15.2. Objectives At the end of this lecture, you should be able to

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1 Lecture 15: Molecular Structure of the Cell Membrane Introduction Welcome to this lecture on molecular structure of the cell membrane. In this lecture, we are going to look at the molecules that make up the composition of the cell membrane. We will discuss the fluid-mosaic model of the cell membrane. We will also look at the different arrangements of the membrane proteins. Finally, we will end this lecture by looking at the major functions of the membrane integral proteins. I hope that you will enjoy the lecture and it assists you in learning about the cell membrane Objectives At the end of this lecture, you should be able to 1.Explain why the fluid-mosaic model of the cell membrane is the widely accepted model of the cell membrane. 2. Describe the arrangements of peripheral and integral membrane proteins. 3. List the 5 major functions of integral proteins Fluid-Mosaic Model of the Cell Membrane In this section of the lecture, we will discuss the fluid-mosaic model of the cell membrane. The fluid-mosaic model, is based on the paper published by Singer and Nicolson in 1972, in which they reviewed their studies as well as other studies on the composition and functional properties of the cell membrane. They proposed that the only model of the cell membrane, consistent with the experimental evidence was a fluid-mosaic model. In this model, the lipids formed the matrix of the cell membrane, within which were embedded protein molecules. In this sea of lipids, there were proteins that either spanned the entire cell membrane or were associated either with the inner or outer leaflet of the phospholipid bilayer of the cell membrane. This shown in Figure 15.1 below. Figure The lipid-globular mosaic model with the lipid providing the matrix. Source: Singer SJ and Nicolson GL. The Fluid Model of the Structure of Cell Membranes. Science 175: (1972). 1

2 As we can see in figure 15.1, the large potato shaped structures are protein molecules. These can either span the whole cell membrane or be associated with one or other leaflet of the cell membrane. The round balls with tails are the phospholipid molecules. The phospholipids make the membrane fluid and the proteins provide the mosaic, hence the term Fluid-Mosaic Model. Question: Which molecule makes the matrix of the cell membrane? Structure and chemical properties of the phospholipids So now we will look at the lipids to understand how these make the matrix of the cell membrane and arrange themselves in a bilayer formation. The majority of the lipids in the cell membrane are phospholipids. A phospholipid is made up of 3 parts as shown in figure (1) It has a central backbone made up of a glycerol molecule, which is make up of 3 carbon atoms. (2) Attached to 2 of the 3 carbons of the glycerol molecule are acyl chains (Acyl chains are made up of chains of carbon atoms linked by covalent bonds). (3) The remaining 3 third carbon has a phosphate group attached that is why it is called a phospholipid. Now to the phosphate group different molecules can be linked. If we look at figure 2, we can see that an inositol sugar group has been attached to the phosphate group making a phospholipid called phosphatidylinositol. Similarly, other phospholipids are named according to the group attached to the phosphate. Figure Structure of a phospholipid molecule showing the glycerol backbone, the acyl chains, the phosphate to which different molecules can be 2

3 attached. In this case, it is an inositol molecule making a phosphatidylinositol molecule. Question: In figure 15.2, what part of the phospholipid is shown as round balls in fig 15.1? Now we have to consider how phospholipids behave in a watery environment, i.e., an aqueous solution. Phospholipid molecules are called amphipathic because one part of it fears water (hydrophobic) and other part likes water (hydrophilic). The acyl chains provide the hydrophobic character while the phosphate part provides the hydrophilic character. These characteristics cause a conflict when a phospholipid is in an aqueous environment, i.e., how can the fear of and like of water both be satisfied. In figure 15.3, we that that phospholipid molecule will arrange themselves on the surface of the water by having the hydrophilic part in the water and hydrophobic part out of the water. This forms a monolayer. Figure When phospholipids are added to water, they will form a monolayer on the water surface with hydrophobic lipid tails out of the water and the hydrophilic head group in the water. This satisfies the needs of the hydrophobic and hydrophilic parts of the phospholipids. But what happens when the phospholipids are completely submerged in an aqueous solution. In this situation, the conflict is resolved by forming a bilayer arrangement. This molecular arrangement is thermodynamically stable, meaning no energy input is necessary to maintain its conformation. As we see in figure 15.4, by this arrangement, the acyl chain (lipid part) form the interior of the bilayer where they can interact with other acyl chains. As water is not welcome in this region, this region makes a hydrophobic region. 3

