32.1. Uptake of Water and Minerals  

        1. Minerals follow the same path as water.
            a. Some mineral ions diffuse in between the cells.
            b. Because of the impermeable Casparian strip, water must eventually enter the cytoplasm of endodermal cells.
            c. Water can move directly into cytoplasm of root hair epidermal cells and is transported across
                cortex and endodermis.
            d. In contrast to water, minerals are actively taken up by plant cells.
            e. Mineral nutrient concentration in roots may be 10,000 times more than in surrounding soil.
            f. During transport throughout a plant, minerals can exit xylem and enter cells that require them.
        2. Mineral ions cross plasma membranes by a chemiosmotic mechanism.
            a. Plants absorb minerals in ionic form: nitrate (NO3-), phosphate (HPO4-) and potassium ions (K+);
                all have difficulty crossing a charged plasma membrane.
            b. It has long been known plants expend energy to actively take up and concentrate mineral ions.
            c. Proton pump hydrolyzes ATP to transport H+ ions out of cell; this sets up an electrochemical gradient
                that causes positive ions to flow into cells.
            d. Negative ions are carried across the plasma membrane in conjunction with H+ ions as H+ ions diffuse
                down their concentration gradient.

    E. Adaptations of Roots for Mineral Uptake

        1. Two symbioticrelationships are known to assist roots in acquiring nutrients.
        2. Legumes have nodules infected with the bacterium Rhizobium.
            a. Plants cannot use atmospheric nitrogen because they lack enzymes to break the tripple N bonds.
            b. Rhizobium makes nitrogen compounds available to plants in exchange for carbohydrates.
            c. Bacteria live in root nodules-structures on plant roots that contain nitrogen-fixing bacteria.
            d. Rhizobial bacteria reduce atmospheric nitrogen (N2) to ammonium (NH4+) (nitrogen-fixation).
            e. Other plants have a relationship with free-living, nitrogen-fixing microorganisms in soil.
        3. Most plants have mycorrhizae; those lacking mycorrhizae are limited in where they can grow.
            a. Mycorrhizae are a mutualistic symbiotic relationship between soil fungi and plant roots.
            b. The fungal hyphae may enter the cortex of roots but do not enter plant cells.
            c. Ectomycorrhizae form a mantle exterior to the root, and they grow between cell walls.
            d. Fungus increases surface area for mineral and water uptake and breaks down organic matter.
            e. In return the root furnishes fungus with sugars and amino acids..
            f. Orchid seeds are small with limited nutrients; they germinate only when invaded by mycorrhizae.
            g. Nonphotosynthetic plants (e.g. Indian pipe) use mycorrhizae to extract nutrients from nearby trees.
        4. Some plants have poorly developed or no roots; other mechanisms supply minerals and water.
            a. Epiphytes take nourishment from air; their attachment to other plants gives them support.
            b. Parasitic plants (e.g., dodders, broomrapes, pinedrops) send out haustoria (root-like projections)
                that grow into host and tap into xylem and phloem of host.
            c. Venus's-flytrap and sundew obtains nitrogen and minerals as leaves capture and digest insects.

32.2. Transport Mechanisms in Plants

    A. Transport Tissues

        1. Vascular plants have transport tissues as an adaptation to living on land.
        2. Xylem vascular tissue passively conducts water and mineral solutes from roots to leaves; it contains
            two types of conducting cells: tracheids and vessel elements.
            a. Tracheids are hollow, nonliving cells with tapered overlapping ends; thinner and longer than vessel
                elements; water crosses end and sidewalls because of pits in secondary cell wall.
            b. Vessel elements are hollow, nonliving cells that lack tapered ends; wider and shorter than tracheids;
                lack transverse end walls; form a continuous pipeline for water and mineral transport.
        3. Phloem is vascular tissue that conducts organic solutes in plants mainly from leaves to roots; contains
           sieve-tube cells and companion cells.
            a. Sieve-tube cells lack a nucleus, are arranged end to end and have channels in end walls (thus,
                the name "sieve-tube") through which plasmodesmata extend from one cell to another.
            b. Companion cells connect to sieve-tube cells by numerous plasmodesmata, are smaller and more
                generalized than sieve-tube cells; they have a nucleus.
        4. These transport systems rely on mechanical properties of water.
            a. Diffusion moves molecules from higher to lower concentrations.
            b. Water potential considers both water pressure and osmotic pressure.
        5. They also rely on chemical properties of water: polarity of water and hydrogen bonding.

    B. Water Potential

        1. Water flows from a region of higher water potential (the potential energy of water) to a region of
            lower water potential.
        2. Water potential is a measure of the capacity to release or take up water; in cells, water potential
            includes the following:
            a. Pressure potential, the effect that pressure has on water potential; water will move from a region
                of higher pressure to a region of lower pressure; and
            b. Osmotic potential, the effect that solutes have on water potential; water tends to move by osmosis
                from an area of lower solute concentration to area of higher solute concentration.
        3. Water flows by osmosis into a plant cell with greater solute concentration than a surrounding solution.
            a. As water enters, pressure increases inside cell; strong plant cell wall allows water pressure to build up.
            b. Pressure potential inside cell increases and balances osmotic potential outside cell; water stops entering.
            c. Turgor pressure is pressure potential that increases due to process of osmosis; critical to plants,
                since plants depend on it to maintain turgidity of their bodies.
        4. Wilted plant cells have insufficient turgor pressure and the plant droops.

