Question

What percentage of photosynthetically active radiation (PAR) is typically used by autotrophs for primary production (photosynthesis)?

• A. A) Around 1 to 5%
• B. B) Around 10 to 20%
• C. C) Around 50 to 60%
• D. D) Around 90 to 95%

Answer 1

Answer:

A. A) Around 1 to 5%

Understanding Photosynthetically Active Radiation (PAR) and Primary Production

• What is Photosynthetically Active Radiation (PAR)?

  • PAR refers to the specific range of the electromagnetic spectrum (light) that photosynthetic organisms are able to use in the process of photosynthesis.
  • This range typically spans from 400 to 700 nanometers (nm), which corresponds to the visible light spectrum.
  • It is the portion of sunlight that plants, algae, and cyanobacteria primarily absorb to drive their metabolic processes.

• Autotrophs and Primary Production:

  • Autotrophs, also known as producers, are organisms that can produce their own food using light, water, carbon dioxide, or other chemicals. The most common form is photoautotrophs, which use light energy.
  • Primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide, primarily through the process of photosynthesis, which converts light energy into chemical energy. This forms the base of almost all food webs on Earth.

• Efficiency of PAR Utilization:

  • While the sun emits a vast amount of energy, autotrophs typically utilize only a very small percentage of the total incident solar radiation for photosynthesis.
  • Specifically, around 1 to 5% of the photosynthetically active radiation (PAR) that actually reaches the plant canopy is converted into chemical energy through primary production.
  • This efficiency can vary based on factors like plant species, environmental conditions (e.g., temperature, water availability, CO₂ concentration), and nutrient availability.

• Reasons for Low Efficiency:

  • Only a fraction of solar radiation is PAR: Not all sunlight is in the PAR range; much of it is in infrared or ultraviolet spectrums, which plants cannot use for photosynthesis.
  • Reflection and Transmission: A significant portion of incident PAR is reflected from the leaf surface or transmitted through the leaf without being absorbed by photosynthetic pigments.
  • Inefficient Absorption: Not all absorbed light energy is efficiently converted. Some energy is lost as heat or fluorescence.
  • Limiting Factors: Photosynthesis is often limited by other factors besides light, such as carbon dioxide concentration, water availability, nutrient levels (e.g., nitrogen, phosphorus), and optimal temperature range.
  • Saturation Point: Plants have a light saturation point beyond which increasing light intensity does not increase the rate of photosynthesis.

• Ecological Significance:

  • Despite the low efficiency, this small percentage of energy conversion forms the entire energy foundation for nearly all life on Earth.
  • The chemical energy stored by primary producers is then transferred through food webs to heterotrophs (consumers).
  • This concept is fundamental to understanding energy flow and biomass accumulation in ecosystems, often illustrated by ecological pyramids (pyramids of energy, biomass, or numbers), where energy decreases significantly at each successive trophic level.