Many of the proteins of this type can be released from the membrane by relatively gentle extraction procedures, such as exposure to solutions of very high or low ionic strength or of extreme pH, which interfere with protein–protein interactions but leave the lipid bilayer intact; these proteins are often referred to as peripheral membrane proteins, and their association with membranes is often regulated by the cell as we discuss next. Transmembrane proteins and many proteins held in the bilayer by lipid groups or hydrophobic polypeptide regions that insert into the hydrophobic core of the lipid bilayer cannot be released in these ways.

这一类蛋白中，许多可以用相对温和的抽提方法从膜上释放下来，例如用离子强度极高或极低的溶液、或极端 pH 的溶液处理；这些条件干扰蛋白质—蛋白质相互作用，却不破坏脂双层。它们常被称为周边膜蛋白，其与膜的结合往往受细胞调控，下文即将讨论。跨膜蛋白，以及许多靠脂基团或靠插入脂双层疏水内核的疏水肽段而固定在双层中的蛋白，都无法用这些方法释放。

The fluidity of a lipid bilayer depends on both its composition and its temperature, as is readily demonstrated in studies of synthetic lipid bilayers. A synthetic bilayer made from a single type of phospholipid changes from a liquid state to a two-dimensional rigid crystalline (or gel) state at a characteristic temperature. This change of state is called a phase transition, and the temperature at which it occurs is lower (that is, the membrane becomes more difficult to freeze) if the hydrocarbon chains are short or have double bonds.

脂双层的流动性既取决于组成，也取决于温度，这一点在合成脂双层的研究中很容易证明。由单一种类磷脂制成的合成双层，会在某一特征温度下由液态转变为二维刚性晶态（凝胶态）。这种状态改变称为相变；如果烃链较短或含有双键，相变发生的温度就更低（也就是说，膜更难被冻住）。

The lipid compositions of the two monolayers of the lipid bilayer in many membranes are strikingly different. In the human red blood cell (erythrocyte) membrane, for example, almost all of the phospholipid molecules that have choline—(CH3)3N+CH2CH2OH—in their head group (phosphatidylcholine and sphingomyelin) are in the outer monolayer, whereas almost all that contain a terminal primary amino group (phosphatidylethanolamine and phosphatidylserine) are in the inner monolayer (Figure 10–15). Because the negatively charged phosphatidylserine is located in the inner monolayer, there is a significant difference in charge between the two halves of the bilayer. We discuss in Chapter 12 how membrane-bound phospholipid translocators generate and maintain lipid asymmetry.

许多膜中，脂双层两个单层的脂质组成差异显著。以人红细胞膜为例，几乎所有头部基团含胆碱—(CH3)3N+CH2CH2OH—的磷脂分子（磷脂酰胆碱和鞘磷脂）都位于外单层，而几乎所有含末端伯氨基的磷脂（磷脂酰乙醇胺和磷脂酰丝氨酸）都位于内单层（图 10–15）。由于带负电的磷脂酰丝氨酸位于内单层，脂双层两半之间存在明显的电荷差。第 12 章将讨论膜结合的磷脂转位酶如何建立并维持脂质的不对称性。

The characteristic biconcave shape of a red blood cell (Figure 10–37), for example, results from interactions of its plasma membrane proteins with an underlying cytoskeleton, which consists mainly of a meshwork of the filamentous protein spectrin. Spectrin is a long, thin, flexible rod about 100 nm in length. As the principal component of the red blood cell cytoskeleton, it maintains the structural integrity and shape of the plasma membrane, which is the cell’s only membrane, as the cell has no nucleus or other organelles. The spectrin cytoskeleton is attached to the membrane through various membrane proteins.

