Introduction to the Human Cell. Danton PhD O'Day

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СКАЧАТЬ mutants, quantify specific proteins, analyze a cell’s molecular composition, carry out physiological assessments and use various microscopic analyses. Some of the most common techniques are summarized in Appendix I (Some Common Techniques Used in Cell Biology) and II (SDS-PAGE, and Western Blotting). This volume attempts to integrate this information into a cohesive story of how normal cells work and how this work can be undermined in different ways by disease.

      Chapter 2

      The Human Cell Membrane

      The human cell membrane is the interface between the inside and the outside of the cell. It is the dividing line between cells. The cell membrane is also the mediator of how cells communicate and how they stick together to form tissues. When cells can’t communicate effectively and if they don’t adhere properly this can lead to a number of diseases of which cancer is of central concern. The cell membrane also binds to various components to bring essential molecules inside for normal cell function or processing for other uses. It is a dynamic and vital component that needs to be understood before we can effectively prevent diseases associated with its structure and function. Here we will focus on its structure which will set the stage for later analyses of how it plays critical roles in a diverse number of diseases.

      Membranes and Biomembranes

      The body contains all sorts of “membranes.” Though they share the same name, not all membranes are the same. Membranes of the body are typically collections of cells, proteins and carbohydrates that form various types of connective tissue, mucus membranes, etc. They serve to protect or hold tissues and organs together or to support them, among other functions. Biological membranes or biomembranes all consist of a bilayer of phospholipids within which various types of proteins and varying amounts of cholesterol are present. A single continuous cell membrane completely surrounds each and every cell of the body. It is a specialized biological membrane that not only keeps the cell intact but also regulates what gets in and out of cells.

      The cell membrane has other functions as well: it allows cells communicate and to stick together. It has receptors for hormones and other molecules that allow it to receive and interpret messages sent from other parts of the body. Inside the cell, there is a diversity of biomembranes each specialized for certain functions. Biological membranes in the cytoplasm compartmentalize cellular functions: the endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles and vacuoles are each surrounded by a single biological membrane and each of these organelles has specific duties to perform. In contrast, mitochondria and the nucleus are surrounded by two biomembranes. For now, we’ll focus on the cell membrane.

      All cells are surrounded by a cell membrane (also called the plasma membrane). This is a biological membrane or biomembrane consisting of a double layer of lipids in which proteins are located. The cell membrane keeps the components of the cell isolated from the external environment. It serves as the communication interface between the cell and its environment. The cell membrane is involved in regulating the flow of materials into and out of the cell, mediating intercellular communication and adhesion and a multitude of other functions.

      The structure and functions of the cell membrane have been defined by over a half a century of research using biochemical, physiological, cellular and molecular techniques. In spite of the important and diverse work that has been done in the past to bring us to this understanding, this chapter does not present a complete or historical review. Instead it shows us what we understand about the cell membrane today.

      The Fluid Mosaic Model of the Cell Membrane

      The most widely accepted model of the cell membrane is the “Fluid Mosaic Model” (Figure 2.1). By this concept the cell membrane consists of a continuous, double layer of phospholipids.

      Figure 2.1. The fluid mosaic model of the cell membrane.

      Proteins either are embedded in the bilayer or associated with either the cytoplasmic or extracellular face. Carbohydrates are linked to the proteins (glycoproteins) or lipids (glycolipids) only on the extracellular side. The phospholipid profiles of the cytoplasmic and extracellular layers differ. Cholesterol, in varying amounts depending on the cell type, lies within the membrane serving to stabilize it. In plants and bacteria, but not humans, the cell membrane is covered by a cell wall. In this chapter, our goals are to learn the major constituents of the biomembranes and to understand how they are organized into a functional cell membrane. The information is designed to set the stage for the chapters that follow.

      Structure of Phospholipids: The Amphipathic Nature of Phospholipids

      As you might have realized from the previous paragraphs, the fluid mosaic model emphasizes the fact that the cell membrane is flexible (fluid) and made up of many specific components (mosaic) that define its functions. The basic structure of all biomembranes is defined by their continuous component—the phospholipid bilayer. Phospholipids are fats that have important attributes that allow them to form membranes. One part of the lipid molecule associates with water while the other part shuns it. This “amphipathic” nature (having both hydrophilic and hydrophobic components) of phospholipid molecules underlies the basic structural organization of biological membranes (Figure 2.2).

      Figure 2.2. The structure of phospholipids and their orientation within the cell membrane.

      •Hydrophilic head—“likes water”—the polar end of the lipid molecule

      •Hydrophobic tail—“hates water”—non-polar chain of fatty acids—Tail length and the number of double bonds varies

      Asymmetry of Lipid Bilayer

      The types of lipids that are present in the phospholipid bilayer define many of the physical attributes of the membrane (e.g., how fluid it is at any temperature). The phospholipid bilayer also contains lipids that are involved in cell communication (signal transduction) as detailed in future chapters. The figure below (Figure 2.3) reveals that there is an uneven distribution of phospholipids in the cell membrane with certain types preferentially on one side and others on the opposite side. These “sides” are called leaflets: the outer leaflet, where glycolipids can be found, faces the environment while the inner leaflet, rich in phosphatidylserine, is oriented towards the cytoplasm.

      Figure 2.3. Types of phospholipids and their localization within the cell membrane.

      Some typical examples of phospholipid localization:

      •Outer leaflet: more phosphatidylcholine and sphingomyelin

      •Inner leaflet: more phosphatidylserine and phosphatidylethanolamine

      •Glycolipids: only on outside face

      Micelles: An Alternative Lipid Conformation

      The amphipathic structure of phospholipids allows them to form other configurations such as micelles and liposomes (Figure 2.4). The role of micelles in normal cell functions remains open to discussion. СКАЧАТЬ