4 Figure In an aqueous environment, the phospholipids will form a bilayer, with water on both sides and water free area in the middle. Also we can see in figure 15.4, the outside and inside face of the membrane is in contact with the water molecules, which is what the hydrophilic portion of the phospholipids like to do. So by this arrangement the conflict is resolved. Both the fears of and likes of water of the phospholipid molecule have been satisfied. In the next lecture, when we discuss how ions and molecules move across the cell membrane, we will learn that this hydrophobic interior of the cell membrane forms a formidable energy barrier for the movement of ions, charged molecules, and water-soluble molecules to cross the membrane. Now we may be wondering why some parts of the phospholipid molecule like water and another part do not? The answer lies in the covalent bonds between the atoms. The acyl chains of the phospholipid have non-polar covalent bonds; hence these carbon chains cannot interact with water molecules, which have polar bonds. On the other hand, the phosphate and group attached to it has polar covalent bonds that can interact with polar water molecules. What is the difference between a polar and non-polar covalent bond? Between which atoms that are linked by covalent bonds can there there be polar bonds and non-polar bonds? So in water, the phospholipid molecule has to find a way so that the hydrophilic part can interact with water and the hydrophobic part can stay away from water. The result, in a watery fluid, is a phospholipid bilayer, which makes the matrix of the cell membrane. Question: Why can oils and fat not dissolve in water? The average width of a cell membrane is 7.5 nm with the two layers of the phospholipids making its width. 4

5 While so far, we have focused on the phospholipids as these make up the majority of lipid molecules found in the cell membrane, there are other lipids. In figure 15.5, we can see the structure of the other lipids: glycolipids and cholesterol. Shown in the same figure are 3 other major types of phospholipids: phosphatidylinositols, phosphatidylserine, and phosphatidylcholine. (Phosphatidylethanolamine, the 4 th major type is missing in this figure). Remember in naming phospholipids, the long first part of the name (phosphatidyl-) is for the acyl chain part and the second part is for the molecule attached to its phosphate group. Glycosphingolipid- Figure The 6 different types lipids that are found in most cell membranes. (the image for phosphatidylethanolamine is not shown but it is a major phospholipid found in the cell membrane) There is a difference in composition of lipids making the outer leaflet of the cell membrane, i.e., the one facing the extracellular fluid and the inner leaflet facing the intracellular fluid. The outer leaflet is made up mainly of phosphatidylcholine with sphingomyelin and glycolipids. The inner leaflet has phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol. We will come across the phospholipid, phosphatidylinositol, in lectures on Cell Signaling. There we will learn that the bond between the carbon of the glycerol 5

6 backbone of the phospholipid and phosphate is broken by an enzyme, phospholipase C, to produce 2 molecules: inositol triphosphate (IP3) and diacylglyerol (DAG). These act as second messengers. On a final point with regard to phospholipids, their acyl chains can be saturated or not. We read in newspaper and magazine articles about heart disease linked to whether the fats we eat are saturated or not. It seems that saturated fats are not good for our heart. (Fats and oils contain a lot of triglycerides, i.e., glycerol molecule with 3 acyl chains). So what is a saturated fat? It is a fat where the bonds between the carbon atoms in the acyl chain are all single. In unsaturated fats, one or more bonds in the acyl carbon chain can have a double bonds. Because the saturated acyl chains can interact closely with each other, the membrane fluidity is reduced. For unsaturated acyl chains, the kinks or bends in the carbon chain, prevent close interaction so the membrane is more fluid. Cholesterol, which we have heard about a lot about in relation to heart disease, also has an effect on the cell membrane fluidity. Due to its rigid planar ring structure, it easily slips in between the acyl chains of neighboring phospholipids. (see figure to review the structure). When the cholesterol concentration is low, the membrane fluidity is reduced but at higher concentration cholesterol increases membrane fluidity. In the lectures on reproductive physiology, we will also come across cholesterol as it is a precursor molecule for the major sex steroid hormones: estrogen, progesterone, estradiol-17β, and testosterone. Cholesterol will also be discussed in lectures on gastrointestinal physiology and in cardiovascular physiology. It has now become common to have one s cholesterol level measured. Why? Permeability of a phospholipid bilayer From what we have discussed so far, we know that the phospholipid bilayer has a middle part, which has no water it is a hydrophobic environment. Figure 15.6 is an electron microscope of two cell membranes. Between the cell membranes is the intracellular space. When we look closely at the cell membranes, we can see two dark lines with a light line between them. The dark thin outside line is due to the hydrophilic head groups of the phospholipids, and the region between these lines, the light area, is due to the acyl chains of the phospholipid molecules; this is hydrophobic region of the cell membrane. 6

