Membrane Composition and Structure

The physical organization and functioning of all biological membranes depend on their constituents: lipids, proteins, and carbohydrates. The lipids establish the physical integrity of the membrane

Stephen Hawking The effects of motor neuron disease have confined the famous physicist to a wheelchair. A major cellular manifestation of this disease is the lack of ability of the nerve cells to stimulate the opening of channels through muscle cell membranes that would result in normal muscle function.

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and create an effective barrier to the rapid passage of hy-drophilic materials such as water and ions. In addition, the phospholipid bilayer serves as a lipid "lake" in which a variety of proteins "float" (Figure 5.1). This general design is known as the fluid mosaic model.

Proteins embedded in the phospholipid bilayer have a number of functions, including moving materials through the membrane and receiving chemical signals from the cell's external environment. Each membrane has a set of proteins suitable to the specialized function of the cell or organelle it surrounds.

The carbohydrates associated with membranes are attached either to the lipids or to protein molecules. They are located on the outside of the plasma membrane, where they protrude into the environment, away from the cell. Like some

5.1 The Fluid Mosaic Model The general molecular structure of biological membranes is a continuous phospholipid bilayer in which proteins are embedded.

5.1 The Fluid Mosaic Model The general molecular structure of biological membranes is a continuous phospholipid bilayer in which proteins are embedded.

of the proteins, carbohydrates are crucial in recognizing specific molecules.

Lipids constitute the bulk of a membrane

Most of the lipids in biological membranes are phospholipids. Recall from Chapter 2 that some compounds are hy-drophilic ("water-loving") and others are hydrophobic ("water-hating") and from Chapter 3 that phospholipids have both hydrophilic regions and hydrophobic regions:

► Hydrophilic regions: The phosphorus-containing "head" of the phospholipid is electrically charged and hence associates with polar water molecules.

► Hydrophobic regions: The long, nonpolar fatty acid "tails" of the phospholipid associate with other nonpolar materials, but they do not dissolve in water or associate with hydrophilic substances.

Outside of cell

Outside of cell

Membrane Composition

As a consequence of these properties, one way in which phospholipids can coexist with water is to form a bilayer, with the fatty acids of the two layers interacting with each other and the polar regions facing the outside aqueous environment (Figure 5.2).

In the laboratory, it is easy to make artificial bilayers with the same organization as natural membranes. In addition, small holes in a bilayer seal themselves spontaneously. This capacity of lipids to associate and maintain a bilayer organization helps biological membranes fuse during vesicle formation, phagocytosis, and related processes.

All biological membranes have a similar structure, but membranes from different cells or organelles may differ greatly in their lipid composition. Phospholipids differ in terms of fatty acid chain length, degree of unsaturation (double bonds) in the fatty acids, and the polar (phosphate-containing) groups present. In addition to phospholipids, membranes may contain cholesterol, a different type of lipid. In some membranes, 25 percent of the lipid is cholesterol (see Chapter 3), but other membranes have no cholesterol at all. When present, cholesterol is important to membrane integrity; most cholesterol in membranes is not hazardous to your health. A molecule of cholesterol is commonly situated next to an unsaturated fatty acid (see Figure 5.1).

The phospholipid bilayer stabilizes the entire membrane structure, but leaves it flexible, not rigid. At the same time, the fatty acids of the phospholipids make the hydrophobic interior of the membrane somewhat fluid—about as fluid as lightweight machine oil. This fluidity permits some molecules to move laterally within the plane of the membrane. A given phospholipid molecule in the plasma membrane may travel from one end of the cell to the other in a little more than a second. On the other hand, seldom does a phospho-lipid molecule in one half of the bilayer flip over to the other side and trade places with another phospholipid molecule. For such a swap to happen, the polar part of each molecule would have to move through the hydrophobic interior of the membrane. Since phospholipid flip-flops are rare, the inner and outer halves of the bilayer may be quite different in the kinds of phospholipids they contain.

The amount of cholesterol present in membranes, along with the degree of saturation of the fatty acids present, can increase or decrease membrane fluidity. Shorter fatty acid chains make for a more fluid membrane, as do unsaturated fatty acids. Adequate membrane fluidity is essential for many membrane functions. Since molecules move more slowly and fluidity decreases at reduced temperatures, membrane functions may decline in organisms that cannot keep their bodies warm. To address this problem, some organisms simply change the lipid composition of their membranes under cold conditions, replacing saturated with unsaturated fatty acids and using fatty acids with shorter tails. Such changes play a part in the survival of plants and hibernating animals and bacteria during the winter.

Aqueous environment

Aqueous environment

Hydrophilic Fatty Acyl Side Chains

The nonpolar, hydrophobic fatty acid "tails" interact with one another in the interior of the bilayer.

The charged, or polar, hydrophilic "head" portions interact with polar water.

5.2 A Phospholipid Bilayer Separates Aque°us arvirOTment Two Aqueous Regions The eight phospholipid molecules shown here represent a small cross section of a membrane bilayer.

