Cell Membrane and Transport Mechanisms PDF

Emmanuel Resurreccion Congressional Integrated High School Senior High School Department GENERAL BIOLOGY 01 CELL MEMBRANE AND TRANSPORT MECHANISMS Mr. John Christian A. Olor Special Science Teacher I Learning Objectives At the end of the discussion, we are expected to: Descri...

Emmanuel Resurreccion Congressional Integrated High School Senior High School Department GENERAL BIOLOGY 01 CELL MEMBRANE AND TRANSPORT MECHANISMS Mr. John Christian A. Olor Special Science Teacher I Learning Objectives At the end of the discussion, we are expected to: Describe and discuss the structural components and functions of the cell membrane. Distinguish the relation of the cell structure and composition to cell membrane's function. Examine and explain the different transport mechanisms in cell. Differentiate between exocytosis and endocytosis. The Cell Membrane What is it? All prokaryotes and eukaryotes are surrounded by the cell membrane which acts as a selective barrier to control the movement of essential substances for our survival as it protects its cellular components from the outside environment. semipermeable controls how, when, and how much materials can enter and leave the cell.  Functions of a Cell Membrane Why is it important for the cell membrane to be selective in allowing materials into and out of the cell? SELECTIVE PERMEABILITY HELPS IDENTIFY THE CELLS TO Acts as a barrier separating inside and outside OTHER CELLS of the cell. Controls the flow of substance into and out of PARTICIPATES IN INTERCELLULAR the cell. SIGNALING  What is the Fluid Mosaic Model?  Fluid Mosaic Model S.J. Singer and Garth L. Nicolson (1972) All cell membranes are dynamic in nature. They are not fixed sheets of organic molecules. Cell membranes are represented according to a fluid-mosaic model, since they are: Fluid – the phospholipid bilayer is viscous and individual phospholipids can move position. Mosaic – the phospholipid bilayer is embedded with proteins, resulting in a mosaic of components.  Structure and Composition OF THE CELL MEMBRANE The cell membrane is a flexible yet sturdy barrier that surrounds and contains the cytoplasm of a cell. Structure and Composition OF THE CELL MEMBRANE WHAT STRUCTURES DO YOU THINK COMPOSES THE CELL MEMBRANE? Lipid Bilayer The basic structural framework of the cell membrane is the lipid bilayer, which is two back-to-back layers made up of three types of lipid molecules—phospholipids, cholesterol, and glycolipids. Phospholipids are about 75% of the lipid bilayer. It exists as a stable boundary between two aqueous compartments. A phospholipid is an amphipathic molecule, it has both hydrophobic and hydrophilic regions. The hydrophilic heads are exposed to the water while sheltering the hydrophobic tails away from the water. Structure of the Phospholipid water is a polar molecule, which part of the phospholipid do you think is also polar? Structure of the Phospholipid hydrophilic head polar, phosphate containing regions. hydrophobic tail non-polar, fatty hydrocarbon chains. Structure of the Phospholipid What makes up a phospholipid? Glycerol backbone Phosphate group 2 fatty acids Saturated fat Unsaturated fat R (alcohol) group Membrane Proteins Like membrane lipids, most membrane proteins are amphipathic. Hydrophilic: parts that extend through and out the lipid bilayer. Hydrophobic: parts that are within the lipid bilayer. MEMBRANE PROTEINS Integral (Transmembrane) and Peripheral The major differences between the integral and peripheral membrane protein lie in terms of their: function, location, and the nature of interaction with the lipid bilayer. Looking at the picture, how do you think these membrane proteins differ from one another based on their location? MEMBRANE PROTEINS Integral (Transmembrane) and Peripheral Integral proteins are proteins embedded in the cell membrane as it extends into or through the bilayer. Transmembrane proteins are integral proteins that cross the lipid bilayer and act as pathways for ions and molecules. MEMBRANE PROTEINS Integral (Transmembrane) and Peripheral Peripheral proteins are proteins that are not as firmly embedded in the membrane. Where are they attached?  FUNCTIONS OF MEMBRANE PROTEINS Transport Carrier and Ion-channels These integral proteins are selective allowing certain solutes, specific ions or molecules to pass across the membrane. Enzymes Enzymatic Activities Some integral proteins are enzymes that catalyze specific chemical reactions that occur inside, outside, or at the surface of the cell. Signal Transduction Cell-surface receptors Since membranes are exquisitely sensitive to chemical messages, some membrane proteins function in detecting these signals. Signal Transduction Cell-surface receptors A receptor protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The signaling molecule may cause the protein to change shape, allowing it to relay the message to the inside of the cell, usually by binding to a cytoplasmic protein. Cell-cell recognition Cell-surface identity markers Membranes carry cell-surface markers that identify them to other cells. Some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells. Cell-cell recognition Cell-surface identity markers Membrane carbohydrates (glyco-) are usually short and covalently bonded with other molecules. Glycoproteins: “self” recognition; to recognize and respond to potentially dangerous foreign cells. Glycolipids: tissue formation recognition. Intercellular Joining Linkers; cell-cell adhesion Some integral proteins serve as linkers that anchor neighboring cells together. Membrane proteins of adjacent cells may hook together in various kinds of junctions. Attachment Attachment to cytoskeleton and ECM Surface proteins that interact with other cells are often anchored to the cytoskeleton by linking proteins. Microfilaments helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that can bind to ECM molecules can coordinate extracellular and intracellular changes. Gradient across the Cell Membrane A concentration gradient is a region along which density of a chemical substance increases or decreases. It is the difference of chemical concentration from one Whatplace to another, such is a concentration as from the gradient? inside and outside of the cell membrane. Gradient across the Cell Membrane A concentration gradient is a region along which density of a chemical substance increases or decreases. It is the difference of chemical concentration from one place to another, such as from the inside and outside of the cell membrane. Gradient across the Cell Membrane There are three important characteristics of molecules that affect their ability to cross the membrane: Size (micro-molecule or macromolecules), Charge (nonpolar or polar molecules), and Solubility (lipid soluble or not)  Transport Mechanisms Cellular transport moves substance within the cell and moves substances into and out of the cell. Essential materials must cross the cell membrane and waste products must be removed for survival. What materials can be transported through the cell membrane and under what conditions? Transport of materials across the plasma membrane is essential to the life of a cell. Certain substances must move into the cell to support metabolic reactions while other substances that have been produced by the cell for export or as waste products must move out of the cell. This movement of substances across the cell membrane can be classified as passive or active, depending on whether they require cellular energy. Transport Mechanisms These two mechanisms may also differ in the amount of substance in its origin and its destination. In passive transport, molecules move along the concentration gradient from high concentration to low concentration. Transport Mechanisms While in active transport, molecules move against the concentration gradient from low concentration to high concentration—which is why this mechanism requires energy for it to occur. Passive Processes Passive transport; any substances can move in and out of the cell without the cell having to expend energy. Some ions and molecules can pass through the membrane easily and do so because of a concentration gradient. Some substances also move in response to a gradient but do so through specific channels formed by membrane proteins. Diffusion It is the movement of molecules from a higher to a lower concentration, down a concentration gradient, until equilibrium is achieved, and the molecules are distributed equally. Diffusion is a physical process that results from the random molecular motion (no net movement) that can be observed with any type of molecule. Diffusion Diffusion: Gas Exchange Factors affecting Diffusion Rate Steepness of the concentration gradient: The greater the difference in concentration between the two sides of the membrane, the higher the rate of diffusion. This can also happen in difference in density: whereas the denser the solvent, the slower the diffusion rate. Temperature the higher the temperature, the faster the rate of diffusion. Factors affecting Diffusion Rate Mass of the diffusing substance The larger the mass of the diffusing particle, the slower its diffusion rate. Smaller molecules diffuse more rapidly than larger ones. Surface area The larger the membrane surface area for diffusion, the faster is the diffusion rate. Diffusion distance The greater the distance over which diffusion must occur, the longer it takes. Membrane Transport Systems Movement of materials across the membrane Types of Diffusion Simple Diffusion It is a passive diffusion process in which one moves freely through the lipid bilayer, from high concentration to lower concentration, without the help of membrane transport proteins. Non-polar, hydrophobic, lipid-soluble molecules and gases move through the bilayer in this manner. Small uncharged polar molecules such as water, urea, and small alcohols can also pass through the lipid bilayer via simple diffusion.  Properties of Types of Diffusion facilitated diffusion Concentration gradient is Facilitated Diffusion required because facilitated diffusion cannot transport Many important molecules required molecules from low to high by the cells cannot easily cross the concentration. plasma membrane as they are either Energy is not needed. electrically charged (polar) or are large molecules. Transport molecules are specific to the type of molecules they can transport across the membrane.  Properties of Types of Diffusion facilitated diffusion Concentration gradient is Facilitated Diffusion required because facilitated diffusion cannot transport These molecules can still enter the molecules from low to high cell by diffusion through specific concentration. channel and/or carriers proteins Energy is not needed. embedded in the plasma membrane, provided there is a concentration Transport molecules are specific gradient. to the type of molecules they can transport across the membrane. Channel-mediated Facilitated Diffusion Channels are integral membrane proteins that allow specific, small, inorganic ions to pass across the membrane by facilitated diffusion. It contains tunnels or openings that serve as a passageway for molecules. Channel-mediated Facilitated Diffusion Most membrane channels are ion channels, integral transmembrane proteins that allow passage of small, inorganic ions that are too hydrophilic to penetrate the nonpolar interior of the lipid bilayer. Channel-mediated facilitated diffusion of Potassium ions (K+) through a gated K+ channel. Carrier-mediated Facilitated Diffusion Carriers are integral membrane proteins that undergo undergo temporary binding to the molecule, resulting in a conformational