HESI A2 Biology Practice Test 2025 - Free HESI A2 Biology Practice Questions and Study Guide

Oxygen is produced as a byproduct during photosynthesis, which occurs primarily in plants, algae, and some bacteria. In this process, organisms convert light energy, usually from the sun, into chemical energy stored in glucose. The overall reaction of photosynthesis can be summarized as carbon dioxide and water being transformed, using light energy, into glucose and oxygen. The oxygen is released into the atmosphere as a byproduct, which is essential for the survival of aerobic organisms that rely on it for cellular respiration.

During cellular respiration, organisms break down glucose to release energy, consuming oxygen in the process and producing carbon dioxide and water, which is fundamentally different from the oxygen-producing role of photosynthesis. Fermentation, on the other hand, occurs in the absence of oxygen and produces byproducts such as lactic acid or alcohol, rather than oxygen.

Thus, photosynthesis is the correct answer for the process that produces oxygen as a byproduct.