Question

Glycolysis is a series of 10 linked reactions that convert one molecule of glucose into two molecules of pyruvate with the concomitant synthesis of two molecules of ATP (Chapter 16 ). The \(\Delta G^{\circ \prime}\) for this set of reactions is \(-35.6 \mathrm{kJ} \mathrm{mol}^{-1}\left(-8.5 \mathrm{kcal} \mathrm{mol}^{-1}\right),\) whereas the \(\Delta G^{\circ}\) is \(-90 \mathrm{kJ} \mathrm{mol}^{-1}\left(-22 \mathrm{kcal} \mathrm{mol}^{-1}\right) .\) Explain why the free-energy release is so much greater under intracellular conditions than under standard conditions.

Short answer

Intracellular conditions involve different reactant/product concentrations than standard conditions, enhancing energy release.

Step-by-step solution

Understanding Standard Conditions

Standard conditions (\(\Delta G^{\circ \prime} \)) are characterized by concentrations of 1 M for each of the reactants and products, a pressure of 1 atm, and a temperature of 298 K (25°C). These are idealized conditions not reflective of the complex environments inside living cells.

Evaluate Intracellular Conditions

Intracellular conditions differ significantly from standard conditions. In the cell, concentrations of reactants and products can be far from 1 M. This can influence the actual Gibbs free energy change, making it more negative or positive depending on the concentration gradients.

Apply the Reaction Gibbs Free Energy Equation

The actual Gibbs free energy change in cells (\(\Delta G \)) is calculated using the equation \(\Delta G = \Delta G^{\circ \prime} + RT \ln \frac{[products]}{[reactants]}\). Since \(R\) is the universal gas constant and \(T\) is the temperature in Kelvin, the concentration terms largely influence this value.

Account for Cellular Concentrations

Inside cells, the concentration of reactants and products like glucose, pyruvate, ADP, and ATP deviate from standard conditions. The buildup of glycolytic intermediates decreases the \([products]/[reactants]\) ratio, effectively reducing \(\Delta G \). This difference results in a greater free-energy release, around \(-90 \, \text{kJ/mol}\), than the \(-35.6 \, \text{kJ/mol}\) under standard conditions.

Key concepts

Free Energy Change

In biochemical reactions, especially glycolysis, the term free energy change comes up frequently. Free energy change is represented by \(\Delta G\), and it tells us whether a reaction can happen by itself, or if it needs help. In simple terms, it predicts the spontaneity of a reaction.
- A negative \(\Delta G\) means the reaction releases energy and can occur spontaneously. - A positive \(\Delta G\) indicates the reaction requires energy to proceed.
Glycolysis has an actual free energy change (\(\Delta G\)) that is affected by many factors inside cells. The standard free energy change given as \(\Delta G^{\circ '}\) is theoretical, usually less negative, and refers to reactions under ideal conditions. However, inside cells, the environment is very different, leading to larger amounts of energy being released.

Intracellular Conditions

Intracellular conditions refer to the actual environment inside cells, which can significantly affect chemical reactions. Cells are busy places where many processes happen simultaneously, and the concentration of various substances constantly changes.
- Concentrations of molecules inside cells, like glucose and pyruvate, differ greatly from their standard reference of 1 M.
- The cellular concentration balance affects the reaction's free energy change, making it either more favorable or less favorable.
For glycolysis, intracellular conditions often lead to a more negative \(\Delta G\), meaning more energy is released compared to theoretical calculations. This happens because of the differing concentrations of reactants and products compared to standard conditions.

Standard Conditions

Standard conditions in biochemistry establish a baseline for comparing reactions, but they're not reflective of what's inside a cell. They include elements such as:
- 1 M concentration for all reactants and products.
- Temperature set at 298 K (25°C).
- Pressure at 1 atmosphere.
These conditions are more of a guideline than reality. They help researchers compare reactions scientifically. However, within actual cellular environments, deviations from these conditions can cause free energy changes (\(\Delta G\)) that differ greatly from calculated standard conditions (\(\Delta G^{\circ '}\)). As a result, biochemical reactions may release more energy in living cells than expected.

ATP Synthesis

One of the primary goals of glycolysis is ATP synthesis. ATP, or adenosine triphosphate, acts as the energy currency of the cell.
- It stores energy in its high-energy phosphate bonds.
- During glycolysis, ATP synthesis occurs when phosphate groups are transferred onto ADP (adenosine diphosphate).
The synthesis of ATP in glycolysis is directly linked to the free energy released by the breakdown of glucose into pyruvate. The more negative the free energy change (\(\Delta G\)), the more favorable ATP production is. This energy-rich molecule then fuels various cellular processes, enabling cells to perform essential functions efficiently.