例如，红细胞特有的双凹形（图 10–37），源自其质膜蛋白与下方细胞骨架的相互作用，而这一骨架主要由丝状蛋白血影蛋白构成的网络组成。血影蛋白是一根细长而柔韧的杆，长约 100 nm。作为红细胞骨架的主要成分，它维持质膜的结构完整性与形状——质膜是红细胞唯一的膜，因为该细胞既无细胞核也无其他细胞器。血影蛋白骨架再通过多种膜蛋白附着到膜上。

Fluidity is also favored by cis-double bonds because they produce kinks in the chains that make them more difficult to pack together (Figure 10–11). The makeup of membranes as a complex mix of many different lipid species further adjusts most membranes so that they remain liquids just above the phase-transition point. Bacteria, yeasts, and other organisms whose temperature fluctuates with that of their environment adjust the fatty acid composition of their membrane lipids to maintain a relatively constant fluidity. As the temperature falls, for instance, the cells of those organisms synthesize fatty acids with more cis-double bonds, thereby avoiding the decrease in bilayer fluidity that would otherwise result from the temperature drop.

顺式双键同样有利于流动性，因为它们使烃链产生扭结，链与链之间更难紧密堆积（图 10–11）。膜由多种脂质混合构成，这一点进一步把大多数膜调节到刚好高于相变点、保持液态。细菌、酵母以及其他体温随环境波动的生物，会调整膜脂的脂肪酸组成，以维持相对恒定的流动性。例如温度下降时，这些生物的细胞会合成含更多顺式双键的脂肪酸，从而避免降温本会造成的双层流动性下降。

Two differently labeled antibodies were used to distinguish selected mouse and human plasma membrane proteins. Although at first the mouse and human proteins were confined to their own halves of the newly formed heterokaryon, the two sets of proteins diffused and mixed over the entire cell surface in about half an hour (Figure 10–32).

研究者用两种带不同标记的抗体，区分选定的小鼠质膜蛋白和人质膜蛋白。起初小鼠蛋白与人蛋白各自局限在新形成的异核体的一半上，但约半小时后，两组蛋白就扩散并混匀到整个细胞表面（图 10–32）。

The most useful of these for the membrane biochemist are detergents, which are small amphiphilic molecules of variable structure (Movie 10.4). Detergents are much more soluble in water than in lipids. Their polar (hydrophilic) ends can be either charged (ionic), as in sodium dodecyl sulfate (SDS), or uncharged (nonionic), as in b-octylglucoside and Triton X-100 (Figure 10–26A). At low concentration, detergents are monomeric in solution, but when their concentration is increased above a threshold, called the critical micelle concentration (CMC), they aggregate to form micelles (Figure 10–26B, C, and D).

对研究膜的生化学家来说，这类试剂中最有用的是去垢剂——一类结构多样的两亲性小分子（动画 10.4）。去垢剂在水中的溶解度远高于在脂质中的溶解度。它们的极性（亲水）端可以带电荷，即离子型，如十二烷基硫酸钠（SDS）；也可以不带电荷，即非离子型，如 β-辛基葡糖苷和 Triton X-100（图 10–26A）。浓度低时，去垢剂在溶液中以单体形式存在；一旦浓度升高越过某一阈值——称为临界微团浓度（CMC）——它们就聚集形成微团（图 10–26B、C、D）。

The lateral diffusion rates of membrane proteins and lipids can be measured by using the technique of fluorescence recovery after photobleaching (FRAP). The method usually involves marking the membrane protein of interest with a specific fluorescent group. This can be done either with a fluorescent ligand such as a fluorophore-labeled antibody that binds to the protein or with recombinant DNA technology to express the protein fused to a fluorescent protein such as green fluorescent protein (GFP; discussed in Chapter 9). The fluorescent group is then bleached in a small area of membrane by a laser beam, and the time taken for adjacent membrane proteins carrying unbleached ligand or GFP to diffuse into the bleached area is measured (Figure 10–33).

膜蛋白和膜脂的侧向扩散速率，可用光漂白后荧光恢复技术（FRAP）测定。该方法通常先给待测膜蛋白标上特定的荧光基团。标记既可以用荧光配体完成，例如结合该蛋白的荧光基团标记抗体；也可以用重组 DNA 技术，让该蛋白与绿色荧光蛋白（GFP，见第 9 章）之类的荧光蛋白融合表达。随后用激光束漂白膜上一小块区域内的荧光基团，再测量周围携带未漂白配体或 GFP 的膜蛋白扩散进入漂白区所需的时间（图 10–33）。

The plasma membrane of a cell is exposed to the often harsh and constantly changing extracellular environment. How are the membrane and its embedded proteins protected from damage? One answer is that many cells have a layer of oligosaccharide and polysaccharide chains that are attached to the lipids and protein domains facing the outside.

