Metabolism is an over-arching term to describe all of the different pathways involved in energy acquisition and conversion. In this section, we will introduce one of these specific pathways, the respiratory pathway. Respiration refers to the acquisition of energy from food. It is a process common to all eukaryotic cells. In studying respiration, we will focus on the metabolic pathways that convert glucose into ATP. Among the components of the respiratory pathway are the processes of glycolysis, the citric acid cycle, and oxidative phosphorylation.

If we think about respiration in day-to-day terms, we immediately think of breathing and oxygen. From an early age, we learn how crucial oxygen is to our survival and that breathing is how we obtain oxygen from the air. We will now begin to see exactly how oxygen plays a role inside our bodies. Using oxygen, organisms can break down ingested foods more easily through oxidation, an energetically favorable reaction. However, though oxygen is a vital component of respiration, respiration can occur without oxygen. Respiration that involves oxygen is called aerobic respiration while that lacking oxygen is called anaerobic. Of the two, aerobic respiration is far more efficient, producing more energy per gram of glucose. We will explore the specifics of how these processes differ in the coming sections.

The energy gained from glucose breakdown can be used to synthesize ATP, which in turn can help with the synthesis of other, more complex molecules that would otherwise be too unfavorable to occur. The mitochondria is the main cellular structure involved in respiration.