3. CELL MEMBRANE
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Membranes are indispensable for cells.
Functions: physical barrier, allow gradients, transduction and transmission of information, ATP synthesis, selective transport, receptors, enzymes and other molecules are part of membranes.
Properties: semipermeable, fluid, flexible and malleable, permanent recycling.
Composition: lipids, proteins, and saccharides. The proportion of these molecules depends on the type of membrane and determines the membrane properties.
Traveling through the extracellular matrix toward the cell, the first cell structure to be found is the plasma membrane. This is an essential part of the cell. Cells die several seconds after the plasma membrane integrity is lost. Cell membranes are physical barriers, but, as we will see later, not just that . Plasma membrane separates the intracellular environment from the external one. In eukaryotes, and some prokaryotes, there are also internal membranes that form organelles, separating the interior of the organelle from the rest of the cytoplasm. Examples of organelles are endoplasmic reticulum, Golgi complex, but also the cell nucleus has a external doble membrane.
Function. Depending on where they are located, membranes have different functions. They generate and maintain electro-chemical gradients, which are used for several purposes: respond to stimuli, transmit information, selectively transport of molecules, and synthesize ATP. Cells "feel" the environment by receptors located in the plasma membrane. The abilities of neuron and muscle cells depend on the properties of their membranes. Membranes contain many enzymes. For example, cellulose and hyaluronan, essential molecules for plant and animal extracelular matrix, respectively, are synthesized by plasma membrane. But there are also phosphorylases, ATPases, lipases, and many more. The integrity of animal tissues relies on cell adhesion molecules, and cell-cell and cell-extracellular matrix adhesions are carried out by adhesion molecules located in the plasma membrane.
Functions of membranes are determined by their physicochemical functions: a) membranes are fluid layers of lipids and proteins, allowing the lateral movement of molecules, as if they were a viscous liquid layer; b) membranes are semipermeable, which means that they work as a selective barrier for diffusion of molecules that "want" to cross from one side to another; c) membranes are malleable and flexible, and are repaired after small breaks, thus recovering their integrity; d) membranes show continuous recycling by removing molecules and synthesizing new ones, which allows adaptation to different physiological requirements.
Components and structure. Membranes are made up of lipids, proteins and carbohydrates. Structure, organization, as well as physical properties of membranes, mostly rely on amphipathic lipids, i.e., having hydrophilic and hydrophobic parts. Lipids are arranged as a bilayer with the hydrophobic part in the middle, which is made up of the fatty chains trying to avoid the hydrophilic environment, and the hydrophilic part in contact with water. Proteins can be associated to cell membranes in several ways. There are transmembrane proteins which have sequences of hydrophobic amino acids located among the lipid fatty chains and two hydrophilic domains located at both surfaces of the membrane, respectively. Proteins anchored to one monolayer of the membrane are also found, as well as others linked to membrane molecules such as lipids. Carbohydrates are not always present in cell membranes, particularly in intracellular membranes, but are abundant in the external monolayer of plasma membrane. Carbohydrates are chemically bound to lipids and proteins.
Drawing showing the organization of plasma membrane according to the fluid model of Singer and Nicolson (1972). Lipids are the main responsible for membrane structure and heterogeneity. Some lipids are associated together to form more dense areas known as lipid rafts. In these rafts, some proteins are included more frequently by electrochemical affinity. Cholesterol is part of cell membranes, and is located among the fatty acid chains, close to hydrophilic heads of the lipids. Transmembrane proteins communicate extracellular (upper part of the figure) and intracellular environments (lower part of the figure). Carbohydrates are located in the outer monolayer, and all of them form the so-called glycocalyx. In this figure, the interactions of cell membrane molecules with cytoskeleton and with extracellular matrix are not depicted (Modified from Edidin, 2003: Nicolson, 2014).
Membranes are large thin sheets. In transversal sections at transmission electron microscopy, they show a trilaminar organization: two external dark lines and one internal clear line. The dark lines correspond to the hydrophilic heads of membrane lipids of both monolayers, whereas the clear line corresponds to the fatty acid chains. This dark-clear-dark organization, known as the unit membrane, is found in every membrane of all cells studied so far. Membrane thickness may be from 6 to 10 nm, which means that not all membranes are equal.
Physiology and structure of cell membrane depend on the proportion of lipids, proteins and carbohydrates. They change according to the cell type and membrane location. For example, plasma membrane of erythrocytes contain 50 % of lipids, 40 % of proteins and 10 % of carbohydrates. A similar composition is found in most of the plasma membranes of other cell types, with some exceptions. Myelin, cell membrane of glial cells that wraps axons, is composed of 80 % of lipids and 20 % of proteins, and almost no carbohydrates. Intracellular membranes usually show a higher proportion of proteins than plasma membrane. A remarkable example is the inner mitochondrial membrane, where proteins are up to 80 %. Furthermore, lipids, proteins, and carbohydrates are diverse, and membranes do not only differ in the proportion of these three molecular groups, but also in the different types of lipids, proteins, and carbohydrates that are present. Moreover, as mentioned above, membranes are continuously recycled, and are able to change the types and proportion of molecules to fit physiological requirements.
In the following pages, we will deal with the different types of membrane molecules, and then with membrane properties and functions. In later sections, the role of membranes in the physiology of organelles will be studied. For example, how they work on intracellular molecular movement by vesicular traffic, endocytosis, exocytosis, energy production, etcetera.
Edidin M. 2003. Lipids on the frontier: a century of cell-membrane bilayers
. 2003. Nature reviews in molecular and cell biology. 4:414-418.
Nicolson GL. 2014. The fluid-mosaic model of membrane structure: still relevant to understanding the structure, function and dynamics of biological membranes after more than 40 years. Biochimica and biophysica acta. 1838:1451-1466. ☆
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Updated: 2016-03-30. 11:56
Atlas of Plant and Animal Histology
Dep. of Functional Biology and Health Sciences.
Faculty of Biology.
University of Vigo