细胞的质膜暴露在常常严酷且不断变化的胞外环境中。质膜及其中嵌入的蛋白如何免受损伤？一个答案是：许多细胞在朝向外侧的膜脂和蛋白质结构域上，连接着一层寡糖链和多糖链。

The terms cell coat or glycocalyx are sometimes used to describe the carbohydrate-rich zone on the cell surface. This carbohydrate layer can be visualized by various stains, such as ruthenium red (Figure 10–25A), as well as by its affinity for carbohydrate-binding proteins called lectins, which can be labeled with a fluorescent dye or some other visible marker.

细胞外被（cell coat）或糖萼（glycocalyx）这两个术语，有时用来描述细胞表面富含糖类的区域。这层糖类外被可用多种染料显示，例如钌红（图10–25A）；也可利用它对凝集素（lectin，一类糖结合蛋白）的亲和力来显示，凝集素可以用荧光染料或其他可见标记物标记。

The plasma membrane encloses the cell, defines its boundaries, and maintains the essential differences between the cytosol and the extracellular environment. Without plasma membranes, cells could not have evolved as individual self-replicating units. Inside eukaryotic cells, the membranes of the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and other membrane-enclosed organelles maintain the characteristic differences between the contents of each organelle and the cytosol.

质膜包被细胞、划定细胞边界，并维持胞质溶胶与胞外环境之间的本质差别。没有质膜，细胞就不可能演化成独立的自我复制单位。在真核细胞内部，细胞核、内质网、高尔基体、线粒体以及其他膜围绕细胞器的膜，维持着各细胞器内含物与胞质溶胶之间的特征性差别。

In all cells, the plasma membrane also contains proteins that act as sensors of external signals, allowing the cell to change its behavior in response to environmental cues, including signals from other cells; these protein sensors, or receptors, transfer information—rather than molecules—across the membrane. Despite their differing functions, all biological membranes have a common general structure: each is a very thin film of lipid and protein molecules, held together mainly by noncovalent interactions

在所有细胞中，质膜还含有一类充当外界信号感受器的蛋白，使细胞能够针对环境线索（包括来自其他细胞的信号）改变自身行为；这些蛋白感受器即受体，它们跨膜传递的是信息而不是分子。尽管功能各异，所有生物膜都有共同的一般结构：每一种膜都是一层极薄的脂分子与蛋白质分子薄膜，主要靠非共价相互作用维系在一起。

A synthetic bilayer made from a single type of phospholipid changes from a liquid state to a two-dimensional rigid crystalline (or gel) state at a characteristic temperature. This change of state is called a phase transition, and the temperature at which it occurs is lower (that is, the membrane becomes more difficult to freeze) if the hydrocarbon chains are short or have double bonds.

由单一种类磷脂制成的人工脂双层，在某一特征温度下由液态转变为二维的刚性晶态（或凝胶态）。这种状态改变称为相变；若烃链较短或含有双键，相变发生的温度就更低（也就是膜更难冻结）。

The makeup of membranes as a complex mix of many different lipid species further adjusts most membranes so that they remain liquids just above the phase-transition point. Bacteria, yeasts, and other organisms whose temperature fluctuates with that of their environment adjust the fatty acid composition of their membrane lipids to maintain a relatively constant fluidity. As the temperature falls, for instance, the cells of those organisms synthesize fatty acids with more cis-double bonds, thereby avoiding the decrease in bilayer fluidity that would otherwise result from the temperature drop.

