Miller and Levine (Digestion and Respiration)

Miller and Levine (Digestion and Respiration) How do animals obtain food? Most filter feeders catch algae and small animals by using modified gills or other structures as nets that filter food items out of water. Detritivores feed on detritus, often obtaining extra nutrients from the bacteria, algae, and other microorganisms that grow on and around it. Carnivores eat other animals. Herbivores eat plants or parts of plants in terrestrial and aquatic habitats. Many animals rely upon symbiosis for their nutritional needs. How does digestion occur in animals? Some invertebrates break down food primarily by intracellular digestion, but many animals use extracellular digestion to break down food. Invertebrate Digestive Systems One-way digestive tracts often have specialized structures, such as a stomach and intestines, that perform different tasks as food passes through them. In some animals, the mouth secretes digestive enzymes that start the chemical digestion of food. Mechanical digestion may occur as specialized mouthparts or a muscular organ called a gizzard breaks food into small pieces. Chemical digestion continues in a stomach and intestines, sometimes aided by secretions from other organs such as a liver or pancreas. Intestines also absorb the nutrients released by digestion. Specializations for Different Diets How are mouthparts adapted for different diets? Carnivores typically have sharp mouthparts or other structures that can capture food, hold it, and “slice and dice” it into small pieces. Herbivores typically have mouthparts adapted to rasping or grinding. Specialized Digestive Tracts Carnivorous invertebrates and vertebrates typically have short digestive tracts that produce fast-acting, meat-digesting enzymes that can digest most cell types found in animal tissues. No animal produces digestive enzymes that can break down the cellulose in plant tissue. However, some herbivores have very long intestines or specialized pouches in their digestive tracts that harbor microbial symbionts that digest cellulose. Cattle, for example, have a pouchlike extension of their stomach called a rumen (plural: rumina), in which symbiotic bacteria digest cellulose. Animals with rumina, or ruminants, regurgitate food that has been partially digested in the rumen, chew it again, and reswallow it. This process is called “chewing the cud.” Gas Exchange What characteristics do the respiratory structures of all animals share? Respiratory structures provide a large surface area of moist, selectively permeable membrane. Respiratory structures maintain a difference in the relative concentrations of oxygen and carbon dioxide on either side of the respiratory membrane, promoting diffusion. Respiratory Surfaces of Aquatic Animals How do aquatic animals breathe? Many aquatic invertebrates and most aquatic chordates other than reptiles and mammals exchange gases through gills. Aquatic reptiles and aquatic mammals, such as whales, breathe with lungs and must hold their breath underwater. Respiratory Surfaces of Terrestrial Animals What respiratory structures enable land animals to breathe? Respiratory structures in terrestrial invertebrates include skin, mantle cavities, book lungs, and tracheal tubes. But all terrestrial vertebrates—reptiles, birds, mammals, and the land stages of most amphibians— breathe with lungs. Lung Structure in Vertebrates Although lung structure in terrestrial vertebrates varies, the processes of inhaling and exhaling are similar. Alveoli provide an enormous surface area for gas exchange, and enable mammals to take in the large amounts of oxygen required by their high metabolic rates. Bird Lungs In birds, the lungs are structured so that air flows mostly in only one direction, so no stale air gets trapped in the system. Gas exchange surfaces are continuously in contact with fresh air. This highly efficient gas exchange helps birds obtain the oxygen they need to power their flight muscles at high altitudes for long periods of time.