1 Membrane Structure and Membranous Organelles L. Andrew Staehelin Introduction 1.1 Common properties Cells, the basic units of life, require membranes for their and inheritance of cell existence. Foremost among these is the plasma membrane, membranes which defines each cell’s boundary and helps create and maintain electrochemically distinct environments within and outside the cell. Other membranes enclose eukaryotic orga­ 1.1.1 Cell membranes possess common nelles such as the nucleus, chloroplasts, and mitochondria. structural and functional properties Membranes also form internal compartments, such as the endoplasmic reticulum (ER) in the cytoplasm and thylakoids All cell membranes consist of a bilayer of polar lipid mol­ in the chloroplast (Fig. 1.1 ). ecules and associated proteins. In an aqueous environment, The principal function of membranes is to serve as a barrier membrane lipids self‐assemble with their hydrocarbon to diffusion of most water‐soluble molecules. Cellular compart­ tails clustered together, protected from contact with water ments delimited by membranes can differ in chemical compo­ (Fig. 1.2 ). Besides mediating the formation of bilayers, this sition from their surroundings and be optimized for a particular property causes membranes to form closed compartments. activity. Membranes also serve as scaffolding for certain pro­ As a result, every membrane is an asymmetrical structure, teins. As membrane components, proteins perform a wide with one side exposed to the contents inside the compart­ array of functions: transporting molecules and transmitting ment and the other side in contact with the external signals across the membrane, processing lipids enzymatically, solution. assembling glycoproteins and polysaccharides,COPYRIGHTED and providing The MATERIALlipid bilayer serves as a general permeability barrier mechanical links between cytosolic and cell wall molecules. because most water‐soluble (polar) molecules cannot readily This chapter is divided into two parts. The first is devoted traverse its nonpolar interior. Proteins perform most of the to the general features and molecular organization of mem­ other membrane functions and thereby define the specificity branes. The second provides an introduction to the architecture of each membrane system. Virtually all membrane molecules and functions of the different membranous organelles of are able to diffuse freely within the plane of the membrane, plant cells. Many later chapters of this book focus on metabolic permitting membranes to change shape and membrane mol­ events that involve these organelles. ecules to rearrange rapidly. Biochemistry & Molecular Biology of Plants, Second Edition. Edited by Bob B. Buchanan, Wilhelm Gruissem, and Russell L. Jones. © 2015 John Wiley & Sons, Ltd. Published 2015 by John Wiley & Sons, Ltd. Companion website: www.wiley.com/go/buchanan/biochem 2 0002271299.INDD 2 6/24/2015 10:39:19 AM CHAPTER 1 MEmBRANE STRUcTURE AND MEmBRANOUs ORGANELLEs 3 PM Nuclear Nucleus Nucleolus V membrane N M G ER CW A B Vacuole Peroxisome Golgi body Smooth endoplasmic reticulum Chloroplast Air space Mitochondrion Rough endoplasmic reticulum Middle lamella Plasma membrane Cellulose/ Cell wall hemicellulose wall A Pectin-rich middle lamella FIGURE 1.1 (A) Diagrammatic representation of a mesophyll leaf cell, depicting principal membrane systems and cell wall domains of a differentiated plant cell. Note the large volume occupied by the vacuole. (B) Thin‐section transmission electron micrograph (TEM) through a Nicotiana meristematic root tip cell preserved by rapid freezing. The principal membrane systems shown include amyloplast (A), endoplasmic reticulum (ER), Golgi stack (G), mitochondrion (M), nucleus (N), vacuole (V), and plasma membrane (PM). Cell wall (CW). Source: (B) Micrograph by Thomas Giddings Jr., from Staehelin et al. (1990). Protoplasma 157: 75–91. 1.1.2 All basic types of cell membranes the next in a functionally active form. Membrane inheritance are inherited follows certain rules: ●● Daughter cells inherit a complete set of membrane types Plant cells contain approximately 20 different membrane from their mother. systems. The exact number depends on how sets of related ●● membranes are counted (Table 1.1). From the moment they Each potential mother cell maintains a complete set of are formed, cells must maintain the integrity of all their membranes. membrane‐bounded compartments to survive, so all mem­ ●● New membranes arise by growth and fission of existing brane systems must be passed from one generation of cells to membranes. 