Question

The synthesis of glucose directly from \(\mathrm{CO}_{2}\) and \(\mathrm{H}_{2} \mathrm{O}\) and the synthesis of proteins directly from amino acids are both nonspontaneous processes under standard conditions. Yet it is necessary for these to occur for life to exist. In light of the second law of thermodynamics, how can life exist?

Short answer

Life can exist despite the nonspontaneity of glucose and protein synthesis because living organisms are not closed systems. They exchange energy and matter with their surroundings, enabling nonspontaneous processes to occur through metabolic coupling. Processes like photosynthesis and cellular respiration harness energy from spontaneous reactions or external sources (e.g., sunlight) to drive nonspontaneous processes such as glucose and protein synthesis. This energy is stored and utilized in the form of ATP, the primary energy currency in living cells, ensuring that life can thrive within the constraints of the second law of thermodynamics.

Step-by-step solution

Understand standard conditions and spontaneous processes

Standard conditions refer to a fixed set of conditions (298 K temperature and 1 atm pressure) under which the spontaneity of processes is determined. A spontaneous process occurs naturally without any external energy input, while a nonspontaneous process requires an outside influence to occur. Under standard conditions, both the synthesis of glucose and proteins from their building blocks are nonspontaneous processes.

Second law of thermodynamics

The second law of thermodynamics states that the total entropy of an isolated system can only increase over time, making spontaneous reactions more probable than nonspontaneous ones. This law is essential because it governs energy transformations in all living systems.

Energy input in living organisms

Living organisms are not closed systems. They constantly exchange energy and matter with their surroundings, which allows nonspontaneous processes to occur inside them. Through a process called metabolic coupling, organisms harness the energy released in spontaneous reactions to drive nonspontaneous processes such as the synthesis of glucose and proteins.

Example: Photosynthesis

Photosynthesis is a primary example of how the synthesis of glucose can occur in living organisms, despite its nonspontaneity under standard conditions. During photosynthesis, plants absorb sunlight and use its energy to convert \(\mathrm{CO}_{2}\) and \(\mathrm{H}_{2}\mathrm{O}\) into glucose and oxygen. This light-dependent reaction provides the necessary energy input for glucose synthesis, allowing it to occur even though it is a nonspontaneous process.

Cellular Processes and ATP

Adenosine triphosphate (ATP) is the primary energy currency of cells, used to facilitate various cellular processes, including the synthesis of proteins. The high-energy phosphate bonds in ATP molecules are typically formed by coupling the energy-releasing reactions, such as the breakdown of glucose during cellular respiration. Therefore, living cells can perform nonspontaneous processes such as protein synthesis by using the energy stored in ATP molecules. In conclusion, the second law of thermodynamics does not prevent life from existing. Even though the synthesis of glucose and proteins are nonspontaneous processes, living organisms manage to perform these tasks using energy from spontaneous reactions or external sources like sunlight. Understanding these processes is fundamental to comprehend the interplay between energy transformation and biological complexity.