Membrane lipids: where they are and how they behave

Cellmembranes protect and organize cells. All cells have sầu an outer plasma membranethat regulates not only what enters the cell, but also how much of any givensubstance comes in. Unlike prokaryotes, eukaryotic cells also possess internalmembranes that encase their organelles và control the exchange of essentialcell components. Both types of membranes have a specialized structure thatfacilitates their gatekeeping function.

Bạn đang xem: Membrane lipids: where they are and how they behave


With few exceptions, cellular membranes — including plasma membranes & internal membranes — are made of glycerophospholipids, molecules composed of glycerol, a phosphate group, and two fatty acid chains. Glycerol is a three-carbon molecule that functions as the backbone of these membrane lipids. Within an individual glycerophospholipid, fatty acids are attached to lớn the first và second carbons, và the phosphate group is attached to the third carbon of the glycerol backbone. Variable head groups are attached lớn the phosphate. Space-filling models of these molecules reveal their cylindrical shape, a geometry that allows glycerophospholipids to lớn align side-by-side to size broad sheets (Figure 1).


(A) The plasma membrane of a cell is a bilayer of glycerophospholipid molecules. (B) A single glycerophospholipid molecule is composed of two major regions: a hydrophilic head (green) and hydrophobic tails (purple). (C) The subregions of a glycerophospholipid molecule; phosphatidylcholine is shown as an example. The hydrophilic head is composed of a choline structure (blue) and a phosphate (orange). This head is connected to lớn a glycerol (green) with two hydrophobic tails (purple) called fatty acids. (D) This view shows the specific atoms within the various subregions of the phosphatidylcholine molecule. cảnh báo that a double bond between two of the carbon atoms in one of the hydrocarbon (fatty acid) tails causes a slight kink on this molecule, so it appears bent.
© 2010 vuialo.net Education All rights reserved.
th carbon in the chain is bound to lớn the 10th carbon in the chain by a double bond, causing a kink. When carbon atoms are attached to neighboring carbons by single bonds, they are also bound to lớn two hydrogene molecules each. The two carbons bound khổng lồ one another by a double-bond in this schematic are bound khổng lồ only one hydrogene molecule each as a result." href="javascript:void(0)" onclick="callNewShowInformConceptAfterPublish("true","true","Y","/scitable/content/ne0000/ne0000/ne0000/ne0000/14706174/U3CP1-1_Phospholipid.jpg", "The lipid bilayer and the structure và composition of a glycerophospholipid molecule", "Figure 1", "(A) The plasma membrane of a cell is a bilayer of glycerophospholipid molecules. (B) A single glycerophospholipid molecule is composed of two major regions: a hydrophilic head (green) and hydrophobic tails (purple). (C) The subregions of a glycerophospholipid molecule; phosphatidylcholine is shown as an example. The hydrophilic head is composed of a choline structure (blue) & a phosphate (orange). This head is connected to a glycerol (green) with two hydrophobic tails (purple) called fatty acids. (D) This view shows the specific atoms within the various subregions of the phosphatidylcholine molecule. lưu ý that a double bond between two of the carbon atoms in one of the hydrocarbon (fatty acid) tails causes a slight kink on this molecule, so it appears bent.", "600", "http://www.vuialo.net/vuialo.net_education", "A phospholipid bilayer is shown in panel A. Each lipid molecule is shown with a spherical, green, polar "head" group attached to two zigzagging, purple, hydrophobic tails. A top row of 15 phospholipids is arranged opposite a bottom row of 15 phospholipids, so that the hydrophobic tails of the top row meet the hydrophobic tails of the bottom row in the middle of the bilayer with the hydrophobic heads on the top & bottom surfaces. In panel B, a single phospholipid is magnified to show its basic structure. Purple lines represent the two hydrophobic tails, which are attached lớn the green, spherical, polar "head" group. The tail on the left is straight, & the tail on the right is kinked at its kết thúc to lớn size an "L"-shape. A ball-and-stick diagram in panel C shows the molecular structure of the lipid phosphatidylcholine. Colored highlighting is used to distinguish each of the four structural subregions. The phospholipid head is shown with the choline region highlighted in blue at the top, & the phosphate group is highlighted in orange below it. The glycerol region that link the phosphate lớn the two lipid tails is shown in green, và each of the two lipid tails is shown in purple. In panel D, the chemical symbol for each atom that makes up the phosphatidylcholine molecule has been juxtaposed over the molecular ball-and-stichồng Model shown in panel C. The choline group (blue) is comprised of a nitrogene molecule attached by single bonds khổng lồ three methyl groups (CH3) & one methylene group (CH2). A second methylene group is attached by a single bond lớn the first methylene group, & lớn an oxygene molecule that is part of the phosphate group. The phosphate group is comprised of a phosphate molecule attached by single bonds khổng lồ four oxygen molecules in total. One of these oxygen molecules is attached by a single bond khổng lồ a terminal methylene group of a glycerol molecule. The glycerol molecule is a 3-carbon molecule. The central carbon is attached to lớn a hydrogen molecule by a single bond, & the two terminal carbon molecules are both attached to lớn two hydrogen molecules. One fatty acid tail is attached lớn the glycerol"s terminal carbon that is not attached to the phosphate head, và a second fatty acid tail is attached to the glycerol"s central carbon. Each fatty acid is comprised of a terminal carboxyl group (COO-) that is attached khổng lồ a long carbon chain. The carbon of each carboxyl group forms a double bond with one oxyren molecule & a single bond with the other oxyren molecule, which is connected by a single bond to the carbon of the glycerol backbone, & a single bond with a carbon from the backbone of the long carbon chain. In phosphatidylcholine, each fatty acid tail contains 18 carbons, including the carbon of the carboxyl group. The carbons that 3D the first tail are attached to each other by single bonds. In the fatty acid chain bound lớn the glycerol"s central carbon, the 9th carbon in the chain is bound lớn the 10th carbon in the chain by a double bond, causing a kink. When carbon atoms are attached lớn neighboring carbons by single bonds, they are also bound khổng lồ two hydrogen molecules each. The two carbons bound to lớn one another by a double-bond in this schematic are bound to lớn only one hydrogen molecule each as a result.")" class="inlineLinks">Figure Detail

