What do cells look like?
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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 200 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. General features of a cell are shown in the Cell Figure. The unique characteristics of different types of cells such red blood cells, muscle cells and fat cells will be described elsewhere. |
Let’s begin by examining the outer wall, or more scientifically the plasma membrane of cells. 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.
What types of substances are found in the intracellular and extracellular fluids?
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In our body fluids we would find small dissolved substances such as ions, amino acids, and the carbohydrate glucose, as well as larger proteins. The major ions (or electrolytes) would include potassium (K+), sodium (Na+), chloride (Cl–), calcium (Ca2+), magnesium (Mg2+), phosphate (PO43–), and bicarbonate (HCO3–). |
As demonstrated in the Plasma Membrane Figure, all of these and other substances will be found in both the intracellular and extracellular fluids. However, the concentration of substances dissolved in either fluid varies and the plasma membrane is bestowed with the awesome responsibility of functioning as a barrier between the two mediums. The concentration of sodium (Na+) and chloride (Cl-) is more abundant in the extracellular fluid while potassium (K+) is more concentrated in the intracellular fluid. These electrolytes move down there concentration gradients through channels and are pumped against their concentration gradient by energy (ATP) requiring pumps.
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