Biology Regents Practice Exam 2026 - Free Practice Questions and Study Guide

Aquatic animals primarily remove oxygen molecules from their environment for cellular respiration. Cellular respiration is the process by which cells convert glucose and oxygen into energy in the form of ATP, while releasing carbon dioxide and water as byproducts. In aquatic environments, oxygen is dissolved in water, and aquatic animals, such as fish, extract this dissolved oxygen using specialized structures like gills.

These animals need a continuous supply of oxygen to sustain their metabolic processes, making it crucial for them to efficiently extract oxygen from the water. While carbon dioxide is a waste product that they need to expel, the focus of cellular respiration is on obtaining oxygen for energy production. Therefore, the emphasis on oxygen as the primary substance is essential in understanding the respiratory needs of aquatic animals.