What are the major classes of membrane transport proteins, and how do they transport biologically important molecules and ions across the plasma membrane?
4.
What is the electrochemical gradient, and how it is used to determine whether the transport of a molecule or ion across the plasma membrane is active or passive?
5.
What are the driving forces for movement of water across cell membrane and the capillary wall?
In addition, the student should be able to define and understand the following properties of physiologically important solutions and fluids: he human body is composed of billions of cells. Although cells can perform different functions, they share certain common elements. This chapter provides an overview of these common elements and focuses on the important function of the transport of molecules and water into and out of the cell across its plasma membrane.
Overview of Eukaryotic Cells
Eukaryotic cells are distinguished from prokaryotic cells by the presence of a membrane-delimited nucleus. With the exception of mature human red blood cells and cells within the lens of the eye, all cells within the human body contain a nucleus. The cell is therefore effectively divided into two compartments: the nucleus and the cytoplasm. The cytoplasm is an aqueous solution containing numerous organic molecules, ions, cytoskeletal elements, and a number of organelles. Many of the organelles are membrane-enclosed compartments that carry out specific cellular function. An idealized eukaryotic cell is depicted in
Fig. 1.1 , and the primary function of some components and compartments of the cell are summarized in . Readers who desire a more in-depth presentation of this material are encouraged to consult one of the many textbooks on cell and molecular biology that are currently available.
The Plasma Membrane
The cells within the body are surrounded by a plasma membrane that separates the intracellular contents from the extracellular environment. Because of the properties of this membrane and, in particular, the presence of specific membrane proteins, the plasma membrane is involved in a number of important cellular functions, including the following:
Selective transport of molecules into and out of the cell. A function carried out by membrane transport proteins.
Cell recognition through the use of cell surface antigens.
Cell communication through neurotransmitter and hormone receptors and through signal transduction pathways.
Tissue organization, such as temporary and permanent cell junctions, and interaction with the extracellular matrix, with the use of a variety of cell adhesion molecules.
Membrane-dependent enzymatic activity.
Determination of cell shape by linkage of the cytoskeleton to the plasma membrane. In this chapter, the structure and function of the plasma membrane of eukaryotic cells are considered. More specifically, the chapter focuses on the transport of molecules and water across the plasma membrane. Only the principles of membrane transport are presented here. Additional details that relate to specific cells are presented in the various sections and chapters of this book.
The plasma membrane of eukaryotic cells consists of a 5-nm-thick lipid bilayer with associated proteins (
Fig. 1.2 ). Some of the membrane-associated proteins are integrated into the lipid bilayer; others are more loosely attached to the
CHAPTER 1 Principles of Cell and Membrane Function 3 • Fig. 1.1 Schematic drawing of a eukaryotic cell. The top portion of the cell is omitted to illustrate the nucleus and various intracellular organelles. See text for details.
Primary Functions of Some Eukaryotic Cellular Components and Compartments 1.1