Lipids and Cell Membranes
CHAPTER
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OUTLINE
The boundaries of all cells are defined by biological membranes (Figure 12.1), dynamic structures in which proteins float in a sea of lipids. The lipid component prevents molecules generated inside the cell from leaking out and unwanted molecules from diffusing in, while the protein components act as transport systems that allow the cell to take up specific molecules and remove unwanted ones. Such transport systems confer on membranes the important property of selective permeability. We will consider these transport systems in greater detail in the next chapter.
In addition to an external cell membrane (called the plasma membrane), eukaryotic cells also contain internal membranes that form the boundaries of organelles such as mitochondria, chloroplasts, peroxisomes, and lysosomes. Functional specialization in the course of evolution has been closely linked to the formation of such compartments. Specific systems have evolved to allow the targeting of selected proteins into or through particular internal membranes and, hence, into specific organelles. External and internal membranes share essential properties; these features are the subject of this chapter.
Biological membranes serve several additional functions indispensable for life, such as energy storage and information transduction, that are dictated by the proteins associated with them. In this chapter, we will examine the properties of membrane proteins that enable them to exist in the hydrophobic environment of the membrane while connecting two hydrophilic environments. In the next chapter, we will discuss the functions of these proteins.
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Many Common Features Underlie the Diversity of Biological Membranes
Membranes are as diverse in structure as they are in function. However, they do have in common a number of important attributes:
Membranes are sheetlike structures, only two molecules thick, that form closed boundaries between different compartments. The thickness of most membranes is between 60 Å (6 nm) and 100 Å (10 nm).
Membranes consist mainly of lipids and proteins. The mass ratio of lipids to proteins ranges from 1:4 to 4:1. Membranes also contain carbohydrates that are linked to lipids and proteins.
Membrane lipids are small molecules that have both hydrophilic and hydrophobic moieties. These lipids spontaneously form closed bimolecular sheets in aqueous media. These lipid bilayers are barriers to the flow of polar molecules.
Specific proteins mediate distinctive functions of membranes. Proteins serve as pumps, channels, receptors, energy transducers, and enzymes. Membrane proteins are embedded in lipid bilayers, which create suitable environments for their action.
Membranes are noncovalent assemblies. The constituent protein and lipid molecules are held together by many noncovalent interactions, which act cooperatively.
Membranes are asymmetric. The two faces of biological membranes always differ from each other.
Membranes are fluid structures. Lipid molecules diffuse rapidly in the plane of the membrane, as do proteins, unless they are anchored by specific interactions. In contrast, lipid molecules and proteins do not readily rotate across the membrane. Membranes can be regarded as two-
dimensional solutions of oriented proteins and lipids .Most cell membranes are electrically polarized, such that the inside is negative [typically −60 millivolts (mV)]. Membrane potential plays a key role in transport, energy conversion, and excitability (Chapter 13).