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Transport Across the Cell Membrane L4 Transport Across the Cell Membrane Edited by; Dr. Ahmed Abdelmonaem Assistant professor of medical physiology Dr. Taha Sadig Ahmed Objectives At the end of this lecture the s...
Transport Across the Cell Membrane L4 Transport Across the Cell Membrane Edited by; Dr. Ahmed Abdelmonaem Assistant professor of medical physiology Dr. Taha Sadig Ahmed Objectives At the end of this lecture the student should be able to : Outline the structure of the human cell-membrane. Define membrane permeability and membrane transport Describe the types of membrane channels. Discuss simple diffusion , facilitated diffusion, osmosis, tonicity, active transport ( primary and secondary ), vesicular transport ( exocytosis & pinocytosis ). Made of a phospholipid bilayer (double layer). Hydrophilic phospholipid Restricts passage of H20 heads and H20 soluble ions. Hydrophobic Proteins span or partially phospholipid span the membrane. tails Hydrophilic phospholipid Functions of Membrane heads Proteins they act as → Integral provide structural integrity of the proteins cell membrane. Protein Layers of the Cell provide route for diffusion of water Channel soluble substances like glucose and proteins electrolytes. Carrier help in transport of substances proteins across the cell membrane. Membrane Receptor serve as receptor sites for hormones proteins and neurotransmitters Enzymes control chemical reactions within the cell membrane Definitions : ❑Cell-membrane transport : is the collection of mechanisms that regulate the passage of solutes ( eg., ions) and molecules through the cell-membrane. ❑ Permeability : Is the ease with which substances can pass through the cell membrane. ✓ Nothing passes through an impermeable barrier. ✓ Anything can pass through a freely permeable barrier. ✓ The cell-membrane is selectively permeable i.e. → it is permeable to certain substances but not to others. Membrane Channels The cell-membrane contains several types of channels , e.g. → (1) Leak channels (pores) Continuously open membrane pores. (2) Voltage-gated channels : Open by changes electrical changes ( membrane potential) (3) Ligand-gated channels : Open by binding to a receptor in the ECF as neurotransmitter and hormones Transport through the cell membrane : (A) Directly through the lipid bilayer → Fat-soluble substance (e.g., O2, CO2, OH) (B) Through the proteins channels → water-soluble substances e.g. glucose , Iron. Types of Membrane Transport : (A) Passive Transport : (B) Active Transport : (1) Simple diffusion. (1) Primary active transport. (2) Facilitated diffusion. (2) Secondary active transport. (3) Osmosis. (3)Vesicular Transport : (i) Exocytosis. (ii) Endocytosis. ❑ Simple Diffusion Passage of ions and molecules through the cell membrane according to concentration gradient & No need for energy Transport of substances through (a) chemical gradient) From high concentration to low concentration (b) Electrical Gradient from positively charged region to negatively charged one Or (c) Electrochemical Gradient No energy ( ATP) is needed. Lipid-soluble & non-polar(uncharged) molecules ( e.g., O2, CO2, fat-soluble vitamins ) Factors Influencing the Rate of Simple Diffusion 1- Chemical concentration difference of the substance : Ions ( sodium , potassium & calcium ) and large water-soluble molecules diffuse through protein channels. The bigger the concentration difference of the substance between the two sides of the membrane → the faster will be its diffusion rate. 2- Molecular size of the substance : The smaller the molecular size of the substance in question →the easier is its diffusion across the membrane. 3- Number of open channels for the substance in the membrane e.g., opening of more sodium channels by acetylcholine at the neuromuscular junction causes → massive diffusion of sodium ions from the ECF ( extracellular fluid ) into the ICF ( intracellular fluid ). 4- Fat-solubility of the substance : ✓ Lipid-soluble substances diffuse freely and easily through the cell the lipid bilayer of the cell membrane. ✓ Examples of Lipid -soluble substances are → oxygen, carbon-dioxide and fat-soluble vitamins. 5 - Electrical potential difference across the membrane : ✓ The bigger the electrical gradient → the faster is the rate of diffusion. 6- Temperature : ✓ The higher the temperature → the faster is the rate of diffusion. 