The cell. 3. Cell membrane.
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Electrically charged molecules are not allowed to cross cell membranes freely. This permits to set different environments at both sides of the membrane.
This property is needed to maintain ionic and molecular gradients in cell membranes.
Molecules located in membranes can move laterally.
The function of membrane is influenced by its fluidity.
Lateral movement of membrane molecules may be restricted by interactions with other molecules located in the same membrane, but also by interactions with molecules located outside of the membrane.
The essential functions of cell membranes are determined by their molecular composition. The type and proportion of these molecules will set the membrane physicochemical features: semipermeability, asymmetry, fluidity, repairing, and recycling.
Membrane semipermeability is a consequence of the internal hydrophobic environment of membranes created by the lipid fatty acid chains, which is difficult to be crossed by molecules having electric charges. Thus, by preventing free diffusion of molecules, membranes can create compartments that keep distinct internal and external environments. However, permeability is selective, and small molecules without electric charges, such as CO2, N2, O2, and molecules with high solubility in fat such as ethanol, can cross membranes almost freely by passive diffusion. Permeability is lower for molecules that have electric charges when the number of positive charges equals negative charges, known as uncharged molecules, such as water and glycerol. It might be thought that water can cross membranes freely but there are some restrictions, and that is why some membranes contain aquaporins, a type of transmembrane protein that have a channel for water. The ability of large uncharged molecules, such as glucose, to cross membranes is low. Membranes are highly impermeable to ions and charged molecules.
The size and electric features of molecules affect their ability to cross cell membranes (Modified from Alberts et al., 2002).
The unequal distribution of ions and molecules between both sides of membranes makes possible create and maintain electrochemical gradients. The difference between the inner and the outer concentration of electric charges is known as membrane potential. This gradient is used for many cell functions. Semipermeability also produces osmotic processes, which are water movement across membranes from a less concentrated solution in one side to a more concentrated solution in the other side, in order to equal both concentrations. Different concentrations between both sides are possible because some solutes cannot cross the membrane. In this way, plant cells are able to increase in size thanks to an higher intracellular concentration of some solutes that cause water to enter the cell, which in turn produces a inner hydrostatic pressure (called turgor pressure) that pushes cell walls outward. Molecules that do not cross membranes freely are useful for cells because they can create gradients that may work as information mechanisms or as energy stores. For example, neurons make use of gradients to process information, and ATP is synthesized from the energy of a proton gradient in mitochondria. Cells have transmembrane proteins that have the ability to allow or move charged molecules and ions between both sides of cell membranes. For example, muscle contraction is triggered by the opening of channels that reduce an existing ionic gradient.
Movements that lipids may have in membranes because of fluidity. Lateral movements are frequent but flip-flop movements are rare and have not been observed for proteins.
Fluidity is a property of membranes. It is related to the ability of membrane molecules to move inside the membrane. Higher fluidity means more movements of molecules. Cell membranes are actually a sheet of fat, where molecules are in a semi liquid viscous state. Thus, it could be thought that molecules can diffuse without restrictions. For example, a glycerophospholipid located in the external monolayer of the plasma membrane may have two type of movements: lateral, i.e. in the same monolayer, and flip-flop, i.e. jumping to the inner monolayer. In artificial membranes, these types of movements have been found, being lateral movement much more frequent than flip-flop, because in the flip-flop movement the hydrophilic head of the lipid molecule must cross the internal layer of fatty acid chains, and this is thermodynamically difficult. By diffusion, lipids can travel laterally 30 µm in 20 seconds, they can travel the whole circumference of a medium size cell in a minute. However, jumping from one monolayer to the other is very infrequent, being the probability of around one time per month. But cholesterol is different and can do flip-flop quite easily.
Membrane fluidity may change depending on the chemical composition. Generally, shorter lengths of fatty acid chains and higher amount of unsaturated bonds between carbons of fatty acids increase membrane fluidity. The amount of cholesterol also influences the membrane fluidity but the net effect depends on temperature and type of lipids in the membrane. In general, it can be said that increase in cholesterol concentration decreases membrane fluidity, although at low temperatures the effect is the opposite. Therefore, cells can change the membrane fluidity by regulating the chemical composition. For example, some bacteria at low temperature environments are able to increase the concentration of unsaturated fatty acids to increase membrane fluidity, whereas they synthesize more membrane saturated fatty acids at higher temperatures. Low temperature decreases membrane fluidity.
Movement of molecules by diffusion may be restricted by interactions with extracellular matrix and cytoskeleton, although membrane local densities such as lipid rafts may affect as well. The organization of the cytoskeleton underneath the membrane determines the membrane area where molecules can diffuse (image on the right).
Transmembrane proteins may also have lateral movements, but they are more restricted than lipids, mostly due to interactions with extracellular matrix and cytoskeleton through their extracellular and intracellular domains, respectively. These interactions may keep proteins in small areas of the membrane during more time than just by diffusion. Cells have other mechanisms to confine proteins to specific domains. For example, gut epithelial cells have some transporters and enzymes located in the apical plasma membrane, but not in the basolateral domain, because of a belt of tight junctions, a molecular zipper that prevent the diffusion between the apical and basolateral domains. Such asymmetrical distribution is essential for the epithelial cell physiology.
Another constrain to free diffusion is mediated by electrochemical interactions of the membrane molecules between each other. Proteins and lipids participate in these interactions, that result in membrane domains and contribute to the heterogeneity of membranes.
Sphingolipids and cholesterol may become associated spontaneously between each other, reducing their motility and increasing the molecular density when compared to neighbour areas. A distinct group of molecules is formed as it was a small raft in a sea of lipids. Actually, these molecular associations are known as lipid rafts, and are thought to be very abundant in cell membranes. Lipid rafts show a dynamic behavior, they can move, grow, diminish, appear and disappear. Indeed, membranes are made up of many dense domains traveling among the glycerophospholipids, so that membrane is quite heterogeneous when considering the molecular distribution. Some experiments suggest that certain types of proteins "feel" more comfortable inside lipid rafts. These proteins spend more time inside than outside the lipid rafts, and thus they travel some time inside these densities. This causes a segregation of molecules along the membrane, and increases the probability of different molecules to be close to each other more time than just by chance (diffusion), increasing in this way the probability of certain molecular reactions. Furthermore, it is suggested that a high concentration of certain types of lipids in the lipid rafts creates a distinct chemical environment that makes easier some chemical reactions or molecular interactions. Lipid rafts have been proposed to be present in the outer monolayer of the plasma membrane because it is in this side where sphingolipids are abundant. Membrane domains have been suggested to exist also in the membranes of organelles, and it is thought that some of their functions rely in these membrane domains.
Membrane proteins, both integral and associated, may also interact between each other and form macromolecular scaffolds intended for transmission of information, cell-cell recognition, start certain enzyme activities, cell movement, and more. There are also multimeric proteins that are active only when all the subunits are hold together. For example, the insulin receptor is made up of four subunits. Proteins and lipids may also interac to form membrane domains.
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Updated: 2017-09-11. 13:02
Atlas of Plant and Animal Histology
Dep. of Functional Biology and Health Sciences.
Faculty of Biology.
University of Vigo