Membrane Transport 1 PDF

## Transport Across Cell Membrane ### I.L.O.S - List the major cation intra-and extracellular. - List the different mechanisms of transport across the cell membrane. - Compare the different membrane transport mechanism. - Demonstrate the mechanism of action and the physiological importance of sodi...

## Transport Across Cell Membrane ### I.L.O.S - List the major cation intra-and extracellular. - List the different mechanisms of transport across the cell membrane. - Compare the different membrane transport mechanism. - Demonstrate the mechanism of action and the physiological importance of sodium potassium pump. - Demonstrate the physiological importance of other primary active carriers. - Identify the properties of secondary active transport and recognize the types of its carriers. - Identify methods of macromolecules transport across the cell membrane. - Describe the forces affecting water transport across the cell membrane: osmosis, and filtration. - Apply the concept of the effective osmotic pressure to explain edema. - Interpret the effect of plasma osmolarity changes on cell volume. ### Introduction Electrolytes are substances that dissociate in solution and can conduct an electrical current. These substances are in the extracellular and intracellular fluid. Within the extracellular fluid, the major cation is sodium, and the major anion is chloride. The major cation in the intracellular fluid is potassium. These electrolytes play an important role in maintaining homeostasis. ### Classification of transport across the cell membrane The transport of across the cell membrane is classified into two main mechanisms: #### Passive transport - Solutes move from high concentration to low concentration (Downhill) - Needs no energy. #### Types - Simple diffusion - Diffusion through ion channels - Facilitated diffusion #### Active transport - Solutes move against concentration gradient from the low concentration to the high concentration (Uphill). - Needs energy (ATP). #### Types - Primary active transport - Secondary active transport ### 1. Passive transport “diffusion” **Definition**: It is a passive process by which a substance in solution expands to fill all the available volume. **Mechanism**: It is a passive continuous random kinetic movement of the molecules downhill From High concentration to Low concentration consuming no energy. **Factors affecting Passive transport “diffusion rate”**: Diffusion rate is directly proportionate to: Fick’s law - Concentration and electrical gradient (electrochemical gradient) - Lipid solubility of the substance. - Temperature: Heat increases the motion of molecules - Membrane surface area - Membrane permeability Diffusion rate is inversely proportionate to: - Molecular weight of diffusing substances - Thickness of the membrane: **There are three mechanisms of “Passive transport”** 1. Simple diffusion. 2. Facilitated diffusion. 3. Diffusion through ion channels. **A-Simple diffusion**: Simple diffusion for lipid and water-soluble molecules: - **Lipid soluble**: as alcohol, oxygen, N2 pass easily across cell membrane. - **Water soluble**: small water-soluble uncharged molecules are of high kinetic energy, passes easily and penetrates the membrane like bullet. **B- Facilitated Diffusion**: Large water-soluble uncharged molecules as glucose diffuse slowly and so need a carrier (Facilitated diffusion) **Characteristics**: It is the process of diffusion using carrier proteins that move substances in the direction of their chemical or electrical gradients and no energy input is required. Facilitated diffusion has a Transport maximum which is the maximum amount of a substance which can be transported per unit of time. It is determined by number and activity of the carrier which are affected by hormones, e.g. Insulin hormone increases the number of glucose transporters. **C- Diffusion through Ion channels**: Charged molecules as ions are transported through ion channels figl, and are diffuse slowly due to: - The formation of hydrated ions with water, which have large size. - The interaction between their charges and the charges on the cell membrane. Ion channels are selective passage ways for simple diffusion of ions down their electrochemical gradients across cell membranes. **Types of ion channels**: - **Leakage channels**: Always open. - **Gated channels**: Ion channels that have gates (Doors) that can change their shape to open or to close the channel in response to various signals. - **Ligand gated ion