Which Statements About The Makeup Of Biological Membranes Are True

The structure and role of cells are critically dependent on membranes, which not only split up the interior of the jail cell from its surround but likewise define the internal compartments of eukaryotic cells, including the nucleus and cytoplasmic organelles. The germination of biological membranes is based on the properties of lipids, and all cell membranes share a common structural organization: bilayers of phospholipids with associated proteins. These membrane proteins are responsible for many specialized functions; some act as receptors that permit the cell to respond to external signals, some are responsible for the selective transport of molecules beyond the membrane, and others participate in electron transport and oxidative phosphorylation. In addition, membrane proteins control the interactions betwixt cells of multicellular organisms. The common structural arrangement of membranes thus underlies a diversity of biological processes and specialized membrane functions, which will exist discussed in detail in later chapters.

Membrane Lipids

The fundamental edifice blocks of all prison cell membranes are phospholipids, which are amphipathic molecules, consisting of two hydrophobic fatty acid chains linked to a phosphate-containing hydrophilic head group (encounter Figure ii.seven). Considering their fat acid tails are poorly soluble in water, phospholipids spontaneously form bilayers in aqueous solutions, with the hydrophobic tails buried in the interior of the membrane and the polar head groups exposed on both sides, in contact with water (Effigy 2.45). Such phospholipid bilayers form a stable barrier between 2 aqueous compartments and stand for the basic construction of all biological membranes.

Figure two.45

A phospholipid bilayer. Phospholipids spontaneously form stable bilayers, with their polar caput groups exposed to h2o and their hydrophobic tails cached in the interior of the membrane.

Lipids institute approximately fifty% of the mass of most cell membranes, although this proportion varies depending on the blazon of membrane. Plasma membranes, for example, are approximately fifty% lipid and 50% protein. The inner membrane of mitochondria, on the other hand, contains an unusually high fraction (most 75%) of protein, reflecting the abundance of protein complexes involved in electron transport and oxidative phosphorylation. The lipid composition of different cell membranes likewise varies (Table ii.iii). The plasma membrane of East. coli consists predominantly of phosphatidylethanolamine, which constitutes eighty% of total lipid. Mammalian plasma membranes are more complex, containing four major phospholipids—phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and sphingomyelin—which together establish fifty to 60% of full membrane lipid. In addition to the phospholipids, the plasma membranes of animate being cells contain glycolipids and cholesterol, which mostly represent to almost 40% of the full lipid molecules.

Tabular array ii.3

Lipid Composition of Cell Membranes ^a.

An important holding of lipid bilayers is that they behave every bit two-dimensional fluids in which private molecules (both lipids and proteins) are gratis to rotate and move in lateral directions (Figure 2.46). Such fluidity is a disquisitional belongings of membranes and is determined past both temperature and lipid composition. For case, the interactions betwixt shorter fatty acid chains are weaker than those betwixt longer chains, so membranes containing shorter fatty acrid chains are less rigid and remain fluid at lower temperatures. Lipids containing unsaturated fatty acids similarly increase membrane fluidity because the presence of double bonds introduces kinks in the fat acid chains, making them more hard to pack 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 construction (see Figure 2.9), cholesterol plays a distinct role in determining membrane fluidity. Cholesterol molecules insert into the bilayer with their polar hydroxyl groups close to the hydrophilic head groups of the phospholipids (Figure 2.47). The rigid hydrocarbon rings of cholesterol therefore collaborate with the regions of the fatty acid chains that are side by side to the phospholipid head groups. This interaction decreases the mobility of the outer portions of the fatty acid chains, making this part of the membrane more rigid. On the other hand, insertion of cholesterol interferes with interactions between fat acrid chains, thereby maintaining membrane fluidity at lower temperatures.

Figure 2.47

Insertion of cholesterol in a membrane. Cholesterol inserts into the membrane with its polar hydroxyl grouping close to the polar caput groups of the phospholipids.

Membrane Proteins

Proteins are the other major constituent of cell membranes, constituting 25 to 75% of the mass of the various membranes of the jail cell. The current model of membrane construction, proposed by Jonathan Singer and Garth Nicolson in 1972, views membranes as a fluid mosaic in which proteins are inserted into a lipid bilayer (Effigy 2.48). While phospholipids provide the basic structural organization of membranes, membrane proteins carry out the specific functions of the different membranes of the cell. These proteins are divided into two general classes, based on 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 but are associated with the membrane indirectly, generally by interactions with integral membrane proteins.

Figure ii.48

Fluid mosaic model of membrane structure. Biological membranes consist of proteins inserted into a lipid bilayer. Integral membrane proteins are embedded in the membrane, ordinarily via α-helical regions of xx to 25 hydrophobic amino acids. Some (more...)

