Respiration occurs when gases are exchanged between the outside environment and the inside of an organism. This is what humans do when they inhale oxygen and exhale carbon dioxide.
A second type of respiration you might hear about in biology is called cellular respiration. Cellular respiration refers to the metabolic processes cells utilize to breakdown carbohydrates into ATP, the energy currency that allows organisms to function.
We have already discussed cellular respiration in Chapters 3 and 4. In this Chapter, we will focus on the first type of respiration (i.e., gas exchange). An organism’s respiratory system allows respiration to occur
Respiration in Kingdom Animalia will be discussed in greater detail in the Diversity of Life chapter. This section will focus mostly on human respiration; however, we will briefly discuss respiration in some animals.
Cnidaria is an animal phylum that is made up of extremely small animals, such as hydra. These animals are invertebrates that utilize simple diffusion to transport gas molecules across tissues that are just two cells thick. They have no circulatory system for transporting gas molecules.
Simple diffusion is a form of passive transport, whereby different molecules travel across a selectively permeable membrane by their concentration gradients. An organism that exchanges gases via simple diffusion usually has a large surface area (despite being a small animal), with almost all cells being in direct contact with a moist environment. The cells that are not in direct contact with the environment will be close to it nonetheless.