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Cooper GM. The Cell: A molecule Approach. 2nd 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 interior of the cabinet from its setting but also define the internal compartments of eukaryotic bio cells, consisting of the nucleus and cytoplasmic organelles. The development of organic membranes is based upon the nature of lipids, and also all cell membranes re-superstructure a usual structural organization: bilayers that phospholipids with connected proteins. This membrane proteins are responsible for many dedicated functions; some act as receptors that allow the cell to answers to exterior signals, some room responsible for the selective carry of molecules throughout the membrane, and also others participate in electron transport and also oxidative phosphorylation. In addition, membrane proteins control the interactions between cells of multicellular organisms. The usual structural organization of membranes thus underlies a range of organic processes and specialized membrane functions, which will be questioned in detail in later chapters.

Membrane Lipids

The an essential building blocks of every cell membranes are phospholipids, which room amphipathic molecules, consisting of 2 hydrophobic fatty acid chains connected to a phosphate-containing hydrophilic head group (see number 2.7). Due to the fact that their fatty acid tails space poorly dissolve in water, phospholipids spontaneously type bilayers in aqueous solutions, with the hydrophobic tails buried in the internal of the membrane and also the polar head groups exposed top top both sides, in call with water (Figure 2.45). Such phospholipid bilayers type a stable obstacle between two aqueous compartments and also represent the straightforward structure the all biological membranes.


Figure 2.45

A phospholipid bilayer. Phospholipids spontaneously form stable bilayers, through their polar head teams exposed to water and their hydrophobic tails buried in the inner of the membrane.

Lipids constitute around 50% the the fixed of many cell membranes, return this relationship varies depending on the type of membrane. Plasma membranes, because that example, are about 50% lipid and also 50% protein. The within membrane of mitochondria, on the other hand, contains an abnormally high fraction (about 75%) of protein, mirroring the variety of protein complexes affiliated in electron transport and oxidative phosphorylation. The lipid ingredient of different cell membranes likewise varies (Table 2.3). The plasma membrane the E. Coli is composed predominantly that phosphatidylethanolamine, i beg your pardon constitutes 80% of total lipid. Mammalian plasma membranes are an ext complex, comprise four major phospholipids—phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and also sphingomyelin—which together constitute 50 come 60% of complete membrane lipid. In addition to the phospholipids, the plasma membranes of pet cells save glycolipids and also cholesterol, which typically correspond to around 40% of the total lipid molecules.


An essential property the lipid bilayers is that they behave together two-dimensional fluids in i m sorry individual molecule (both lipids and also proteins) are free to rotate and also move in lateral direction (Figure 2.46). Such fluidity is a critical property of membranes and also is figured out by both temperature and also lipid composition. For example, the interaction between shorter fatty mountain chains room weaker 보다 those between longer chains, so membranes containing much shorter fatty mountain chains are much less rigid and also remain fluid at lower temperatures. Lipids include unsaturated fatty acids an in similar way increase membrane fluidity since the existence of twin bonds introduce kinks in the fatty mountain chains, making castle more difficult to fill together.


Figure 2.46

Mobility of phospholipids in a membrane. Individual phospholipids can rotate and move laterally within a bilayer.

Because of its hydrocarbon ring framework (see number 2.9), cholesterol dram a distinct role in identify membrane fluidity. Cholesterol molecule insert into the bilayer v their polar hydroxyl teams close to the hydrophilic head groups of the phospholipids (Figure 2.47). The rigid hydrocarbon ring of cholesterol as such interact with the regions of the fatty acid chains the are nearby to the phospholipid head groups. This interaction decreases the mobility that the outer portions that the fatty acid chains, make this part of the membrane much more rigid. On the various other hand, insertion that cholesterol interferes through interactions 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 with its polar hydroxyl team close come the polar head teams of the phospholipids.

Membrane Proteins

Proteins are the other significant constituent of cabinet membranes, constituting 25 come 75% the the fixed of the various membranes the the cell. The present model that membrane structure, proposed by Jonathan Singer and Garth Nicolson in 1972, views membranes together a liquid mosaic in which proteins are placed into a lipid bilayer (Figure 2.48). If phospholipids administer the an easy structural organization of membranes, membrane proteins bring out the details functions the the various membranes of the cell. This proteins are split into two general classes, based on the nature of their association through the membrane. Integral membrane proteins room embedded directly within the lipid bilayer. Peripheral membrane proteins are not put into the lipid bilayer but are associated with the membrane indirectly, typically by interactions v integral membrane proteins.

