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Cooper, George M. "The Cell: A Molecular Approach".2.Southampton: Sinauer Associates.



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It is crucial for cells to have membranes, which not only separate the interior from the outside, but also define the interior compartments within them, including the nucleus and cytoplasmic organelles.Membrane formation is based on properties of lipids, and all cell membranes have the same structure: bilayers of phospholipids with associated proteins.These membrane proteins perform a variety of functions; some may function as receptors that allow cells to respond to outside signals, some may transport molecules across membranes selectively, and still others may facilitate electron transport as well as oxidative phosphorylation in cells.Membrane proteins also control interactions between cells in organisms with more than one cell.Membranes are thus involved in a wide range of biological processes and specialized membrane functions, which we will cover in more detail in later chapters.


Membrane Lipids

Phospholipids, the units of all cell membranes, are complex molecules consisting of two hydrophobic chains linked to a phosphate-containing hydrophilic head group (figure 2.7). .It is these phospholipid bilayers that render all biological membranes stable between two aqueous compartments.


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This bilayer consists of phospholipids.These molecules form stable bilayers by spontaneously exposing their polar head groups to water and covering their hydrophobic tails with water.


.For example, plasma membranes consist of approximately 50% lipids and 50% proteins.The inner membrane of mitochondria, on the other hand, contains an unusually high proportion of protein (roughly 75%), reflecting the abundance of protein complexes involved in electron transport and oxidative phosphorylation.Different cell membranes also differ in their lipid composition (Table 2.3).Bacillus E.E. E.coli consists primarily of phosphatidylethanolamine, which accounts for 80% of its total lipid content.As of 2012, mammalian plasma membranes contain four major phospholipids, including phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and sphingomyelin. These phospholipids contribute 50 to 60% of total membrane lipids.Aside from phospholipids, animal cells also contain glycolipids and cholesterol, which together produce about 40% of the total amount of lipid molecules.


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Among the important properties of lipid bilayers is that they behave as fluids in which individual molecules (both lipids and proteins) are free to rotate and move lateral to one another (Figure 2.46).Membrane fluidity is determined both by temperature and by lipid composition.Due to the weaker interactions between shorter fatty acid chains than between longer chains, membranes containing shorter fatty acid chains are less rigid and remain flexible at lower temperatures.Unsaturated fatty acids also enhance membrane fluidity because their double bonds cause kinks in the chains, which makes it harder for them to pack.


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The movement of phospholipids in a membrane.Phospholipid molecules can rotate and move lateraly within a bilayer.


Having a structure of hydrocarbon rings (see Figure 2.9), cholesterol plays a very important role in membrane fluidity.Cholesterol molecules attach to the bilayer with their polar hydroxyl groups close to the hydrophilic head groups of the phospholipids (Figure 2.47).Therefore, cholesterol interacts with the regions of fatty acids chains that are next to phospholipid head groups because its stiff hydrocarbon rings are rigid.Due to this interaction, the outer portions of fatty acid chains become more rigid.Cholesterol, on the other hand, interferes with interactions between fatty acid chains thereby maintaining membrane fluidity at low temperatures.


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Adding cholesterol to a membrane.The polar hydroxyl group of cholesterol is close to the polar head group of phospholipids when it resides in the membrane.


Membrane Proteins

Proteins, constituting 25 to 75% of the mass of the various membranes of the cell, are the other major constituent of their membranes.Current views of membrane structure, made popular by Jonathan Singer and Garth Nicolson in 1972, suggest that membranes are fluid mosaics in which proteins are inserted into a lipid bilayer (Figure 2.48).Phospholipids form the structural organization of membranes while membrane proteins perform the specific functions of each membrane of the cell.Two general classes of membrane proteins are distinguished by the nature of their association with the membrane.Integral membrane proteins are inserted directly into the bilayer.The proteins in peripheral membranes are not directly inserted into the lipid bilayer, but are associated with it indirectly through interactions with integral membrane proteins.


Membrane mosaic model.Proteins are inserted into lipid bilayers in order to form biological membranes.Membrane proteins are usually embedded in the membrane through *-helical regions of 20 to 25 hydrophobic amino acids each.There are (more..)


