2-12 Membranes surround the cell and hold it in

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• Membranes are thin, highly conserved structures found in all cells.
• Membranes are stabilized by hydrophobic interactions, hydrogen bonds and ionic interactions.
• Membranes are composed of lipids and proteins. The lipids form a bilayer, with the hydrophilic portion of the lipids facing the aqueous environment, and the hydrophobic portions clustering together inside the membrane. The majority of membrane proteins are involved in moving small molecules across the membrane or in energy generation.

General properties

The cytoplasmic membrane immediately surrounds the inside of the cell and is perhaps the most conserved structure in living cells. Membranes are thin structures, measuring about 8 µm thick and every living thing on this planet has some type of membrane They are the major barrier separating the inside of the cell from the outside and allow cells to selectively interact with their environment. Membranes are highly organized and asymmetric. This asymmetry comes from the fact that the membrane that faces the environment performs very different functions than does the side that faces the cytoplasm. Membranes are also dynamic, constantly adapting to changing environmental conditions.

Physical structure

Membranes are composed of lipids and proteins. The majority of lipids are phospholipids as described earlier, but about 50% of all know bacterial species also contain hopanoids as shown in Figure 2-75. These molecules have a similar structure to sterols found in eukaryotic membranes and serve to help stabilize the membrane. Proteins are more numerous in bacterial membranes than in eukaryotic membranes. This is because bacteria in general only contain a single membrane in contact with the cytoplasm and this has to carry out all the functions of the cell. In eukaryotes these functions are divided amongst the cytoplasmic membrane and the other organelles.

Much of the general behavior of membranes is dictated by the behavior of lipids in water. Because phospholipids are amphipathic, they tend to congregate when placed in an aqueous environment. This is done in a very specific fashion such that the hydrophilic portions face the water and the hydrophobic portions are buried inside. Under the cell's direction lipids are organized into a bilayer, where there are two sheets of lipids oriented so that the hydrophobic faces of each sheet face each other as shown in Figure 2-20. Lipid bilayers can be almost any size and can form vesicles spontaneously, if lipids are placed in an aqueous environment. In the cell, however, the synthesis of membranes is performed by specific enzymes and is tightly controlled.

The cytoplasmic membrane is held together by a number of forces. Hydrophobic interactions between the alkyl chains of neighboring lipids are a major component of membrane stability. Hydrogen bonds between lipids and between membrane proteins and lipids also hold a membrane together. Further stability comes 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.

The hydrophobic region of the membrane provides a critical function: it prevents polar compounds, such as ions and most biological molecules, from passing through it. This allows the cell to create and maintain gradients of ions and small molecules across the membrane by mechanisms described below.

So how do polar molecules ever cross this membrane, since this represents an important interaction between the cell and its environment? This transfer across the membrane comes about through the specific functioning of proteins that are imbedded in the membrane. Some proteins span the membrane while others are exposed on the outside or the inside. These proteins may move within the plane of the membrane or they may be anchored to structures in or near the membrane. Many of the membrane-spanning proteins are involved in transport of the polar molecules that must pass through the membranes. A subset of these proteins are also involved in energy generation as discussed below.

The membrane is fluid and has the consistency of a light grade oil. It has been termed a fluid mosaic: "fluid" because the lipids are free to move about on each side of the membrane and "mosaic" because there is a definite pattern to it. Lipids do not generally switch from outside to inside or vice versa, because of the problem of trying to move the hydrophilic group through the hydrophobic core of the bilayer.

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