{"id":581,"date":"2017-04-16T02:54:31","date_gmt":"2017-04-16T02:54:31","guid":{"rendered":"https:\/\/pressbooks.hccfl.edu\/bio1\/chapter\/the-plasma-membrane-3\/"},"modified":"2025-08-29T17:43:12","modified_gmt":"2025-08-29T17:43:12","slug":"the-plasma-membrane-3","status":"publish","type":"chapter","link":"https:\/\/pressbooks.hccfl.edu\/bio1\/chapter\/the-plasma-membrane-3\/","title":{"raw":"The Plasma Membrane","rendered":"The Plasma Membrane"},"content":{"raw":"Cells closely control the exchange of substances in and out of the cell.\u00a0 Some substances are excluded, others are taken in, and still others are excreted \u2013 all in controlled quantities. Although the plasma membrane<\/strong> encloses the cell\u2019s borders, it is far from being a static barrier; it is dynamic and constantly in flux. The plasma membrane must be sufficiently flexible to allow certain cells, such as red blood cells and white blood cells, to change shape as they pass through narrow capillaries. In addition to these more obvious functions, the surface of the plasma membrane carries markers which allow cells to recognize one another.\u00a0 This is vital as these markers play a role in the \u201cself\u201d versus \u201cnon-self\u201d distinction of the immune response.\n

Fluid Mosaic Model<\/h1>\nIn 1972, S. J. Singer and Garth L. Nicolson proposed a new model of the plasma membrane. This theory, compared to earlier theories, best explains both microscopic observations and the function of the plasma membrane. This theory is called the fluid mosaic model<\/strong>. The model has evolved somewhat over time, but still best accounts for the structure and functions of the plasma membrane as we now understand them. The fluid mosaic model describes the structure of the plasma membrane as comprised of diverse components\u2014including phospholipids, cholesterol, proteins, and carbohydrates\u2014that are able to flow and change position, while maintaining the basic integrity of the membrane. Both phospholipid molecules and embedded proteins are able to move laterally in the membrane. The fluidity of the plasma membrane is necessary for the activities of certain enzymes and transport molecules within the membrane.\n\nPlasma membranes range from 5\u201310 nm thick. As a comparison, human red blood cells, visible via light microscopy, are approximately 8 \u03bcm thick, or approximately 1,000 times thicker than a plasma membrane.\n\n[caption id=\"attachment_656\" align=\"alignnone\" width=\"899\"]\"\" Figure 1<\/strong> The fluid mosaic model of the plasma membrane structure describes the plasma membrane as a fluid combination of phospholipids, cholesterol, proteins, and carbohydrates.[\/caption]\n

Components of the Plasma Membrane<\/h1>\nThe plasma membrane is made up primarily of a bilayer of phospholipids with embedded proteins, carbohydrates, glycolipids, and glycoproteins, and, in animal cells, cholesterol (Figure 1<\/strong>).\n

Phospholipids<\/h2>\nThe main fabric of the membrane is composed of two layers of phospholipid molecules, and the polar ends of these molecules (which look like a collection of balls in an artist\u2019s rendition of the model) (Figure 2<\/strong>) are in contact with aqueous fluid both inside and outside the cell. Thus, both surfaces of the plasma membrane are hydrophilic<\/strong> (\"water loving\"). In contrast, the interior of the membrane, between its two surfaces, is a hydrophobic<\/strong>\u00a0(\"water fearing\") or nonpolar region because of the fatty acid tails. This region has no attraction for water or other polar molecules.\n\n[caption id=\"attachment_258\" align=\"alignnone\" width=\"300\"]\"Figure Figure 2<\/strong> Phospholipid bilayer. \"Extracellular\" = outside the cell; \"Intracellular\" = inside the cell. Photo credit: OpenStax Anatomy and Physiology<\/a>.[\/caption]\n

A phospholipid molecule (Figure 3<\/strong>) consists of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon. This arrangement gives the overall molecule an area described as its head (the phosphate-containing group), which has a polar character or negative charge, and an area called the tail (the fatty acids), which has no charge. The head can form hydrogen bonds, but the tail cannot.<\/p>\n\n\n[caption id=\"attachment_386\" align=\"alignnone\" width=\"267\"]\"diagram Figure 3<\/strong> This phospholipid molecule is composed of a hydrophilic head and two hydrophobic tails. The hydrophilic head group consists of a phosphate-containing group attached to a glycerol molecule. The hydrophobic tails, each containing either a saturated or an unsaturated fatty acid, are long hydrocarbon chains.[\/caption]\n

