Consider the following statements about "oxygen toxicity": 1. Oxygen is essential for the survival of every living organism. 2. For some organisms, oxygen is toxic. 3. Breathing pure oxygen for long periods is always beneficial for humans. Which of the statements given above is/are correct?

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).

NCERT Class 7 Science — Chapter 2: Respiration in organisms

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Consider the following statements about "oxygen toxicity":

Oxygen is essential for the survival of every living organism.
For some organisms, oxygen is toxic.
Breathing pure oxygen for long periods is always beneficial for humans.

Which of the statements given above is/are correct?

Official previous-year question

Verified 1 Aug 2026.

Options

A1 only
B2 only
C1 and 3 only
D1, 2 and 3

Did you get this right?

Solution

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).

Your note

More from Chapter 2: Respiration in organisms

With reference to cellular respiration, consider the following statements: 1. It is the process of synthesis of food within the cell using energy. 2. It takes place in the cells of all living organisms. 3. The breakdown of food always occurs in the absence of oxygen. Which of the statements given above is/are correct?

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Consider the following statements regarding yeast: 1. They are single-celled organisms. 2. They respire anaerobically to produce lactic acid. 3. They can survive in the absence of air. Which of the statements given above is/are correct?

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What are the end-products of anaerobic respiration (fermentation) in yeast?

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Consider the following statements about the diaphragm: 1. It is a rigid bony plate located in the chest. 2. It forms the floor of the chest cavity. 3. It moves downwards during the process of inhalation. Which of the statements given above is/are correct?

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The correct option is (b).

Explanation

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).