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Organisms need cellular respiration to harvest energy from glucose and other biomolecules for cellular functions like growth, repair, and movement. Cellular respiration breaks down glucose using oxygen to produce ATP, the energy currency cells can readily use. The process also releases heat as a byproduct, making energy available throughout the organism.

Aerobic respiration requires oxygen and completely breaks down glucose into carbon dioxide and water, producing significant ATP. Anaerobic respiration occurs without oxygen and produces fewer ATP molecules. Fermentation, a type of anaerobic respiration, generates ethanol or lactic acid as byproducts instead of carbon dioxide and water.

A respirometer measures oxygen consumption by organisms using the ideal gas law principle. Potassium hydroxide inside the respirometer traps carbon dioxide as potassium carbonate. As cells consume oxygen and release carbon dioxide, the total gas volume decreases, causing pressure to drop. A manometer attached to the respirometer detects this pressure change, allowing scientists to quantify oxygen used.

In eukaryotes, the mitochondria is the primary site where cellular respiration occurs. Enzymes in the mitochondria carry out the breakdown of glucose through the Krebs cycle and oxidative phosphorylation, generating most of the ATP used by cells. Mitochondrial health directly affects an organism's energy production and overall cellular function.

Photosynthesis and cellular respiration are opposite processes. Photosynthesis produces glucose and oxygen from carbon dioxide and water using sunlight energy. Cellular respiration uses that glucose and oxygen to produce carbon dioxide, water, and ATP. The products of one process serve as reactants for the other, creating a continuous cycle of energy and matter in ecosystems.

Germinating seeds are actively respiring because they lack green parts and cannot photosynthesize, so they depend entirely on stored energy. They break down glucose through cellular respiration to produce ATP for growth and development. This active respiration consumes oxygen and releases carbon dioxide, which is why oxygen levels decrease measurably in a sealed respirometer.

The ideal gas law states that the number of gas molecules in a container can be determined from pressure, volume, and temperature. In a respirometer, as organisms consume oxygen through respiration, the total number of gas molecules decreases, lowering pressure inside the sealed chamber. By measuring pressure changes, scientists can calculate exactly how much oxygen was consumed using this mathematical relationship.