 Hello, my name's Broderick and I'm a science communicator. We're going to be having a closer look at the organ, tissue and cell structure and function for a few animal systems. First, the digestive system. The digestive system is the way that animals get their nutrients and get rid of some wastes. Animals need energy in the form of carbohydrates, proteins and fats. They also need small amounts of minerals and vitamins. The digestive system is a system of organs that work together to break down food into the useful and not useful. The useful is broken down until they're small enough to enter cells. If you think of animals as being lined on the outside by an epidermal layer, the skin, along their digestive system, the gastrointestinal tract, is a layer of cells called the epithelial layer. This is the border to entry into the rest of the body. In humans and most animals, the gastrointestinal tract, or GI, starts with a mouth, which leads into an esophagus, which leads into the stomach. After the stomach, the tract becomes the small intestine, then the large intestine, the rectum and the anus. There are also some accessory organs. In humans, the important ones are the salivary glands, the liver, the pancreas and the gallbladder. They provide specialised functions, including producing digestive juices, which aid digestion. In different animals, the organs, or size of organs, tends to change a bit depending on what the animal eats. You might have heard that cows have four chambers to their stomachs. These house a lot of symbiotic bacteria to help digest grass and hay. In humans, the cecum is hardly worth mentioning. It has the appendix which can be removed with no ill effects. Koalas, though, have a really long cecum, about two metres. This provides an extra chamber for fermentation of their eucalyptus leaf food by symbiotic bacteria, which increases the nutritional value of the leaves. Even most birds and grasshoppers have multiple stomach chambers. Many carnivores have large, stretchy stomachs that allow them to eat a lot at once, like when a lion catches a springbok or a snake catches a possum. The bane job of the GI tract is breaking down food. This is done mechanically at a large scale by chewing, churning and peristalsis. Peristalsis is the autonomic squeezing and relaxing of muscles that moves the food along the tract. At a smaller scale, to bring nutrition into a size that the cells can handle, the digestive system uses chemicals and enzymes. In general, the epithelial tissues in the digestive system have thin membranes to allow the inward flow of nutritional molecules and are equipped to create and release enzymes and chemicals for breaking down food. Acids, salts and enzymes are used to break down food chemically. Enzymes are biological catalysts. They are little proteins made by the body that enable the breaking of bonds or turning big molecules into smaller component ones. Even before you start eating, your salivary glands start making saliva in the mouth. One of the active ingredients in saliva is salivary amylase. Amylases are enzymes that break down carbohydrates into simpler sugars. The stomach produces gastric juice. Gastric juice includes hydrochloric acid, which helps to break down the matter between cells in your food. It also includes the enzyme pepsin, which is a protease. Proteases break down proteins into smaller polypeptides. Pepsin works especially well in the acidic environment. The liver and gallbladder produce bile, which contains salts, which help break down fats and oils. The small intestine works together with the pancreas to produce and release more amylases, proteases, nucleases, which break down nucleic acids like DNA, salts and lipases, which break down fats.