Figure 2.48

Fluid mosaic model of membrane structure. Biological membranes consist of proteins put into a lipid bilayer. Integral membrane proteins are embedded in the membrane, generally via α-helical areas of 20 to 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 portions of these proteins are usually α-helical regions of 20 to 25 nonpolar amino acids. The hydrophobic side chains of this amino acids interact with the fatty acid chains the membrane lipids, and the development of an α helix neutralizes the polar personality of the peptide bonds, as discussed earlier in this chapter v respect to protein folding. Like the phospholipids, transmembrane proteins room amphipathic molecules, v their hydrophilic sections exposed come the aqueous environment on both political parties of the membrane. Part transmembrane proteins span the membrane only once; others have multiple membrane-spanning regions. Many transmembrane proteins of eukaryotic bio plasma membranes have been modification by the enhancement of carbohydrates, which are exposed top top the surface ar of the cell and may take part in cell-cell interactions.

Proteins can likewise be anchored in membrane by lipids that are covalently attached to the polypeptide chain (see thing 7). Distinctive lipid modifications anchor protein to the cytosolic and also extracellular faces of the plasma membrane. Proteins can be anchored come the cytosolic challenge of the membrane either by the addition of a 14-carbon fatty mountain (myristic acid) to their amino terminus or through the enhancement of one of two people a 16-carbon fatty mountain (palmitic acid) or 15- or 20-carbon prenyl groups to the next chains of cysteine residues. Alternatively, proteins space anchored to the extracellular challenge of the plasma membrane by the addition of glycolipids to your carboxy terminus.

Transport throughout Cell Membranes

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

Figure 2.49

Permeability the phospholipid bilayers. Small uncharged molecules deserve to diffuse easily through a phospholipid bilayer. However, the bilayer is impermeable to larger polar molecules (such as glucose and amino acids) and also to ions.

Although ions and also most polar molecules can not diffuse throughout a lipid bilayer, plenty of such molecule (such as glucose) are able come cross cabinet membranes. These molecules pass throughout membranes via the action of particular transmembrane proteins, i m sorry act as transporters. Such move proteins recognize the selective permeability of cell membranes and thus beat a an essential role in membrane function. Lock contain many membrane-spanning areas that type a passage v the lipid bilayer, enabling polar or fee molecules to overcome the membrane v a protein pore without interacting with the hydrophobic fatty acid chains that the membrane phospholipids.

As debated in information in thing 12, there room two general classes that membrane deliver proteins (Figure 2.50). Channel proteins form open pores with the membrane, allowing the complimentary passage of any molecule that the ideal size. Ion channels, because that example, enable the i of not natural ions such as Na+, K+, Ca2+, and Cl- throughout the plasma membrane. Once open, channel proteins type small pores through which ions of the ideal size and also charge deserve to cross the membrane by complimentary diffusion. The pores developed by this channel proteins room not permanently open; rather, they can be selectively opened and also closed in an answer to extracellular signals, allowing the cell to regulate the motion of ions across the membrane. Such regulated ion networks have been particularly well studied in nerve and also muscle cells, whereby they mediate the transmission of electrochemical signals.

Figure 2.50

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

In contrast to channel proteins, carrier proteins selectively bind and also transport specific tiny molecules, such as glucose. Quite than developing open channels, transport proteins act prefer enzymes come facilitate the passage of certain molecules across membranes. In particular, carrier proteins bind particular molecules and then undergo conformational changes that open channels through i beg your pardon the molecule to be transported have the right to pass across the membrane and be exit on the other side.

As described so far, molecule transported by one of two people channel or transport proteins cross membranes in the energetically favorable direction, as figured out by concentration and electrochemical gradients—a process known together passive transport. However, carrier proteins also carry out a system through i beg your pardon the power changes linked with delivering molecules across a membrane deserve to be coupled to the use or production of other creates of metabolic energy, just as enzymatic reactions deserve to be combination to the hydrolysis or synthetic of ATP. Because that example, molecules deserve to be transported in one energetically unfavorable direction across a membrane (e.g., versus a concentration gradient) if their move in that direction is coupled to ATP hydrolysis as a source of energy—a procedure called active transport (Figure 2.51). The totally free energy stored together ATP have the right to thus be supplied to regulate the internal composition that the cell, and to drive the biosynthesis of cabinet constituents.

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Figure 2.51

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

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