Plasma Membrane Mechanisms of Cell Membrane Transport PDF

Plasma membrane Mechanisms of Cell Membrane Transport First year: Physical therapy By: Dr/ Rasha Mohammed Ali Lecturer of Medical physiology, Faculty of medicine, Assiut university  Objectives...

Plasma membrane Mechanisms of Cell Membrane Transport First year: Physical therapy By: Dr/ Rasha Mohammed Ali Lecturer of Medical physiology, Faculty of medicine, Assiut university  Objectives to: Describe the fluid mosaic model of membrane structure and function. At the end of this Define permeability and list session, the students factors influencing permeability. should be able Identify and describe carried- mediated transport processes: Primary active transport, secondary active transport, facilitates diffusion. All animal cells are enveloped by a cell membrane known as plasma membrane that separates the cytoplasmic contents of the cell, or the intracellular fluid (ECF), from the fluid outside the cell, the extracellular fluid (ECF). Character of the plasma membrane 1. Its thickness is about 75-100 Angstrom 2. It is composed of : ✓Lipid bilayer (fats):phospholipids and cholesterol 45 % ✓ Proteins 50 % ✓ Carbohydrates 5 % 3. Selectively permeable: allows some substances to cross more easily than others so controls exchange of materials such as nutrients and waste between cells and their environment. The phospholipids bilayer form a thin, flexible sheet, while the proteins "float" in the phospholipid and the carbohydrates extend out from the proteins.  Structure of the cell membrane I- The phospholipids are arranged in a bilayer, with their polar, hydrophilic phosphate heads facing outwards, and their non- polar, hydrophobic fatty acid tails facing each other in the middle of the bilayer. This hydrophobic layer acts as a barrier to all but the smallest molecules, effectively isolating the two sides of the membrane. Animal cell membranes also contain cholesterol linking the fatty acids together and so stabilizing and strengthening the membrane. Mention the functions of cell membrane proteins Membrane proteins Integral Peripheral Span the Are attached to thickness of the one surface of cell membrane. the membrane. Function as: Function as: 1. Channels (or Enzymes pores). 2. Carrier proteins. 3. Receptors. (Guyton and Hall Textbook of Physiology. 13th ed. Chapter-2) Cell membrane carbohydrates: The carbohydrates forms a minor components (5-8%) They present on the outer surface of cell membranes. It either bound to phospholipids (glycolipids) or proteins (glycoproteins). Never present in the free form. Formed from monosaccharides and constitute a cell coat or glycocalyx outside the cell membrane. The glycocalyx is involved in protection and cell recognition, and antigens such as the ABO antigens on blood cells are usually cell-surface glycoproteins.  Functions of Membrane carbohydrates 1- Receptor 2- Cell Attachment 3- Immune Reactions  Transport through the cell membrane Passive Transport Weeee!!! cell doesn’t use energy 1. Diffusion 2. Facilitated Diffusion high 3. Osmosis low Active Transport cell does use energy This is 1. Primary and secondary active transport gonna be hard 2. Bulk or vesicular transport high work!! (Endocytosis, Exocytosis) low Passive transport: The molecules move across the membrane by their kinetic energy. It needs no energy. 3 Types of passive transport 1. Simple diffusion 2. Facilitated diffusion 3. Osmosis  Diffusion Definition: It means random movements of molecules from the area of high conc. to the area of low conc (It depends on the concentration Gradient) It continues until the molecules are equally distributed across the cell membrane Has 2 subtypes - Simple diffusion - Facilitated diffusion.  Simple diffusion: Definition: the movements of molecules or ions through a membrane opening without the necessity of binding with carrier proteins in the membrane. Occurs through two pathways: A) Lipid bilayer B) Protein channels DIFFUSION A) Diffusion through lipid bilayer: Lipid soluble substances: They are immediately dissolved in the membrane lipids and transported to the other side of the cell membrane. CO2, O2, N2. Water molecules: Although it is insoluble in the membrane lipids but it pass directly through the lipid bilayer. This is due to: 1. the small size of the water molecules and 2. their high kinetic energy. so, it penetrate the membrane very rapidly like bullets before the hydrophobic character of the lipids can stop them. Ions can not pass through the lipid bilayer because of their charges which: ❖react with water and form hydrated ions. ❖Interacts with the charges of the lipid bilayer resulting in their repulsion. B) Diffusion through the protein channels (For electrolytes): Characters of protein channels: 1.Selective permeability. 2.Open and close by gates. Types of channels: A. Classification according to their selective permeability: 1.Positively charged channels for negatively charged ions e.g. chloride ions. 2.Negatively charged channels for positively charged ions e.g. sodium and potassium channels. 3.Uncharged channels for water. B. Classification according to presence or absence of gate: 1. Leak channels: continuously open. 2. Gated channels: can open or close channels. a-Voltage gated channels: open and close by alternation in membrane potential. b- Chemical-gated channels: open and close by chemical.  