channels**: that open or close in response to a ligand as a neurotransmitter. e.g.: ligand gated sodium channels. - **Voltage gated ion channels**: that open or close due to alterations in membrane voltage. e.g.: voltage gated sodium channels, voltage gated sodium channels. ### 2. Active transport It is the form of transport that requires energy to transport the molecule against concentration gradient (from low concentration to high concentration). It is either primary or secondary active transport. **Requirements:** a. Source of energy (ATP). b ATPase enzyme for hydrolysis of ATP. c. Carrier protein **1) The Primary active transport** such as: - Na*- K+ pump - Ca++ ATPase pump - H+- K* pump #### Sodium-Potassium Pump: **Site**: in all cell membranes **Mechanism**: Pump 3 sodium ions (positive charges) outside the cell for only 2 potassium ions into the cell using one ATP molecule hydrolysis **ATP-----ADP+ P +Energy** **Functions**: Maintain cell volume and share in Resting Membrane Potential "RMP" establishment: because it is an electrogenic pump. It also Provide energy for secondary active transport. #### Resting membrane potential: - Difference in electric potential across the membrane at rest. - Inside the cell is negative relative to outside #### Calcium ATPase pump: **Site**: Excitable cells On the cell membrane to pump calcium ions out, or on the sarcoplasmic reticulum surface to pump calcium back to it. **Function**: Primary active pump (ATP ase) to keep intracellular calcium at extremely low concentration in the intracellular cytosol for relaxation of the skeletal and cardiac muscle. #### H+- K+ pump: **Site**: Stomach **Function**: Help in HCL formation in gastric cavity **2) Secondary (indirect) active transport**: The electrochemical gradients set up by primary active transport store energy, which can be released as the ions move back down their gradients. Secondary active transport uses the energy stored in these gradients to move other substances against their own gradients. As an example, let’s suppose we have a high concentration of sodium ions in the extracellular space (thanks to the hard work of the sodium-potassium pump). If a route such as a channel or carrier protein is open, sodium ions will move down their concentration gradient and return to the interior of the cell. In secondary active transport, the movement of the sodium ions down their gradient is coupled to the uphill (against electrochemical gradient) transport of other substances by a shared carrier protein (a cotransporter). For instance, in the figure 3, a carrier protein lets sodium ions move down their gradient, but simultaneously brings a glucose molecule up its gradient into the cell. The carrier protein uses the energy of the sodium gradient to drive the transport of glucose molecules. ### There are two Types of carriers in Secondary active transport: 1. **Symport carrier “Co-transport”:** In the same direction. Such as Sodium Glucose co-transport. The binding of glucose and Na+ to the carrier binding sites triggers the conformational change in the protein molecule to transport both particles to the cell interior. ### Site: renal tubular cells, intestinal cells. 2. **Antiport carrier “Counter-transport” :** In opposite direction Such as Sodium -Calcium counter transport, and Sodium-Hydrogen counter transport The binding of Na+ ions to the carrier binding sites at the cell outer surface is simultaneously accompanied by binding of Ca+ or H+ ions to the carrier at the cell interior. **Site**: Sodium -hydrogen exchanger in the renal tubular cells and Sodium-calcium exchanger in the cardiac muscle. | | Primary active transport | Secondary active transport | |---|---|---| | Energy source | Energy is derived from the direct breakdown of the ATP | Energy is derived from the sodium concentration gradient that was created by the Na+-K+ ATPase pump | | Carrier type | A single molecule is transported by (uniport carrier) More complicated primary active-transport mechanisms involve the transfer of two different passengers in opposite directions, the most important example being the Na-K pump. | Two molecules are transported at the same time by: Symport carrier: If they are moving in the same direction Antiport carrier: if they are moving in opposite directions | | Examples | Na+ - K+ pump. Ca++ ATPase pump H+ - K pump | **Symport carriers:** Sodium-Glucose coTransporter Sodium-dependent amino acid transporters   **Antiport carriers:** Sodium calcium exchanger Sodium hydrogen exchanger |