The Biological Membrane

In many areas of science, it is at the interface where things get interesting, and difficult. Chemical reactions in homogeneous gas or liquid phases are complex enough, but when an interface is present, even the notion of chemical concentration is oversimplified. For example, the pH near the surface of a colloid can vary with distance from the surface. Surface-active agents will distribute themselves differently between the surface and the bulk fluid. Many surfaces act as catalysts. They can be gatekeepers, affecting the transport of substances between phases. The phospholipid bilayer membrane is the major interface formed by living things. It is the structure that separates "inside" from "outside." By increasing the complexity of the system, it also increases its possibilities.

The biological membrane is a flexible sheet forming a closed surface, whose basic structure is formed of a phospholipid bilayer (see Figure 4.3). The capability of phospholipids to form enclosed bilayer vesicles spontaneously was described in Section 3.7.2. The outer membrane of all cells is called the plasma membrane or cell membrane. Similar membranes also enclose cell organelles such as the mitochondria or the nucleus in eukaryotic cells.

Figure 4.2 Animal (a) and plant (b) cells. (From Fried, 1990. © The McGraw-Hill Companies, Inc. Used with permission.)

Besides being flexible, the phospholipid molecules can move freely within the plane of the membrane, a behavior described as a two-dimensional fluid. Other molecules are intimately associated with the membrane. Eukaryotic membranes can contain large amounts of cholesterol, which increase the fluidity of the membrane. Fatty acids serve the same function in prokaryotes. Globular proteins are embedded in the membrane, somewhat like icebergs floating in the sea. Some penetrate both surfaces of the membrane and participate in the transport of substances across it. Others are embedded in one or the

Proteins

Phospholipid bilayer

Proteins

Phospholipid bilayer

Membrane pore

Figure 4.3 Plasma membrane structure. (From Van De Graaff and Rhees, 1997. © The McGraw-Hill Companies, Inc. Used with permission.)

Membrane pore

Figure 4.3 Plasma membrane structure. (From Van De Graaff and Rhees, 1997. © The McGraw-Hill Companies, Inc. Used with permission.)

other surface and function as chemical receptors or catalytic sites. Biological membranes are asymmetrical. The external surface of the plasma membrane and the internal surface of organelles have carbohydrates bonded to them, forming glycolipids and glycoproteins.

The plasma membrane of archaean cells is chemically distinct from eubacterial or eukaryotic cell membranes. Instead of being composed of lipids made from straight-chain fatty acids bonded to glycerol by ester bonds, archean membrane lipids are made of the branched hydrocarbon isoprene bonded to glycerol by ether bonds. This structure is thought to give archeans greater physical and chemical resistance to the relatively unfavorable environmental conditions in which they are often found.

Membranes are typically less than 10% carbohydrate by mass; the rest of the membrane mass is about equally divided between protein and lipid. In animals, about half the lipid is phospholipid and half is cholesterol.

The membrane controls the transport across it of both substances and information. Information is transported in the sense that substances, called ligands, can bind to receptors composed of transmembrane proteins (proteins that penetrate both sides) on one side of the membrane, producing a change in its conformation on the other side. The altered protein can then affect other reactions. This sends a signal across the membrane without a substance actually crossing over. Examples of this are the intercellular messengers called hormones. Insulin, for example, binds to a receptor and causes two separate effects. Primarily, it stimulates plasma membrane mechanisms for the transport of glucose, some ions, and amino acids. Second, it results in changes in intracellular metabolism that result in increased synthesis and storage of protein, glycogen, and lipid. Some toxic substances act by binding with receptors, either by stimulating an inappropriate response directly, or by competing with normal ligands.

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