7- Pressure Gradient. Large molecule Needs for carrier No energy It is similar to simple diffusion in that : (1) Involves movement of particles down the concentration gradient (2) No energy ( ATP) is needed for this type of transport. Example - Glucose (GLUT ) - Amino acid ✓ It transports large , lipid-insoluble substances {e.g., sugars , amino acids} (whereas simple diffusion transports small, lipid-soluble substances ). Osmosis: It is diffusion of a solvent (usually water molecules) through a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration. ( in other words : diffusion of from the side with the lower solute concentration to the side with the higher solute concentration ). The highest concentration of water is pure water Osmotic pressure: It is pressure which forces the water to move from where there is little dissolved solute to where there is lots dissolved solutes. It is determined by the number of particles per unit volume of fluids The amount of osmotic pressure exereted by a solute is proportional to the number of molecules or ions. The membrane is selectively permeable to water but not to the solute ( e.g., glucose ). Osmoles is the unit used to express the concentration in term of numbers of particles. 1) Hydrostatic pressure 2) osmotic pressure 1) Hydrostatic pressure: It is physiological processes that regulate fluids intake & output as well as movement of water & substances dissolved in it between the body compartments 2) osmotic pressure: The pressure exerted by the flow of water through a semi-permeable membrane separating two solutions with different conc. of solute Illu capillary microcirculation.jpg Osmotic Pressure = The force that needs to be applied to stop Concentrated solute on this osmosis Dilute solute on this side side Osmotic forces acting Osmotic pressure: the pressure required to prevent the osmosis. Water passively diffuses from the Selectively dilute side to the permeable concentrated side in membrane order to equalize the concentration on the 2 sides Ability of solution to change shape of cells by changing water volume Isotonic = equal concentration solutes Hypertonic = higher conc. of solutes Hypotonic = lower conc. of solutes * Serum plasma osmalarity (280-300 m osmol). I- Isotonic Solutions: A solution with the same osmalality as serum and other body Fluids. e.g. N/S 0.9%, Ringer Lactate, D5W. II- Hypotonic Solutions: A solution with an osmolality lower than that of serum plasma. e.g. half strength saline (0.45% sodium chloride). III- Hypertonic Solution: A solution with an osmalality higher than that of serum. e.g. D/S 0.9%, D/S 0.18%, D/S 0.45%, D10W, D25W. *Hypotonic Solutions (0.45% saline) Decreases intravascular osmolarity. Results in intracellular expansion. Used for cellular dehydration. Complications include shock and increased ICP. Contraindications include cerebral edema, and hypotension. *Hypertonic Solutions (D5%.45% saline, D5% NS, D5%LR.) Increases intravascular osmolarity. Results in intracellular and interstitial dehydration. Used for intravascular expansion by shifting intracellular and interstitial fluids. Complications include circulatory overload. Contraindications include intracellular dehydration and hyperosmolar states. *Isotonic Solutions (NS, Lactated Ringers, D5%W.) Does not change osmolarity. Results in TBW expansion. Used to increase intravascular space. Complications include circulatory overload. Contraindications include circulatory overload and LR in alkalosis and liver disease. Definition :Its transport of substance across the cell membrane against an electro chemical gradient It needs 1)Carrier protein 2)Energy (ATPase Enzyme) *Types 1)Primary Active transport 2)Secondary active transport Primary Active Transport : (1) Moves molecules of one substance against the concentration gradient. (2) Energy source used: ATP. Example : Na+-K+ pump ( 3 sodium ions transported out of the cell in exchange for 2 K ions transported into the cell ). Na+ (1) Moves molecules of two or more substance against the concentration gradient (2) Energy source utilized : other than ATP : Energy is supplied indirectly form primary transport. Example ; Na+ - Ca² exchanger Ca²+ Na is moving to the interior causing calcium to move out. 1) Uniport transport one substance in one direction (Ca pump) 2) Symport transport two substances simultaneously in the same direction (Na-Glucose transport) 3) Antiport (counter transport ) transport one substance in one direction and another substance in the opposite direction (Na – K pump) 2&3 called cotrasport Vesicular transport (A) Exocytosis : Process by which cellular products (eg., hormones, mucus) are secreted into extracellular environment. AS, ❑The product released into small membranous vesicle or sac. ❑The vesicle migrates to the cell-membrane , fuses with it and then ruptures , spilling its contents out of the cell. Definition: - Movement of large molecules or particles - enclosed in side cell membrane (vesicles ) - from out side the cell to inside the cell 3 types 1) Pinocytosis (Cell drinking) 2) Phagocytosis(Cell eating) 3) Receptor mediated endocytosis. (1) Pinocytosis ( cell-drinking ) Process that permits transport of fluid from outside the cell into it. Cell-membrane invaginates, fuses, vesicle containing ECF pinches off, and vesicle enters cell. Celleating Large particles (bacteria) Substance not in solution Seen by microscope Only phagocytic cells - neutrophils - monocytes - The tissue macrophages ) Cell eating of 1)Bacteria 2)Dead tissue 3)Polymoph nucular leuko cytes - Transport macromolecules - More specific - More rapid - Recycled Thank You !
