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Components and Architecture of Cell Membrane

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Components and Architecture of Cell Membrane Paper 12: Membrane Biophysics Module 1: Components and Architecture of Cell Membrane Learning objectives: In this module, prime focus is mainly on the plasma membrane, however, majority of the concepts discussed below are also applicable to the membranes of various cells organelles. Following are the learning objectives of this module. 1. Introduction and importance of cell membrane 2. Various components of cell membrane 3. Structural features of membrane 4. Functions of cell membrane 1. INTRODUCTION A Membrane can be described as the boundary which define specific region from rest of the surrounding. It is not only gives a definite boundary but also control the composition of the enclosed space. In living systems each cell is covered by a 5 to 10 nm wide layer of phospholipids and proteins, which encloses whole cell and protects cell cytoplasm from extracellular environment and this biological membrane is ‘cell-membrane’ or commonly known as the plasma membrane (see figure 1). In addition to the plasma membrane, a eukaryotic cell contains various intracellular organelles like golgi apparatus, mitochondria etc. and these organelles are surrounded by their intracellular membranes. These membrane coverings maintain the characteristic differences between the cell cytoplasm and contents of each respective organelle. Another important property of cell membrane is its semi-permeable nature, through which selective transport and retention of cell nutrients takes place and toxic or unwanted substances cannot pass into the cell. Figure 1: General and ultra-structure cell membrane. Reference: Derivative work: Dhatfield (talk) (Cell_membrane_detailed_diagram.svg) creative commons 2. BASIC COMPONENTS OF CELL MEMBRANE Three bio-molecules namely lipid, protein and carbohydrates are the main components of a plasma membrane. Their compositions and relative proportions in membranes may vary in different types of cells (see table no. 1). The variation in proportion of each component is provided for the special requirements of the cell for example in general plasma membranes have approximately equal proportions of lipids and proteins (45-50%). Whereas, mitochondrial inner membrane contains approximately 75% proteins which are required in electron transport chain. Lipids are present ubiquitously in different forms like phospholipids, cholesterol etc. in cell membranes and constitute approximately 50% of the mass of most cell membranes. Proteins may range from 20% to 70% of the total mass of a particular membrane. The percentage of carbohydrates in cell membrane is approximately 5-8% and these are generally associated with either to lipid (glycolipids) of various classes, or with proteins (as glycoproteins). A common feature of all biomembranes is the presence of a bilayer of phospholipids; however, there is the presence of several unique proteins in certain cellular membranes for distinctive functions. Table 1: Percentage distribution of lipids, proteins, and carbohydrates in different bio-membranes. S.N. Type membrane % Lipid % Protein % Carbohydrate 1 Human erythrocyte plasma membrane 43 49 8 2 Amoeba plasma membrane 42 54 4 3 Mitochondrial inner membrane 24 79 0 4 Myelin 79 18 3 2.2 LIPIDS: THE MAIN CONSTITUENTS OF CELL MEMBRANE The lipids components of membranes are further divided into glycerophospholipids, sphingolipids, glycolipids and sterols. In general, glycerophospholipids and sphingolipids constitute the largest proportions of lipids in all biological membranes. 2.2.1. Phospholipids With few exceptions, the most abundant lipid components present in membranes are phospholipids. Phospholipids are basically composed of glycerol backbone at which two fatty acid chains and a phosphate group are attached (Figure 2). Fatty acids are attached to the first and second carbons, while the phosphate group is attached to the third carbon of the glycerol backbone. The fatty acyl side chains may be saturated or may contain one or more double bonds. The phosphate group and glycerol backbone makes the head portion of phospholipid as hydrophilic, whereas the fatty acid tails have a nature of hydrophobicity. Therefore, phospholipids are considered as amphipathic molecules with a polar head and a hydrophobic tails (see figure 2). Figure 2: Basic structure of a phospholipid. Source: Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/ ( Creative Commons Attribution 3.0 Unported) One more special characteristic of phospholipids make them a crucial molecule in membranes. The phosphate group of a phospholipid can be attached to variable functional groups(R group) and give rise to different types of phospholipids. For example Phosphatidic acid has the hydrogen