Question

Net primary productivity is equal to
A. The amount of light energy converted to chemical energy by photosynthesis per unit time.
B. The amount of energy used by primary producers for respiration.
C. The amount of light energy converted to chemical energy by photosynthesis per unit time plus the amount of energy used by primary producers for respiration.
D. The amount of energy used by primary producers for respiration minus the amount of light energy converted to chemical energy by the primary producers.
E. The amount of light energy converted to chemical energy by photosynthesis per unit time minus the energy used by primary producers for respiration.

Answer 1

Hint: Primary production is the synthesis of organic compounds from atmospheric or aqueous carbon-dioxide through the process of photosynthesis. The ecologists distinguish primary productivity as either net or gross primary productivity.

Complete answer:
Net primary productivity or NPP = Gross primary productivity (GPP) $-$Respiration (by plants).
Thus, it can be explained that gross primary productivity stands for the total amount of chemical energy (typically carbon biomass) that is produced by primary producers through photosynthesis, and from this GPP; some amount of energy is used by primary producers to carry out cellular respiration and maintenance of existing tissues. So, the remaining energy after subtracting the energy used in respiration from the total chemical energy produced by primary producers is known as net primary productivity or NPP that is available to the herbivores.
Option (A) is incorrect. The amount of light energy converted to chemical energy by photosynthesis per unit time is called the gross primary productivity.
Option (B) is incorrect. The amount of energy used by primary producers for respiration is not the NPP because NPP is GPP minus respiration.
Option (C) is incorrect. NPP = GPP$-$respiration but not the amount of light energy converted to chemical energy by photosynthesis per unit time plus the amount of energy used by primary producers for respiration.
Option (D) is incorrect. The quantity of energy used by primary producers for respiration minus the quantity of light energy converted into chemical energy by the primary producers is not net primary productivity but it is the remaining energy that was left after the chemical energy (produced by photosynthesis) was utilized to carry out respiration.
Option (E) is correct. The amount of light energy converted to chemical energy by photosynthesis per unit time minus the energy used by primary producers for respiration is called net primary productivity.

Hence, the correct option is (E).

Note:
Net primary productivity is the rate at which all autotrophs produce net useful chemical energy that provides energy for all heterotrophic activity. Four factors usually limit the amount of NPP on land – light, water, temperature, and mineral nutrients.