakdown products can be converted to glucose or fructose.

• Fats: Glycerol can enter glycolysis; fatty acids are oxidized to acetyl groups and enter the Krebs cycle.

• Proteins: Amino acids are deaminated and their carbon skeletons enter at pyruvate or later steps.

Fermentation

Fermentation is an anaerobic process that occurs when oxygen is not present. It allows glycolysis to continue by regenerating NAD+, but does not produce ATP beyond that generated in glycolysis.

• No ATP production beyond glycolysis.

• Partial degradation of sugars.

• Regenerates NAD+ for glycolysis.

Types of Fermentation

1. Lactic Acid Fermentation:

   • Pyruvic acid is reduced to lactic acid.

   • Occurs in muscle cells under anaerobic conditions.

2. Alcohol Fermentation:

   • Pyruvic acid loses CO2 and is converted to ethanol.

   • Occurs in yeast and some bacteria.

Example: Many bacteria and yeast carry out alcohol fermentation under anaerobic conditions. Muscle cells perform lactic acid fermentation when oxygen is scarce.