Many integral membrane proteins (called transmembrane proteins) span the lipid bilayer, with portions exposed on both sides of the membrane. The membrane-spanning portions of these proteins are usually α-helical regions of xx to 25 nonpolar amino acids. The hydrophobic side chains of these amino acids interact with the fatty acrid chains of membrane lipids, and the formation of an α helix neutralizes the polar graphic symbol of the peptide bonds, as discussed earlier in this chapter with respect to protein folding. Like the phospholipids, transmembrane proteins are amphipathic molecules, with their hydrophilic portions exposed to the aqueous surroundings on both sides of the membrane. Some transmembrane proteins span the membrane only one time; others have multiple membrane-spanning regions. Virtually transmembrane proteins of eukaryotic plasma membranes have been modified past the addition of carbohydrates, which are exposed on the surface of the cell and may participate in jail cell-cell interactions.

Proteins can also be anchored in membranes by lipids that are covalently attached to the polypeptide chain (see Chapter seven). Distinct lipid modifications anchor proteins to the cytosolic and extracellular faces of the plasma membrane. Proteins tin can be anchored to the cytosolic face up of the membrane either by the addition of a fourteen-carbon fat acrid (myristic acrid) to their amino terminus or by the add-on of either a 16-carbon fatty acid (palmitic acid) or 15- or 20-carbon prenyl groups to the side 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 Jail cell Membranes

The selective permeability of biological membranes to minor molecules allows the cell to control and maintain its internal composition. Simply small uncharged molecules can diffuse freely through phospholipid bilayers (Effigy 2.49). Pocket-size nonpolar molecules, such as O₂ and CO₂, are soluble in the lipid bilayer and therefore can readily cantankerous jail cell membranes. Small uncharged polar molecules, such as H₂O, also can diffuse through membranes, but larger uncharged polar molecules, such every bit glucose, cannot. Charged molecules, such as ions, are unable to diffuse through a phospholipid bilayer regardless of size; fifty-fifty H⁺ ions cannot cross a lipid bilayer by gratis diffusion.

Figure 2.49

Permeability of phospholipid bilayers. Small uncharged molecules can diffuse freely through a phospholipid bilayer. Nonetheless, the bilayer is impermeable to larger polar molecules (such every bit glucose and amino acids) and to ions.

Although ions and most polar molecules cannot diffuse beyond a lipid bilayer, many such molecules (such every bit glucose) are able to cross prison cell membranes. These molecules pass beyond membranes via the action of specific transmembrane proteins, which act equally transporters. Such transport proteins determine the selective permeability of cell membranes and thus play a critical office in membrane function. They contain multiple membrane-spanning regions that form a passage through the lipid bilayer, allowing polar or charged molecules to cantankerous the membrane through a protein pore without interacting with the hydrophobic fatty acrid chains of the membrane phospholipids.

As discussed in detail in Chapter 12, at that place are two general classes of membrane transport proteins (Figure 2.l). Channel proteins class open pores through the membrane, allowing the free passage of whatever molecule of the appropriate size. Ion channels, for example, allow the passage of inorganic ions such equally Na⁺, Chiliad⁺, Caⁱⁱ⁺, and Cl^- across the plasma membrane. One time open up, aqueduct proteins grade small pores through which ions of the appropriate size and charge tin cross the membrane by gratuitous diffusion. The pores formed by these channel proteins are not permanently open; rather, they can exist selectively opened and closed in response to extracellular signals, allowing the cell to control the movement of ions across the membrane. Such regulated ion channels accept been particularly well studied in nerve and musculus cells, where they mediate the manual of electrochemical signals.

Effigy 2.fifty

Aqueduct and carrier proteins. (A) Channel proteins grade open up pores through which molecules of the appropriate size (e.m., ions) can cross the membrane. (B) Carrier proteins selectively bind the small-scale molecule to be transported and and so undergo a conformational (more...)

In contrast to channel proteins, carrier proteins selectively bind and send specific pocket-sized molecules, such every bit glucose. Rather than forming open up channels, carrier proteins act like enzymes to facilitate the passage of specific molecules across membranes. In particular, carrier proteins bind specific molecules and then undergo conformational changes that open channels through which the molecule to be transported tin can pass across the membrane and be released on the other side.

As described and then far, molecules transported by either aqueduct or carrier proteins cross membranes in the energetically favorable management, equally adamant past concentration and electrochemical gradients—a process known as passive transport. Yet, carrier proteins likewise provide a machinery through which the energy changes associated with transporting molecules beyond a membrane can be coupled to the utilise or production of other forms of metabolic energy, just equally enzymatic reactions can be coupled to the hydrolysis or synthesis of ATP. For example, molecules can be transported in an energetically unfavorable management beyond a membrane (e.g., against a concentration gradient) if their transport in that direction is coupled to ATP hydrolysis as a source of energy—a process called active send (Figure 2.51). The free energy stored as ATP can thus be used to command the internal composition of the cell, as well every bit to drive the biosynthesis of cell constituents.

Figure 2.51

Model of agile transport. Model of active transportEnergy derived from the hydrolysis of ATP is used to ship H⁺ against the electrochemical gradient (from depression to high H⁺ concentration). Bounden of H⁺ is accompanied by phosphorylation of the carrier (more...)

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