branch ANIMALS

=The Need For Gas Exchange= Cellular respiration is the process by which cells convert energy rich molecules (food) into a form of energy that is easily utilized by cells, called ATP. Aerobic respiration yields large amounts of cellular energy (ATP) but requires oxygen. Carbon dioxide is produced as a waste product. Efficient gas exchange ensures enough oxygen is supplied / carbon dioxide is removed in order to maintain cellular energy levels. [image:] =Requirements for efficient gas exchange= ===Moist Surface=== Efficient gas exchange relies on a moist surface as oxygen must first dissolve into water before it can diffuse through a cell membrane. ===Concentration Gradient=== [image:]Cellular respiration ultimately relies on the diffusion of oxygen into the cell and carbon dioxide out of the cell. Diffusion is the net movement of particles down their concentration gradient. In order for oxygen to diffuse into the cell there must be a higher concentration outside the cell. Similarly, in order for carbon dioxide to diffuse out of the cell there must be a lower concentration of carbon dioxide surrounding the cell. ===Thin and Permeable Membrane=== The gas exchange surface needs to be thin in order to reduce the distance over which gasses have to diffuse. ===Large Surface Area to Volume Ration (SA:VOL)=== The greater the surface area that is in contact with the environment, the greater the rate of diffusion (increases the rate of oxygen supply and carbon dioxide removal). However, increasing volume increases oxygen demand and the diffusion distance from the surface to the center. Gas exchange structures increase the SA:Vol to ensure oxygen supply is sufficient to meet the organisms oxygen demands. It also ensures that carbon dioxide is released at a greater rate than it is produced (a build up of carbon dioxide within the cells would be toxic) As an organism increases in size its volume increases more rapidly than it's surface area. The small size of singled celled organisms ensures they have a great enough SA:Vol ratio to satisfy their needs. Larger organisms rely on gas exchange structures that increase their surface area without significantly increasing their volume. [image:]
Credit: Ben Himme