Structure and Functions of the Cell Membrane PDF

Unit 4: Cellular Transport Mechanisms Lesson 4.1 Structure and Functions of the Cell Membrane Contents Introduction 1 Learning Objectives 2 Warm Up 3...

Unit 4: Cellular Transport Mechanisms Lesson 4.1 Structure and Functions of the Cell Membrane Contents Introduction 1 Learning Objectives 2 Warm Up 3 Learn about It! 4 Structural Components of the Plasma Membrane 5 Phospholipid Bilayer 5 Molecules Embedded in the Membrane 6 Membrane Proteins 8 Carbohydrate Chains 10 Permeability of the Plasma Membrane 11 Key Points 12 Check Your Understanding 13 Challenge Yourself 15 Photo Credit 15 Bibliography 15 Unit 4: Cellular Transport Mechanisms Lesson 4.1 Structure and Functions of the Cell Membrane Introduction What is your notion of cholesterol? You most probably know that this lipid class can cause cardiovascular diseases. Yet, we still enjoy various cholesterol-rich foods such as fries, fatty beef patties, eggs, and cheese. The low-density lipoprotein (LDL) cholesterol, which is also dubbed as the “bad cholesterol,” is particularly dangerous because it can accumulate in arterial walls and can result in clogging. Normal cholesterol levels in the body should be less than 200 mg/dL in adults. This lipid, despite its negative health impacts, is still required by the body to perform its various biological functions. Such functions include the 4.1. Structure and Functions of the Cell Membrane 1 Unit 4: Cellular Transport Mechanisms syntheses of hormones, bile acids, and vitamin D. In addition, cholesterol is also an important structural component of the plasma membrane of cells, especially in the aspect of membrane fluidity. You can imagine how stiff a membrane will be in the absence of cholesterol, and a stiff and rigid membrane then implies impairment of various biological functions. Note, however, that our bodies produce the cholesterol needed for plasma membrane structure; this is not obtained from our diet. The plasma membrane, being the primary barrier of cells, is very important because the impaired function may put an organism’s life at risk. For instance, diabetes is an example of a defect in the cell’s plasma membrane. A diabetic’s plasma membrane does not respond properly to insulin, the hormone that lowers blood sugar level. Consequently, diabetic individuals need insulin maintenance through intravenous injection. Aside from regulating what goes into and out of the cell, the plasma membrane is also responsible for the communication of cells with one another. It is also responsible for the different chemical processes happening inside the cell because of some membrane- associated enzymes. The membrane’s efficiency in doing its role is very important. What do you think is the relationship between the structure and function of the membrane? How does it contribute to the cell being the fundamental unit of life? Learning Objectives DepEd Competencies In this lesson, you should be able to the Describe the structural following: components of the cell membrane Identify the different structural components (STEM_BIO11/12-Ig-h-11). Relate the structure and of the plasma membrane. composition of the cell membrane Describe the diverse roles of proteins in the to its function membrane. (STEM_BIO 11/12-Ig-h-12). Compare membrane permeability for polar and nonpolar molecules. 4.1. Structure and Functions of the Cell Membrane 2 Unit 4: Cellular Transport Mechanisms Warm Up Build a Cell Membrane 15 minutes One of the discussions in Unit 1 involves the different proteins found in the plasma membrane. In this activity, the students will apply what they have learned about the different molecules of the plasma membrane by assembling cut-outs of its components. Materials printed cut-outs and worksheet on plasma membrane structure scissors glue Procedure 1. Form a group with three members each. 2. Prior to the activity, access the link below and print the cell membrane components and the worksheet. Plasma Membrane Structure Worksheet Quipper, “Plasma Membrane Structure Worksheet,” (2020), https://drive.google.com/open?id=17mdOVKMCW8kGv5E8ojZ 1e7SewSSmx3Xy, last accessed 22 June 2020. 3. Cut all of the structural components of the cell membrane in the printed worksheet. 4. Thereafter, form the phospholipid bilayer alongside all other molecules, namely, proteins, carbohydrates, and cholesterol. In assembling the components, consider the extracellular and intracellular sides of the lipid bilayer (second page of the worksheet). 5. Label the components of the plasma membrane. Note that proteins are classified here as integral, peripheral, and transmembrane. You may review the figure from the previous unit to accomplish this activity. 6. Determine which among these proteins are channel, carrier, and receptor proteins. 4.1. Structure and Functions of the Cell Membrane 3 Unit 4: Cellular Transport Mechanisms 7. After completing this task, you may be asked to present your work in class. 