Harvesting Energy: Glycolysis and Cellular Respiration

Harvesting Energy: Glycolysis and Cellular Respiration

Introduction to Energy Flow in Ecosystems

Energy flows through ecosystems while chemicals are recycled. The process of harvesting energy in cells involves converting organic molecules into ATP and heat, which is essential for cellular activities.

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

• Photosynthesis: Converts light energy to organic molecules.

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

Overall Cellular Respiration Equation:

$\mathrm{C_6H_{12}O_6 + 6\ O_2 \rightarrow 6\ CO_2 + 6\ H_2O + \text{Energy (ATP + heat)}}$

Glycolysis

Overview of Glycolysis

Glycolysis is a catabolic pathway that breaks down glucose (a six-carbon sugar) into two molecules of pyruvic acid. This process occurs in the cytoplasm and can take place with or without oxygen.

• Location: Cytoplasm

• Oxygen Requirement: Can occur with or without O2

• Products: 2 pyruvic acid, 2 ATP (net), 2 NADH per glucose

Phases of Glycolysis

• 1. Glucose Activation Phase:

  • Uses ATP to phosphorylate intermediates

  • Costs 2 ATP per glucose

• 2. Energy Harvest Phase:

  • Produces ATP

  • Yields 4 ATP per glucose (net gain: 2 ATP)

  • Produces 2 NADH per glucose

Key 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 step; forms fructose-6-phosphate.

4. Splitting: 6-carbon sugar split into two 3-carbon sugars.

5. Rearrangement of 3-carbon sugars: Only one form continues through glycolysis.

6. Oxidation and Phosphorylation: NAD+ is reduced to NADH; high-energy phosphate bonds are formed.

7. Substrate-Level Phosphorylation: Phosphate transferred to ADP to form ATP.

8. End Products: 2 pyruvic acid molecules, 2 ATP (net), 2 NADH per glucose.

Cellular Respiration

Overview

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

• Overall Equation: $\mathrm{Organic\ compounds + O_2 \rightarrow CO_2 + H_2O + Energy}$

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

The Krebs Cycle (Citric Acid Cycle)

The Krebs cycle completes the breakdown of glucose by oxidizing acetyl CoA to CO2. It occurs in the mitochondrial matrix and is essential for harvesting high-energy electrons for the electron transport chain.

• Location: Mitochondrial matrix

• Key Steps:

  • Removal of CO2

  • Production of NADH from NAD+

  • Attachment of coenzyme A to form acetyl CoA (bridge reaction)

• Products per glucose: 6 NADH, 2 FADH2, 2 ATP, 4 CO2

Note: Two turns of the Krebs cycle are required to completely oxidize one glucose molecule.

Electron Transport Chain and Oxidative Phosphorylation

The electron transport chain (ETC) is located in the inner mitochondrial membrane. It uses electrons from NADH and FADH2 to create a proton gradient, which drives ATP synthesis via oxidative phosphorylation.

• Location: Inner mitochondrial membrane

• Function: Accepts electrons from NADH and FADH2

• ATP Production: Produces most (about 90%) of the ATP in cellular respiration

Chemiosmosis

Chemiosmosis is the mechanism by which the energy from the electron transport chain is used to generate ATP. The ETC creates a proton gradient across the inner mitochondrial membrane, and ATP synthase uses this gradient to synthesize ATP as protons flow back into the matrix.

• Proton gradient is formed by the ETC across the inner mitochondrial membrane.

• ATP synthase uses the energy of the proton gradient to make ATP.

• The folding of the inner mitochondrial membrane increases the surface area for chemiosmosis.

ATP Yield from Cellular Respiration

The total ATP yield from the complete oxidation of one glucose molecule is approximately 36 ATP.

Metabolic Process	Substrate-Level Phosphorylation	Coenzyme Reduced	Oxidative Phosphorylation	Total ATP
Glycolysis	Net 2 ATP	2 NADH	4(6)-2	6
Oxidation of Pyruvate		2 NADH	6	6
Krebs Cycle	2 ATP	6 NADH, 2 FADH2	18, 4	24
Total				36

Other Fuels in Respiration

Cellular respiration can utilize carbohydrates, fats, and proteins as fuel sources. These macromolecules enter the pathway at different points.

• Carbohydrates: Polysaccharides are broken down to glucose or fructose.

• Fats: Glycerol enters glycolysis; fatty acids are converted to acetyl CoA for the Krebs cycle.

• Proteins: Amino acids are deaminated and enter at pyruvate, acetyl CoA, or Krebs cycle intermediates.

Fermentation

Overview

Fermentation is an anaerobic process that allows glycolysis to continue in the absence of oxygen by regenerating NAD+. It results in the partial degradation of sugars and does not produce ATP beyond glycolysis.

• Occurs without O2

• No additional ATP production

• Regenerates NAD+

Types of Fermentation

1. Lactic Acid Fermentation: Pyruvic acid is reduced to lactic acid.

   • Equation: Glucose → pyruvic acid → lactic acid

   • Occurs in muscle cells and some bacteria under anaerobic conditions

2. Alcohol Fermentation: Pyruvic acid loses CO2 and is converted to ethanol.

   • Equation: Glucose → pyruvic acid → ethanol + CO2

   • Carried out by yeast and some bacteria

Summary Equation for Cellular Respiration:

$\mathrm{C_6H_{12}O_6 + 6\ O_2 \rightarrow 6\ CO_2 + 6\ H_2O}$

Summary Equation for Glycolysis, Krebs Cycle, and Electron Transport Combined:

$\mathrm{C_6H_{12}O_6 + 36\ ADP + 6\ O_2 \rightarrow 6\ CO_2 + 36\ ATP + 42\ H_2O}$