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You are watching: Which molecules prevent cell membranes from dissolving in water

Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland also (MA): Sinauer Associates; 2000.

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The structure and attribute of cells are critically dependent on membranes, which not just separate the interior of the cell from its atmosphere yet likewise define the inner compartments of eukaryotic cells, including the nucleus and cytoplasmic organelles. The formation of organic membranes is based on the properties of lipids, and all cell membranes share a prevalent structural organization: bilayers of phospholipids with linked proteins. These membrane proteins are responsible for many kind of specialized functions; some act as receptors that allow the cell to respond to outside signals, some are responsible for the selective carry of molecules throughout the membrane, and others participate in electron transfer and oxidative phosphorylation. In addition, membrane proteins control the interactions between cells of multicellular organisms. The common structural company of membranes hence underlies a variety of biological procedures and also specialized membrane functions, which will certainly be debated in information in later chapters.

Membrane Lipids

The fundamental building blocks of all cell membranes are phospholipids, which are amphipathic molecules, consisting of two hydrophobic fatty acid chains attached to a phosphate-containing hydrophilic head team (see Figure 2.7). Since their fatty acid tails are poorly soluble in water, phospholipids spontaneously create bilayers in aqueous services, with the hydrophobic tails hidden in the internal of the membrane and the polar head groups exposed on both sides, in contact with water (Figure 2.45). Such phospholipid bilayers create a secure obstacle in between 2 aqueous compartments and recurrent the standard framework of all biological membranes.


Figure 2.45

A phospholipid bilayer. Phospholipids spontaneously develop steady bilayers, via their polar head groups exposed to water and their hydrophobic tails hidden in the inner of the membrane.

Lipids constitute roughly 50% of the mass of the majority of cell membranes, although this propercent varies depending on the type of membrane. Plasma membranes, for example, are roughly 50% lipid and also 50% protein. The inner membrane of mitochondria, on the other hand also, contains an unusually high fraction (about 75%) of protein, mirroring the abundance of protein complexes associated in electron deliver and oxidative phosphorylation. The lipid complace of various cell membranes also varies (Table 2.3). The plasma membrane of E. coli is composed primarily of phosphatidylethanolamine, which constitutes 80% of complete lipid. Mammalian plasma membranes are more complicated, containing four significant phospholipids—phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and sphingomyelin—which together constitute 50 to 60% of complete membrane lipid. In addition to the phospholipids, the plasma membranes of animal cells contain glycolipids and cholesterol, which generally correspond to about 40% of the complete lipid molecules.


An vital residential or commercial property of lipid bilayers is that they behave actually as two-dimensional fluids in which individual molecules (both lipids and proteins) are cost-free to turn and move in lateral directions (Figure 2.46). Such fluidity is a crucial property of membranes and is figured out by both temperature and also lipid complace. For instance, the interactions between shorter fatty acid chains are weaker than those between much longer chains, so membranes containing shorter fatty acid chains are much less rigid and also remajor liquid at reduced temperatures. Lipids containing unsaturated fatty acids similarly rise membrane fluidity because the visibility of double bonds introduces kinks in the fatty acid chains, making them more challenging to pack together.


Figure 2.46

Mobility of phospholipids in a membrane. Individual phospholipids deserve to turn and move laterally within a bilayer.

Due to the fact that of its hydrocarbon ring structure (check out Figure 2.9), cholesterol plays a distinct duty in determining membrane fluidity. Cholesterol molecules insert into the bilayer with their polar hydroxyl teams close to the hydrophilic head teams of the phospholipids (Figure 2.47). The rigid hydrocarbon rings of cholesterol therefore connect via the areas of the fatty acid chains that are surrounding to the phospholipid head groups. This interaction decreases the mobility of the external parts of the fatty acid chains, making this part of the membrane more rigid. On the various other hand, insertion of cholesterol interferes via interactions in between fatty acid chains, thereby preserving membrane fluidity at reduced temperatures.


Figure 2.47

Insertion of cholesterol in a membrane. Cholesterol inserts into the membrane via its polar hydroxyl group close to the polar head groups of the phospholipids.

Membrane Proteins

Proteins are the various other major constituent of cell membranes, constituting 25 to 75% of the mass of the miscellaneous membranes of the cell. The current model of membrane framework, proposed by Jonathan Singer and also Garth Nicolkid in 1972, views membranes as a fluid mosaic in which proteins are inserted into a lipid bilayer (Figure 2.48). While phospholipids administer the standard structural company of membranes, membrane proteins lug out the particular functions of the various membranes of the cell. These proteins are divided right into 2 general classes, based upon the nature of their association with the membrane. Integral membrane proteins are embedded directly within the lipid bilayer. Peripheral membrane proteins are not inserted into the lipid bilayer yet are connected with the membrane indirectly, mostly by interactions through integral membrane proteins.

Figure 2.48

Fluid mosaic version of membrane structure. Biological membranes consist of proteins placed right into a lipid bilayer. Integral membrane proteins are installed in the membrane, generally using α-helical regions of 20 to 25 hydrophobic amino acids. Some (even more...)