Facilitated Diffusion (Carrier Mediated Diffusion) Definition: It is the process of passive transport of molecules across a cell's membrane via specific transmembrane integral proteins (carrier proteins). Molecules move down their concentration gradient; it does not require chemical energy. Large polar molecules such as glucose and amino acids are among the most important substances that transported across the cell membrane by facilitated diffusion. Mechanism 1- The transported molecule bind to specific site (receptor) in the protein channel, then 2- Conformational changes in the carrier protein, so the channel opens in the opposite side of the membrane. 3- The kinetic motion of attached molecule causes it to break away from its receptor site and to be released on the opposite side.  Facilitated diffusion is different from simple diffusion in: 1) The transport depend a carrier protein. 2) The rate of facilitated diffusion depend on the saturation of carrier protein while simple diffusion is directly proportional to the concentration difference. Characters of the carrier: 1. Specificity: Each carrier protein is specialized to transport a specific substance. 2. Saturation Limits: When all carrier proteins are saturated, the speed of transport reaches its maximum. This limit is known as the transport maximum (Tm). - Until Tm is reached, the substances rate of transport across the membrane is directly related to its concentration - When Tm is reached, the carrier is saturated and the rate of substance's transport across the membrane is maximal. Increase in the substance’s concentration is not accompanied by corresponding increase in the rate of transport. 3. Competition: Several closely related compounds may compete for a ride on the same carrier.  Factors affecting net rates of diffusion i-Permeability of the membrane: 1.Thickness of the membrane: inversely related to the rate of diffusion. 2. Lipid solubility of the substances in the cell membrane increases the quantity of molecules transported. 3. Number of protein channels: directly related to the rate of diffusion. 4. Temperature: the greater the temperature, the greater the thermal motion of the molecules and ions, so the diffusion increase. 5.The molecular diameter: the smaller the diameter, the greater the permeability of the channel. ii- Effect of concentration difference: The substance diffuses from high to low concentration. iii- The pressure difference: the molecules moves from the high- pressure side toward the low-pressure side. iv - Effect of electrical potential: if an electrical potential is applied across the membrane, because of their electrical charges, ions will move through the membrane even though no concentration difference exists to cause their movements. Factors affecting rate of diffusion Lipid solubility Molecular Molecular size & wt. Temperature Thickness of membrane Membrane related Surface area Concentration gradient Gradients Pressure gradient Electrical gradient  Fick’s law of diffusion – ΔC∙P∙A Q α ──── MW∙ ΔX Q = net rate of diffusion ΔC = conc. gradient of a substance P = permeability of membrane to the sub. A = surface area of a membrane MW = molecular wt. of sub. ΔX = thickness or distance  Osmosis Osmosis: This net diffusion of water down its concentration gradient through a selectively permeable membrane is known as osmosis. In other words, water moves by osmosis to the area of higher solute concentration. Osmotic pressure: an indication of the force with which pure water moves into that solution because of its solute concentration. The magnitude of this pressure depends on the number of solute particles present. A solution with a high concentration of nonpenetrating solute exerts greater osmotic pressure than a solution with a lower concentration of nonpenetrating solute does.  Semi-permeable membrane is permeable to water, but not to sugar Osmotic pressure in cells: Isotonic solution: A solution that contain an equal amount of solutes. It does not cause an osmotic flow. It has the same osmotic pressure with red blood cells. The red blood cells are suspended in the plasma(isotonic solution). Water enters and leaves cells in equal amounts. Therefore, their sizes remain unchanged. Hypotonic Solution: it contains a lower amount of solutes and in which Water will flow into the cell. - If RBCs are placed in hypotonic solution (lower osmotic pressure) more water enters than leaves. Therefore, the cells swells and may burst or rupture. Hypertonic Solution: it contain a higher concentration of solutes. The cell will lose water by osmosis.