The correct option is 1 and 4 only
Explanation
Cellular respiration is an enzyme-controlled catabolic process involving the biological oxidation of organic compounds (respiratory substrates) to release energy. While carbohydrates are the primary source, the metabolic pathway is amphibolic, meaning other molecules can enter the cycle for breakdown or synthesis.
Statement-wise Analysis:
- Statement 1 is Correct: Carbohydrates (specifically glucose) are the most common and preferred respiratory substrates in living organisms. They are the first to be utilized for energy production through glycolysis.
- Statement 2 is Incorrect: Fats and proteins can be used as respiratory substrates. When carbohydrates are unavailable, fats are broken down into fatty acids and glycerol, and proteins are degraded into amino acids. These intermediates enter the respiratory pathway at various stages, such as Acetyl CoA or Pyruvate.
- Statement 3 is Incorrect: Organic acids can serve as respiratory substrates. In certain plants (e.g., CAM plants) and bacteria, organic acids like malic acid, citric acid, or oxalic acid are oxidized to release energy under specific physiological conditions.
- Statement 4 is Correct: Respiration involves a series of multiple stepwise reactions (Glycolysis, Krebs Cycle, Electron Transport Chain). This controlled, stepwise breakdown ensures that the energy released is coupled to the synthesis of ATP rather than being dissipated rapidly as heat, which would occur in a single-step combustion.
Key Takeaway:
Respiration is a stepwise oxidation process. While carbohydrates are the obligate (usual) substrate, fats, proteins, and organic acids act as facultative substrates under specific conditions.