MCQ Detailed View

Plants carry out two main physiological processes: photosynthesis and respiration. During the daytime, in the presence of sunlight, plants perform photosynthesis, a process in which they take in carbon dioxide (CO₂) and release oxygen (O₂). This occurs because chlorophyll in plant leaves absorbs sunlight, using it to convert CO₂ and water into glucose and oxygen.However, during the nighttime, photosynthesis stops because there is no sunlight available to drive the reaction. In the absence of light, plants cannot fix carbon dioxide or produce oxygen via photosynthesis. Instead, they continue performing cellular respiration, a process that occurs 24 hours a day in all living cells.

What Happens During Respiration in Plants:

1. 
   

   Oxygen intake: Plants take in oxygen (O₂) from the atmosphere for respiration.

   
   


2. 
   

   Glucose breakdown: Stored glucose is oxidized to produce energy in the form of ATP for plant growth, maintenance, and repair.

   
   


3. 
   

   Carbon dioxide release: As a by-product of respiration, plants release carbon dioxide (CO₂) back into the atmosphere.

   
   

This is why plants give off carbon dioxide at night instead of oxygen. The total oxygen they release during the day (via photosynthesis) is much greater than the amount they consume at night, ensuring a net oxygen gain in the environment.   Why Other Options Are Incorrect:

• 
  

  Take in Carbon Dioxide: Happens during the day for photosynthesis, not at night.

  
  


• 
  

  Give off Oxygen: Only occurs in light-dependent photosynthesis.

  
  


• 
  

  None of these: Incorrect because plants clearly release CO₂ at night.

  
  

Therefore, the correct answer is Option C – Give off Carbon Dioxide, as plants respire continuously but photosynthesize only in the presence of sunlight.