Structural Components of the Cell Membrane PDF

STRUCTURAL COMPONENTS OF THE CELL MEMBRANE After going through this lesson, you are expected to: 1. identify the three major constituents of the cell membrane; 2. distinguish the different composition of the cell membrane; 3. explain the importance of the structural components of the cell membrane;...

STRUCTURAL COMPONENTS OF THE CELL MEMBRANE After going through this lesson, you are expected to: 1. identify the three major constituents of the cell membrane; 2. distinguish the different composition of the cell membrane; 3. explain the importance of the structural components of the cell membrane; 4. explain how the arrangement of molecules provides fluidity to the cell; The Cell Membrane Cell Membrane (also known as plas ma membrane) is a physical and chemical barrier which separates the inside a nd outside of the cell providing fixed environment inside the cell. It is a bilayer of li pid with embedded proteins, in which the proteins and lipids. Figure 1. The cell membrane The Structural Component of the Cell Membrane The cell membrane is composed of three main components: lipids, proteins, and carbohydrates. The ratio of lipids and proteins in the cell membrane is 1:1 or 50% lipids and 50% proteins. Membrane protein in the cell membrane is several times larger than the lipid molecule, but lipid molecules are 50 times more than protein molecules. The ratio is not absolute and varies from membrane to membrane Phospholipid Bilayer The fundamental building block of cell membrane is the phospholipid which is an amphipathic molecule, consisting of both hydrophobic and hydrophilic regions. The hydrophilic or “water loving” (polar) region is the globular head containing phosphate group; the hydrophobic or “water-fearing” (nonpolar) regions are their fatty acid tails. The membrane lipids are organized into a continuous bilayer in which the hydrophobic regions of the phospholipids are shielded from the aqueous environment since it is poorly soluble in water and constitute a barrier impenetrable to almost all substances, while the hydrophilic regions are exposed to high water content region. Proteins are found inserted into this lipid bilayer and are classified into integral proteins and peripheral proteins. Figure 2. The Phospholipid Bilayer Figure 3. The Amphipathic nature of the phospholipid 1  It is also semi-permeable in nature, where it is impermeable to water-soluble molecule but not to water. Approximately, the phospholipid to phospholipid thickness of the cell membrane is about 5-10nm. Protein Protein, the second major component of the cell membrane is grouped into three distinct classes depending on their relationship to the lipid bilayer. Figure 4. The Membrane Proteins 1. Integral proteins as their name suggests, integrated into the membrane proteins that penetrate the lipid bilayer. They pass entirely through the lipid bilayer and protrude from both the extracellular and cytoplasmic sides of the cell membrane. 2. Peripheral proteins are membrane proteins that are associated within the surface of the cell membrane and found either on the cytoplasmic or extracellular side. Unlike integral protein, they do not stick into the hydrophobic core of the membrane and they tend to be more loosely attached. Carbohydrates Carbohydrates are the third major component of the cell membrane. In general, they are found on the outside surface of the cells and are bound either in protein forming glycoproteins or to lipids forming glycolipids. These carbohydrates may consist of 2-60 monosaccharide units and can either be straight or branched. The Fluid Mosaic Model The fluid mosaic model describes the cell membrane as several molecules (phospholipid, cholesterol and proteins) that are constantly moving. This movement helps the cell membrane maintains its role as a barrier between the inside and outside of the cell environment. The fluidity of a cell membrane depends on the lipid composition of the membrane, the density of integral proteins, and the temperature. The fatty acids and cholesterol play an important role in the fluidity of the cell membrane. 2  Figure 5. The Fluid Mosaic Model Role of Fatty Acids The structure of the fatty acid tails of the phospholipid is important in determining how fluid is the membrane. Saturated fatty acids have no double bonds, so they are relatively straight while unsaturated fatty acids contain one or more double bond, often resulting in a bend or kink. A long chain of saturated fatty acids have greater interactions among themselves making the cell membrane stiffer. While more unsaturated fatty acids in the lipid tails make the membrane becomes less tightly packed resulting to the increase of the cell membrane fluidity. Thus, at cooler temperature the straight tails of saturated fatty acids can pack tightly together, making a dense and fairly rigid cell membrane while unsaturated fatty acid tails cannot pack together as tightly because of the bent structure of the tails making the cell membrane to stay fluid at lower temperature. Role of Cholesterol The presence of cholesterol in the membrane makes it possible for the cell membrane to maintain its fluidity across a wide range of temperatures. It helps to minimize the effects of temperature on fluidity. At low temperature, cholesterol increases the fluidity by keeping the phospholipids from packing tightly together while at high temperature, it reduces fluidity. In this way, cholesterol expands the range of the temperatures at which a membrane maintains a functional healthy fluidity. The number of cholesterol molecules in the membrane can be as high as the number of phospholipids. A high amount of cholesterol in the phospholipid bilayer 3 makes the cell membrane remains fluid. While having a high density of integral proteins makes the cell membrane have less fluid. The Importance of Membrane Fluidity Membrane fluidity provides a perfect compromise between a rigid structure which makes mobility absent and a completely fluid where mechanical support would be lacking. It also allows interactions to take place within the membrane. Because of membrane fluidity, molecules that interact can come together, carry out the necessary reaction, and move apart. Basic cellular processes, including cell movement, cell growth, cell division, formation of intercellular junctions, secretion, and endocytosis, depend on the fluidity of the cell membrane. 4  TRANSPORT MECHANISMS After going through this lesson, you are expected to: 1. Describe a structure for transport of mechanisms. 2. Explain the transport mechanisms of cells through passive and active transport. 