Proteins<\/h2>\nProteins make up the second major chemical component of plasma membranes (see Figure 1<\/strong>). Integral proteins<\/strong> are embedded in the plasma membrane and may span all or part of the membrane (Figure 1<\/strong>). Integral proteins may serve as channels or pumps to move materials into or out of the cell. Peripheral proteins<\/strong> are found on the exterior or interior surfaces of membranes, attached either to integral proteins or to phospholipid molecules (Figure 1<\/strong>). Both integral and peripheral proteins may serve as enzymes, as structural attachments for the fibers of the cytoskeleton, or as part of the cell\u2019s recognition sites.\n\nThe recognition sites on the plasma membrane are called\u00a0receptors,<\/strong> which are attachment sites for substances that interact with the cell. Each receptor is structured to bind with a specific substance. The binding of a specific substance to its receptor on the plasma membrane can activate processes within the interior of the cell \u2013 such as activating enzymes involved in metabolic pathways. These metabolic pathways might be vital for providing the cell with energy, making substances for the cell, or breaking down cellular waste or toxins for disposal. Likewise, extracellular hormones and neurotransmitters bind to plasma membrane receptors which transmit a signal into the cell to intracellular molecules. Some recognition sites are used by viruses as attachment points. Although they are highly specific, pathogens like viruses may evolve to exploit receptors to gain entry to a cell by mimicking the specific substance that the receptor is meant to bind. This specificity helps to explain why human immunodeficiency virus (HIV) or any of the five types of hepatitis viruses invade only specific cells.\n
\n\nCystic Fibrosis is caused by a defect in an integral protein in the cell membrane which acts as a channel. The CFTR protein moves ions from one side of the membrane to another. When it is not functioning correctly, this causes very thick mucus to build up in the lungs and digestive tract.\n\n[caption id=\"attachment_259\" align=\"alignnone\" width=\"300\"]\"cartoon When the CFTR channel protein is functioning correctly (1), ions (small balls) are able to pass through the membrane. When it is not functioning correctly (2), ions are unable to cross the membrane. Photo credit: LBudd14<\/a>,\u00a0\u00a0May, 2013. Wikimedia<\/a>.[\/caption]\n\n<\/div>\n

Carbohydrates<\/h2>\nCarbohydrates are the third major component of plasma membranes. They are always found on the exterior surface of cells and are bound either to proteins (forming glycoproteins<\/strong>) or to lipids (forming glycolipids<\/strong>). These carbohydrate chains may consist of 2\u201360 monosaccharide units and may be either straight or branched. Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that allow cells to recognize each other.\u00a0These sites have unique patterns that allow the cell to be recognized, much the way that the facial features unique to each person allow him or her to be recognized. This recognition function is very important to cells, as it allows the immune system to differentiate between body cells (called \u201cself\u201d) and foreign cells or tissues (called \u201cnon-self\u201d). Similar types of glycoproteins and glycolipids are found on the surfaces of viruses and may change frequently, preventing immune cells from recognizing and attacking them.\n
The carbohydrates that make up glycoproteins are responsible for determining human A, B, O blood types. These glycoproteins are recognized by the immune system, which leads to incompatibility in blood types.<\/div>\n
\n\n[caption id=\"attachment_260\" align=\"alignnone\" width=\"1024\"]\"\" ABO Blood types. In this figure, the membrane carbohydrate is represented by the \"lollipops\". They are termed \"antigens\". Photo credit: InvictaHOG, 2006. Wikimedia<\/a>.[\/caption]\n\n<\/div>\n

Membrane Fluidity<\/h1>\n

The mosaic characteristic of the membrane, described in the fluid mosaic model, helps to illustrate its nature. The proteins and other components that exist in the membrane can move with respect to each other, rather like boats floating on a lake. The membrane is not like a balloon, however, that can expand and contract; rather, it is fairly rigid and can burst if penetrated or if a cell takes in too much water. However, because of its mosaic nature, a very fine needle can easily penetrate a plasma membrane without causing it to burst, and the membrane will flow and self-seal when the needle is extracted.<\/p>\n

The mosaic characteristics of the membrane explain some but not all of its fluidity. There are two other factors that help maintain this fluid characteristic. One factor is the nature of the phospholipids themselves. The structure of the fatty acid tails in each phospholipid can make the membrane more dense and rigid, or less dense and flexible. \u00a0The relative fluidity of the membrane is particularly important in a cold environment. A cold environment tends to make membranes less fluid and more susceptible to rupturing. Many organisms (fish are one example) are capable of adapting to cold environments by changing the proportion of different types of fatty acids in their membranes in response to the lowering of the temperature.<\/p>\nAnimals have an additional membrane constituent that assists in maintaining fluidity. Cholesterol,<\/strong> which lies alongside the phospholipids in the membrane, tends to dampen the effects of temperature on the membrane. Thus, this lipid functions as a buffer, preventing lower temperatures from inhibiting fluidity and preventing increased temperatures from increasing fluidity too much. Thus, cholesterol extends, in both directions, the range of temperature in which the membrane is appropriately fluid and consequently functional. Cholesterol also serves other functions, such as organizing clusters of transmembrane proteins into lipid rafts.\n\n[h5p id=\"128\"]\n

