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

Cooper GM. The Cell: A molecule Approach. Second edition. Sunderland (MA): Sinauer Associates; 2000.

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The structure and role of cells are critically dependency on membranes, which not only separate the internal of the cell from its setting but also define the inner compartments of eukaryotic bio cells, consisting of the nucleus and also cytoplasmic organelles. The development of organic membranes is based on the properties of lipids, and also all cell membranes re-publishing a usual structural organization: bilayers that phospholipids with connected proteins. This membrane proteins are responsible for many devoted functions; part act together receptors that allow the cell to respond to outside signals, some room responsible for the selective deliver of molecules throughout the membrane, and also others take part in electron transport and oxidative phosphorylation. In addition, membrane proteins regulate the interactions between cells of multicellular organisms. The common structural organization of membranes for this reason underlies a range of organic processes and specialized membrane functions, which will certainly be debated in information in later chapters.

Membrane Lipids

The an essential building blocks of every cell membranes are phospholipids, which are amphipathic molecules, consisting of two hydrophobic fatty mountain chains linked to a phosphate-containing hydrophilic head group (see number 2.7). Due to the fact that their fatty mountain tails are poorly soluble in water, phospholipids spontaneously type bilayers in aqueous solutions, with the hydrophobic tails buried in the internal of the membrane and the polar head groups exposed top top both sides, in contact with water (Figure 2.45). Such phospholipid bilayers kind a stable obstacle between two aqueous compartments and represent the basic structure of all organic membranes.


Figure 2.45

A phospholipid bilayer. Phospholipids spontaneously kind stable bilayers, with their polar head groups exposed come water and their hydrophobic tails hidden in the inner of the membrane.

Lipids constitute approximately 50% of the fixed of most cell membranes, although this ratio varies relying on the form of membrane. Plasma membranes, for example, are around 50% lipid and also 50% protein. The inner membrane the mitochondria, ~ above the other hand, contains an abnormally high portion (about 75%) the protein, showing the variety of protein complexes connected in electron transport and oxidative phosphorylation. The lipid composition of various cell membranes also varies (Table 2.3). The plasma membrane of E. Coli consists predominantly the phosphatidylethanolamine, which constitutes 80% of total lipid. Mammalian plasma membranes are more complex, include four major phospholipids—phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and sphingomyelin—which with each other constitute 50 to 60% of full membrane lipid. In enhancement to the phospholipids, the plasma membranes of pet cells save glycolipids and cholesterol, which usually correspond to around 40% that the total lipid molecules.


An crucial property of lipid bilayers is that they behave as two-dimensional fluids in which individual molecule (both lipids and proteins) are totally free to rotate and also move in lateral direction (Figure 2.46). Such fluidity is a an important property the membranes and also is figured out by both temperature and lipid composition. For example, the interactions between much shorter fatty mountain chains are weaker than those in between longer chains, so membrane containing much shorter fatty mountain chains are less rigid and also remain liquid at reduced temperatures. Lipids include unsaturated fat acids likewise increase membrane fluidity due to the fact that the presence of twin bonds introduce kinks in the fatty acid chains, making them more difficult to fill together.


Figure 2.46

Mobility that phospholipids in a membrane. Individual phospholipids can rotate and move laterally in ~ a bilayer.

Because the its hydrocarbon ring structure (see figure 2.9), cholesterol dram a distinct function in determining membrane fluidity. Cholesterol molecule insert into the bilayer through their polar hydroxyl groups close come the hydrophilic head teams of the phospholipids (Figure 2.47). The rigid hydrocarbon rings of cholesterol because of this interact with the areas of the fatty acid chains that are nearby to the phospholipid head groups. This interaction decreases the mobility of the external portions that the fatty mountain chains, make this part of the membrane an ext rigid. Top top the other hand, insertion of cholesterol interferes v interactions between fatty acid chains, thereby maintaining membrane fluidity at lower temperatures.


Figure 2.47

Insertion of cholesterol in a membrane. Cholesterol inserts right into the membrane v its polar hydroxyl team close to the polar head teams of the phospholipids.

Membrane Proteins

Proteins are the other significant constituent of cell membranes, constituting 25 to 75% of the fixed of the miscellaneous membranes the the cell. The existing model of membrane structure, proposed by Jonathan Singer and also Garth Nicolson in 1972, views membranes as a fluid mosaic in which protein are placed into a lipid bilayer (Figure 2.48). When phospholipids provide the basic structural company of membranes, membrane proteins lug out the details functions that the different membranes the the cell. These proteins are split into two basic classes, based on the nature of your association through the membrane. Integral membrane proteins space embedded straight within the lipid bilayer. Peripheral membrane proteins space not inserted into the lipid bilayer yet are linked with the membrane indirectly, usually by interactions through integral membrane proteins.

Figure 2.48

Fluid mosaic design of membrane structure. Organic membranes covers proteins inserted into a lipid bilayer. Integral membrane protein are installed in the membrane, generally via α-helical regions of 20 come 25 hydrophobic amino acids. Part (more...)

