2. Cell Membrane and Transport of Substance Across The Cell Membrane.pdf

Cell Membrane and Membrane Transport Systems Dr. Arzu Temizyürek Altınbaş Universtiy Faculty of Medicine https://www.gettyimages.com.au/photos/cell-membrane Dep. Of Physiology  Cells are th...

Cell Membrane and Membrane Transport Systems Dr. Arzu Temizyürek Altınbaş Universtiy Faculty of Medicine https://www.gettyimages.com.au/photos/cell-membrane Dep. Of Physiology  Cells are the basic functional units of the body. They come in a variety of shapes and sizes. This diversity reflects their diverse functions. Principal parts of cells a.Plasma membrane – selectively permeable, gives form, and separates from the external environment b.Cytoplasm and organelles – fluid part of cell and little organs that do the functions c.Nucleus – contains DNA and directs cell activities The general functions of the cell membrane include: 1)Physical isolation The cell membrane is a physical barrier that separates intracellular fluid inside the cell from the surrounding extracellular fluid. 2)Regulation of exchange with the environment The cell membrane controls the entry of ions and nutrients into the cell, the elimination of cellular wastes and release of products from the cell. 3)Communication between the cell and its environment The cell membrane contains proteins that enable the cell recognize and respond to molecules or to changes in its external environment. Any alteration in the cell membrane may affect the cell’s activities. 4)Structural support Proteins in the cell membrane hold the cytoskeleton, the cell’s interior structural scaffolding, in place to maintain cell shape. Membrane proteins also create specialized junctions between adjacent cells or between cells and extracellular matrix. Cell-cell and cell-matrix junctions stabilize the structure of tissues.  THE FLUID MOSAIC MODEL Proteins and phospholipids are not trapped in the membrane but constantly move laterally; known as the fluid mosaic model.  Cell Membrane: Composition Lipids Proteins Phospholipids Integral Sphingolipids Transmembrane Cholesterol Lipid-anchored Peripheral ▪ Carbohydrates ▪ Glycoproteins ▪ Glycolipids  COMPOSITION OF THE CELL MEMBRANE Proteins: 55 % Phospholipids: 25 % Cholesterol: 13 % Other lipids: 4 % Carbohydrates: 3 % Membranes Are Mostly Lipid and Protein  Lipid Bilayer In an aqueous environment these molecules form lipid bilayer (two layers of phospho- lipids). Polar region is oriented toward the outer surface of the membrane -- interacts with water; Nonpolar, hydrophobic fatty acids are in the center of the membrane away from the water.  Membrane Lipids Create a Hydrophobic Barrier  Three main types of lipids make up the cell membrane:  Phospholipids. ▪ The glycerol phosphate head of the molecule is polar and thus hydrophilic. ▪ The fatty acid tail is nonpolar and thus hydrophobic. Functional Significance of the Lipid Bilayer A semi- or selective-permeable barrier Lipophilic, or non-water-soluble substances simply pass through its lipid core gases, such as oxygen and carbon dioxide, alcohol Most hydrophilic, or water-soluble substances are repelled by the hydrophobic interior → specialized transport mechanisms. nutrient molecules, such as glucose and amino acids ions (Na+, Ca++, H+, Cl–, and HCO3–). UREA Sphingolipids Functions: Protection from harmful environmental factors Signal transmission Serve as adhesion sites for extracellular proteins Cholesterol Cholesterol molecules insert themselves between hydrophilic heads of the phospholipids and helps make membranes impermeable to small water soluble molecules and keeps membranes flexible over a wide range of temperatures. They mainly help determine the degree of permeability (or impermeability) of the bilayer to water-soluble constituents of body fluids. Cholesterol controls much of the fluidity of the membrane as well. Structural Proteins The structural proteins have three major roles: The first is to connect the membrane to the cytoskeleton to maintain the shape of the cell. The second role is to create cell junctions that hold tissues together, such as tight junctions and gap junctions. The third role is to attach cells to the extracellular matrix There are two types of cell membrane proteins: 1. Integral Proteins: Structural channels (or pores) through which watersoluble substances, can diffuse. Carrier Proteins, transport substances in the direction opposite to their electrochemical gradients for diffusion Enzymes Receptor peptide hormones 2. Peripheral Proteins: function almost entirely as enzymes or as controllers of transport of substances through pores.  Membrane carbohydrates occur almost in combination with proteins or lipids in the form of glycoproteins or glycolipids. Many other carbohydrate compounds, called proteoglycans—which are mainly carbohydrate substances bound to small protein cores. The outside surface of the cell often has a loose carbohydrate coat called the glycocalyx.  Function of Membrane Carbohydrates 1. Because of negative surface charge it repels other negatively charged substances 2. The glycocalyx of some cells attaching cells to one another 3. Can act as receptor substances for binding hormones activate a cascade of intracellular enzymes 4. Some of them enter into immune reactions  The lipid layer in the middle of the membrane is impermeable to the usual water-soluble substances, such as ions, glucose, and urea. fat-soluble substances, such as oxygen, carbon dioxide, and alcohol, can penetrate this portion of the membrane with ease.  