as R group. The naming of phospholipid is based upon by naming the derivatives for the head-group alcohol (R, Figure 2) with “phosphatidyl-” as a prefix. Phosphatidyl-choline, phosphatidyl-ethanolamine, and phosphatidyl-serine are other common membrane phospholipids having choline, ethanolamine and serine respectively. Table 2 is showing different types of phospholipids which are formed on the basis of modification of R group in phosphate. Table 1: Phospholipids with different head groups. S.N. Name of R group Formula of R Phospholipid type 1 ─ ─H Phosphatidic acid + 2 Ethanolamine ─CH2─CH2─N H3 Phosphytidyletahnolamine + 3 Choline ─CH2─CH2─N (CH3)3 Phosphtidylcholine 4 Serine Phosphtidylserine 5 Glycerol Phosphtidylglycerol 6 Myo-inositol 4,5, Phosphtidylinositol 4,5, bisphosphate bisphosphate 7 Phosphtidylglycerol Cardiolipin Phosphatidylinositol (PI) are generally present on the cytoplasmic leaflet of membranes and get phosphorylated into PI phosphate (PIP), PI bisphosphate (PIP2), and PI trisphosphate (PIP3). These phospholipids are important for regulating cell growth, their proliferation, and apoptosis. In general, majority of phospholipids are neutral in nature (e.g. phosphatidylcholine, phosphatidylethalamine and spingomyelin). However, several phospholipids like phophatidylionositol and phosphotidylserine shows acidic nature with negative charge. When phospholipids are placed in an aqueous solution they will self assemble into micelles or bilayers, structures that exclude water molecules from the hydrophobic tails while keeping the hydrophilic head in contact with the aqueous solution. The same phenomenon is observed in cell membrane where water content is present inside and outside of the cell and phospholipids are packed as bilayer confirmations. Moreover, the cylindrical shape of phospholipids, allows phospholipids to align side-by-side to form broad sheets (see Figure 3). Figure 3. Orientation of phospholipids as bilayer in membrane 22.2. Sphingolipids and Glycolipids Another class of phospholipids found mainly in plasma membranes are sphingolipids which lack the basic glycerol backbone and have a sphingosine backbone fatty acid, and headgroup of a sphingolipid. In sphingolipids, the terminal hydroxyl group of sphingosine is esterified to phosphocholine, so its hydrophilic head is similar to that of phosphatidylcholine (Figure 4). Ceramide is the most basic sphingolipid which only a hydrogen in the Csp-3 position and an amide-linked fatty acid (table 3). Sphingomyelin is another most common sphingolipid, widely present in myelin sheaths of neurons. Figure 4. General structure of a Spingolipid Table 3: Spingolipids with different head groups. S.N. Name of R group Formula of R Spingolipid type 1 ─ ─H Ceramide - + 2 Phosphocholine PO 4─CH2─CH2─N (CH3)3 Spingomyelin 3 Glucose Glucosylcerebroside 4 Di/tri/tetrasaccharide Lectosylceramide 5 Complex Ganglioside GM2 oligosaccharides Glycolipids are the sub-group of sphingolipids and generally present approximately 2% of the total lipid content of plasma membranes. These sugar containing lipids have a spingosine derived backbone and mono- or oligosaccharide bound to the Csn-3 position. For example, Glucosylcerebroside is a glycolipid, abundant in myelin sheaths and it consists of the ceramide and oleic acid linked to a single glucose residue (table 3). These are generally found at the outer leaflet of the plasma membrane and their carbohydrate moieties are exposed on cell surface. 2.2.3. Sterols Another important class of membrane lipids is the Sterols and their derivatives. These sterols are present in the cell membrane for important hormones and signaling molecules. In general, all steroids contain four hydrocarbon rings (A, B, C, and D), with the carbons numbered as shown figure 5a. Cholesterol is the major steroidal constituent of plasma membranes. It accounts for near-about 20% and 30-50% of the of total lipid content in animal cell and plant cell, respectively. However, cholesterol is generally absent in prokaryotic membranes and mitochondrial membrane. Cholesterol has the entire hydrocarbon structure, but also has a hydroxyl substituent on one ring which makes this molecule as amphipathic (see figure 5b). Cholesterol maintains the rigidity and stability of the membrane. A separate module is placed in the membrane chapter for the detailed study of the cholesterol, where the properties and related functions of cholesterol will be given in detail. Figure 5 (a): The structure of sterols and (b) structure of cholesterol. The major portion of the molecule is hydrophobic (grey), the hydroxyl group is present at 3rd position. 2.3 MEMBRANE PROTEINS Proteins also contributed the major proportion (50% of total mass of membrane) in biological membrane and these proteins participated in several biological activities like
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