3. Explain how different factors affect the rate of diffusion and osmosis CROSSING PLASMA MEMBRANES As a flexible fatty boundary studded with proteins and carbohydrates, the cell’s plasma membrane tends to keep the watery cell contents in and moisture, chemicals, and other elements of the external environment out. Recall, though, that nutrients must pass into cells and waste products must pass out. To be infective, the viruses must gain entry to the cell. The plasma membrane is selectively permeable, that is, permeable (penetrable) to certain substances but not all. So what accounts Fig.1. Features of plasma membrane for the selectively that allows Source: Image retrieved from https://upload.wikimedia.org/ wikipedia/commons/3/3a/Cell_membrane detailed_diagram_4.svg nutrients, wastes, and viruses to pass through plasma membranes while most other substances are barred? Phospholipids are the foundation of all biological membranes. The lipid bilayer forms as a result of the interaction between the nonpolar (hydrophobic or “water-fearing”) phospholipid tails, the polar (hydrophilic or “water-loving”) phospholipid heads, and the surrounding water. The nonpolar tails face toward the water. Transmembrane proteins float within the bilayer and serve as channels through which various molecules can pass. 5 MOVEMENT OF MATERIALS IN THE CELL Cells, just like any other life forms, must obtain needed materials from the outside environment for survival. Furthermore, materials such as waste products that are no longer needed by the cell must be eliminated from its interior to the outside environment. It has been stated that the cell structure responsible for directing the traffic of materials in and out of the cell is the cell membrane. But how does the cell membrane perform such function? There are two major ways of directing materials in and out of the cells, namely; passive transport and active transport. Passive Transport Passive transport involves the movement of materials through the cell membrane without the expenditure or use of energy. This process does not require energy because the materials move along the concentration gradient, that is moving form a region of high concentration to a region of low concentration. A type of passive transport is simple diffusion. Examples of materials that pass through the cell membrane by this process are small molecules like carbon dioxide (CO2) and oxygen (O). Fig.2. Solute molecules move from high to low concentration 6  A kind of simple diffusion of great importance is osmosis. This is the diffusion of water across the cell membrane. The direction of the movement of water depends on the amount of dissolved substances or solute concentration of the solution surrounding a cell. If the solute concentration of the solution is greater than that of a cell (hypertonic solution), water will move out from the cell, causing the cell to shrink. Conversely, if the solute concentration is less than that of a cell (hypotonic solution), water will tend to move into the cell, causing it to expand, and even burst. However, if the solute concentration is equal to that of a cell (isotonic solution), then there will be no net water movement. Thus, the cell remains intact. Fig.2. Solute molecules move from low to high solute concentration Fig.3. Osmosis demonstration with red blood cells (animal cell) and plant cell walls places in a hypertonic, isotonic, and hypotonic solution 7  Another form of passive transport is facilitated diffusion. In this type of diffusion, protein molecules in the cell membrane act as carriers for certain materials. Molecules such as glucose and steroid hormones may be too large to penetrate the cell membrane, thus requiring the help of protein carriers to bring them into the cell. Fig.4. Facilitated diffusion Source: Image retrieved from https://ib.bioninja.com.au/_ Media/facilitated-diffusion_med.jpeg Active Transport This involves the expenditure of energy by the cell, since the movement of materials goes against the concentration gradient, that is from a low concentration area to a high concentration area. Protein carriers in the cell membrane are provided with energy by the cell to perform such function. Cells can move substances in across the plasma membrane by the import process of endocytosis or out across the plasma membrane via the export process of exocytosis. Some cells discharge wastes this way or secrete proteins, such as hormones or digestive enzymes, into the bloodstream or into a food-digesting organ like the stomach or small intestine. Endocytosis is the process by which a cell membrane invaginates and forms a pocket around a cluster of molecules. This pocket pinches off and forms a vesicle that transports the molecules into the cell. There are three types of endocytosis, namely; phagocytosis, pinocytosis, and receptor-mediated endocytosis  Phagocytosis- known as “cell eating”, the type of endocytosis through which a cell takes in food particles. 8   Pinocytosis- known as “cell-drinking”, the type of endocytosis by which a cell absorbs small particles outside and brings them inside.  Receptor-mediated endocytosis- process by which cells absorb metabolites, hormones, proteins by the inward budding of the plasma membrane. It is a form of endocytosis in which receptor proteins on the cell surface use to capture a specific target molecule. Fig.5. Active Transport Mechanisms: Phagocytosis, Pinocytosis, and Receptor-mediated endocytosis Source: Image retrieved from https://www.scienceabc.com/wp-content/uploads/2018/10/Endocytosis.jpg What Factors Affect the Rate of Diffusion? Particles will always move around in a medium, but the overall rate of diffusion can be affected by many factors. 1. Concentration. If the difference in concentration is higher, then the molecules will go down the concentration gradient faster. If there is not as great of a difference in concentration, the molecules will not move as quickly, and the rate of diffusion will decrease. 2. Temperature. Particle move due to the kinetic energy associated with them. as temperature increases, the kinetic energy associated with each particle also increases. As a result, particles will move faster. If they can move faster, then they can also diffuse faster. 3. Mass of Particle. Heavier particles will move more slowly and so will have a slower rate of diffusion. Smaller particles on the other hand will diffuse faster. 4. Solvent Properties. Viscosity and density greatly affect diffusion. If the medium that a given particle must diffuse through is very dense or viscous, then the particles will have a harder time diffusing through it. So, the rate of diffusion will be lower. If the medium is less dense or 9  less viscous, then the particles will be able to move more quickly and will diffuse faster. Factors that Affect Osmosis 1. Concentration gradient. The greater the concentration difference, the faster the rate of osmosis. 2. Temperature. The higher the temperature, the faster the rate of osmosis. We will be looking at how diffusion occurs in hot and cold mixtures. Then, we can explore how the shape of a container affects the movement of particles. In the second activity, we will observe the size and weight of the samples put into different solutions. 10