Several integral membrane proteins (known as transmembrane proteins) span a lipid bilayer, with portions exposed on both sides.These membrane-spanning proteins contain 20 to 25 nonpolar amino acids in their *-helical regions.In addition, the hydrophobic side chains of these amino acids interact with the fatty acid chains in membrane lipid chains, forming an * helix that neutralizes the polar character of the peptide bonds, as discussed earlier in this chapter.In the same way as phospholipids, transmembrane proteins also are amphipathic molecules because their hydrophilic portions are exposed to the aqueous environment on both sides of the membrane.Transmembrane proteins span the membrane only once; others have multiple membrane-spanning segments.

Proteins can also be covalently attached to lipid chains, which anchor them in membranes (see Chapter 7).Various modifications of lipids anchor proteins to the cytosolic and extracellular faces of the plasma membrane. .Alternatively, glycolipids attach proteins to the extracellular face of the plasma membrane by attaching to their carboxy termini.


Transport across Cell Membranes

Selective permeability allows biological membranes to regulate and maintain cellular composition.Through the bilayers of phospholipids, only small, uncharged molecules can diffuse freely (Figure 2.49).The small nonpolar molecules O2 and CO2 are soluble in lipid bilayers and can pass readily across membranes.A large polar molecule, like glucose, cannot diffuse through membranes, but smaller uncharged polar molecules, like H2O, can.The free diffusion of charged molecules such as ions is impossible in a phospholipid bilayer regardless of size; even free-moving H+ ions cannot cross a lipid bilayer.


Bilayer permeability of phospholipids.Phospholipid bilayers allow uncharged molecules to diffuse freely.Bilayers (such as glucose and amino acids) and ions can pass through, but larger polar molecules (such as glucose and amino acids) cannot.


Glucose is able to cross the cell membrane, even though ions and most other polar molecules cannot.Transmembrane proteins function as transporters for these molecules to cross membranes.Transport proteins determine cellular membrane permeability and play a vital role in membrane function.These membrane-spanning domains provide a passageway for polar or charged molecules to travel through the lipid bilayer without interacting with the hydrophobic fatty acid chains of the membrane lipids.

In Chapter 12, we discussed in detail the two main kinds of membrane transport proteins (Figure 2.5).Proteins in the channel form open pores inside the membrane, allowing any molecule of the right size to pass through freely.The plasma membrane is home to inorganic ions such as Na+, K+, Ca2+, and Cl-.As soon as channel proteins open, they form small pores by which ions of the right size and charge can pass freely across the membrane.Channel proteins form pores that are not permanently open; rather they can be selectively opened and closed in response to extracellular signals, thus allowing the cell to control the movement of ions across the membrane.Ion channels that regulate the flow of ions in nerve and muscle cells are particularly well studied, since they control the transfer of electrochemical signals.


Proteins that act as transport channels and carriers.It is believed that channel proteins comprise open pores that allow molecules of the correct size (e.g., ions) to cross the membrane.(B) Carrier proteins bind small molecules and undergo conformational changes (more..)


Carrier proteins, in contrast to channel proteins, selectively bind and transport specific small molecules, such as glucose.It is these carrier proteins that facilitate the passage of specific molecules across membranes, rather than creating open channels.A carrier protein binds specific molecules and then undergoes conformational changes to open channels through which the transported molecule can pass through the membrane and be released on the other side.

.In addition, carrier proteins are also capable of coupling the energy changes involved in transporting molecules across a membrane with the use or production of other forms of metabolic energy, just as enzymatic reactions can be coupled with the synthesis or hydrolysis of ATP.Active transport occurs when ATP hydrolysis supplies energy for transportation in an energy-inefficient direction across a membrane (e.g., against a gradient of concentration).Free energy stored in ATP can be used to control cellular composition, as well as drive biosynthesis of cell components.


Transport model that is active.Model of active transportATP hydrolysis generates energy for transporting H+ against the electrochemical gradient (from low to high H+ concentration).As H+ binds to the carrier, it is phosphorylated (more..)


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