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Lecture 4


8 Pages

Biological Sciences
Course Code
Connie Soros

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Plant Physiology- Lecture 4  The biologically regulated movement of molecules and ions from one location to another is known as transport  Plants exchange solutes with their environment and among their tissues and
 organs  Both Iocal and long-distance transport processes in plants are controlled largely by cellular membranes  Transport must occur across the plasma membrane, which is a thin layer of two lipids (lipid bilayer, each layer is hydrophilic towards the outside and hydrophobic towards the inside, forming a hydrophobic barrier to diffusion)  The plasma membrane also detects information about the physical environment, about molecular signals from other cells, and about the presence of invading pathogens and often relays these signals by changes in ion flux across the membrane  Remember that plants, unlike animals and some other organisms are immobile and are highly dependent on cell to cell signaling to trigger defense mechanisms against changes within the plant, within the environment, presence of pathogens, etc. since they can not just get up and move, hide, or run to shelter PASSIVE AND ACTIVE TRANSPORT  Passive transport is spontaneous movement (diffusion) of a solute across a membrane in the direction of a gradient of electrochemical potential (from higher to lower potential), also known as downhill transport - Once equilibrium is reached, no further net movements of solutes can occur unless there is the addition of a driving force  Active transport is the use of energy to move a solute across a membrane against a concentration gradient, a potential gradient, or both (electrochemical potential), also known as uphill transport - Active transport is not spontaneous, because it requires energy to be applied A common way of applying energy is to couple the transport to the hydrolysis of ATP HYDROLYSIS OF ATP  ATP hydrolysis is a reaction in which chemical energy that has been stored and transported in the high-energy phosphoanhydridic bonds in ATP (Adenosine triphosphate) is released by water - The product is ADP (Adenosine diphosphate) and an inorganic phosphate, orthophosphate (Pi)  It is not simply breaking a bond, the bond is hydrolysed (hydro=water, lysis=to separate), a water molecule has to come in to break this bond  Because the water molecule comes in, some bonds are formed, as well as the bond being broken  So it’s not just a case of energy being required to break bonds; energy is also released by the bonds that are formed  Hydrolysis of the phosphate groups in ATP is especially exemonic (energy releasing), because the resulting orthophosphate group is greatly stabilized, making the products (ADP and PJ much lower in energy than the reactant (ATP)  The negative charge density associated with the three adjacent phosphate units of ATP also destabilizes the molecule, making it higher in energy  Hydrolysis relieves some of these electrostatic repulsions as well, liberating useful energy in the process  ATP hydrolysis is the final link between the energy derived from sunlight and useful work such as the establishment of ion gradients across membranes, and biosynthetic processes necessary to maintain life TRANSPORT OF IONS ACROSS MEMBRANE BARRIERS  The extent to which a membrane permits or restricts movement of a substance is called membrane permeability  The permeability depends on both the composition of the membrane and on the chemical nature of the solute CATION AND ANIONS AND DIFFUSION POTENTIAL  When salts diffuse across a membrane, an electrical membrane potential (voltage) can develop (see Fig. 6.2)  If two salt solutions of different concentrations are separated by a membrane, the different ions of the salt will permeate the membrane differently (unless the membrane is very porous) as they diffuse down their respective gradients of electrochemical potential (ie. They each will have different permeabilities.) - For example if the concentration of [KCl] on side A is higher than on side B
 the potassium and chloride ions will diffuse into side B - If the membrane is more permeable to potassium (K+) than to chloride (CI-), the potassium ions will diffuse faster than chloride ions causing the cell to develop a negative electrical charge with respect to the extracellular medium and a charge separation (+ and -) will develop, resulting in the establishment of a diffusion potential - As the system comes to equilibrium, the concentration gradient and diffusion potential will both disappear  The Nernst equation is an equation that predicts the electrical potential at which a charged ion will be in equilibrium across a membrane, as a function of the relative concentrations of the ion on each side  The values in Table 6.1 (oversimplified) compare the predicted ion concentrations calculated by substituting the concentration of each ion in the external bathing solution and the measured membrane potential into the Nernst equation  These values are compared to the observed experimental measurements of ion concentrations at steady state in pea root cells  For all the ions measure only K+ is near equilibrium  The anions NO ,3Cl , H 2O 4- and SO 42-all have higher internal concentrations that what was predicted  This would indicate that there is active transport moving these anions across the membrane  The cations Na , Mg 2+ and Ca2+ all have internal concentrations that are lower than those predicted by the Nernst equation  These cations enter the cell by diffusion but are then actively exported back out  Ion concentrations in the cytosol and vacuole are controlled by passive and active transport processes  Control of the ion concentrations in the cytosol is important for the regulation of metabolic enzymes  The cell wall surrounding the plasma membrane is not a barrier to permeability (le. Very permeable) and therefore is not a factor in solute transport.  To summarize the results of a number of studies on the green algae Chara and Nitella (perfect model organisms for this type of study because the cells are several inches long and contain a lot of cytoplasm in comparison to higher plants) and many higher plants: + - Potassium ions (K ) are accumulated passively by both the cytosol and the vacuole (when + + extracellular K concentrations are very low, K may be taken up actively - Cations, Sodium (Na ) and Calcium (Ca ) are pumped actively out of the cytosol into the extacellular space and the vacuole, at both the plasma membrane and the tonoplast + - Excess protons (H ) which can be generated by intermediary metabolism are also actively pumped out of the cytoplasm (this process helps to maintain a neutral pH in the cytoplasm, the vacuole and extracellular medium tend to be more acid by one or two pH untis) - - - - Anions Chloride(Cl ), Nitrate (N3 ) and Hydrogen phosphate (H 2O )4are taken up actively into the cytosol MEMBRANE TRANSPORT PROCESSES  There are three classes of
 membrane transport proteins: channels carriers and pumps  Scientists first discovered these when they did permeability studies comparing artificial phospholipid bilayers with that of biological membranes - Scientists first discovered these when they did permeability studies comparing artificial phospholipid bilayers with that of biological membranes - They found that although biological and artificial membranes have similar permeabilities to nonpolar molecules and many small polar molecules, biological membranes are much more permeable to ions, some large polar molar molecules as sugars, and to water than artificial bilayers - This is because of the presence of transport proteins that facilitate the passage of selected ions and in biological membranes CHANNELS  Channels are transmembrane proteins that enhance and greatly increase passive diffusion across membranes.  The proteins function as selective pores through which molecules or ions can diffuse across the membrane  Channels are selective for different densities and surface charges on ions in addition to having different pore sizes.  Transport through these channels is always passive  Usually these channels do not involve selective
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