Glycerophospholipids are by far the most abundant lipids in cell membranes. Like all lipids, they are insoluble in water, but their chất lượng geometry causes them lớn aggregate inkhổng lồ bilayers without any energy đầu vào. This is because they are two-faced molecules, with hydrophilic (water-loving) phosphate heads and hydrophobic (water-fearing) hydrocarbon tails of fatty acids. In water, these molecules spontaneously align — with their heads facing outward và their tails lining up in the bilayer"s interior. Thus, the hydrophilic heads of the glycerophospholipids in a cell"s plasma membrane face both the water-based cytoplasm and the exterior of the cell.

Altogether, lipids trương mục for about half the mass of cell membranes. Cholesterol molecules, although less abundant than glycerophospholipids, tài khoản for about trăng tròn percent of the lipids in animal cell plasma membranes. However, cholesterol is not present in bacterial membranes or mitochondrial membranes. Also, cholesterol helps regulate the stiffness of membranes, while other less prominent lipids play roles in cell signaling and cell recognition.


© 2010 vuialo.net Education All rights reserved.
In addition lớn lipids, membranes are loaded with proteins. In fact, proteins account for roughly half the mass of most cellular membranes. Many of these proteins are embedded inkhổng lồ the membrane và stiông xã out on both sides; these are called transmembrane proteins. The portions of these proteins that are nested amid the hydrocarbon tails have hydrophobic surface characteristics, and the parts that stiông chồng out are hydrophilic (Figure 2).

