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This review will discuss the current understanding of the control of carbon partitioning from the cellular to whole-plant levels, focusing on (i) the pathways employed for phloem loading in source leaves, particularly in grasses, and the routes used in sink organs for phloem unloading; (ii) the transporter proteins responsible for sugar efflux and influx across plasma membranes; and (iii) the key enzymes regulating sucrose metabolism, signalling, and utilization. Search for related content PubMed PubMed citation Articles by Braun, D. Stomata open up during the day to let CO2 in and inadvertently let H2O escape Water vapor leaves the air spaces of the plant via the stomates This water is replaced by evaporation of the thin layer of water that clings to the mesophyll cells Remember, water has strong cohesive properties - as the water leaves, it is replaced by water clinging to the inside of the cell walls This creates a tension (pulling) on the water in the xylem and gently pulls the water toward the direction of water loss The cohesion of water is strong enough to transmit this pulling force all the way down to the roots Adhesion of water to the cell wall also aids in resisting gravity As we said before, the water column in the tallest trees can be 100m - the tension created by evaporation of water coupled with the cohesive and adhesive forces is enough to support this column against the forces of gravity All of this movement also flows down the water potential gradient between the soil, plant, and air . Articles by Wang, L. The Author 2013. Water flowing through the apoplast contains many minerals that the plant needs - it may also contains toxins and substances that the plant may not want. In an animal cell, water flows from hypotonic to hypertonic solutions, but in a plant cell, there is the added presence of the pressure created by the cell wall The combination of solute concentration differences and physical pressure are incorporated into the plant total water potential, abbreviated with the Greek letter psi ( which is also abbreviated as w) Water will flow through a membrane from a solution of high water potential to a solution of low water potential Water potential is measured in units of megapascals (MPa) Pure water has a water potential of 0 MPa (s = 0 MPa) The addition of solutes lowers water potential ( = -0.23 MPa for instance) Solute potential is the tendency of water to move by osmosis An increase in pressure (by lowering a piston for example) will raise water potential Pressure potential is the tendency of water to move in response to pressure These two forces combine to form the following equation: w =p +s w = total water potential p = water potential due to pressure May be positive or negative s = water potential due solute concentration (also known as Osmotic Potential) Always negative or zero Absorption of Water and Minerals by Roots Absorption is a surface area phenomenon - the more surface area there is, the more absorption there will be. While phloem loading mechanisms have been extensively studied in dicot plants, there is less information about phloem loading in monocots. This active transport lowers the water potential within the stele Water passively flows into the roots, pushing the water up against gravity Water that reaches the leaves is often forced out, causing a beading of water upon the leaf tips known as guttation In most plants, however, root pressure is not the primary mechanism for transporting the xylem Tall trees generate almost no root pressure (the weight of the water pushing down on the xylem more than counteracts any generated root pressure) The Control of Transpiration Water is needed for photosynthesis - it is also lost as a product of obtaining carbon by this very same process.