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 within them. The process of harvesting energy involves converting light energy into chemical energy (ATP and heat) through a series of metabolic pathways.

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

• Photosynthesis: Converts light energy to organic molecules.

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

Overall equation for cellular respiration:

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

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 glucose 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 the pathway.

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.

Additional info: Substrate-level phosphorylation refers to the direct transfer of a phosphate group to ADP to form ATP, as opposed to oxidative phosphorylation, which occurs in the mitochondria.

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 named after Hans Krebs.

• Location: Mitochondrial matrix

• Inputs: Acetyl CoA, NAD+, FAD, ADP

• Outputs (per glucose): 6 NADH, 2 FADH2, 2 ATP, 4 CO2

• Bridge Reaction: Converts pyruvate to acetyl CoA, producing NADH and CO2

Key Functions:

• Removes CO2

• Produces NADH and FADH2

• Generates a small amount of ATP by substrate-level phosphorylation

Additional info: Two turns of the Krebs cycle are required to fully oxidize one glucose molecule (since each glucose yields two pyruvate molecules).

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 process by which the energy from the electron transport chain is used to pump protons across the inner mitochondrial membrane, creating a proton gradient. ATP synthase uses this gradient to synthesize ATP as protons flow back into the mitochondrial matrix.

• Proton gradient drives ATP synthesis

• ATP synthase is the enzyme complex that makes ATP

• Inner mitochondrial membrane's folds (cristae) increase surface area for chemiosmosis

ATP Yield:

• For every NADH: 3 protons moved, 1 ATP synthesized

• For every FADH2: 2 protons moved, 1 ATP synthesized

Overall Summary Equation

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

ATP Yield Table

The following table summarizes ATP production from each stage of cellular respiration:

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

Additional info: The actual ATP yield may vary depending on cell type and shuttle mechanisms.

Other Fuels in Respiration

Cellular respiration can utilize carbohydrates, fats, and proteins as fuel sources. Glycolysis and the Krebs cycle are metabolic crossroads for these macromolecules.

• 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 glycolysis or the Krebs cycle at various points.

Fermentation

Overview

Fermentation is an anaerobic process (occurs without oxygen) that allows glycolysis to continue by regenerating NAD+. It results in the partial degradation of sugars and does not produce additional ATP beyond glycolysis.

• No ATP production (beyond glycolysis)

• Regenerates NAD+

• Occurs in the absence of O2

Types of Fermentation

1. Lactic Acid Fermentation:

   • Pyruvic acid is reduced to lactic acid.

   • $\mathrm{Glucose \rightarrow pyruvic\ acid \rightarrow lactic\ acid}$

   • Occurs in muscle cells under anaerobic conditions and in some bacteria.

2. Alcohol Fermentation:

   • Pyruvic acid loses CO2 and is converted to ethanol.

   • $\mathrm{Glucose \rightarrow pyruvic\ acid \rightarrow ethanol + CO_2}$

   • Occurs in yeast and some bacteria.

Additional info: Under anaerobic conditions, muscle cells switch from cellular respiration to lactic acid fermentation to continue producing ATP.