Ultimate energy source for life on Earth Across ecosystems and trophic levels, what is the fundamental, original source of energy that sustains living systems?

Introduction / Context:
Energy flow in biology begins with primary production. While organisms use immediate fuels like glucose and electron acceptors like oxygen, the ultimate source that powers photosynthesis and, by extension, most food webs is solar radiation. This question distinguishes proximate energy carriers from the primordial energy input to the biosphere.

Given Data / Assumptions:

• Most ecosystems depend on photosynthetic organisms.
• Light energy is converted to chemical energy via photosystems and the Calvin cycle.
• Exceptions (e.g., deep-sea vent communities) exist but do not overturn the global rule.

Concept / Approach:

Photosynthesis captures photon energy to form ATP and NADPH, driving carbon fixation to produce carbohydrates. Consumers and decomposers ultimately derive energy from these products. Oxygen is a product and electron acceptor in respiration, not an energy source; glucose stores energy but is synthesized using energy input from sunlight.

Step-by-Step Solution:

Verification / Alternative check:

Global energy budgets and biosphere productivity models attribute the vast majority of biological energy input to solar radiation captured by photosynthesis.

Why Other Options Are Wrong:

Glucose is a derived energy store; oxygen and carbon dioxide are reactants/products, not energy sources. Geothermal energy supports niche ecosystems but not the planetary majority.

Common Pitfalls:

Confusing “fuel molecules” with the original energy source that created them.

Final Answer:

Sunlight