8. Answer the guide questions below. Guide Questions 1. What are the different protein membranes in the structure of the plasma membrane that you have formed? 2. What are the strategies that you considered to form the cut-outs properly? 3. Why are these membrane proteins important? Learn about It! We can recall from the discussion in Unit 1 that the plasma membrane (as shown in Fig. 4.1.1) is one of the fundamental cellular structures. Aside from being known to be the “gatekeeper” or the primary barrier of the cell, it plays other important functions because of its structural components. Fig. 4.1.1. The cell membrane consists of a diversity of lipids, carbohydrates, and proteins. 4.1. Structure and Functions of the Cell Membrane 4 Unit 4: Cellular Transport Mechanisms As the primary barrier of the cell, the cell membrane prevents the cellular components from being mixed up with chemicals of the external environment. It also receives information or stimulus from the environment to initiate any necessary response. Lastly, the cell membrane is also flexible enough that it allows the cell to move and grow. In this unit, one will deeply understand the structure of the plasma membrane and relate it to its various functions. Structural Components of the Plasma Membrane Phospholipid Bilayer Based on our previous knowledge, the plasma membrane is mostly composed of phospholipids (as shown in Fig. 4.1.2) which are composed of two distinct regions. This nature of the plasma membrane was first proposed in 1925 by Dutch scientists Evert Gorter and Francois Grendel. A phospholipid is an amphipathic molecule that has both a hydrophilic region (the “water-loving” phosphate head) and a hydrophobic region (the “water-fearing” hydrophobic tails). This nature of the phospholipid molecules explains why they form a bilayer in water or an aqueous solution. Fig.4.1.2. The aqueous nature of the intracellular and extracellular environments energetically favors the formation of the phospholipid bilayer. 4.1. Structure and Functions of the Cell Membrane 5 Unit 4: Cellular Transport Mechanisms Since the external and internal environment of the cell is mostly composed of water, the hydrophilic polar heads of the phospholipid molecules naturally associate with it. Likewise, the hydrophobic nonpolar tails associate with each other. A phospholipid layer, alone, will be troubled because of the presence of these two conflicting forces, i.e., one face has a natural affinity with water, while the other face repels it. The formation of two layers of phospholipids resolves this conflict. It becomes energetically stable for the bilayer that the fatty acids face each other, while the polar phosphate heads face the naturally aqueous internal and external cellular environments. Detergents, such as soaps, are also amphipathic in nature. Because of the similarity in nature, high concentrations of detergents can “solubilize” the cell membrane, thus disrupting them. This explains why in experiments that require the disruption of cell membranes and other internal membranes such as the nuclear membrane (or nuclear envelope), amphipathic substances such as detergents are used. How is the plasma membrane described by the fluid mosaic model? Molecules Embedded in the Membrane Membranes are not rigid but rather fluid and flexible structures. Biologists use the fluid mosaic model to describe a membrane’s structure as composed of diverse protein molecules suspended in a fluid phospholipid bilayer. This model was proposed by S. Jonathan Singer and Garth Nicholson in 1972. The lipid content of the membrane is responsible for its fluidity which prevents it from solidifying as external temperatures drop. The presence of cholesterol molecules (as shown in Fig. 4.1.1 earlier) specifically helps maintain membrane fluidity particularly by preventing the membrane from becoming too fluid at higher temperatures and too solid at lower temperatures. This fluidity of the membrane is important for two reasons. First, it imparts flexibility to the membrane which is important for cells that particularly move such as an amoeba performing cytoplasmic streaming in Fig. 4.1.3. Second, the fluidity allows the synthesized membrane proteins and phospholipids to be easily incorporated into the membrane. 4.1. Structure and Functions of the Cell Membrane 6 Unit 4: Cellular Transport Mechanisms Fig. 4.1.3. The unicellular amoeba is capable of moving using its pseudopods or false feet because of the natural fluidity of the plasma membrane. Aside from the cholesterol embedded in the hydrophobic portion of the lipid bilayer, the fatty acids of the phospholipids themselves contribute to the fluidity of the membrane. As shown in Fig. 4.1.4, some of the phospholipids have unsaturated bonds (double bonds) that result in kinks or bent portions. If all of the fatty acid tails will have saturated bonds (no double bond), they will push against each other at low temperatures making the membrane more viscous and rigid. This is addressed by the presence of kinks that push adjacent phospholipids to maintain fluidity at low temperatures. Fig. 4.1.4. The presence of double bonds in fatty acids helps maintain membrane fluidity. 