change in shape in order to move substances or molecules across the membrane by facilitated diffusion. Carrier-mediated Facilitated Diffusion Carrier-mediated facilitated diffusion of glucose across a plasma membrane. (1) Glucose binds to a specific type of carrier protein on the outside surface of the membrane. (2) As the transporter undergoes a change in shape, glucose passes through the membrane. (3) The transporter releases glucose on the other side of the membrane. Osmosis It is a special type of diffusion in which there is a net movement of solvent through a selectively permeable membrane. It is the passive movement of water molecules across a selectively permeable membrane from an area of higher to lower water concentration until equilibrium is reached. Focuses on solvent movement rather than solute when solute conc. is high, water conc. is low when solute conc. is low, water conc. is high Osmotic pressure is the pressure that develops due to osmosis; a measure of difference in the levels of two solutions after osmosis. Osmosis Tonicity It refers to the strength of a solution in relation to osmosis, as cells placed in solutions of different concentration can lead to various conditions. Tonicity Hypotonic Is when a cell is placed in: a solution with more water outside than inside the cell. solute concentration is lower than inside the cell. As a result of this water concentration gradient, the water moves into the cell causing it to lyse. It causes turgor pressure in plants. Tonicity Hypertonic Is when a cell is placed in: a solution with less water concentration than inside the cell. solute concentration is higher than inside the cell. Results in the net movement of water molecules from cytoplasm to the external environment. Crenation Plasmolysis Tonicity Isotonic Is when a cell is placed in: a solution with the same concentration of water and other solutes in the cell. no net gain or loss of water. In this type of solution, there is an equal amount of solvent that goes in and out of the cell.  Clinical Connection of Isotonic, Hypertonic, and Hypotonic Solutions. RBCs and other body cells may be damaged or destroyed if exposed to hypertonic or hypotonic solutions since most body cells are in isotonic solution. For this reason, intravenous solutions (IVs) are isotonic. Sometimes infusion of a hypertonic solution such as mannitol (sugar alcohol) is useful to treat patients who have cerebral edema, excess interstitial fluid in the brain. Infusion of such a solution relieves fluid overload by causing osmosis of water from interstitial fluid into the blood. The kidneys then excrete the excess water from the blood into the urine. Hypotonic solutions, given either orally or through an IV, can be used to treat people who are dehydrated. The water in the hypotonic solution moves from the blood into interstitial fluid and then into body cells to rehydrate them. Water and most sports drinks that you consume to “rehydrate” after a workout are hypotonic relative to your body cells. Active Processes Active Transport; It is the movement of molecules against the concentration gradient across the cell membrane that requires energy. Two sources of cellular energy can be used to drive active transport: 1. Energy obtained from hydrolysis of ATP which is the source of energy in primary active transport, and 2. Energy stored in an ionic concentration gradient which is a source of energy in secondary active transport. Properties of Active Transport: Energy is needed in the form of ATP Transport proteins are highly specific to the type of molecules they can transport across the membrane. Primary Active Transport A substance moves across the membrane against its concentration gradient by carriers that underwent a change in shape with the use of energy supplied by hydrolysis of ATP. Secondary Active Transport The energy stored in Na+ or H+ concentration gradient is used to drive other substances across the membrane against their own concentration gradients. Antiporters move Na (or H+) and another substance in opposite directions across the membrane Symporters move Na (or H+) and another substance in the same direction across the membrane. A carrier protein simultaneously binds to Na+ (or H+) and another substance and then changes its shape so that both substances across the membrane at the same time. TRANSPORT IN VESICLES How do macromolecules enter and exit the cell? Bulk-phase Endocytosis Bulk Transport Mechanisms Large molecules, such as proteins, are transported into and out of the cell using certain transport mechanisms as they cannot enter and exit the cell through carrier proteins. They enter and exit the cell in a different process that also requires energy. Endocytosis: It is a general term for the various types of active transport on how macromolecules enter the cell. The cell membrane bends inward (invaginates) forming a vesicle containing the macromolecule to be transported. Phagocytosis Cell eating, it is a process by which cells take in large solid materials through infoldings of the cell membrane to form endocytic vesicles. pseudopods Pinocytosis Cell drinking, it is a process of taking in fluids into the cell. Any solute or small particles in the fluid will be moved into the cell as well. Receptor-mediated A special and specific form of pinocytosis, receptor-mediated endocytosis is highly selective because it only takes up specific ligands by forming ligand-receptor complexes. Exocytosis Materials and other molecules for export are secreted out of the cell through this process. Macromolecules to be transported are carried by the vesicles to the cell membrane. The membrane surrounding the vesicle then fuses with the cell membrane and breaks off. Therefore, the macromolecule is released out of the cell. Waste materials and secretion of hormones are expelled via exocytosis.