Bioenergetics terminology — endergonic reactions: In cellular metabolism, an endergonic reaction is best described as a process that requires an input of energy to proceed and does not occur spontaneously under standard conditions. Which statement captures this definition most accurately?

Introduction / Context:
Understanding the terms endergonic and exergonic is foundational to biochemistry and microbiology. These labels describe the direction of free-energy change in a reaction and predict whether a process can proceed spontaneously inside cells. Mastery of this concept helps explain why cells couple certain reactions to adenosine triphosphate (ATP) hydrolysis to drive biosynthesis, transport, and movement.

Given Data / Assumptions:

• Endergonic reactions have a positive change in Gibbs free energy (Delta G > 0).
• Exergonic reactions have a negative change in Gibbs free energy (Delta G < 0).
• Cellular conditions are near physiological temperature, pressure, and pH.

Concept / Approach:
In plain language, an endergonic reaction is not energetically favorable on its own. It requires an energy input to proceed in the forward direction. Cells achieve this by coupling the endergonic step to an exergonic process with a larger magnitude negative free-energy change, most commonly ATP → ADP + Pi. When the summed Delta G of the coupled steps is negative, the overall pathway becomes thermodynamically favorable.

Step-by-Step Solution:
Identify the definition: endergonic implies Delta G > 0. Translate definition into practical terms: energy must be supplied. Connect to cellular strategy: coupling to ATP hydrolysis or ion gradients. Select the statement that explicitly states the need for input energy.

Verification / Alternative check:
Common examples include polymerization (e.g., amino acids to proteins), active transport against concentration gradients, and many biosynthetic steps. Each is driven by coupling to ATP hydrolysis or similar exergonic events, confirming the energy requirement.

Why Other Options Are Wrong:

• Releases energy / gives off heat / loses energy: These descriptions fit exergonic reactions, not endergonic ones.
• No change in free energy: Would describe an equilibrium process (Delta G ≈ 0), not endergonic.

Common Pitfalls:
Confusing heat release with spontaneity; spontaneity is governed by free energy change, not temperature change alone. Also, a reaction can be endergonic yet proceed in cells if it is coupled to a sufficiently exergonic step.

Final Answer:
Requires energy in order to proceed.