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|Chapter Three: Cell Structures|
Surrounding every cell is some sort of covering that keeps what's inside the cell inside and prevents harmful particles in the external environment from diffusing into the cell. There are two types of organelles which serve as a covering for the cell: the cell membrane and the cell wall. All cells have a cell membrane, and certain cells also have a cell wall. We'll discuss each of these separately, because they perform very different functions.
The cell membrane's main purpose is to regulate the movement of materials into and out of the cell. By doing so, the internal environment of the cell can be different than the external environment, since only certain materials can pass through. Scientists say that the cell membrane is selectively permeable, which means that only certain substances can permeate (go through) the membrane. The next section discusses how the cell membrane accomplishes this very important function, but for now, let's discuss what the cell membrane is made of.
Cell membranes consist mainly of phospholipids. Recall from Chapter Two that phospholipids have two parts: a polar phosphate "head" and two nonpolar fatty acid "tails." These molecules are arranged in what is called a bilayer (a double layer) so that the polar heads face the external and the internal environments of the cell and the fatty acids form the inside of the membrane. The phospholipids are free to move around, often switching places with their neighbors. This allows for the membrane to stretch and change shape.
Also contained within the membrane are large protein molecules. Some of them jut out on either side of the membrane, while others are only on one side.
Unlike the cell membrane which is found in all cells, not all cells have a cell wall, another structure which surrounds the cell membrane. In particular, prokaryotes usually have a cell wall of some sort, and a type of eukaryotes called algae may also have a cell well. The cell walls in each of these two types of organisms is different.
For prokaryotes, the cell wall usually contains large polymers called peptidoglycans. These molecules provide for the strength of the bacterial cell wall. Some prokaryotic cell walls have two layers: an inner layer made of peptidoglycans and an outer layer composed of lipoproteins and lipopolysaccharides.
In eukaryotes, the cell wall has three main parts: the primary cell wall, the middle lamella, and the secondary cell wall. The primary cell wall is located closest to the inside of the cell. It is made mostly of cellulose which allows the wall to stretch as the cell grows. The middle lamella is composed of polysaccharides called pectins. Outside of the middle lamella is the secondary cell wall, which contains both cellulose and a strong material called lignin. Lignin strengthens the wall and gives the cell a somewhat rectangular shape.