膜由多种脂类混合组成，这种组成进一步把大多数膜调节到略高于相变点的液态。细菌、酵母以及其他体温随环境波动的生物，会调整膜脂中脂肪酸的组成，以维持相对恒定的流动性。例如温度下降时，这些生物的细胞就合成含更多顺式双键的脂肪酸，从而避免降温本应带来的膜流动性下降。

Although many lipids and membrane proteins are not distributed uniformly, large-scale lipid phase segregations are seen rarely in living cell membranes. Instead, specific membrane proteins and lipids are seen to concentrate in a more temporary, dynamic fashion facilitated by protein–protein interactions that allow the transient formation of specialized membrane regions

虽然许多脂类和膜蛋白的分布并不均匀，但活细胞膜上很少见到大尺度的脂相分离。相反，特定的膜蛋白和膜脂是以更短暂、更动态的方式聚集起来的：蛋白质—蛋白质相互作用促成这种聚集，使特化的膜区域得以瞬时形成。

The tendency of mixtures of lipids to undergo phase transitions, as seen in artificial bilayers (see Figure 10–12), may help create rafts in living cell membranes—organizing and concentrating membrane proteins either for transport in membrane vesicles (discussed in Chapter 13) or for working together in protein assemblies, such as when they convert extracellular signals into intracellular ones (discussed in Chapter 15).

正如人工脂双层中所见（见图10–12），脂类混合物发生相变的倾向可能有助于在活细胞膜上形成脂筏——把膜蛋白组织并聚集起来，或者用于膜泡运输（见第13章），或者使它们在蛋白质复合体中协同工作，例如把胞外信号转换成胞内信号时（见第15章）。

One of the many functions of a slippery carbohydrate layer is to protect cells against mechanical and chemical damage; it also keeps various other cells at a distance, preventing unwanted cell–cell interactions.

这层滑润的糖类外被有多种功能，其一是保护细胞免受机械损伤和化学损伤；它还使其他各种细胞与本细胞保持距离，从而阻止不需要的细胞—细胞相互作用。

Spectrin is a long, thin, flexible rod about 100 nm in length. As the principal component of the red blood cell cytoskeleton, it maintains the structural integrity and shape of the plasma membrane, which is the cell’s only membrane, as the cell has no nucleus or other organelles. The spectrin cytoskeleton is attached to the membrane through various membrane proteins.

血影蛋白是一根长而细的柔性杆，长约100 nm。作为红细胞骨架的主要成分，它维持质膜的结构完整性和形态——质膜是红细胞唯一的膜，因为红细胞既无细胞核也无其他细胞器。血影蛋白骨架通过多种膜蛋白连接到质膜上。

The lateral diffusion rates of membrane proteins and lipids can be measured by using the technique of fluorescence recovery after photobleaching (FRAP). The method usually involves marking the membrane protein of interest with a specific fluorescent group. This can be done either with a fluorescent ligand such as a fluorophore-labeled antibody that binds to the protein or with recombinant DNA technology to express the protein fused to a fluorescent protein such as green fluorescent protein (GFP; discussed in Chapter 9).

膜蛋白和膜脂的侧向扩散速率可用荧光漂白恢复（FRAP）技术测定。该方法通常先用特定荧光基团标记待测膜蛋白：标记途径一是用荧光配体，例如能结合该蛋白的荧光标记抗体；二是用重组DNA技术，使该蛋白与绿色荧光蛋白（GFP，见第9章）等荧光蛋白融合表达。

The fluorescent group is then bleached in a small area of membrane by a laser beam, and the time taken for adjacent membrane proteins carrying unbleached ligand or GFP to diffuse into the bleached area is measured

随后用激光束漂白膜上一小块区域内的荧光基团，并测量带有未漂白配体或GFP的相邻膜蛋白扩散进入该漂白区所需的时间。

The lateral diffusion rates of membrane proteins and lipids can be measured by using the technique of fluorescence recovery after photobleaching (FRAP). The method usually involves marking the membrane protein of interest with a specific fluorescent group. This can be done either with a fluorescent ligand such as a fluorophore-labeled antibody that binds to the protein or with recombinant DNA technology to express the protein fused to a fluorescent protein such as green fluorescent protein (GFP; discussed in Chapter 9).

