Structure of the Plasma Membrane

Because both the intracellular and extracellular environments (or "compartments") are aqueous, a barrier must be present to prevent the loss of enzymes, nucleotides, and other cellular molecules that are water-soluble. Since this barrier surrounding the cell cannot itself be composed of water-soluble molecules, it is instead composed of lipids.

The plasma membrane (also called the cell membrane), and indeed all of the membranes surrounding organelles within the cell, are composed primarily of phospholipids and proteins. Phospholipids, described in chapter 2, are polar (and hydrophilic) in the region that contains the phosphate group and nonpolar (and hydrophobic) throughout the rest of the molecule. Since the environment on each side of the membrane is aqueous, the hydrophobic parts of the molecules "huddle together" in the center of the membrane, leaving the polar parts exposed to water on both surfaces. This results in the formation of a double layer of phospholipids in the cell membrane.

The hydrophobic middle of the membrane restricts the passage of water and water-soluble molecules and ions. Certain of these polar compounds, however, do pass through the membrane. The specialized functions and selective transport properties of the membrane are believed to be due to its protein content. Membrane proteins are described as peripheral or integral. Peripheral proteins are only partially embedded in one face of the membrane, whereas integral proteins span the membrane from one side to the other. Since the membrane is not solid—phospholipids and proteins are free to

Secretory vesicle

Centriole

Nucleolus

Agranular endoplasmic reticulum

Secretory vesicle

Centriole

Nucleolus

Agranular endoplasmic reticulum

Nucleus

Agranular Endoplasmic Reticulum

Golgi complex

Nuclear envelope

Mitochondrion

Lysosome

Chromatin

-Plasma membrane

Microtubule

Granular endoplasmic reticulum

Cytoplasm (cytosol)

Ribosome

Golgi complex

Nuclear envelope

Mitochondrion

Lysosome

Chromatin

-Plasma membrane

Nucleus

Microtubule

Granular endoplasmic reticulum

Cytoplasm (cytosol)

Ribosome

■ Figure 3.1 A generalized human cell showing the principal organelles. Since most cells of the body are highly specialized, they have structures that differ from those shown here.

Extracellular side

Glycoprotein Glycolipid

Extracellular side

Fluid Mosaic Model Cell Membrane

Nonpolar_ end

Polar end

Phospholipids -

Proteins

Cholesterol

Intracellular side

Nonpolar_ end

Polar end

Phospholipids -

Proteins

Cholesterol

Intracellular side

■ Figure 3.2 The fluid-mosaic model of the plasma membrane. The membrane consists of a double layer of phospholipids, with the polar regions (shown by spheres) oriented outward and the nonpolar hydrocarbons (wavy lines) oriented toward the center. Proteins may completely or partially span the membrane. Carbohydrates are attached to the outer surface.

move laterally—the proteins within the phospholipid "sea" are not uniformly distributed. Rather, they present a constantly changing mosaic pattern, an arrangement known as the fluid-mosaic model of membrane structure (fig. 3.2).

The proteins found in the plasma membrane serve a variety of functions, including structural support, transport of molecules across the membrane, and enzymatic control of chemical reactions at the cell surface. Some proteins function as receptors for hormones and other regulatory molecules that arrive at the outer surface of the membrane. Receptor proteins are usually specific for one particular messenger much like an enzyme that is specific for a single substrate. Other cellular proteins serve as "markers" (antigens) that identify the blood and tissue type of an individual.

ft The plasma membranes of all higher organisms contain cholesterol. The cells in the body with the highest ^ Ji i~i content of cholesterol are the Schwann cells, which form insulating layers by wrapping around certain nerve fibers (see chapter 7). Their high cholesterol content is believed to be important in this insulating function. The ratio of cholesterol to phospholipids also helps to determine the flexibility of a plasma membrane. When there is an inherited defect in this ratio, the flexibility of the cell may be reduced. This could result, for example, in the inability of red blood cells to flex at the middle when passing through narrow blood channels, thereby causing occlusion of these small vessels.

Cell Structure and Genetic Control

Pseudopod

Pseudopod

Plasma Membrane Red Blood Cell Phagocytosis

Pseudopods > forming food vacuole

Pseudopods > forming food vacuole

■ Figure 3.3 Scanning electron micrographs of phagocytosis. (a) The formation of pseudopods and (b) the entrapment of the prey within a food vacuole.

In addition to lipids and proteins, the plasma membrane also contains carbohydrates, which are primarily attached to the outer surface of the membrane as glycoproteins and glycolipids. These surface carbohydrates have numerous negative charges and, as a result, affect the interaction of regulatory molecules with the membrane. The negative charges at the surface also affect interactions between cells—they help keep red blood cells apart, for example. Stripping the carbohydrates from the outer red blood cell surface results in their more rapid destruction by the liver, spleen, and bone marrow.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • URSULA
    Where are phospholipid in fluid mosaic model of membrane?
    5 years ago
  • fred
    What are the cells in the body that contain plasma membrane with highest cholesterol content?
    4 years ago
  • darcy
    Are proteins in plasma membrane polar or nonpolar?
    4 years ago
  • franziska
    Is the cytsol of red blood cell intracellular?
    4 years ago
  • Roberta
    Where is plasma found in the human heart diagram?
    3 years ago

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