References<\/h1>\nUnless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.\n\nText adapted from: OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016 http:\/\/cnx.org\/contents\/b3c1e1d2-839c-42b0-a314-e119a8aafbdd@9.10","rendered":"

Cells closely control the exchange of substances in and out of the cell.\u00a0 Some substances are excluded, others are taken in, and still others are excreted \u2013 all in controlled quantities. Although the plasma membrane<\/strong> encloses the cell\u2019s borders, it is far from being a static barrier; it is dynamic and constantly in flux. The plasma membrane must be sufficiently flexible to allow certain cells, such as red blood cells and white blood cells, to change shape as they pass through narrow capillaries. In addition to these more obvious functions, the surface of the plasma membrane carries markers which allow cells to recognize one another.\u00a0 This is vital as these markers play a role in the \u201cself\u201d versus \u201cnon-self\u201d distinction of the immune response.<\/p>\n

Fluid Mosaic Model<\/h1>\n

In 1972, S. J. Singer and Garth L. Nicolson proposed a new model of the plasma membrane. This theory, compared to earlier theories, best explains both microscopic observations and the function of the plasma membrane. This theory is called the fluid mosaic model<\/strong>. The model has evolved somewhat over time, but still best accounts for the structure and functions of the plasma membrane as we now understand them. The fluid mosaic model describes the structure of the plasma membrane as comprised of diverse components\u2014including phospholipids, cholesterol, proteins, and carbohydrates\u2014that are able to flow and change position, while maintaining the basic integrity of the membrane. Both phospholipid molecules and embedded proteins are able to move laterally in the membrane. The fluidity of the plasma membrane is necessary for the activities of certain enzymes and transport molecules within the membrane.<\/p>\n

Plasma membranes range from 5\u201310 nm thick. As a comparison, human red blood cells, visible via light microscopy, are approximately 8 \u03bcm thick, or approximately 1,000 times thicker than a plasma membrane.<\/p>\n

\"\"
Figure 1<\/strong> The fluid mosaic model of the plasma membrane structure describes the plasma membrane as a fluid combination of phospholipids, cholesterol, proteins, and carbohydrates.<\/figcaption><\/figure>\n

Components of the Plasma Membrane<\/h1>\n

The plasma membrane is made up primarily of a bilayer of phospholipids with embedded proteins, carbohydrates, glycolipids, and glycoproteins, and, in animal cells, cholesterol (Figure 1<\/strong>).<\/p>\n

Phospholipids<\/h2>\n

The main fabric of the membrane is composed of two layers of phospholipid molecules, and the polar ends of these molecules (which look like a collection of balls in an artist\u2019s rendition of the model) (Figure 2<\/strong>) are in contact with aqueous fluid both inside and outside the cell. Thus, both surfaces of the plasma membrane are hydrophilic<\/strong> (“water loving”). In contrast, the interior of the membrane, between its two surfaces, is a hydrophobic<\/strong>\u00a0(“water fearing”) or nonpolar region because of the fatty acid tails. This region has no attraction for water or other polar molecules.<\/p>\n

\"Figure
Figure 2<\/strong> Phospholipid bilayer. “Extracellular” = outside the cell; “Intracellular” = inside the cell. Photo credit: OpenStax Anatomy and Physiology<\/a>.<\/figcaption><\/figure>\n

A phospholipid molecule (Figure 3<\/strong>) consists of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon. This arrangement gives the overall molecule an area described as its head (the phosphate-containing group), which has a polar character or negative charge, and an area called the tail (the fatty acids), which has no charge. The head can form hydrogen bonds, but the tail cannot.<\/p>\n

\"diagram
Figure 3<\/strong> This phospholipid molecule is composed of a hydrophilic head and two hydrophobic tails. The hydrophilic head group consists of a phosphate-containing group attached to a glycerol molecule. The hydrophobic tails, each containing either a saturated or an unsaturated fatty acid, are long hydrocarbon chains.<\/figcaption><\/figure>\n

Proteins<\/h2>\n

Proteins make up the second major chemical component of plasma membranes (see Figure 1<\/strong>). Integral proteins<\/strong> are embedded in the plasma membrane and may span all or part of the membrane (Figure 1<\/strong>). Integral proteins may serve as channels or pumps to move materials into or out of the cell. Peripheral proteins<\/strong> are found on the exterior or interior surfaces of membranes, attached either to integral proteins or to phospholipid molecules (Figure 1<\/strong>). Both integral and peripheral proteins may serve as enzymes, as structural attachments for the fibers of the cytoskeleton, or as part of the cell\u2019s recognition sites.<\/p>\n