- If RBCs are placed in a hypertonic solution (higher osmotic pressure) more water leaves than enters. Therefore, the cells shrink. - Both of these conditions would damage the red blood cells and disrupt function. Normal Saline is 0.9 percent solution of sodium chloride (NaCl) What type of solution are these cells in? A B C Hypertonic Isotonic Hypotonic  Active transport It is the movement of molecules against concentration gradient from low to high concentration. Active transport uses cellular energy It depend on the presence of carrier protein. It is divided into two types according to the source of the energy used: 1. Primary active transport 2. Secondary active transport 1- Primary active transport: The energy derived directly from breakdown of Adenosine triphosphate (ATP) for example sodium potassium exchange pump. The carrier protein called ion pumps. e.g. 1. Sodium-potassium ATPase pump (all cells) 2. Ca++ ATPase pump (Cardiac muscle) 3. H+ ATPase pump (Stomach- renal tubules)  Primary active transport (sodium potassium exchange pump) Sodium ion concentrations are high in extracellular fluids but low in cytoplasm. Potassium is the opposite. The sodium potassium exchange pump ejects sodium ions and recaptures lost potassium. On average for each ATP molecule consumed, three sodium ions are ejected and two potassium ions are obtained by the cell. This demands 40 % of the ATP in a resting cell. Because of the counter active transport, the carrier protein is called an exchange pump.  Na+ -K + pump- electrogenic pump - Attachment of 2K+ on outer side & 3 Na+ on inner side Activation of ATPase 3Na+ Conformational change ATPa 2K+ Efflux of 3 Na+ & influx of 2K+ es Creates negativity inside the cell Helps in maintaining cell volume 1- Electrogenic 2- Energy used nature of the for secondary pump: It establish active Na+ and K+ concentration transport: gradient These The steep Na+ gradients are gradient is used Importance of critically important to provide in the ability of the sodium potassium energy for nerve and muscle pump secondary cells to generate electrical signals active transport. 3- It helps regulate cell volume by controlling the concentrations of the solutes inside the cell Primary active transport of hydrogen ions: At two places in the body, primary active transport of hydrogen ions is very important: 1- In the gastric gland of the stomach: this is the basis for secreting hydrochloride acid in the stomach digestive secretion. 2- In the late distal tubules and cortical collecting ducts of the kidneys Primary active transport of calcium ions: Cell membrane and pumps calcium to the outside of the cell. The other pumps calcium ions into one or more of the intracellular organelles of the cell, such as sarcoplasmic reticulum of the muscle cells and the mitochondria in all cells 2- Secondary active transport: The energy is derived indirectly from ionic concentration gradient that have been induced by primary active transport e.g. Na+ - glucose cotransporter Na+ - H+ exchanger  Vesicular transport  The movement of macromolecules such as proteins or polysaccharides into or out of the cell is called vesicular transport (bulk transport).  There are two types of vesicular transport, exocytosis and endocytosis, and both require the expenditure of energy (ATP).  EXOCYTOSIS: It is the process by materials are exported OUT OF THE CELL via secretory vesicles. It is important in expulsion of waste materials out of the cell and in the secretion of cellular products such as digestive enzymes or hormones. ENDOCYTOSIS: It is the process by which materials move INTO THE CELL. There are two types of endocytosis: 1. phagocytosis (cellular eating) for whole particle e. macrophages engulf and digest bacteria, 2. pinocytosis for macromolecules (cell drinking), 3. Receptor-mediated endocytosis: It is a process by which cells absorb metabolites, hormones, other proteins - and in some cases viruses - by the inward budding of plasma membrane vesicles containing proteins with receptor sites specific to the molecules being absorbed. It is highly selective process, used for the specific uptake of certain substances required by the cell in high concentrations (examples include LDL via the LDL receptor or iron via transferrin).  bacterium Pseudopodia internalization Phagoso- some Fusion Residual body absorption digestion Phagocytosis Exocytosis Endocytosis Channel vs. Carrier Proteins Channel proteins Carrier proteins selectively form open pores bind the small molecule to be through which transported and then undergo molecules of the a conformational change to appropriate size release the molecule on the (e.g., ions) can cross other side of the membrane. the membrane.  Types of carriers: a- Uniporters are carriers that transport a single particle in one direction, such as facilitated diffusion of glucose. b- Symporters transport two particles in the same direction, such as the Na co-transport of glucose and amino acids. c- Antiporters transport molecules in the opposite direction, such as the Na + -Ca ++ and Na+-H+ exchangers. Passive transport Active transport No expenditure of Expenditure of energy energy molecules mol. ( ATP ) Takes place along Can take place against conc., electrical, & conc. Gradient pressure gradient Carrier may or may Carrier is always not be required required Rate is proportional to Rate is proportional to conc. difference availability of carrier & energy. (Vmax) Simple Diffusion Facilitated Diffusion Passive transport Passive transport For small molecules For large molecules No carrier required Carrier mediated Rate of transport is Initially rate is directly proportional to proportional to conc. conc. gradient gradient till Vmax Examples – ( saturation of carriers) Lipid soluble – Examples – O2, CO2, alcohol glucose, amino acids Lipid insoluble – urea, Na+, K+