Stacked, Folded, and Bent Lipid Membranes
- PDF / 654,641 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 25 Downloads / 197 Views
Stacked, Folded, and Bent Lipid Membranes Darryl Y. Sasaki and Mark J. Stevens Abstract Lipid membranes are generally thought of as flat or spherical structures, much as we would view the plasma membrane of a cell. Within the cell, however, there exists a wide variety of stacked, folded, and other forms of bent structures that support and enable such functions as photosynthesis, light-sensing, protein synthesis, molecular shuttling, chemical uptake and release, and cell division. These functions benefit from the high asymmetry of the membrane. Stacked or folded structures provide a highly concentrated and ordered assembly for facile energy and molecular transport, while bent structures facilitate budding, division, and fusion events. In this article, we survey the progress made in understanding the formation of these membrane architectures, the development of synthetic forms of stacked and folded assemblies, and the unique materials issues they present. Keywords: biological, biomimetic, cellular, simulation, structural.
Introduction Lipid membranes typically conjure images of either a flat, borderless surface, or vesicular structures with minimal structural complexity. Such structures are the synthetic equivalent of the plasma membrane, which on the nanometer scale is a nearly planar material containing multiple two-dimensional (2D) substructures. However, a lipid membrane1 can exist as an array of 3D structures that include infinite assemblies such as cubic, hexagonal (HI), and inverted hexagonal (HII) phases as well as tubules, tapes, and cochleates. Metastable structures, such as toroids, budding, and Genus 2 vesicles, are also known.2–4 If we look to biology, the major constituents of membrane structures are in 3D architectures existing as stacked, folded, and bent assemblies.5 Although the plasma membrane physically separates the cell from the environment and is the gateway to the cell, it is these 3D cytoplasmic structures that provide for all the chemical synthesis, molecular distribution, energy collection, and respiration that make life happen.
MRS BULLETIN • VOLUME 31 • JULY 2006
The familiar flat and spherical structures are also the most straightforward constructs of synthetic lipid membranes. By simply dispersing lipids in aqueous solution, lipid bilayer sheets form spontaneously, with subsequent transformation into vesicles to balance the free-energy costs of entropy and edge energy. Under the microscope, the plasma membrane seems almost akin to the synthetic lipid membrane, with a nearly structureless surface enveloping the cell. It is, in fact, riddled with bumps, bulges, and pits that are part of the endocytotic and exocytotic processes that allow the cell to obtain nutrients, expel waste, and gather environmental information. Looking deeper into the cell (Figure 1), we find the stacked and folded structures of such organelles as the Golgi bodies, the endoplasmic reticulum, the mitochondria, and, in plant cells, the thylakoid membrane of the chloroplast (Figure 2). Here, the cell captures and co
Data Loading...
