Problem 3 - Campbell Biology

Question

Which of the following metabolic processes can occur without a net influx of energy from some other process? $$(A)\mathrm{ADP}+(\mathrm{P}) \rightarrow \mathrm{ATP}+\mathrm{H}_{2} \mathrm{O}$$ $$(B)\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}+6 \mathrm{O}_{2} \rightarrow 6 \mathrm{CO}_{2}+6 \mathrm{H}_{2} \mathrm{O}$$ $$(C) 6 \mathrm{CO}_{2}+6 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}+6 \mathrm{O}_{2}$$ $$ (D)Amino acids\rightarrow Protein$$

Step-by-Step Solution

Answer

Reaction B: Cellular respiration (\text{C}_6 \text{H}_{12} \text{O}_6 + 6 \text{O}_2 \rightarrow 6 \text{CO}_2 + 6 \text{H}_2 \text{O}) is exergonic and occurs without a net influx of energy.

Step 1: Understand Chemical Reactions Types

Identify the metabolism type for each reaction. Exergonic reactions release energy, while endergonic reactions require energy input.

Step 2: Analyze Reaction A

The reaction \(\text{ADP} + \text{P} \rightarrow \text{ATP} + \text{H}_2 \text{O}\) is an endergonic process. It requires energy to form ATP from ADP and a phosphate group.

Step 3: Analyze Reaction B

The reaction \(\text{C}_6 \text{H}_{12} \text{O}_6 + 6 \text{O}_2 \rightarrow 6 \text{CO}_2 + 6 \text{H}_2 \text{O}\) is also known as cellular respiration. It is exergonic, meaning it releases energy and does not require an energy input.

Step 4: Analyze Reaction C

The reaction \(6 \text{CO}_2 + 6 \text{H}_2 \text{O} \rightarrow \text{C}_6 \text{H}_{12} \text{O}_6 + 6 \text{O}_2\) is photosynthesis. This is an endergonic process, requiring energy input from sunlight to produce glucose.

Step 5: Analyze Reaction D

The formation of proteins from amino acids \( \text{Amino acids} \rightarrow \text{Protein} \) is an endergonic process. It requires energy to bond amino acids together.

Step 6: Identify Reaction Without Energy Input

Based on the analysis, reaction B is the only metabolic process that releases energy rather than requiring an energy input.

Key Concepts

Exergonic Reactions

Exergonic reactions are chemical processes that release energy into the surroundings. In these reactions, the energy of the products is lower than the energy of the reactants, leading to a net release of energy. An important example of an exergonic reaction is cellular respiration, where glucose molecules are broken down to release energy in the form of ATP. Because exergonic reactions release energy, they can occur spontaneously without an external energy source.
Key characteristics of exergonic reactions:

• Release energy.
• Negative change in Gibbs free energy (\( \triangle G < 0 \)).
• Spontaneous processes.

Endergonic Reactions

Endergonic reactions require an input of energy to proceed. These reactions are not spontaneous and the energy of the products is higher than the energy of the reactants. A classic example of an endergonic reaction is the synthesis of ATP, where ADP and a phosphate group combine using energy obtained from cellular respiration.
Key characteristics of endergonic reactions:

• Require energy input.
• Positive change in Gibbs free energy (\( \triangle G > 0 \)).
• Non-spontaneous processes.

Cellular Respiration

Cellular respiration is a vital metabolic process that converts glucose and oxygen into carbon dioxide, water, and energy in the form of ATP. This process is exergonic because it releases energy. During cellular respiration, the breakdown of glucose occurs in several stages:

• Glycolysis - Glucose is broken down into pyruvate, producing a small amount of ATP and NADH.
• Krebs Cycle - Pyruvate is further broken down, generating ATP, NADH, and FADH₂.
• Electron Transport Chain - NADH and FADH₂ donate electrons to the chain, creating a large amount of ATP.

Photosynthesis

Photosynthesis is the process by which green plants, algae, and some bacteria convert carbon dioxide and water into glucose and oxygen using sunlight energy. This process is endergonic because it requires energy input from sunlight. Photosynthesis can be divided into two main stages:

• Light-dependent reactions - Capture light energy to produce ATP and NADPH.
• Calvin cycle (light-independent reactions) - Use ATP and NADPH to convert carbon dioxide into glucose.

Protein Synthesis

Protein synthesis is the biological process by which cells build proteins from amino acids. This is an endergonic process as it requires energy input to form peptide bonds between amino acids. Protein synthesis occurs in two main stages:

• Transcription - DNA is transcribed into mRNA in the nucleus.
• Translation - mRNA is translated into a polypeptide chain at the ribosome, forming the protein.

ATP provides the necessary energy for peptide bond formation and other steps in this process.