The recognition sites on the plasma membrane are called\u00a0receptors,<\/strong> which are attachment sites for substances that interact with the cell. Each receptor is structured to bind with a specific substance. The binding of a specific substance to its receptor on the plasma membrane can activate processes within the interior of the cell \u2013 such as activating enzymes involved in metabolic pathways. These metabolic pathways might be vital for providing the cell with energy, making substances for the cell, or breaking down cellular waste or toxins for disposal. Likewise, extracellular hormones and neurotransmitters bind to plasma membrane receptors which transmit a signal into the cell to intracellular molecules. Some recognition sites are used by viruses as attachment points. Although they are highly specific, pathogens like viruses may evolve to exploit receptors to gain entry to a cell by mimicking the specific substance that the receptor is meant to bind. This specificity helps to explain why human immunodeficiency virus (HIV) or any of the five types of hepatitis viruses invade only specific cells.<\/p>\n

\n

Cystic Fibrosis is caused by a defect in an integral protein in the cell membrane which acts as a channel. The CFTR protein moves ions from one side of the membrane to another. When it is not functioning correctly, this causes very thick mucus to build up in the lungs and digestive tract.<\/p>\n

\"cartoon
When the CFTR channel protein is functioning correctly (1), ions (small balls) are able to pass through the membrane. When it is not functioning correctly (2), ions are unable to cross the membrane. Photo credit: LBudd14<\/a>,\u00a0\u00a0May, 2013. Wikimedia<\/a>.<\/figcaption><\/figure>\n<\/div>\n

Carbohydrates<\/h2>\n

Carbohydrates are the third major component of plasma membranes. They are always found on the exterior surface of cells and are bound either to proteins (forming glycoproteins<\/strong>) or to lipids (forming glycolipids<\/strong>). These carbohydrate chains may consist of 2\u201360 monosaccharide units and may be either straight or branched. Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that allow cells to recognize each other.\u00a0These sites have unique patterns that allow the cell to be recognized, much the way that the facial features unique to each person allow him or her to be recognized. This recognition function is very important to cells, as it allows the immune system to differentiate between body cells (called \u201cself\u201d) and foreign cells or tissues (called \u201cnon-self\u201d). Similar types of glycoproteins and glycolipids are found on the surfaces of viruses and may change frequently, preventing immune cells from recognizing and attacking them.<\/p>\n

The carbohydrates that make up glycoproteins are responsible for determining human A, B, O blood types. These glycoproteins are recognized by the immune system, which leads to incompatibility in blood types.<\/div>\n
\n
\"\"
ABO Blood types. In this figure, the membrane carbohydrate is represented by the “lollipops”. They are termed “antigens”. Photo credit: InvictaHOG, 2006. Wikimedia<\/a>.<\/figcaption><\/figure>\n<\/div>\n

Membrane Fluidity<\/h1>\n

The mosaic characteristic of the membrane, described in the fluid mosaic model, helps to illustrate its nature. The proteins and other components that exist in the membrane can move with respect to each other, rather like boats floating on a lake. The membrane is not like a balloon, however, that can expand and contract; rather, it is fairly rigid and can burst if penetrated or if a cell takes in too much water. However, because of its mosaic nature, a very fine needle can easily penetrate a plasma membrane without causing it to burst, and the membrane will flow and self-seal when the needle is extracted.<\/p>\n

The mosaic characteristics of the membrane explain some but not all of its fluidity. There are two other factors that help maintain this fluid characteristic. One factor is the nature of the phospholipids themselves. The structure of the fatty acid tails in each phospholipid can make the membrane more dense and rigid, or less dense and flexible. \u00a0The relative fluidity of the membrane is particularly important in a cold environment. A cold environment tends to make membranes less fluid and more susceptible to rupturing. Many organisms (fish are one example) are capable of adapting to cold environments by changing the proportion of different types of fatty acids in their membranes in response to the lowering of the temperature.<\/p>\n

Animals have an additional membrane constituent that assists in maintaining fluidity. Cholesterol,<\/strong> which lies alongside the phospholipids in the membrane, tends to dampen the effects of temperature on the membrane. Thus, this lipid functions as a buffer, preventing lower temperatures from inhibiting fluidity and preventing increased temperatures from increasing fluidity too much. Thus, cholesterol extends, in both directions, the range of temperature in which the membrane is appropriately fluid and consequently functional. Cholesterol also serves other functions, such as organizing clusters of transmembrane proteins into lipid rafts.<\/p>\n

\n