Question

Energy Calculate and compare the total energy in kJ that is converted to ATP during the processes of cellular respiration and fermentation.

Short answer

In cellular respiration, 36 ATP molecules are produced per glucose molecule, resulting in a total energy release of 1800 kJ. In fermentation, only 2 ATP molecules are produced per glucose molecule, resulting in a total energy release of 100 kJ. Cellular respiration is more efficient at converting energy to ATP when compared to fermentation, with a difference of 1700 kJ.

Step-by-step solution

Cellular Respiration: ATP Production

Cellular respiration consists of three main pathways: glycolysis, the citric acid cycle (also known as the Krebs cycle), and oxidative phosphorylation (electron transport chain). Each of these pathways produces a specific number of ATP molecules. 1. Glycolysis: 2 ATP molecules per glucose molecule 2. Citric Acid Cycle: 2 ATP molecules per glucose molecule 3. Oxidative Phosphorylation: 32 - 34 ATP molecules per glucose molecule Total ATP production for cellular respiration = Glycolysis ATP + Citric Acid Cycle ATP + Oxidative Phosphorylation ATP = 2 + 2 + 32 (taking the lower value) = 36 ATP molecules

Fermentation: ATP Production

Fermentation is an anaerobic process that occurs when oxygen is not available for cellular respiration. The primary goal of fermentation is to regenerate NAD+ to continue glycolysis. Fermentation does not generate additional ATP beyond glycolysis. So, the total ATP produced by fermentation is equal to the ATP produced during glycolysis, which is 2 ATP molecules per glucose molecule.

Energy Released per ATP Molecule

The energy released during the synthesis of one ATP molecule is approximately 50 kJ/mol.

ATP Energy for Cellular Respiration

To calculate the total energy in kJ released in ATP during cellular respiration, multiply the total ATP production by the energy released per ATP molecule: Total_energy_respiration = Total_ATP_respiration × Energy_per_ATP = 36 ATP × 50 kJ/mol = 1800 kJ

ATP Energy for Fermentation

Similarly, calculate the total energy in kJ released in ATP during fermentation: Total_energy_fermentation = Total_ATP_fermentation × Energy_per_ATP = 2 ATP × 50 kJ/mol = 100 kJ

Comparison of ATP Energy

Finally, compare the total energy in kJ for cellular respiration and fermentation: Cellular respiration: 1800 kJ Fermentation: 100 kJ Difference: 1800 - 100 = 1700 kJ Cellular respiration is more efficient and converts more energy to ATP molecules compared to fermentation, with a difference of 1700 kJ.

Key concepts

ATP Production

ATP, or Adenosine Triphosphate, serves as the primary energy currency of the cell, powering countless biological processes. It's produced through a meticulous sequence of reactions during the process of cellular respiration.

Cellular respiration can be divided into three major stages: **Glycolysis**, **Citric Acid Cycle**, and **Oxidative Phosphorylation**. Each of these stages contributes varying amounts of ATP:

• Glycolysis: Produces 2 ATP per glucose molecule.
• Citric Acid Cycle: Adds another 2 ATP per glucose.
• Oxidative Phosphorylation: Yields the majority, adding 32 to 34 ATP per glucose.

Summing these, a single molecule of glucose can generate approximately 36 ATP through cellular respiration. This considerable output contrasts with the energy yield from fermentation, which is significantly lower and only produces 2 ATP per glucose. Understanding the difference in ATP production highlights the efficiency of cellular respiration in energy conversion.

Fermentation

Fermentation is an anaerobic process, meaning it occurs without oxygen. When oxygen is scarce or absent, cells can switch to fermentation to produce ATP. This process ensures that glycolysis continues by regenerating NAD+, a crucial cofactor enabling glycolysis under anaerobic conditions.

During fermentation, particularly lactic acid fermentation or alcohol fermentation, NADH produced in glycolysis is converted back to NAD+, allowing glycolysis to produce ATP.

• **Lactic Acid Fermentation:** Typical in muscle cells, this process converts pyruvate into lactic acid.
• **Alcohol Fermentation:** Found in yeast, it converts pyruvate into ethanol and carbon dioxide.

However, fermentation is significantly less efficient in generating energy compared to cellular respiration, with only 2 ATP molecules produced per glucose molecule. It's often employed as a short-term solution during rapid energy demands or low oxygen availability.

Oxidative Phosphorylation

Oxidative phosphorylation is the third and final stage of cellular respiration, occurring in the mitochondria. It's where the majority of ATP is produced: 32 to 34 ATP per glucose molecule.

This process begins with the electron transport chain, where electrons are passed along a series of proteins embedded in the inner mitochondrial membrane. These electrons originate from NADH and FADH2, generated in earlier stages.

• **Electron Transport Chain (ETC):** Electrons moving through ETC establish a proton gradient across the membrane.
• **Chemiosmosis:** Protons flow back into the mitochondrial matrix through ATP synthase, a protein that facilitates the synthesis of ATP from ADP and inorganic phosphate.

This coupling of electron transport and ATP production is essential for meeting the cellular energy demand efficiently. Because of oxidative phosphorylation, cells can yield an impressive amount of ATP, reinforcing the powerhouse reputation of mitochondria.

Glycolysis

Glycolysis serves as the gateway for glucose metabolism and occurs in the cytoplasm of cells. This "sugar-breaking" process does not require oxygen (anaerobic) and produces ATP directly by substrate-level phosphorylation.

Here, a single molecule of glucose is split into two molecules of pyruvate, yielding 2 ATP and 2 NADH:

• **Energy Investment Phase:** Initial steps use up to 2 ATP molecules to modify glucose, preparing it for breakdown.
• **Energy Payoff Phase:** Generates 4 ATP molecules, resulting in a net gain of 2 ATP per glucose.

Aside from ATP, glycolysis provides intermediates for other pathways, such as fermentation or aerobic respiration, depending on oxygen availability. Glycolysis is foundational for energy metabolism, its simplicity allowing life to derive energy regardless of environmental conditions.

Citric Acid Cycle

The Citric Acid Cycle, also known as the Krebs Cycle, is a central hub in cellular metabolism, operating in the mitochondrial matrix. Each turn of the cycle oxidizes acetyl-CoA to produce high-energy electron carriers.

Here’s how it breaks down:

• **Inputs:** Each cycle begins with acetyl-CoA, derived from pyruvate from glycolysis.
• **Outputs:** Produces 2 ATP (or GTP), along with 3 NADH and 1 FADH2 per acetyl-CoA molecule.

The cycle doesn't generate much ATP directly, contributing only 2 ATP per glucose molecule. Instead, its main role is to produce NADH and FADH2 which store energy in their electrons. These high-energy electron carriers are crucial for driving oxidative phosphorylation and ultimately converting energy to a much larger amount of ATP.