The nonpolar, hydrophobic fatty acid "tails" interact with one another in the interior of the bilayer.

The charged, or polar, hydrophilic "head" portions interact with polar water.

5.2 A Phospholipid Bilayer Separates Aque°us arvirOTment Two Aqueous Regions The eight phospholipid molecules shown here represent a small cross section of a membrane bilayer.

Membrane proteins are asymmetrically distributed

All biological membranes contain proteins. Typically, plasma membranes have one protein molecule for every 25 phos-pholipid molecules. This ratio varies, however, depending on membrane function. In the inner membrane of the mitochondrion, which is specialized for energy processing, there is one protein for every 15 lipids. On the other hand, myelin, a membrane that encloses some nerve cells and uses the properties of lipids to act as an electrical insulator, has only one protein per 70 lipids.

Many membrane proteins are embedded in, or extend across, the lipid bilayer. Like phospholipids, these proteins have both hydrophilic and hydrophobic regions:

► Hydrophilic regions: Stretches of amino acids with hydrophilic R groups (side chains; see Table 3.2) give certain regions of the protein a polar character. Those regions, or domains, interact with water, sticking out into the aqueous extracellular environment or cytoplasm.

► Hydrophobic regions: Stretches of amino acids with hydro-phobic R groups give other regions of the protein a nonpolar character. Those domains interact with the fatty acid chains in the interior of the lipid bilayer, away from water.

A special preparation method for electron microscopy, freeze-fracturing, reveals proteins embedded in the lipid bi-layer of cellular membranes (Figure 5.3). The bumps that can be seen protruding from the interior of these membranes are not observed in pure lipid bilayers.

According to the fluid mosaic model, the proteins and lipids in a membrane are independent of each other and interact only noncovalently. The polar ends of proteins can in-

5.3 Membrane Proteins Revealed by the Freeze-Fracture Technique This membrane from a spinach chloroplast was first frozen and then separated so that the membrane bilayer was split open.

teract with the polar ends of lipids, and the nonpolar regions of both molecules interact hydrophobically (see Figure 5.1).

There are two general types of membrane proteins:

► Integral membrane proteins have hydrophobic regions and penetrate the phospholipid bilayer. Many of these proteins have long hydrophobic a-helical regions that span the hydrophobic core of the bilayer. Their hydro-philic ends protrude into the aqueous environments on either side of the membrane (Figure 5.4).

► Peripheral membrane proteins lack hydrophobic regions and are not embedded in the bilayer. Instead, they have polar or charged regions that interact with similar regions on exposed parts of integral membrane proteins or phos-pholipid molecules (see Figure 5.1).

Some membrane proteins are covalently attached to fatty acids or other lipid groups. These proteins can be classified as a special type of integral protein, as their hydrophobic lipid component allows them to insert themselves into the lipid bilayer.

Proteins are asymmetrically distributed on the inner and outer surfaces of a membrane. Integral membrane proteins that protrude on both sides of the membrane, known as transmembrane proteins, show different "faces" on the two membrane surfaces. Such proteins have certain specific domains on the outer side of the membrane, other domains within the membrane, and still other domains on the inner side of the membrane. Peripheral membrane proteins are localized on one side of the membrane or the other, but not

Integral Membrane Proteins
5.4 Interactions of Integral Membrane Proteins An integral membrane protein is held in the membrane by the distribution of the hydrophilic and hydrophobic R groups of its amino acids.
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Responses

  • Margaret
    What is the chemical structure of the eight phospholipids?
    7 years ago
  • Girma
    How phospholipids flip from inside layer to outside layer?
    7 years ago
  • kaari
    Why cant fatty acids make membrane bilayers?
    7 years ago
  • Amanda
    What are the functions of INTEGRAL proteins in the cell membrane?
    7 years ago
  • MIKKO YLI-SIRNI
    What are the functions of the protein embedded in the lipid bilayer?
    7 years ago
  • luwam
    Which part of the cell membrane is hydrophilic?
    7 years ago
  • nelma helkovaara
    What is the chemical composition of the plasma membrane?
    6 years ago
  • sofia
    What lipid composes plasma membranes with hydrophilic and hydrophobic regions?
    6 years ago
  • madihah kinfe
    What is the structure and composition of cell membrane?
    6 years ago
  • klaus
    What are the functions of the hydrophilic and hydrophobic regions in a plasma membrane?
    6 years ago
  • bowman
    What is the composition of the cell (plasma) membrane?
    6 years ago
  • MYRTLE
    Are the major lipids of plasma membranes?
    6 years ago
  • GIANNA
    What are some structures related to the plasma membrane?
    6 years ago
  • Lori
    Why do some membranes differ in protein composition?
    6 years ago
  • Esmeralda
    Why does lipid composition in membrane matter?
    6 years ago
  • eddie
    What is the composition of the hydrophilic and hydrophobic regions of the cell membrane?
    2 years ago
  • Yusef
    Is cholesterol hydrophilic or hydrophobic?
    1 year ago
  • niklas
    Is amino acid is component of plasma membrane?
    12 months ago

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