膜蛋白和膜脂的侧向扩散速率可用荧光漂白恢复（FRAP）技术测定。该方法通常先用特定荧光基团标记待测膜蛋白：标记途径一是用荧光配体，例如能结合该蛋白的荧光标记抗体；二是用重组DNA技术，使该蛋白与绿色荧光蛋白（GFP，见第9章）等荧光蛋白融合表达。

The fluorescent group is then bleached in a small area of membrane by a laser beam, and the time taken for adjacent membrane proteins carrying unbleached ligand or GFP to diffuse into the bleached area is measured

随后用激光束漂白膜上一小块区域内的荧光基团，并测量带有未漂白配体或GFP的相邻膜蛋白扩散进入该漂白区所需的时间。

Some intracellular signaling proteins, for example, that help relay extracellular signals into the cell interior are bound to the cytosolic half of the plasma membrane by one or more covalently attached lipid groups, which can be fatty acid chains or prenyl groups (Figure 10–18). In some cases, myristic acid is added to the N-terminal amino group of the protein during its synthesis on a ribosome.

例如，某些把胞外信号向细胞内部传递的胞内信号蛋白，通过一条或多条共价连接的脂基团结合在质膜的胞质侧半层上，这些脂基团可以是脂肪酸链，也可以是异戊二烯基（图10–18）。在有些情况下，蛋白质在核糖体上合成的同时，豆蔻酸就被加到它的N端氨基上。

While still in the ER, the transmembrane segment of the protein is cleaved off and a glycosylphosphatidylinositol (GPI) anchor is added, leaving the protein bound to the noncytosolic surface of the ER membrane solely by this anchor (discussed in Chapter 12); transport vesicles eventually deliver the protein to the plasma membrane

这类蛋白还在内质网中时，其跨膜片段即被切除，同时加上一个糖基磷脂酰肌醇（GPI）锚，于是蛋白仅靠这个锚结合在内质网膜的非胞质面上（见第12章）；随后转运小泡把它送到质膜。

Around 1970, researchers first recognized that individual lipid molecules are able to diffuse freely within the plane of a lipid bilayer. The initial demonstration came from studies of synthetic (artificial) lipid bilayers, which can be made in the form of spherical vesicles, called liposomes (Figure 10–9), or planar lipid films.

1970年前后，研究者首次认识到单个脂分子能在脂双层平面内自由扩散。最初的证据来自对人工合成脂双层的研究：这类脂双层既可做成球形囊泡，即脂质体（图10–9），也可做成平面脂膜。

When phospholipid molecules are exposed to an aqueous environment, they behave as you would expect from the above discussion. They spontaneously pack together to minimize exposure of their hydrophobic tails to water and maximize exposure of their hydrophilic heads to water. Depending on their shape, the optimal packing arrangement is achieved in either of two ways: they can form spherical micelles, with the tails inward, or they can form double-layered sheets, or bilayers, with the hydrophobic tails sandwiched between the hydrophilic head groups

磷脂分子处于水环境中时，其行为正如上文所推论。它们自发聚集堆积，使疏水尾部与水的接触降到最低、亲水头部与水的接触达到最大。依分子形状不同，最佳堆积方式有两种：可形成尾部朝内的球形微团，也可形成双层片状结构即脂双层，疏水尾部夹在两侧亲水头部之间。

By contrast to flip-flop, all lipid molecules rapidly exchange places with their neighbors within a monolayer (∼107 times per second). This gives rise to a rapid lateral diffusion, with a diffusion coefficient (D) of about 10–8 cm2/sec, which means that an average lipid molecule diffuses the length of a large bacterial cell (∼2 μm) in about 1 second. These studies have also shown that individual lipid molecules rotate very rapidly about their long axis and have flexible hydrocarbon chains.

