Passive and Active Transport

Chapter 1 : Cellular Biology Introduction to Pathophysiology Passive transport ● Water and small, electrically uncharged molecules can move easily through pores in the plasma membrane's lipid bilayer ● Passive transport occurs naturally through any semipermeable barrier ● Molecules flow from an area of high concentration to an area of low concentration Active transport ● Some molecules are too large to pass through pores or are ligands bound to receptors on the cell's plasma membrane ● Active transport requires the cell to expend metabolic energy ● Movement of a solute against its concentration gradient occurs by specialized transporters called pumps ● Pumps harness an energy source such as ATP hydrolysis, a transmembrane ion gradient, or sunlight Membrane transport proteins ● Specialized membrane transport proteins are present in all cell membranes ● Transport proteins allow selective passage of molecules ● The two main classes of membrane transport proteins are transporters and channels ● Transporters are specific and undergo conformational changes to enable membrane transport ● Channels form a pore across the lipid bilayer and allow ions and selective polar organic molecules to diffuse across the membrane Cell Volume Regulation ● Cells regulate volume by controlling leaks through pores or protein channels ● Maintaining ionic concentration gradients is necessary for cellular excitation and membrane conductivity ● Intracellular K+ concentrations are important for enzyme activity, including protein synthesis Transport of Macromolecules ● Large molecules and fluids are transported by endocytosis (taking in) and exocytosis (expelling) ● Receptor-macromolecule complexes enter the cell through receptormediated endocytosis Active Transport ● Pumps carry out active transport through three ways: coupled pumps, ATPdriven pumps, and light-driven pumps ● Coupled pumps link the uphill transport of one solute to the downhill transport of another solute ● ATP-driven pumps drive uphill transport through the hydrolysis of ATP ● Light-driven pumps use energy from sunlight to drive uphill transport and are mainly found in bacteria Mediated Transport ● Mediated transport systems can move solute molecules singly or two at a time ● Symport involves the simultaneous movement of two molecules in one direction, while antiport involves movement in opposite directions ● Uniport involves the movement of a single molecule in one direction Electrolytes and Nonelectrolytes ● Body fluids are composed of electrolytes, which dissociate into ions in solution ● Nonelectrolytes, such as glucose and urea, do not dissociate ● Electrolytes make up approximately 95% of solute molecules in body water ● Ions exhibit polarity and are categorized as cations (positively charged) or anions (negatively charged) Electrolyte Balance ● Na+ is the predominant extracellular cation, while K+ is the principal intracellular cation ● The difference in intracellular and extracellular concentrations of these ions is important for electrical impulses in nerve and muscle cells ● Electrolytes are measured in milliequivalents per liter (mEq/L) or milligrams per deciliter (mg/dL) Passive Transport ● Diffusion is the movement of solute molecules from an area of greater concentration to an area of lesser concentration ● Filtration and osmosis are also forms of passive transport Concentration gradient and diffusion ● Particles in a solution move randomly in any direction ● If the concentration of particles in one part of the solution is greater than that in another part, the particles distribute themselves evenly throughout the solution ● If the concentration of particles is greater on one side of a permeable membrane, the particles diffuse spontaneously from the area of greater concentration to the area of lesser concentration until equilibrium is reached ● The diffusion rate is influenced by differences of electrical potential across the membrane, as well as the size and lipid solubility of the substance ● Water-soluble substances and ions diffuse slowly, while nonpolar molecules and lipid-soluble substances diffuse rapidly Filtration and hydrostatic pressure ● Filtration is the movement of water and solutes through a membrane due to a greater pushing pressure on one side ● Hydrostatic pressure is the mechanical force of water pushing against cellular membranes ● Hydrostatic pressure is partially balanced by osmotic pressure, which tends to pull water back into the capillaries