Description
Introduction
Cells, the basic units of life, require membranes for their existence. Foremost among these is the plasma membrane, which defines each cell’s boundary and helps create and maintain electrochemically distinct environments within and outside the cell. Other membranes enclose eukaryotic organelles such as the nucleus, chloroplasts, and mitochondria. Membranes also form internal compartments, such as the endoplasmic reticulum (ER) in the cytoplasm and thylakoids in the chloroplast (Fig. 1.1).
The principal function of membranes is to serve as a barrier to diffusion of most water‐soluble molecules. Cellular compartments delimited by membranes can differ in chemical composition from their surroundings and be optimized for a particular activity. Membranes also serve as scaffolding for certain proteins. As membrane components, proteins perform a wide array of functions: transporting molecules and transmitting signals across the membrane, processing lipids enzymatically, assembling glycoproteins and polysaccharides, and providing mechanical links between cytosolic and cell wall molecules.
This chapter is divided into two parts. The first is devoted to the general features and molecular organization of membranes. The second provides an introduction to the architecture and functions of the different membranous organelles of plant cells. Many later chapters of this book focus on metabolic events that involve these organelles.
Cell membranes possess common structural and functional properties
All cell membranes consist of a bilayer of polar lipid molecules and associated proteins. In an aqueous environment, membrane lipids self‐assemble with their hydrocarbon tails clustered together, protected from contact with water (Fig. 1.2). Besides mediating the formation of bilayers, this property causes membranes to form closed compartments. As a result, every membrane is an asymmetrical structure, with one side exposed to the contents inside the compartment and the other side in contact with the external solution.
The lipid bilayer serves as a general permeability barrier because most water‐soluble (polar) molecules cannot readily traverse its nonpolar interior. Proteins perform most of the other membrane functions and thereby define the specificity of each membrane system. Virtually all membrane molecules are able to diffuse freely within the plane of the membrane, permitting membranes to change shape and membrane molecules to rearrange rapidly.
ll basic types of cell membranes are inherited
Plant cells contain approximately 20 different membrane systems. The exact number depends on how sets of related membranes are counted (Table 1.1). From the moment they are formed, cells must maintain the integrity of all their membrane‐bounded compartments to survive, so all membrane systems must be passed from one generation of cells to the next in a functionally active form. Membrane inheritance follows certain rules:
● Daughter cells inherit a complete set of membrane types from their mother.
● Each potential mother cell maintains a complete set of membranes.
● New membranes arise by growth and fission of existing membranes.