Many kind of integral membrane proteins (referred to as transmembrane proteins) expectations the lipid bilayer, via sections exposed on both sides of the membrane. The membrane-extending parts of these proteins are normally α-helical areas of 20 to 25 nonpolar amino acids. The hydrophobic side chains of these amino acids communicate through the fatty acid chains of membrane lipids, and also the development of an α helix neutralizes the polar character of the peptide bonds, as questioned previously in this chapter with respect to protein folding. Like the phospholipids, transmembrane proteins are amphipathic molecules, with their hydrophilic sections exposed to the aqueous environment on both sides of the membrane. Some transmembrane proteins span the membrane only once; others have multiple membrane-covering areas. Most transmembrane proteins of eukaryotic plasma membranes have actually been modified by the addition of carbohydrates, which are exposed on the surface of the cell and may participate in cell-cell interactions.

Proteins have the right to likewise be anchored in membranes by lipids that are covalently attached to the polypeptide chain (check out Chapter 7). Distinct lipid changes anchor proteins to the cytosolic and also extracellular deals with of the plasma membrane. Proteins deserve to be anchored to the cytosolic confront of the membrane either by the addition of a 14-carbon fatty acid (myristic acid) to their amino terminus or by the addition of either a 16-carbon fatty acid (palmitic acid) or 15- or 20-carbon prenyl groups to the side chains of cysteine residues. Conversely, proteins are anchored to the extracellular challenge of the plasma membrane by the enhancement of glycolipids to their carboxy terminus.

Transport throughout Cell Membranes

The selective permecapability of organic membranes to tiny molecules permits the cell to control and also maintain its internal composition. Only little uncharged molecules have the right to diffuse freely with phospholipid bilayers (Figure 2.49). Small nonpolar molecules, such as O2 and also CO2, are soluble in the lipid bilayer and therefore can conveniently cross cell membranes. Small uncharged polar molecules, such as H2O, also have the right to diffuse with membranes, yet bigger uncharged polar molecules, such as glucose, cannot. Charged molecules, such as ions, are unable to diffuse through a phospholipid bilayer regardmuch less of size; even H+ ions cannot cross a lipid bilayer by free diffusion.

Figure 2.49

Permecapacity of phospholipid bilayers. Small uncharged molecules have the right to diffusage easily via a phospholipid bilayer. However, the bilayer is impermeable to bigger polar molecules (such as glucose and amino acids) and also to ions.

Although ions and also many polar molecules cannot diffuse across a lipid bilayer, many kind of such molecules (such as glucose) are able to cross cell membranes. These molecules pass throughout membranes using the action of particular transmembrane proteins, which act as transporters. Such deliver proteins recognize the selective permecapacity of cell membranes and therefore play a vital duty in membrane attribute. They contain multiple membrane-spanning areas that develop a passage through the lipid bilayer, allowing polar or charged molecules to cross the membrane via a protein pore without communicating with the hydrophobic fatty acid chains of the membrane phospholipids.

As questioned in detail in Chapter 12, there are 2 general classes of membrane transport proteins (Figure 2.50). Channel proteins form open up pores with the membrane, allowing the cost-free passage of any kind of molecule of the correct size. Ion networks, for instance, allow the passage of inorganic ions such as Na+, K+, Ca2+, and Cl- throughout the plasma membrane. Once open up, channel proteins create tiny pores through which ions of the proper size and charge deserve to cross the membrane by complimentary diffusion. The pores formed by these channel proteins are not permanently open; fairly, they can be selectively opened and closed in response to extracellular signals, enabling the cell to regulate the motion of ions throughout the membrane. Such regulated ion channels have actually been especially well stupassed away in nerve and also muscle cells, wbelow they mediate the transmission of electrochemical signals.

Figure 2.50

Channel and carrier proteins. (A) Channel proteins create open pores via which molecules of the correct dimension (e.g., ions) can cross the membrane. (B) Carrier proteins selectively bind the little molecule to be transported and then undergo a conformational (even more...)

In contrast to channel proteins, carrier proteins selectively bind and transport certain small molecules, such as glucose. Rather than creating open networks, carrier proteins act prefer enzymes to facilitate the passage of particular molecules throughout membranes. In particular, carrier proteins bind certain molecules and also then undergo conformational transforms that open up channels through which the molecule to be transported have the right to pass throughout the membrane and also be released on the various other side.

As explained so much, molecules transported by either channel or carrier proteins cross membranes in the energetically favorable direction, as identified by concentration and electrochemical gradients—a process known as passive transfer. However, carrier proteins likewise carry out a device through which the energy transforms connected with delivering molecules across a membrane have the right to be coupled to the use or manufacturing of various other creates of metabolic energy, simply as enzymatic reactions deserve to be coupcaused the hydrolysis or synthesis of ATP. For example, molecules deserve to be transported in an energetically unfavorable direction across a membrane (e.g., against a concentration gradient) if their transport in that direction is coupresulted in ATP hydrolysis as a resource of energy—a process dubbed energetic deliver (Figure 2.51). The free energy stored as ATP have the right to hence be supplied to regulate the inner complace of the cell, and to drive the biosynthesis of cell constituents.

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Figure 2.51

Model of active transfer. Model of active transportEnergy obtained from the hydrolysis of ATP is provided to carry H+ versus the electrochemical gradient (from low to high H+ concentration). Binding of H+ is accompanied by phosphorylation of the carrier (more...)

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