Figure 1: Respiration equation

Differentiating between respiration, gas exchange, and breathing

Cellular respiration is the process that uses O2 to release energy from our food (Figure 1). This process takes place in the mitochondria of cells. The energy produced is then used by our cells to carry out life processes. CO2 and H2O are created as a waste product of respiration (Figure 1).

Gas exchange is the process where different gases are transferred in opposite directions across a specialised respiratory surface (or a moist semi-permeable membrane) (Figure 2). Gas exchange provides a means of supplying an organism with O2 and removing the CO2.

Figure 2: Gas exchange at the alveolus.

Breathing is a physical movement, using muscles, to increase the rate of gas exchange in animals that have lungs. However, many organisms do not breathe. Instead, they ventilate (or ‘air out’) their gas exchange surfaces. This means they make rhythmic body movements to aid gas exchange. Breathing is one form of ventilation. Only animals with lungs ‘breathe’.

Diffusion

O₂ and CO₂ move in and out of cells, and across the specialised respiratory membrane by the passive process called diffusion. 

Respiration produces waste CO₂, causing the amount of CO₂ to increase in the cell. Eventually, the CO₂ concentration in the cell is higher than that in the surrounding blood. So CO₂ diffuses out from the cell membrane and into the blood.

Likewise, respiration uses up O₂, causing the amount of O₂ to decrease in the cell. Eventually, the O₂ concentration in the cell is much lower than that in the surrounding blood. So O₂ diffuses out from the blood and into the cell membrane (Figure 3).