Q2TYU - Campbell Biology

Question

Photoautotrophs use 

 

(A) light as an energy source and CO₂ as a carbon source.

(B) light as an energy source and methane as a carbon source.

(C) N₂ as an energy source and CO₂ as a carbon source.

(D) CO₂ as both an energy source and a carbon source.

Step-by-Step Solution

Answer

(A) The option “light as an energy source and CO₂ as a carbon source” is true.

(B) The option “light as an energy source and methane as a carbon source” is false.

(C) The option “N₂ as an energy source and CO₂ as a carbon source” is false.

(D) The option “CO₂ as both an energy source and a carbon source” is false.

Step 1: Photoautotrophs

Photoautotrophs are living organisms that can produce their own food through photosynthesis. They do so with the help of a pigment called chlorophyll that is present within stacked structures called chloroplasts. Due to the presence of this pigment, they often have a green-colored appearance.

Step 2: Explanation of option ‘(A)’

Photoautotrophs capture light from the sun and use it as an energy source. Additionally, these organisms take up carbon dioxide from the atmosphere and use it to synthesize organic molecules. Carbon dioxide is also used to produce glucose molecules.

 

Thus, photoautotrophs use light as an energy source, and carbon dioxide is used as a carbon source.

Therefore, the given statement is true.

Step 3: Explanation of option ‘(B)’

Photoautotrophs use light as a source of energy. The light energy is captured by the cells of the photoautotrophs and is used for various metabolic pathways. However, methane is not a carbon source; instead, it damages the plants and causes chlorosis.

 

Thus, photoautotrophs use light as an energy source but not methane as a carbon source.

Therefore, the given statement is false.

Step 4: Explanation of option ‘(C)’

Nitrogen is an essential component of plants. However, it does not provide energy or fuel to carry out metabolic pathways within the photoautotrophs. Carbon dioxide is, however, used as a carbon source. Carbon dioxide is used for the synthesis of organic molecules.

 

Thus, photoautotrophs do not use nitrogen as an energy source. 

Therefore, the given statement is false.

Step 5: Explanation of option ‘(D)’

Carbon dioxide is an important source of carbon in plants. They derive it from the atmosphere. Additionally, they also use light energy from the sun to activate metabolic activities within the photoautotrophs. Carbon dioxide can in no way provide energy to the photoautotrophs.

 

Thus, carbon dioxide does not provide energy to photoautotrophs. 

Therefore, the given statement is false.