Chapter 3The Cellular Level of Organization1Okay, the next chapter that we're going to cover is the cellular level of organization. And the firstthing that we need to talk about are cells and cell theory.The cell is the smallest structural and functional living unit and we have over 200 different typesof human cells that differ in size, shape, sub-cellular components, and functions. Each cellmaintains homeostasis at a cellular level and we couldn't examine cells until the invention of themicroscope in the 17th century.But our cells have coordinated activities that allow for homeostasis at a higher organizationallevel and we're going to talk about how cells come from the division of pre-existing cells and thevital functions that cells produce and perform for us every second of every day within the humanbody. Now cells differentiate into specialized cells and you can see here the differentiation of 4different tissue types from a single cell.So from a single cell we get epithelial tissue, which has many different subcategories; connectivetissue–again, has many different subcategories; muscle tissue–there’s 3different types we willdiscuss; and neural tissue–and you can see that each of these cells and tissues are very differentin both their shape and structure. But all cells have some common structures and functions thatwe'll talk about.Now cell differentiation is when all cells in the body come from a single fertilized ovum. Thisovum contains the genetic potential to become any cell in the body. As cellular division occurs,the cytoplasm of the cell is divided into smaller parcels and these parcels differ from one anotherbecause of regional differences in the position of the cytoplasm at fertilization. So this ovum hasthe potential to become any cell in the body, and the first cell division–again–creates thesesmaller portions of the cytoplasm.Those differences in the cytoplasm might result in different daughter cells. So the cytoplasmicdifferences affect the DNA of the cells, which can turn specific genes on or off, and the daughtercell begins to develop specialized structural and functional characteristics. This process ofgradual specialization is called differentiation and it's this process that produces the specializedcells that form the tissues responsible for all of our body functions.Now cells are the smallest units of life and the primary components of the cell are the plasmamembrane and the cytoplasm. The plasma membrane is a bi-molecular layer of lipids andproteins–that you will see in more detail coming up–in a constantly-changing fluid mosaicmodel. It plays a dynamic role in the cellular activity, but it also separates the extracellular fluid–which is sometimes referred to as the ECF–from the interstitial fluid or intracellular fluid, theICF–and the cytoplasm, as you'll see, can be further subdivided into different types oforganelles, both non membranous organelles and membranous organelles, that we will look at inmore detail.