4.1. Structure and Functions of the Cell Membrane 7 Unit 4: Cellular Transport Mechanisms The membrane is very much like a fluid because the molecules can move freely past one another and a mosaic because of the diversity of proteins embedded in it. Many of these protein molecules drift laterally like floating icebergs within the bilayer. The number and kinds of proteins can vary in the plasma membrane and in the membranes of the various organelles. It is important to understand the functions of these membrane proteins especially with regard to medicine because most of the drugs bind to them. For instance, omeprazole (Prilosec) is a drug that relieves heartburn by blocking some of the transport proteins that pump hydrogen ions into the stomach cavity. Another example is the antidepressant drug fluoxetine (Prozac), which prevents receptors on brain cell surfaces from reabsorbing a mood-altering biochemical called serotonin. Membrane Proteins Each type of protein that is associated with the cell membrane has a specific function. All these membrane proteins, however, can be classified into two groups. Peripheral proteins are those that are found in only one side of the membrane. They interact with the internal or external surface of the bilayer. By contrast, integral proteins are those that penetrate the hydrophobic core of the lipid bilayer. They may or may not span the entire lipid bilayer. If they do so, they become transmembrane proteins. These proteins may be functionally classified as channel, carrier, adhesion, recognition, or receptor proteins as shown in Fig. 4.1.5. Transport proteins may either be channel proteins or carrier proteins. Channel proteins, as their name implies, serve as tubes or channels that allow passage of molecules across the membrane. These transport proteins are open channels through which a substance moves on its own across a membrane without requiring energy. Carrier proteins are involved in the transport of molecules, but they change shape as they receive certain substances. This change in shape allows the movement of specific molecules across the membrane. Thus, this protein is very specific to the substance it transports. It will be discussed further in this unit that carrier proteins may or may not require energy to mo ve substances. 4.1. Structure and Functions of the Cell Membrane 8 Unit 4: Cellular Transport Mechanisms Adhesion proteins in the plasma membrane fasten adjacent cells together in animal tissues. Recognition proteins function as unique identity tags for each cell or species. They also help in recognition when the body is being invaded by pathogens so that the necessary immune response may be triggered. Receptor proteins bind to specific extracellular substances such as hormones or toxins, or to molecules on another cell’s plasma membrane. They have certain shapes that allow only specific molecules to bind to them. Binding to these proteins triggers the change in the cell’s shape which brings a cellular response that is related to metabolism, movement, division, or even cell death. Diff erent receptor proteins occur on different cells, but all are critical for homeostasis. Fig. 4.1.5. Membrane proteins may function for (a) transport, (b) cell adhesion, (c) cell recognition, or (d) as receptors. 4.1. Structure and Functions of the Cell Membrane 9 Unit 4: Cellular Transport Mechanisms Carbohydrate Chains Aside from phospholipids and proteins, carbohydrates are also present in the plasma membrane. These carbohydrate chains are attached to either the phospholipids or the proteins. As mentioned in the first unit, phospholipids with attached carbohydrates chains are called glycolipids, and proteins with carbohydrate chains are called glycoproteins. The carbohydrate chains occur only on the outside surface of the lipid bilayer or the peripheral proteins that occur on one surface or the other. The location of glycoprotein and glycolipid on the lipid bilayer In animal cells, the carbohydrate chains attached to proteins give the cell a “sugar coat” called glycocalyx which imparts protection and other functions such as cell-to-cell adhesion, reception of signaling molecules, and cell-to-cell recognition. This carbohydrate in the cell’s exterior can vary in the number and sequence of sugars and in whether the chains are branched. Each cell in an individual has its own identity because of these chains. This is the reason why transplanted tissues are often rejected by recipient individuals. One’s immune system may be able to detect that the foreign tissue’s cells do not have the appropriate carbohydrate chains to be recognized. 