In the case of FLUID TRANSPORT THROUGH PHLOEM, phloem sap can't travel through the phloem conducting cells (sieve cells and/or sieve tube elements) unless the plant uses energy to move certain substances across cell membranes into the conducting cells. To understand this, let's have a look at the three ways substances can move across membranes:

  • Diffusion - a passive form of transport, molecules simply move across the membrane following a potential gradient.
  • Facilitated Diffusion - also passive, substances must pass through a protein "filter" in the membrane, though they are still following the potential gradient.
  • Active Transport - the cell uses energy (stored and "delivered" by ATP, adenosine triphosphate) to pump cells into or out of the cell AGAINST a gradient. "pips" than with molecules moving across membranes.

    A brief description of the magical PROTON PUMP...

  • a PROTON is a positively charged subatomic particle, one of the Big Three that make up matter.
  • When a hydrogen atom (nothing more than a proton (+) and an electron (-) whose electrical charges "cancel" each other out) loses its electron (which can happen for a variety of reasons; hydrogen atoms by themselves are not very stable), it becomes a PROTON (a.k.a. "Hydrogen Ion").

  • Without any energy input, the concentration of hydrogen ions inside and outside the cell would eventually be the same. You know the drill: it's the Second Law of Thermodynamics.

  • But lo. It is not so. Cells are alive, and one of the properties of living things is that they are able to control--to a greater or lesser degree--their internal environment. Cells can expend energy (in the form of a short-lived energy courier molecule named "ATP"--more on that one later) and actually PUSH hydrogen ions out of the cytoplasm, through the plasma membrane and into the extracellular spaces. This requires ENERGY, since it's essentially rolling those protons "UPHILL." The membrane machinery and the process itself are known as the PROTON PUMP

  • Eventually, this causes the outside of the cell to become crowded with hydrogen ions (relative to the inside of the cell), and also to be relatively more positively charged than the inside.

  • The effect is like pushing more and more water behind a dam. The water "wants" to push past the dam, but if the dam is strong enough, it can hold the water back. MOST dams, however, have gates that can be opened to varying degrees, allowing some water through.

  • Ever seen a hydroelectric plant at a waterfall? The falling water is in the process of changing its potential energy (which it got by being placed on the upper side of the waterfall, by the sun as vapor and then as rain) into KINETIC energy. In the process, we use that energy of falling water to turn big turbines, which then convert that water's kinetic energy into ELECTRICAL ENERGY!

  • Pretty much the same thing is happening at the cellular level. Those crowded protons (like the water behind the dam) "want" to pour back into the cell against the plasma membrane (the dam). And the plasma membrane has gates, too. They are the protein channels that allow only *certain* types of molecules into the cell. And many of those protein channels use those eager protons bumping around at the gates to actually "carry" various types of substances into the cell.

  • As a proton zips through the open gate, its potential energy changes to KINETIC energy (like the water coming through the dam), and that energy can be packaged by the cell, not as electricity, but as the chemical bonds of ATP!

    The crowd of hydrogen ions outside the membrane results in the outside of the cell being more positively charged than the inside. This difference is called a MEMBRANE POTENTIAL, and--as described above--it can be used to do controlled work, such as capture energy and store it in the bonds of ATP.

    The membrane potential can be used to do work such as bringing positively charged ions into the cell following the gradient.

    Negatively charged ions can also be brought into the cell via COTRANSPORT: some proteins carry not only hydrogen ions, but also can carry a particular negative ion (such as nitrite or nitrate) into the cell, using the membrane potential energy (the happily inrushing protons) to do the work.

    This is how SUGARS are loaded into plant cells.

    Phloem sap is a thick solution containing up to 30% sugars (sucrose), amino acids, hormones etc.

    (in contrast, xylem sap is relatively thin and watery--contains mostly dissolved inorganics)

    Plants need to mobilize stored carbohydrates in order to perform cellular work via cellular respiration:

    1. convert starches/stored carbs into simple sugars

    2. load simple sugar (usually sucrose) into phloem

    3. transport sugar to wherever it needs to go

    Source: any location where sugar is either produced or stored

    Sink: location where sugar is used

    How do we get the sugars from the source to the sink?

    1. load sugar into sieve elements at the source and into the phloem via either of several pathways:

    a. apoplastic pathway: water travels along the outside of the cell walls (apoplast is the nonliving continuum formed by cell walls touching each other, creating a matrix)

    b. symplastic pathway: water travels from protoplast to protoplast via plasmodesmata (symplast is the cytoplasmic continuum formed by plasmodesmata)

    c. tonoplastic (transcellular) pathway: water travels from cell to cell by passing from vacuole to vacuole (tonoplast is the vacuole membrane)

    The driving force causing the water movement is, of course, water potential!

    Transfer cells (recall these specialized types of parenchyma cells) facilitate the movement of water/solutes from apoplast to symplast and vice versa.

    * Because sugar accumulating in phloem transport cells may concentrate sugar 2-3 times what it is in regular mesophyll, ATP is needed to load the sugar to run the proton pumps. Transfer cells are very metabolically active!

    * Sucrose is cotransported into sieve tubes by transport proteins (see Figures in Chapter 32).

    * Phloem sap can move at a rate of 1m/hour, which is too fast for simple diffusion. It's moved via bulk flow: differences in pressure at opposite ends of a conduit cause movement with the potential gradient.

    * As sugar concentration rises in certain areas of the phloem (sink), water potential drops. This causes water from areas of higher water potential to flow into the sieve tube elements.

    * Result: water flows under pressure, somewhat like water through a hose!

    * Potential gradient goes from source to sink.

    * Once the sugars are unloaded, the water can diffuse into the xylem and be carried throughout the plant, with xylem sap.

    ************************************