Chapter 36 Transport in Vascular Plants Overview Pathways for Survival For vascular plants the evolutionary journey onto land involved the differentiation of the plant body into roots which absorb water and minerals from the soil and shoots which absorb light and atmospheric CO2 for photosynthesis between roots and shoots Xylem transports water and minerals from the roots to the shoots This morphological solution created a new problem the need to transport materials Phloem transports sugars from the site of production to the regions that need them for growth and metabolism Concept 36 1 Physical forces drive the transport of materials in plants over a range of distances The uptake and loss of water and solutes by individual cells such as root hairs Short distance transport of substances from cell to cell at the level of tissues or Transport in plants occurs on three scales 1 2 organs such as the loading of sugar from photosynthetic leaf cells into the sieve tubes of phloem 3 plant Long distance transport of sap within xylem and phloem at the level of the whole Selective Permeability of Membranes A Review The selective permeability of a plant cell s plasma membrane controls the movement of solutes into and out of the cell Molecules tend to move down their concentration gradient Diffusion across a membrane is called passive transport and occurs without the direct expenditure of metabolic energy by the cell Active transport is the pumping of solutes across membranes against their electrochemical gradients the combined effects of the concentration gradient and the solute and the voltage charge difference across the membrane The cell must expend metabolic energy usually in the form of ATP to transport solutes uphill Transport proteins embedded in the membrane are needed for the movement of solutes across the membrane Some transport proteins bind selectively to a solute on one side of the membrane and release it on the opposite side Others act as selective channels providing a selective passageway across the membrane For example the membranes of most plant cells have potassium channels that allow potassium ions K to pass but not other ions such as sodium Na Some channels are gated opening or closing in response to certain environmental or biochemical stimuli proton pump The most important active transport protein in the plasma membrane of plant cells is the The Central Role of Proton Pumps It hydrolyzes ATP and uses the released energy to pump hydrogen ions H out of the cell This results in a proton gradient with a higher H concentration outside the cell than inside Because the proton pump moves positive charges in the form of H out of the cell the pump also contributes to a voltage known as a membrane potential a separation of opposite charges across a membrane Makes inside of plant cell negative in charge relative to outside This voltage is called a membrane potential because the charge separation is a form of potential stored energy that can be harnessed to perform cellular work This potential energy is used to drive the transport of many different solutes For example the membrane potential generated by proton pumps contributes to the uptake of potassium ions K by root cells The proton gradient also functions in cotransport in which the downhill passage of one solute H is coupled with the uphill passage of another such as NO3 or sucrose called chemiosmosis a unifying process in cellular energetics The role of proton pumps in transport is a specific application of the general mechanism In chemiosmosis a transmembrane proton gradient links energy releasing processes to energy consuming processes The ATP synthases that couple H diffusion to ATP synthesis during cellular respiration and photosynthesis function somewhat like proton pumps However proton pumps normally run in reverse using ATP energy to pump H Effects of Differences in Water Potential The survival of plant cells depends on their ability to balance water uptake and loss The net uptake or loss of water by a cell occurs by osmosis the passive transport of water against its gradient across a membrane In the case of a plant cell the direction of water movement depends on solute concentration and physical pressure due to the cell wall The combined effects of solute concentration and pressure are called water potential represented by the Greek letter psi Water will move across a membrane from the solution with the higher water potential to the solution with the lower water potential For example if a plant cell is immersed in a solution with a higher water potential than the cell osmotic uptake of water will cause the cell to swell By moving water can perform work such as expanding the cell Therefore the potential in water potential refers to the potential energy that can be released to do work when water moves from a region with higher to lower Plant biologists measure in units called megapascals MPa where one MPa is equal to about 10 atmospheres of pressure An atmosphere is the pressure exerted at sea level by an imaginary column of air about 1 kg of pressure per square centimeter Plant cells exist at approximately 1 MPa How Solutes and Pressure Affect Water Potential Both pressure and solute concentration affect water potential The combined effects of pressure and solute concentrations on water potential are incorporated into the following equation where p is the pressure potential and s is the solute potential or osmotic potential p s Pressure potential is the physical pressure on a solution can be positive or negative relative to the atmospheric pressure Water in living cells is usually under positive pressure The cell contents press the plasma membrane against the cell wall producing turgor pressure The solute potential s or osmotic potential of a solution is proportional to the number of dissolved solute molecules By definition the solute potential of pure water is 0 The addition of solutes lowers the water potential because the solutes bind water molecules which have less freedom to move than they do in pure water Any solution at atmospheric pressure has a negative water potential and adding solutes always lowers the water potential Quantitative Analysis of Water Potential Water potential affects the uptake and loss of water in plant cells In a flaccid cell p 0 and the cell is limp If this cell is placed in a solution with a higher solute concentration and therefore a lower water will leave the cell by osmosis Eventually