7 Figure Transmission electron microscope picture of a cell membrane. Bloom and Fawcett, 1994, Springer. While the cell membrane is only 5-6 nm in width, it acts as a very large energy barrier for ions, charged molecules, and water-soluble molecules. For us to understand why the cell membrane is such a large energy barrier, we need to know how ions and charged molecules interact with the polar water molecules. In water, ions and charged molecules as well as water-soluble molecules have cloud or shield of water molecules attached to them. If the ion or charged molecule is pass to through a hydrophobic region, these water molecules have to be removed during their passage across the cell membrane. This requires a very large amount of energy, which is not available. So as an ion or a charged molecule or a water soluble molecule cannot shed its water molecules to cross a lipid bilayer, it makes a lipid bilayer energetically impermeable to ions, charged molecules, water-soluble molecules. The lipid bilayer is also impermeable to many water-soluble molecules such proteins, nuclei acids, sugars, and nucleotides due to their large size. However, the cell membrane is permeable to the small-uncharged water-soluble molecules, e.g., oxygen, carbon dioxide, ammonia, urea, and water. Which types of molecules can cross the lipid bilayer? What are the chemical properties of these molecules? It has been mentioned in the previous lectures that ions, charged molecules and large water-soluble molecules do cross the cell membrane. After all, cells need to take in glucose, amino acids and other material and excrete metabolic waste in the interstitial fluid. The mechanism by which this is done will be explained in detail in the next lecture. Here we will look at the role of integral membrane proteins, many of which are involved in regulation of the movement of ions and molecules across the cell membrane Features of peripheral and integral membrane proteins Besides the lipids, the cell membrane also contains proteins. These are divided into 2 classes: peripheral and integral. 7

8 Peripheral proteins can be easily removed from the membrane and are not embedded or covalently linked to the lipid bilayer. Instead, they interact and adhere to proteins that are embedded in or span the cell membrane, i.e., integral proteins. Peripheral proteins can be present both on the extracellular and intracellular face of the cell membrane. Integral proteins on the other hand are embedded in or span the lipid bilayer and very hard to separate from the cell membrane. As you can see from figure 15.7, integral proteins can be found in either the outer or inner leaflet of the cell membrane or spanning the entire width of the cell membrane. Figure Different arrangements of the peripheral and integral membrane proteins in the lipid bilayer of a cell membrane. In figure 15.7, blue colored proteins are peripheral proteins that adhere to integral proteins shown in orange. Integral proteins can have one or more segments spanning the cell membrane. Note also the covalent linkage of proteins with membrane lipids (on the right-side of the figure). We also see that integral proteins have an extracellular domain and an intracellular domain. Also the segments of the protein passing through the membrane have a helical shape. Some proteins have only one segment while others have more than one protein segment passing through the cell membrane. Now we may wonder how a protein is able to interact both with hydrophobic environment in the interior of the membrane and the aqueous solution on either side of the cell. Where does its amphipathic character come from? Proteins are made up of amino acids and some amino acids have hydrophobic properties. These make up the segment that spans the membrane. Their hydrophobic preferring side chains can interact with the acyl chains of the cell 8

9 membrane phospholipids. Their hydrophilic amino acids are positioned on the inside of the helix so they create a hydrophilic environment. List the amino acids that are hydrophobic side groups and those that have hydrophilic side groups? Like phospholipids, membrane proteins are free to diffuse along the plan of the cell membrane. In an elegant experimental study, done in 1970 s, Frye and Edidin labeled cell membrane proteins of mouse cells with one colored marker and human cells with another colored marker. Then they fused the cells together and observed that after mins, the membrane proteins of mouse and human cells were mixed. The process of the experiment is shown in Figure If the membrane proteins were not free to diffuse in the cell membrane, the colors would not have mixed. This experiment and others have shown that, in general, membrane proteins are not anchored nor tied down to particular area of the cell membrane. When needed the protein can be anchored to a particular site on the cell surface, e.g., the localization of acetylcholine receptors (AchR) at the post- synaptic region of the neuromuscular junction. We will learn a lot more about AchR in lectures on synaptic physiology. Figure Schematic representation of Frye and Edidin experiment. They took two mice and human cells and marked the proteins in the cell membrane with a coloured tag. After fusion of the two cells, they found that the coloured tags were intermixed. The only logical conclusion of this result was that proteins are free to diffuse along the plane of the cell membrane. 9