与翻转运动相反，所有脂分子都在同一单层内迅速与相邻分子交换位置（约每秒10⁷次）。这产生了快速的侧向扩散，扩散系数（D）约为10⁻⁸ cm²/秒，也就是说一个平均大小的脂分子约1秒就能扩散完一个大细菌细胞的长度（约2 μm）。这些研究还表明，单个脂分子绕自身长轴旋转极快，其烃链也可弯曲摆动。

Both the diversity and the exposed position of the oligosaccharides on the cell surface make them especially well suited to function in specific cell-recognition processes. Plasma-membrane-bound lectins that recognize specific oligosaccharides on cell-surface glycolipids and glycoproteins mediate a variety of transient cell–cell adhesion processes

寡糖既高度多样，又暴露在细胞表面，这两点使它们特别适合在特异的细胞识别过程中发挥作用。结合于质膜的凝集素能识别细胞表面糖脂和糖蛋白上的特异寡糖，介导多种短暂的细胞—细胞黏着过程

Because the extracellular parts of most plasma membrane proteins are glycosylated, carbohydrates extensively coat the surface of all eukaryotic cells. These carbohydrates occur as oligosaccharide chains covalently bound to membrane proteins (glycoproteins) and lipids (glycolipids). They also occur as the polysaccharide chains of integral membrane proteoglycan molecules.

由于大多数质膜蛋白暴露在外的部分都被糖基化，糖类广泛覆盖着所有真核细胞的表面。这些糖类以寡糖链的形式共价结合在膜蛋白（形成糖蛋白）和膜脂（形成糖脂）上；它们也以多糖链的形式存在于整合膜蛋白聚糖分子上。

A transmembrane protein always has a unique orientation in the membrane. This reflects both the asymmetric manner in which it is inserted into the lipid bilayer in the ER during its biosynthesis (discussed in Chapter 12) and the different functions of its cytosolic and noncytosolic domains.

跨膜蛋白在膜中总有唯一确定的取向。这既反映它在生物合成过程中以不对称方式插入内质网脂双层（见第12章），也反映其胞质侧结构域与非胞质侧结构域的功能不同。

The cytoskeleton is linked to the membrane through two transmembrane proteins: a multipass protein called band 3 and a single-pass protein called glycophorin. The spectrin tetramers bind to some band 3 proteins via ankyrin molecules, and to glycophorin and band 3 (not shown) via band 4.1 proteins.

该骨架通过两种跨膜蛋白连接到膜上：多次跨膜的带3蛋白和单次跨膜的血型糖蛋白。血影蛋白四聚体经锚蛋白与部分带3蛋白结合，并经带4.1蛋白与血型糖蛋白及带3蛋白（图中未示）结合。

The lipid compositions of the two monolayers of the lipid bilayer in many membranes are strikingly different. In the human red blood cell (erythrocyte) membrane, for example, almost all of the phospholipid molecules that have choline—(CH3)3N+CH2CH2OH—in their head group (phosphatidylcholine and sphingomyelin) are in the outer monolayer, whereas almost all that contain a terminal primary amino group (phosphatidylethanolamine and phosphatidylserine) are in the inner monolayer

在许多膜中，脂双层两个单层的脂类组成差别显著。例如在人红细胞膜上，头部含胆碱—(CH3)3N+CH2CH2OH—的磷脂（磷脂酰胆碱与鞘磷脂）几乎全部位于外单层，而含末端伯氨基的磷脂（磷脂酰乙醇胺与磷脂酰丝氨酸）几乎全部位于内单层

In these cases, attractive forces between lipid molecules must outweigh the entropic cost associated with concentrating them. Phase transitions thus break the homogeneity of the bilayer into a patchwork of domains with different properties. There has been a long debate among cell biologists about whether the lipid molecules in the plasma membrane of living cells similarly segregate into specialized domains, called lipid rafts.

在这些情况下，脂分子之间的吸引力必定超过把它们浓集在一起所付出的熵代价。于是相变打破了脂双层的均一性，使其变成由不同性质区域拼成的「补丁」。至于活细胞质膜中的脂分子是否也同样分离成特化区域即脂筏，细胞生物学家长期存在争论。

Two differently labeled antibodies were used to distinguish selected mouse and human plasma membrane proteins. Although at first the mouse and human proteins were confined to their own halves of the newly formed heterokaryon, the two sets of proteins diffused and mixed over the entire cell surface in about half an hour