Many integral membrane protein (called transmembrane proteins) expectancy the lipid bilayer, with parts exposed on both political parties of the membrane. The membrane-spanning sections of this proteins are usually α-helical areas of 20 come 25 nonpolar amino acids. The hydrophobic side chains of these amino acids connect with the fatty acid chains the membrane lipids, and the formation of one α helix neutralizes the polar character of the peptide bonds, as questioned earlier in this chapter v respect come protein folding. Favor the phospholipids, transmembrane proteins space amphipathic molecules, with their hydrophilic sections exposed to the aqueous atmosphere on both political parties of the membrane. Part transmembrane proteins expectancy the membrane only once; others have multiple membrane-spanning regions. Most transmembrane protein of eukaryotic plasma membranes have been modification by the enhancement of carbohydrates, which are exposed ~ above the surface ar of the cell and also may take part in cell-cell interactions.

Proteins can additionally be anchored in membrane by lipids that are covalently attached come the polypeptide chain (see thing 7). Distinct lipid modifications anchor proteins to the cytosolic and also extracellular deals with of the plasma membrane. Proteins can be anchored come the cytosolic face of the membrane one of two people by the addition of a 14-carbon fatty mountain (myristic acid) to your amino terminus or by the addition of one of two people a 16-carbon fatty mountain (palmitic acid) or 15- or 20-carbon prenyl teams to the next chains of cysteine residues. Alternatively, proteins are anchored to the extracellular face of the plasma membrane by the addition of glycolipids to their carboxy terminus.

Transport across Cell Membranes

The selective permeability of organic membranes to tiny molecules enables the cabinet to control and maintain its interior composition. Only little uncharged molecules have the right to diffuse freely through phospholipid bilayers (Figure 2.49). Little nonpolar molecules, such together O2 and also CO2, room soluble in the lipid bilayer and therefore have the right to readily cross cell membranes. Tiny uncharged polar molecules, such as H2O, additionally can diffuse through membranes, yet larger uncharged polar molecules, such together glucose, cannot. Fee molecules, such together ions, room unable come diffuse with a phospholipid bilayer regardless of size; even H+ ions cannot overcome a lipid bilayer by cost-free diffusion.

Figure 2.49

Permeability of phospholipid bilayers. Little uncharged molecules deserve to diffuse openly through a phospholipid bilayer. However, the double layer is impermeable to bigger polar molecules (such as glucose and also amino acids) and to ions.

Although ions and most polar molecules cannot diffuse across a lipid bilayer, many such molecule (such together glucose) room able to cross cell membranes. This molecules pass across membranes via the activity of particular transmembrane proteins, which act as transporters. Such move proteins recognize the selective permeability of cabinet membranes and also thus beat a critical role in membrane function. Castle contain lot of membrane-spanning regions that type a passage v the lipid bilayer, allowing polar or charged molecules to overcome the membrane through a protein sharp without communicating with the hydrophobic fatty acid chains the the membrane phospholipids.

As debated in information in thing 12, there room two general classes the membrane deliver proteins (Figure 2.50). Channel proteins type open pores through the membrane, enabling the free passage of any molecule that the proper size. Ion channels, because that example, enable the i of inorganic ions such together Na+, K+, Ca2+, and Cl- throughout the plasma membrane. When open, channel proteins type small pores through which ions of the suitable size and also charge have the right to cross the membrane by free diffusion. The pores developed by this channel proteins are not permanently open; rather, they can be selectively opened and also closed in solution to extracellular signals, enabling the cabinet to manage the movement of ions throughout the membrane. Such regulated ion networks have been particularly well learned in nerve and also muscle cells, where they adjust the infection of electrochemical signals.

Figure 2.50

Channel and carrier proteins. (A) Channel proteins kind open pores v which molecule of the proper size (e.g., ions) can cross the membrane. (B) transport proteins selectively tie the little molecule to it is in transported and also then undergo a conformational (more...)

In contrast to channel proteins, carrier proteins selectively bind and transport specific tiny molecules, such together glucose. Fairly than creating open channels, carrier proteins act prefer enzymes come facilitate the i of details molecules across membranes. In particular, carrier proteins bind particular molecules and also then experience conformational transforms that open channels through which the molecule to be transported can pass throughout the membrane and be released on the various other side.

As explained so far, molecules transported by one of two people channel or carrier proteins cross membranes in the energetically favorable direction, as established by concentration and also electrochemical gradients—a process known as passive transport. However, carrier proteins also provide a mechanism through i m sorry the energy changes connected with carrying molecules across a membrane can be combination to the usage or manufacturing of other creates of metabolic energy, just as enzymatic reactions deserve to be coupled to the hydrolysis or synthesis of ATP. Because that example, molecules can be transported in one energetically unfavorable direction across a membrane (e.g., versus a concentration gradient) if their transport in that direction is coupled to ATP hydrolysis together a resource of energy—a procedure called energetic transport (Figure 2.51). The free energy stored together ATP can thus be offered to manage the interior composition the the cell, as well as to drive the biosynthesis of cell constituents.

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Figure 2.51

Model of energetic transport. Design of energetic transportEnergy derived from the hydrolysis the ATP is offered to deliver H+ against the electrochemical gradient (from short to high H+ concentration). Binding the H+ is add by phosphorylation that the transport (more...)

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