10 Looking at Figure 15.8, what result would have been expected if membrane proteins were not free to diffuse along the plane of the cell membrane? Many important proteins in the cell membrane are not single protein molecules but are made up of multiple protein molecules called sub-units. These form multimeric protein complexes. These multimeric proteins can be made up of only a single types of protein molecule or can be a mixture of 2 or more different protein molecules. By having subunits, a mulitmeric protein can have the different subunits carry out different functions, e.g., one protein sub-unit can be a binding site for a ligand while another can act as an enzyme, which is activated when the ligand binds to the other sub-unit. When we learn about cell signaling and organ function, we will come across many examples of multimeric proteins in physiological processes Five Major functions carried out by membrane proteins In the following list, the major classes of function that are carried out by membrane proteins is listed. The functions are only explained briefly as in later lectures, we will discuss in more detail the mechanism of their function and the role their function plays in the overall working of physiological systems Role in cell-to-cell communication As we are multicellular organisms, communication between our cells is crucial for coordinating response and changes in function needed to maintain homeostasis. Our cells communicate with each other predominantly with chemical signals. Except for steroid and thyroid hormones, and other lipid soluble signaling molecules that can cross the lipid bilayer, other chemical signals need to have receptor protein in the cell membrane with which they can interact to influence the cellular activity Receptors These are membrane proteins that bind the chemical messengers (signals). They form a very important class of membrane proteins. They convey the information into the cell by modifications of their protein structure. The binding of the signal molecule to the extracellular domain causes conformational changes of the protein arrangement that extend through the cell membrane to the intracellular domain of the receptor. The intracellular domain can become enzymatically active or interact with other cytoplasmic proteins. We will learn a lot more about this when we start to learn about cell signaling and second messengers, topics of later lectures Adhesion molecules Another important class of membrane proteins are adhesion molecules. These are involved in a number of different processes such as directing migration of immune cells, axonal guidance in the developing nervous 10

11 system, regulation of cell shape, and growth. They make physical connections with the extracellular matrix and with other cells. Like protein receptor molecules, adhesion molecules can send signals into the cell. These molecules are also medically important as loss of cell-cell and cellmatrix adhesion is a hallmark of metastatic tumor cells. Integrins or cell matrix adhesion molecules are a large family of transmembrane proteins that link cells to components of the extracellular matrix, e.g., fibronectin and laminin. There are several superfamilies of adhesion molecules: Cadherins - Ca 2+ -dependent cell adhesion molecules which have a large extracellular domain that binds Ca 2+, Ca 2+ -independent neural cell adhesion molecules (N-CAMs) which are members of the immunoglobulin superfamily. What makes metastatic cancer cells such a terror for a person with cancer? If the cancer cells did not loss their cell-to-cell connections, would the tumor be localized or dispersed? Pores, Channels, Carriers, and Pumps. Pores and channels are transmembrane proteins that provide passage way for water, specific ions, and other molecules to flow passively down their electrochemical gradient either into or out of the cell. Carriers can either facilitate the transport of a specific molecule across the membrane or couple the transport of a molecule to that of other solutes. Pumps are enzymes that use the energy derived from adenosine triphosphate (ATP) to transport substances into or out of cells against their electrochemical gradients. You will find that there are many kinds of pumps which are described in the next lecture. Some pumps, like the Na/K ATPase, are vital for cell survival as these are important for the regulation of cell volume Integral signaling proteins located in the cytoplasmic face of the cell membrane These proteins, which are soluble, are linked to lipids in the membrane. They play an important role in cell signaling. Examples include guanosine triphosphate (GTP) binding proteins (G-proteins), kinases, and oncogene Summary In this lecture, we were explained some of the chemistry and physics underlying the bilayer organization of the cell membrane and why fluid-mosaic model is the best model we have of a cell membrane. We also learnt the lipid bilayer is 11

12 impermeable to ions, charged molecules, and large watersoluble molecules, therefore has to be pathways and mechanism for moving these substances into and out of the cell. This is done by integral membrane proteins. We also learnt that integral proteins have other functions besides transport. In the next lecture, we will go into more detailed discussion of how the membrane proteins carry transport function for movement of substances across the cell membrane, and why this is important for us to know, as this will be very useful for us when we study the function of organ systems of the body in the other modules on medical physiology. Figure nicely summaries the structure and composition of a cell membrane. Figure A schematic diagram of the cell membrane showing the lipid bilayer matrix within which are the protein molecules that either span the width of the cell membrane or form peripheral attachments. ( Reading and References Walter F. Boron and Emile L. Boulpaep. Medical Physiology: A Cellular and Molecular Approach. (2012). Chapter 2. Saunders Elsevier Singer SJ and Nicolson GL. The Fluid Mosaic Model of the Structure of Cell Membranes. Science 175 (4023): (1972) 12