Human cells can differ in size and function. Some are bigger and some longer, some will make hormones while others will help our body move. In fact, there are roughly two hundred different types of cells in our body. Although these cells may seem unrelated, most of the general features will be the same from one cell to the next. Therefore, we can discuss cells by describing the features of a single cell. The unique characteristics of different types of cells such red blood cells, muscle cells, and fat cells will be described as they become relevant later in this chapter and book.
Let's begin by examining the outer wall, or more scientifically the plasma membrane of cells. As shown in Figure 2.1, the plasma membrane separates the inside of the cell from the outside of the cell. The watery environment inside the cell is called the intracellular fluid. Meanwhile, the watery medium outside of cells is called the extracellular fluid. Previously, it was noted that our body is about 60 percent water. Of this 60 percent, roughly two-thirds of the water is intracellular fluid while the remaining one-third is extracellular fluid, which would include the plasma of our blood.
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