4.1. Structure and Functions of the Cell Membrane 10 Unit 4: Cellular Transport Mechanisms How does the plasma membrane permit certain molecules to enter selectively into the cell? Permeability of the Plasma Membrane The main function of the plasma membrane is to regulate the passage of molecules into and out of the cell. Although simple to hear, this is very important because the cell needs to maintain its normal composition while dealing with its changing environment, one of the key aspects of homeostasis. As we all know, it is selectively permeable, which means it only allows certain substances into the cell while keeping others out. There is no required energy for molecules that can move freely across the membrane. Substances that are hydrophobic and with nature similar to the hydrophobic core of the lipid bilayer are able to diffuse across membranes without using energy. However, polar and relatively larger molecules which are chemically incompatible with the hydrophobic center of the membrane require energy to drive their transport. Water, despite its polarity, relies on the small size of its molecules to cross the lipid bilayer. Generally, small, non-charged molecules such as carbon dioxide, oxygen, glycerol, and alcohol can freely cross the membrane. They are able to slip between the hydrophilic heads of the phospholipids and pass through the hydrophobic tails of the membrane because these are also nonpolar. The movement of these molecules depends on their concentration gradient—they move from an area where their concentration is high, to an area where their concentration is low. For instance, a cell always uses oxygen to perform cellular respiration. The internal consumption of oxygen in tissues results in low cellular concentration. Since the oxygen concentration is higher outside than inside the cell, oxygen tends to move across the membrane into the cell. On the other hand, the concentration of carbon dioxide is highest inside the cell because it is one of the by -products of cellular respiration. Therefore, carbon dioxide tends to move from the inside to the outside of the cell. 4.1. Structure and Functions of the Cell Membrane 11 Unit 4: Cellular Transport Mechanisms What could be the effect on a cell if one tries to remove the cholesterol in its plasma membrane? Did You Know? The cells of a multicellular organism “talk” to one another by using signaling molecules that are also called chemical messengers. These messengers are transported by the circulatory system to various target sites around the body for animal cells. For example, the pancreas releases a hormone called insulin, which is then transported via blood vessels to the liver, which signals this organ to store glucose as glycogen. Failure of the liver to respond appropriately results in diabetes. Key Points _____________________________________________________________________________________________ The plasma membrane is described by the fluid mosaic model because its structure is composed of diverse protein molecules embedded in a mosaic-like fashion in the fluid phospholipid bilayer. The phospholipids with the attached carbohydrates chains are called glycolipids and proteins with carbohydrate chains are called glycoproteins which can be found on the outside surface of the cell. They usually function for cell-cell recognition. Diverse proteins are associated with the plasma membrane, and they are functionally classified into whether they transport molecules, aid in the recognition of other cells, or whether they determine the presence of substances in the extracellular environment. The phospholipid bilayer is selectively permeable because it only allows certain substances into cells while keeping others out. The size and the chemical nature of 4.1. Structure and Functions of the Cell Membrane 12 Unit 4: Cellular Transport Mechanisms the molecules are important determiners of the permeability of the membrane to certain substances. The molecules that pass through the plasma membrane follow a concentration gradient wherein they move from an area of higher concentration to an area of lower concentration. The structural components and functions of the plasma membrane ___________________________________________________________________________________________ Photo Credit OSC Microbio 03 04 EukPlasMem by CNX OpenStax is licensed under CC BY 4.0 via Wikimedia Commons. Bibliography Hoefnagels, Marielle. Biology: The Essentials. 2nd ed. McGraw-Hill Education, 2016. Mader, Sylvia S., and Michael Windelspecht. Biology. 11th ed. McGraw-Hill Education, 2014. Reece, Jane B, Martha R. Taylor, Eric J. Simon, Jean L. Dickey, and Kelly Hogan. Biology Concepts and Connections. 8th ed. Pearson Education South Asia Pte Ltd., 2016. 4.1. Structure and Functions of the Cell Membrane 13 Unit 4: Cellular Transport Mechanisms Simon, Eric J., and Jane B. Reece. Campbell Essential Biology. 5th ed. Pearson Education Inc., 2013. Starr, Cecie, Christine A. Evers, and Lisa Starr. Biology Today and Tomorrow. 4th ed. Cengage Learning Asia Pte Ltd, 2014. 4.1. Structure and Functions of the Cell Membrane 14