What Is So Unique About Biomembrane Organization and Dynamics?

Biological membranes are complex quasi two-dimensional, supramolecular assemblies of a diverse variety of lipids, proteins and carbohydrates, that compartmentalize living matter into cells and subcellular structures. Membranes are held together by the hydrophobic effect, which is an entropy-driven process originating from strong attractive forces between water molecules. Membrane organization and dynamics are characterized by the absence of intermolecular connectivity among its constituent units, thermodynamically controlled (spontaneous) self assembly, and inherent dynamics characterized by a gradient. Membrane phenomena display a wide range of spatiotemporal scales, thereby making it challenging for experiments and simulations alike. We envision that unraveling the spatiotemporal complexity of biological membranes would enable us to build a more robust membrane model, which would help in addressing unresolved issues in human health and disease. 1.1 Cellular Membranes as Identity Markers A long time back, biochemists used to think that a living cell is a bag full of enzymes. In reality, eukaryotic cells are characterized by a number of compartments separated from each other and the cytoplasm by thin membranes (see Fig. 1.1). The composition, organization and physical dimension of the intracellular organelle membranes exhibit a lot of variation. The outermost membrane in eukaryotic cells is termed the plasma membrane, which separates the interior of the cell from the outer milieu and provides the cell its unique identity. Cellular organization is therefore characterized by morphological compartmentalization S. Pal Academy of Scientific and Innovative Research, New Delhi, India CSIR-Indian Institute of Chemical Technology, Uppal Road, Hyderabad 500 007, India A. Chattopadhyay (*) Academy of Scientific and Innovative Research, New Delhi, India CSIR-Centre for Cellular and Molecular Biology, Uppal Road, Hyderabad 500 007, India e-mail: [email protected] © Springer International Publishing AG 2017 A. Chattopadhyay (ed.),Membrane Organization and Dynamics, Springer Series in Biophysics 20, DOI 10.1007/978-3-319-66601-3_1 1 offered by the membrane. Membranes compartmentalize living matter into cells and subcellular structures. Cells require some mechanism to prevent dissipation (diffusing away) of their genetic information (contained mainly in the nucleus) and therefore “compartmentalization has long been recognized as a physical prerequisite for Darwinian evolution” [1]. Importantly, the membrane is the first organelle in a cell to sense any stress or stimuli [2]. In physical terms, membranes can be described as a complex anisotropic fluid that are deformable and can therefore be treated as soft matter [3, 4]. In molecular terms, this means membranes are optimally fluid to be able to carry out their function while maintaining their characteristic selective barrier properties. Membranes present themselves to macromolecules as highly structured interfaces on which important biochemical processes are carried out and catalyzed. For this reason, the structure and molecular organization of membranes are crucial for membrane function. Biological membranes are complex quasi two-dimensional, supramolecular assemblies of a diverse variety of lipids, proteins and carbohydrates (see Fig. 1.2a). Membranes of eukaryotic cells contain thousands of diverse lipid types [6, 7]. Membranes provide an identity to the cell and its organelles, and represent an ideal milieu for the proper function of membrane proteins. Cells are densely packed with membranes. In fact, 35% of the dry weight of a cell is that of its membranes. The human body is composed of 10 cells which correspond to a total membrane surface area of 3 km. Contrary to textbook descriptions [8], cellular membranes are often crowded [9, 10] with a high protein density (typically 25,000 proteins/μm; [11]). Even a number of years back it was postulated that there could be only a few lipid molecules separating two protein molecules in a biological membrane [12]. Taken together, the model of biological membranes is evolving into one where “the membrane resembles a cobble-stone pavement, with the proteins organized in patches that are surrounded by lipidic rims, rather than icebergs floating in a sea of lipids” [10]. The actin cytoskeletal network underlying the membrane was initially not considered to be an active part of the membrane. However, this has changed in recent years. A number of observations using sophisticated microscopic techniques have established the notion of actin cytoskeleton dependent dynamics of molecules Fig. 1.1 A schematic representation of a eukaryotic cell showing the plasma membrane and membrane bound organelles 2 S. Pal and A. Chattopadhyay