MEMBRANE PROTEINS MEMBRANE PROTEINS can be categorized according to Structure Function Integral Peripheral Membrane Structural Membrane Membrane activate proteins proteins transporters proteins enzymes receptors are found in are active in Carrier Channel proteins proteins are active in Cell junctions Cytoskeleton Receptor- change form mediated conformation endocytosis Open channels Gated channels Metabolism Signal transfer open and close Mechanically Chemically Voltage-gated gated gated channel channel channel Figure 5.8 Map of membrane proteins  CELL TRANSPORT SYSTEMS Cell membranes are selectively permeable. If a membrane allows a substance to pass through it, the membrane is said to be permeable to that substance. If a membrane does not allow a substance to pass, the membrane is said to be impermeable to that substance. 1. Passive Transport: Movement of substances into or out of the cell without expenditure of energy - Diffusion: Molecules move across a membrane from an area of high concentration to an area of low concentration Via gradient - Facilitated Diffusion: Diffusion across the a membrane via interaction of a carrier protein.  The rate of diffusion is determined by the amount of substance available the velocity of kinetic motion the number and sizes of openings in the membrane through which the molecules or ions can move HOW RAPID DIFFUSES A SUBSTANCE THROUGH THE BILAYER ? LIPID SOLUBILITY Simple diffusion Facilitated diffusion Protein “pores” called aquaporins permit rapid passage of water through the membrane. Facilitated Diffusion of Glucose Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Outside of cell Higher concentration Glucose Carrier protein Membrane Inside of cell Lower concentration 2. Active Transport: Movement of substances into or out of the cell with expenditure of energy. Pumps are used, because they force molecules from an area of low concentration to an area of high concentration Example: Sodium (Na) Potasium Pump (K): moves Na ions back out of the cell whereas K ions into the cell OR Calcium (Ca) In primary active transport, the energy is derived directly from breakdown of adenosine triphosphate (ATP) or some other high-energy phosphate compound. In secondary active transport, the energy is derived secondarily from energy that has been stored in the form of ionic concentration differences of secondary molecular or ionic substances between the two sides of a cell membrane, created originally by primary active transport. The protein channels are distinguished by two important characteristics: (1) They are often selectively permeable to certain substances (2) many of the channels can be opened or closed by gates that are regulated by electrical signals (voltage-gated channels) or chemicals that bind to the channel proteins (ligand-gated channels) The opening and closing of gates are controlled in two principal ways: 1. Voltage gating. the gate or of its chemical bonds responds to the electrical potential across the cell membrane. 2. Chemical (ligand) gating by the binding of a chemical substance (a ligand) with the protein, that opens or closes the gate. 3. Bulk Transport: molecules that are too large to be moved by transport proteins are transported via ENDOCYTOSIS Endocytosis & Exocytosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Endocytosis Invagination Formation Formation of pouch of vesicle Extracellular fluid Extracellular substances Cytoplasm now within vesicle Exocytosis Joining of Secretion of vesicle cellular with plasma product membrane Secretion now in extracellular fluid  ENDOCYTOSIS Phagocytosis Pinocytosis Such as bacteria, Liquid or very food particles small particles Adsorptive Receptor-mediated endocytosis endocytosis Molecules bind to spesific receptors Clathrin –mediated end. Caveola- mediated edn.  ENERGY REQUIREMENTS Uses energy of Requires energy molecular motion. MEMBRANE TRANSPORT from ATP. Does not require ATP. Diffusion Endocytosis Secondary creates Primary Exocytosis Simple Facilitated active concentration active diffusion diffusion transport gradient transport Phagocytosis for Molecule Mediated transport Uses a goes through requires a membrane-bound lipid bilayer. membrane protein. vesicle. PHYSICAL REQUIREMENTS  FUNCTIONAL SYSTEMS OF THE CELL Pinocytosis is the only means by which most large macromolecules, such as most protein molecules, can enter cells. Phagocytosis. Phagocytosis occurs in much the same way as pinocytosis occurs, except that it involves large particles rather than molecules.  FUNCTIONAL SYSTEMS OF THE CELL Phagocytosis is initiated when a particle binds with receptors on the surface of the phagocyte In the case of bacteria, each bacterium is attached to a specific antibody The antibody attaches to the phagocyte receptors, dragging the bacterium along with it. This intermediation of antibodies is called opsonization. FUNCTIONAL SYSTEMS OF THE CELL Phagocytosis occurs in the following steps: 1. Receptors attach to the surface ligands of the particle 2. A a closed phagocytic vesicle is formed 3. Actin and other contractile fibrils surround the phagocytic vesicle 4. The vesicle separates from the cell membrane, leaving the vesicle in the cell interior Function of lysosomes Dygestive organs of the cells Removal of damaged portions of cells Bactericidal effect These agents include (1) lysozyme, which dissolves the bacterial cell membrane; (2) lysoferrin, which binds iron and other substances before they can promote bacterial growth (3) acid at a pH of about 5.0 which activates the hydrolases and inactivates bacterial metabolic systems.  Recycling of Cell Organelles—Autophagy Autophagy is a process in which intracellular components and dysfunctional organelles are delivered to the lysosome for degradation and recycling Organelles and large protein aggregates are degraded and recycled Once worn-out cell organelles inside the lysosomes, the organelles are digested and the nutrients are reused by the cell. Contributes to the routine turnover of cytoplasmic components Key mechanism for tissue development, for cell survival Debnath et al. 2023 Thank You