 You're probably already quite familiar with the main systems in the body, digestive, respiratory and circulatory systems, and you'll know these down to an organ level. For example, in the digestive system we have a stomach, mouth, anus, liver and a whole lot more. They are the functional units of organisms. We're going to drill down a bit deeper in the hierarchy to understand how the structure of individual cells and tissues within those organs enable their functions. First, let's see one way that was similar to plants. In her videos, Tori showed us how plants have different functional units and different tissues. One interesting plant tissue is the epidermis, the outer layer. It goes around everything and that same tissue type is found on the leaves, the roots, the stems, the flowers. Some tissue types reappear in different organs or systems. We too have an epidermis. Our skin tissue goes around our feet, our bodies, our faces. Because we have systems on the inside as well, and you can kind of imagine us as very complex long tubes, we have a second kind of protection layer tissue called the epithelial layer. The epithelial layer lines our tubes and our organs. Another way of looking at the complexity of multicellular organisms is through the hierarchy of systems. A hierarchy is a system of organising people or ideas in terms of level, importance or responsibility. Someone might see your school as having a hierarchy with the principal at the top, then the other head staff, the teachers and the students. But that's not to say the students are less important. What's the point of a school without students? This is the same in organisms. We organise the structures in a hierarchy to better understand how they work. But that's not to say the cell is less important than the organ. It's just another layer of detail. Even a blob of lab-grown meat has different types of cells and requires complexity. Researchers are developing ways to grow cultured meat. To grow meat in a lab that is thicker than a few layers of cells, the meat blob needs electrical stimulation to stimulate nerve impulses, scaffolding structures, systems for bringing in nutrients and taking away waste and the introduction of fat cells. In living organisms cells are programmed to differentiate, to become specialists for the needs of the organism. In plants, Mary stem cells can be either shoot or root Mary stems that differentiate into whatever tissues the plant needs. In animals, not all cells can turn into anything. Embryonic stem cells begin as unspecialised cells and essentially can become any type of tissue. Scientists are still working to understand how adult stem cells, for example the blood stem cells from bone marrow, can differentiate into different tissue types while in the body and when in a lab culture. Stem cells that can differentiate into a variety of cell types are called pluripotent. Many animal cells are differentiated as needed to suit specific functions of the tissue or organ they are in. Cell differentiation and growth is controlled by a complex communication system of hormones and enzymes that respond to changes in conditions. In the next few videos we are going to be taking a closer look at the relationship between structure and function in different animal systems across the hierarchy of system, organ, tissue and cell.