The correct option is (b).
Explanation
Oxygen plays a dual role in biology; it is a vital electron acceptor for aerobic respiration but can be chemically reactive and damaging. The requirement for oxygen varies across different life forms, and its concentration must be regulated within physiological limits for humans.
Statement-wise Analysis:
- Statement 1 is Incorrect. Oxygen is not essential for the survival of every living organism. A significant group of organisms, known as anaerobes (e.g., bacteria like Clostridium botulinum), survive and generate energy in the absence of oxygen.
- Statement 2 is Correct. For obligate anaerobes, oxygen is toxic. These organisms lack the necessary enzymes (such as superoxide dismutase and catalase) to neutralize reactive oxygen species (free radicals) produced during oxygen metabolism. Consequently, exposure to oxygen destroys their cellular structures.
- Statement 3 is Incorrect. Breathing pure (100%) oxygen for prolonged periods is not always beneficial; it leads to oxygen toxicity. High concentrations of oxygen cause oxidative stress, leading to lung damage (pulmonary toxicity) and central nervous system complications (seizures). This is medically known as the Paul Bert effect (CNS) or Lorrain Smith effect (pulmonary).
Key Takeaway:
Life is divided into aerobes (require oxygen) and anaerobes (do not require oxygen). Oxygen is toxic to obligate anaerobes and can also be toxic to humans in high concentrations (hyperoxia).