Cell Biology Basics

Three separate cell layers begin to form from the blastula that allow for the differentiation of cells that ultimately form the being. These germ layers or sheets are referred to as the ectoderm, the mesoderm, and the endoderm. Each type of germ layer forms multiple unique cells that together form the embryo, fetus, and infant. This is the first type of specification that forms in the growing blastula. Besides the three germ layers, cells that begin to create the mammalian placenta begin to form, which is the structure than nourishes the embryo. The placenta does not come from any of the germ layers.

As the embryo develops, some cells turn on specific genes, while other cells turn off genes, which results in the specification of cells. The upregulation of specific genes makes proteins in the cytoplasm that do different things inside the cell, creating the molecules and organelles necessary for cell function. The result of the upregulation of certain genes and the downregulation of certain genes is the formation of different cell types that go from the simplest cell types—the ectodermal cells, endodermal cells, and mesodermal cells—to the more complex cells, like nerve cells, muscle cells, and epidermal cells.

As the process of differentiation of the blastula occurs, the ectodermal cells arrive on the outside of the blastula, the mesodermal cells arrive at the middle of the blastula, and the endodermal cells arrive at the inside of the blastula/embryo. These cells communicate with one another, beginning the process of intracellular communication, which will be discussed in chapter 8. The shape of the embryo begins to form with a caudal aspect, mesial aspect, and cephalad aspect, giving the tail, trunk, and head of the embryo, respectively.

Rather quickly, the embryo begins to grow because different DNA molecules are turned on in certain cells that change the growth rate of each specific type of cell. Some cells grow faster than others, which creates the unique shape of the embryo. The head is larger than the tail because these cells grow faster and bigger than the caudal cells so the embryo takes on the shape of a larger head, a medium-sized middle, and a smaller tail. No one knows how these cells communicate with one another to create this unique shape.

The cellular unit consists of numerous different molecules that make organelles—each of which is enclosed by a membrane of some sort.

The organelles are separate from one another but are connected by an intracellular matrix that allows them to stay basically in the same place unless they are needed to travel to one or another part of the cell.

Refer to figure 2 to see what the typical cell looks like:

Figure 2

Cells need to both divide and grow. They can’t do this without some sort of basic cellular nutrition and cellular energy. The cell uses organelles that ingest molecules from outside the cell, using them inside the cell for cellular growth and development. The molecules ingested by the cell are acted upon by enzymes that catalyze biochemical reactions. The biochemical reactions can create cell structures or can be part of the energy pathways necessary for the cell to function. All biochemical reactions in the cells use catalysts to make the reaction happen. The catalyst is necessary for the reaction but doesn’t change during the reaction so it is available for further reactions.

Cellular processes and cellular reactions are not singular events but occur in chains of reactions that go through multiple steps to take a single molecule and, over many steps, transform it into the desired molecule necessary for the cell’s function. In general, small molecules are ingested and, through multiple enzymatic steps, are transformed into bigger molecules that make cell structures. When there are enough big molecules inside the cell to support cell division, the process of cell division occurs, creating two daughter cells that have enough substrate molecules to support themselves. This process happens millions of times to create cells of all types and sizes.