At physiological temperatures, cell membranes are fluid; at cooler temperatures, they become gel-like. Scientists who Mã Sản Phẩm membrane structure và dynamics describe the membrane as a fluid mosaic in which transmembrane proteins can move sầu laterally in the lipid bilayer. Therefore, the collection of lipids và proteins that 3D a cellular membrane relies on natural biophysical properties to form and function. In living cells, however, many proteins are not miễn phí lớn move. They are often anchored in place within the membrane by tethers khổng lồ proteins outside the cell, cytoskeletal elements inside the cell, or both.

Xem thêm: Lỗi #Div/0 Trong Excel Là Gì, Phân Tích Trường Hợp Hỗn Hợp Lỗi


Cell membranes serve as barriers và gatekeepers. They are semi-permeable, which means that some molecules can diffuse across the lipid bilayer but others cannot. Small hydrophobic molecules & gases like oxygen & carbon dioxide cross membranes rapidly. Small polar molecules, such as water & ethanol, can also pass through membranes, but they bởi vì so more slowly. On the other hand, cell membranes restrict diffusion of highly charged molecules, such as ions, & large molecules, such as sugars and amino acids. The passage of these molecules relies on specific transport proteins embedded in the membrane.


Specialized proteins in the cell membrane regulate the concentration of specific molecules inside the cell.
© 2010 vuialo.net Education All rights reserved.

Membrane transport proteins are specific & selective for the molecules they move sầu, và they often use energy khổng lồ catalyze passage. Also, these proteins transport some nutrients against the concentration gradient, which requires additional energy. The ability to maintain concentration gradients & sometimes move sầu materials against them is vital khổng lồ cell health & maintenance. Thanks khổng lồ membrane barriers & transport proteins, the cell can accumulate nutrients in higher concentrations than exist in the environment và, conversely, dispose of waste products (Figure 3).

Other transmembrane proteins have communication-related jobs. These proteins bind signals, such as hormones or immune mediators, lớn their extracellular portions. Binding causes a conformational change in the protein that transmits a signal to intracellular messenger molecules. Like transport proteins, receptor proteins are specific và selective sầu for the molecules they bind (Figure 4).


Transporters carry a molecule (such as glucose) from one side of the plasma membrane to the other. Receptors can bind an extracellular molecule (triangle), và this activates an intracellular process. Enzymes in the membrane can bởi vì the same thing they vị in the cytoplasm of a cell: transform a molecule inkhổng lồ another size. Anchor proteins can physically liên kết intracellular structures with extracellular structures.
© 2010 vuialo.net Education All rights reserved.

Peripheral membrane proteins are associated with the membrane but are not inserted into the bilayer. Rather, they are usually bound to other proteins in the membrane. Some peripheral proteins form a filamentous network just under the membrane that provides attachment sites for transmembrane proteins. Other peripheral proteins are secreted by the cell & khung an extracellular matrix that functions in cell recognition.


Incontrast khổng lồ prokaryotes, eukaryotic cells have sầu not only a plasma membrane thatencases the entire cell, but also intracellular membranes that surround variousorganelles. In such cells, the plasma membrane is part of an extensive endomembrane system that includes theendoplasmic reticulum (ER), the nuclear membrane, the Golgi apparatus, andlysosomes. Membrane components are exchanged throughout the endomembrane systemin an organized fashion. For instance, the membranes of the ER & the Golgiapparatus have different compositions, & the proteins that are found in thesemembranes contain sorting signals, which are lượt thích molecular zip codes thatspecify their final destination.

Mitochondriavà chloroplasts are also surrounded by membranes, but they have sầu unusualmembrane structures — specifically, each of these organelles has two surroundingmembranes instead of just one. The outer membrane of mitochondria andchloroplasts has pores that allow small molecules lớn pass easily. The innermembrane is loaded with the proteins that 3D the electron transport chain& help generate energy for the cell. The double membrane enclosures ofmitochondria & chloroplasts are similar to lớn certain modern-day prokaryotes andare thought khổng lồ reflect these organelles" evolutionary origins.


Membranes are made of lipids and proteins, & theyserve sầu a variety of barrier functions for cells and intracellular organelles.Membranes keep the outside "out" và the inside "in," allowing only certainmolecules to cross và relaying messages via a chain of molecular events