    C. Water Transport

        1. Movement of water and minerals in a plant involves entry into roots, xylem, and leaves.
        2. Water and minerals enter root cells before they reach xylem by two routes already described.
        3. Water entering root cells creates a positive pressure called root pressure.
            a. Root pressure (primarily at night) tends to push xylem sap upward in plant.
            b. Guttation is appearance of drops of water along the edge of leaves, as a result of water being forced
                out of leaf vein endings; it is result of root pressure.
            c. Root pressure is not a sufficient mechanism for water to rise to the tops of trees.

    D. Cohesion-Tension Model of Xylem Transport

        1. Water and dissolved minerals must be transported upward from roots to xylem.
        2. Cohesion-tension model states that transpiration creates a tension (i.e., a negative pressure) that pulls
            water upward in xylem.
        3. Water molecules are cohesive with one another, adhesive with xylem walls. (Fig. 37.9)
        4. Transpiration is a plant's loss of water to atmosphere through evaporation at leaf stomates.
        5. Cohesion is tendency of water molecules to cling together, a result of their forming hydrogen bonds.
        6. Adhesion is ability of water (a polar molecule) to interact with molecules comprising walls of xylem
            vessels; gives a water column extra strength and prevents it from slipping back down.
        7. In daytime, negative water potential created by transpiration extends from leaves to roots; water
            column must be continuous.
        8. If a water column within xylem is broken by cutting a stem, the water column will drop back down the
            xylem vessel away from site of breakage, making it more difficult for conduction to occur.
        9. At least 90% of the water taken up by roots is lost through stomates by transpiration.
        10. With plenty of water, stomates remain open, allowing CO2 to enter leaf and photosynthesis to occur.
        11. Under water stress, more water is lost through a leaf than can be brought up and stomates close.
        12. Photosynthesis requires CO2 enter the leaf; there must be sufficient water so stomates can remain open
            and allow CO2 to enter.

    E. Opening and Closing of Stomates

        1. Each stomate has two guard cells with a pore between them.
        2. Stomates open from turgor pressure when guard cells take up water; when they lose water, turgor
            pressure decreases and stomates close.
        3. Guard cells are attached to each other at their ends; inner walls are thicker than outer walls.
        4. Radial expansion is prevented by cellulose microfibrils in the walls but outer walls can expand lengthwise.
        5. As they take up water, they buckle out, thereby creating an opening between cells.
        6. Since 1968, it has been known that when stomates open, there is accumulation of K+ ions in guard cells.
        7. A proton pump run by breakdown of ATP to ADP and P transports H+ outside the cell; this establishes
            an electrochemical gradient allowing K+ to enter by way of a channel protein.
        8. Blue-light component of sunlight is a signal that can cause stomates to open.
            a. There is evidence that flavin pigments absorb blue light.
            b. This pigment sets in motion a cytoplasmic response activating the proton pump that causes K+ ions to
                accumulate in guard cells.
        9. Evidence suggests a receptor in the plasma membrane of guard cells brings about inactivation of the proton
            pump when CO2 concentration rises, as happens when photosynthesis ceases.
        10. Abscisic acid (ABA) produced by cells in wilting leaves, also causes stomates to close; photosynthesis
            cannot occur but water is conserved.
        11. In plants kept in dark, stomates open and close on a 24-hour basis as if responding to sunlight in daytime
            and absence of sunlight at night; some sort of internal biological clock must keep time.

    F. Organic Nutrient Transport  

        1. Marcello Malpighi (1679) suggested bark transferred sugars from leaves to roots.
            a. He observed results of removing a strip of bark from a tree (girdling).
            b. Bark swells just above the cut and sugar accumulates in swollen tissue.
            c. Today, we know phloem was removed but xylem remained; therefore, phloem transports sugars.
        2. Radioactive tracer studies using C-14 confirm phloem transports organic nutrients.
            a. When C-14 -labeled carbon is supplied to mature leavves, radioactively labeled sugar moves to roots.
            b. Similar studies confirm phloem transports amino acids, hormones, and mineral ions.
        3. Aphids used in study
            a. It is difficult to take samples of sap from just phloem cells without injuring phloem.
            b. Aphids (small insects) drive their mouth stylets into a sieve-tube cell; samples are easily taken.
            c. The aphid body is cut off; the stylet becomes a small needle from which phloem is collected.)
            d. Such research indicates sap can move through phloem from 60-100 cm per hour or more.

    G. Pressure-Flow Model of Phloem Transport  

        1. Pressure-flow model explains transport of sap through sieve tubes by a positive pressure potential.
        2. Buildup of water creates a positive pressure potential within sieve tubes that moves water and sucrose
            to a sink (e.g., at the roots).
        3. Pressure exists from leaves to roots; at roots, sucrose is transported out and water follows.
        4. Consequently, the pressure gradient causes a flow of water from leaves to roots.
        5. Conducting cells of phloem are sieve-tubes lined end to end.
        6. Cytoplasm extends through the sieve plates of adjoining cells to form a continuous tube system.
        7. During the growing season, leaves produce sugar.
        8. Sucrose is actively transported into phloem by an electrochemical gradient established by a H+ pump.
        9. Water consequently flows into sieve tubes by osmosis.
        10. A sink can be at the roots or any other part of the plant that requires nutrients.
        11. Because phloem sap flows from source to sink, sap can move any direction within phloem.