0002271299.INDD 3 6/24/2015 10:39:20 AM 4 PART I COMPARTMENTS TABLE 1.1 Membrane types found in plant cells. Hydrophilic head group Plasma membrane Nuclear envelope membranes (inner/outer) Endoplasmic reticulum Golgi cisternae (cis, medial, trans types) Lipid micelle Trans‐Golgi network/early endosome membranes Clathrin‐coated,COPIa/Ib*, COPII*, secretory and retromer vesicle membranes Autophagic vacuole membrane Hydrophobic Multivesicular body/late endosome membranes tail Tonoplast membranes (lytic/storage vacuoles) Peroxisomal membrane Glyoxysomal membrane Lipid bilayer Chloroplast envelope membranes (inner/ outer) Thylakoid membrane Mitochondrial membranes (inner/outer) *COP, coat protein. FIGURE 1.2 Cross‐sectional views of a lipid micelle and a lipid bilayer in aqueous solution. glucocerebrosides, galactosylglycerides, and sterols (Figs. 1.3 and 1.4). These molecules share an important physico­ chemical property: they are amphipathic, containing both 1.2 The fluid‐mosaic hydrophilic (“water‐loving”) and hydrophobic (“water‐ fearing”) domains. When brought into contact with water, membrane model these molecules spontaneously self‐assemble into higher‐ order structures. The hydrophilic head groups maximize The fluid‐mosaic membrane model describes the molecular their interactions with water molecules, whereas hydropho­ organization of lipids and proteins in cellular membranes bic tails interact with each other, minimizing their exposure and illustrates how a membrane’s mechanical and physio­ to the aqueous phase (see Fig. 1.2). The geometry of the logical traits are defined by the physicochemical character­ resulting lipid assemblies is governed by the shape of the istics of its various molecular components. This model amphipathic molecules and the balance between hydro­ integrates much of what we know about the molecular philic and hydrophobic domains. For most membrane properties of membrane lipids, their assembly into bilayers, lipids, the bilayer configuration is the minimum‐energy the regulation of membrane fluidity, and the different self‐assembly structure, that is, the structure that takes the mechanisms by which membrane proteins associate with least amount of energy to form in the presence of water lipid bilayers. (Fig. 1.5). In this configuration, the polar groups form the interface to the bulk water, and the hydrophobic groups become sequestered in the interior. 1.2.1 The amphipathic nature of Phospholipids, the most common type of membrane lipid, have a charged, phosphate‐containing polar head group membrane lipids allows for the and two hydrophobic hydrocarbon tails. Fatty acid tails con­ spontaneous assembly of bilayers tain between 14 and 24 carbon atoms, and at least one tail has one or more cis double bonds (Fig. 1.6). The kinks introduced In most cell membranes, lipids and glycoproteins make by these double bonds influence the packing of the molecules roughly equal contributions to the membrane’s mass. in the lipid bilayer, and the packing, in turn, affects the overall Lipids belong to several classes, including phospholipids, fluidity of the membrane. 0002271299.INDD 4 6/24/2015 10:39:22 AM CHAPTER 1 MEMBRANE STRUCTURE AND MEMBRANOUS ORGANELLES 5 Choline, FIGURE 1.3 Plant membrane lipids. ethanolamine, P or serine olar (h O ydrophilic) OPO– Galactose Glucose Sphingosine Glycerol O Glycerol O OH O 1 23 1 23 3 21 CH2 CH CH2 CH2 CH CH2 CH CH CH2 O O O O CH NH COC O COC O CH C O Nonpolar (h ydrophobic) Fatty acid Fatty acids Fatty acids Phospholipid Galactosylglyceride Glucocerebroside FIGURE 1.4 Sterols found in plant plasma Cholesterol Campesterol Sitosterol Stigmasterol membranes. OH OH OH OH Hydrophilic Hydrophobic 1.2.2 Phospholipids move rapidly in the Studies of the movement of phospholipids in bilayers have revealed that these molecules can diffuse laterally, rotate, flex plane of the membrane but very slowly their tails, bob up and down, and flip‐flop (Fig. 1.7). The exact from one side of the bilayer to the other mechanism of lateral diffusion is unknown. One theory sug­ gests that individual molecules hop into vacancies (“holes”) Because individual lipid molecules in a bilayer are not bonded that form transiently as the lipid molecules within each mono­ to each other covalently, they are free to move. Within the layer exhibit thermal motions. Such vacancies arise in a fluid plane of the bilayer, molecules can slide past each other freely. bilayer at high frequencies, and the average molecule hops A membrane can assume any shape without disrupting the ~107 times per second, which translates to a diffusional distance hydrophobic interactions that stabilize its structure. Aiding of ~1 μm traversed in a second. Both rotation of individual this general flexibility is the ability of lipid bilayers to close on molecules around their long axes and up‐and‐down bobbing themselves to form discrete compartments, a property that are also very rapid events. Superimposed on these motions