实验用两种带不同标记的抗体，分别识别选定的小鼠质膜蛋白和人质膜蛋白。起初小鼠蛋白和人蛋白各自局限在新形成的异核体的一半上，但两套蛋白在约半小时内就扩散、混匀到整个细胞表面

In this experiment, a mouse and a human cell were fused to create a hybrid cell, which was then stained with two fluorescently labeled antibodies. One antibody (labeled with a green dye) detects mouse plasma membrane proteins, the other antibody (labeled with a red dye) detects human plasma membrane proteins. When cells are stained immediately after fusion, mouse and human plasma membrane proteins are still found in the membrane domains originating from the mouse and human cell, respectively. After a short time, however, the plasma membrane proteins diffuse over the entire cell surface and completely intermix.

本实验把一个小鼠细胞和一个人细胞融合成杂种细胞，再用两种荧光标记抗体染色。一种抗体（标绿色染料）识别小鼠质膜蛋白，另一种抗体（标红色染料）识别人质膜蛋白。融合后立即染色时，小鼠和人的质膜蛋白仍分别位于各自来源的膜区域内；但过一小段时间后，这些质膜蛋白就扩散到整个细胞表面并完全混合。

Such functional reconstitution can determine which proteins are both necessary and sufficient for a particular cell function. For example, the approach provided proof for the hypothesis that the enzymes that make ATP (ATP synthases) use H+ gradients in mitochondrial, chloroplast, and bacterial membranes to produce ATP.

这种功能重建可以确定哪些蛋白对某一细胞功能既是必需的、又是充分的。例如，正是这一方法证明了下述假说：合成ATP的酶（ATP合酶）利用线粒体膜、叶绿体膜和细菌膜上的H⁺梯度来生成ATP。

Many membrane proteins can be solubilized and then purified in an active form by the use of mild detergents. These detergents cover the hydrophobic regions on membrane-spanning segments that become exposed after lipid removal but do not unfold the protein.

许多膜蛋白可用温和去垢剂增溶，并以有活性的形式纯化出来。这类去垢剂覆盖住脂类被除去后暴露出来的跨膜片段疏水区，但不会使蛋白质解折叠（变性）。

From FRAP measurements, we can estimate the diffusion coefficient for the marked cell-surface protein. The values of the diffusion coefficients for different membrane proteins in different cells are highly variable, because interactions with other proteins impede the diffusion of the proteins to varying degrees. Measurements of proteins that are minimally impeded in this way indicate that cell membranes have a viscosity comparable to that of olive oil.

由FRAP测量结果可以估算被标记的细胞表面蛋白的扩散系数。不同细胞中不同膜蛋白的扩散系数差别很大，因为与其他蛋白的相互作用会不同程度地阻碍这些蛋白扩散。对受阻最小的蛋白所作的测量表明，细胞膜的黏度与橄榄油相当。

The fluorescent molecules are bleached in a small area using a laser beam. The fluorescence intensity recovers as the bleached molecules diffuse away and unbleached molecules diffuse into the irradiated area (shown here in side and top views). The diffusion coefficient is calculated from a graph of the rate of recovery: the greater the diffusion coefficient of the membrane protein, the faster the recovery

用激光束漂白一小块区域内的荧光分子。随着被漂白的分子扩散离开、未被漂白的分子扩散进入被照射区域，荧光强度逐渐恢复（此处以侧视图和俯视图表示）。扩散系数由恢复速率作图计算得出：膜蛋白的扩散系数越大，恢复越快。

The fluidity of cell membranes has to be precisely regulated. It allows membrane proteins to interact rapidly and transiently, and certain membrane transport processes and enzyme activities, for example, cease when the bilayer viscosity is experimentally increased beyond a threshold level. The fluidity of a lipid bilayer depends on both its composition and its temperature, as is readily demonstrated in studies of synthetic lipid bilayers.

细胞膜的流动性必须受到精确调控。流动性使膜蛋白能够快速而短暂地相互作用；例如把脂双层黏度人为提高到超过某一阈值时，某些膜转运过程和酶活性就会停止。脂双层的流动性既取决于组成，也取决于温度，这一点在人工脂双层研究中很容易得到证明。