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More Cell Wall
©2001 Timothy Paustian, University of Wisconsin-Madison
The cytoplasmic membrane is that area of the cell immediately surrounding the cytoplasm and is perhaps the most conserved structure in living cells. Every living thing on this planet has some type of membrane. Membranes are thin structures, measuring 8 nm thick. They are the major barrier in the cell, separating the inside of the cell from the outside. It is this structure which allows cells to selectively interact with their environment. Membranes are highly organized and asymmetric having two faces with different topologies and different functions. Membranes are also dynamic, constantly adapting to changing environmental conditions.
Structure of components
Membranes in bacteria are composed of phospholipids and proteins. Phospholipids contain a charged or polar group (often phosphate, hence the name) attached to a 3 carbon glycerol back bone. There are also two fatty acid chains dangling from the other carbons of glycerol. The phosphate end of the molecule is hydrophilic and is attracted to water. The fatty acids are hydrophobic and are driven away from water. The first figure shows the structure of the lipid ethanol amine.
Figure 1 - The structure of a phospholipid
Membranes also contain proteins. Typically 20-30% of membrane associated protein is soluble in water and loosely associated. The other 70-80% is tightly bound to the membrane, often spanning both sides. These proteins are also often amphipathic molecules (contain both hydrophobic and hydrophilic portions) with stretches of hydrophilic amino acids and stretches of hydrophobic amino acids. Most of them are placed in the membranes so that the hydrophobic amino acids associate with the lipids in the membrane and the hydrophilic amino acids are outside the membrane interacting with either the cytoplasm or the periplasm.
Remember, membranes are not static they are always changing.
Because phospholipids have hydrophobic and hydrophilic portions, they do remarkable things. When placed in an aqueous environment, the hydrophobic portions stick together, as do the hydrophilic. A very stable form of this arrangement is the lipid bilayer. This way the hydrophobic parts of the molecule form one layer, as do the hydrophilic. Lipid bilayers form spontaneously if phospholipids are placed in an aqueous environment. These are known as membrane vesicles and are used to study membrane properties experimentally.
Figure 2 - Cartoon of the Lipid Bilayer
Some proteins span the membrane while others are found on the outside or the inside. Many of the membrane spanning proteins are involved in transport or energy generation.
Stabilization of the membrane
The cytoplasmic membrane is stabilized by hydrophobic interactions between neighboring lipids and by hydrogen bonds between neighboring lipids. Hydrogen bonds can also form between membrane proteins and lipids. Further stability come from negative charges on proteins that form ionic interactions with divalent cations such as Mg+2 and Ca+2 and the hydrophilic head of lipids
Some proteins may move within the plane of the membrane while others are anchored to structures in or near the membrane. The result is that the membrane is actually fluid and has the consistency of a light grade oil. It has been termed a fluid mosaic, mosaic because there is a definite pattern to it, fluid because the lipids are free to move about on each side of the membrane. Lipids do not genrally switch sides, moving from outside to inside or inside to outside. This arrangements confers a number of properties on the membrane which allow it to perform many functions.
Retains the cytoplasm.
The concentration of solutes, sugars, ions etc. are much higher within the cell than outside. A fundamental principle of nature, however is that solute concentrations will tend to equilibrate. In this case, causing water to flow into the cell (a process known as osmosis) and the solutes to flow out. The cell membrane prevents free flow of material and thus serves as an osmotic barrier
Since the cell is separated from its environment and needs to get nutrients in and waste out, the membrane must be able to accommodate this. It acts as a selective barrier. Some molecules can cross the membrane without assistance, most cannot. Water, non-polar molecules and some small polar molecules can cross. non-polar molecules penetrate by actually dissolving into the lipid bilayer.
Most polar compounds such as amino acids, organic acids and inorganic salts are not allowed entry, but instead must be specifically transported across the membrane by proteins.
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