Absorption of Mineral Salts

I. Absorbing Surfaces of Roots
   A. Plants produce large root systems to solve the problem of absorbing
       scarce amounts of water and mineral elements from soils. 
   B. The shape of a plant root system is controlled by the genetics 
       of the plant rather than the environment.
      1. Grasses (monocots) have fibrous root systems.
      2. Perennial herbaceous (nonwoody) dicots have taproot systems
      3. Other herbaceous dicots have a taproot but it is 
          difficult to distinguish it from branch roots.
      4. Roots extend outward from the upper stem system much farther 
          than above-ground branches.
   C. Even though the genetic make-up of the plant determines whether
       a taproot or fibrous root system will be developed, the soil
       environment will also influence the plant root system.
      1. Dry soils:  plants put more biomass in roots than in shoots.  This 
           results in a greater root-to-shoot ratio
      2. Roots will grow wherever they can.  Important factors influencing 
           root growth are:  (1) mechanical impedance, (2) temperature, 
           (3) aeration, (4) availability of water, (5) availability of mineral 
      3. If water is more available deep in the ground, plants will have 
           deep roots systems.  The roots will generally grow far below the 
           soil surface.
      4. Shallow root systems take advantage of brief intermittent rains.  
           The root mass of a shallow rooted plant generally spread out in 
           a wide diameter around the plant.
   C. Branch roots of annual plants (plants that germinate, grow, 
        reproduce and die in one season) elongate for only a few 
        days.  Perennial plants (live a year or more) have roots that 
        elongate for a long period of time.
   D. Roots are cylindrical.  A cylinder has more strength per unit cross-
        sectional area than other shapes.  The cylindrical root (protected 
        by root cap) can force soil particles aside without breaking.
      1. If roots are to grow toward water and ions they must explore a 
           large soil volume.
      2. When soils are moist diffusion of water molecules toward roots is 
           rapid, but when soils dry to a water potential near 1.5 MPa 
           (permanent wilting point) diffusion of water and dissolved ions 
           can decrease 1,000-fold.
      3. At the permanent wilting point of soils plants have difficulty 
           obtaining water and mineral ions for two reasons:
          (a) limited exploration of the soil by roots.
          (b) limited diffusion of water and ions into roots.
   E. Root Hairs:  responsible for absorption of ions and water.  Modified 
         epidermal cells in the region of root elongation (a root distance of 
         about 1 cm long).  Root hairs are more frequent and extend over 
         a greater region of the root when soils are moderately dry rather 
         than wet.  Conifers do not have root hairs, instead they depend 
         on mycorrhizae as do some angiosperms.

II. Mycorrhizae (fungus-root)
   A. Mycorrhizae are a symbiotic and mutualistic association 
         between nonpathogenic fungus and living root cells, primarily 
         cortical and epidermal cells.  Fungi receive organic nutrients from 
         the plant.  The fungi improve mineral and water-absorption by the 
         roots.  Young roots become infected by the fungus.
   B. Two main groups of mycorrhizae:
      1. Ectomycorrhizae:  fungal hyphae form a mantel outside the 
           root and within the root in the intercellular spaces of the 
           epidermis and cortex.  No intracellular penetration.  Hartig net:  
           extensive network of fungal hyphae formed between the plant 
           cells.  Common on trees:  Pinaceae (pine, fir, spruce, larch, 
           hemlock), Fagaceae (oak, beech, chestnut), Betulaceae (birch, 
           alder),  Salicaceae (willow, poplar).
      2. Endomycorrhizae:  three sub groups.  Most common are the 
           vesicular arbuscular mycorrhizae (VAM).  Fungi are members of 
           the Endogonacae, internal network of hyphae between cortical 
           cells that extends out into the soil, where the hyphae absorb 
           mineral salts and water.  Hyphae are surrounded by an 
           invaginated plasma membrane of the cortex cell.  Most species 
           of herbaceous angiosperms.  Gymnosperms:  Cupressus, Thuja, 
           Taxodium, Juniperus, Sequoia.  Ferns., lycopods, and 
      3. Ectendotrophic mycorrhizae:  intermediate properties of both
   C. Fungal partner receives sugars from the host plant.  Thus what 
        effects the plant host will affect the fungal partner.
      1. Plants that are grown in shade and are deficient in sugars 
           predictably have poor mycorrhizal development.
      2. Plants grown on fertile soils have less developed mycorrhizae 
           than plants grown on nonfertile soils.
      3. Mycorrhizae greatly increase phosphate absorption by the plant 
      4. Mycorrhizae greatly increase absorption of ions that generally 
           diffuse slowly toward roots or are in high demand (PO4-,  NH4+.
           K+, and NO3-).
      5. Advantageous to trees growing on nonfertile soils:  mine-waste 
           areas, landfills, roadsides.

III. Movement of Ions Into the Plant Root
   A. Mineral elements can reach the root in three ways:
         1. diffusing through the soil solution
         2. carried passively as water moves by bulk flow into the roots
         3. roots growing toward the nutrient elements
   B. Mineral salts are absorbed and transported upward from root 
         regions containing root hairs and by older regions many 
         centimeters from the root tip.  Mycorrhizae readly absorb 
         nutrients near root tips where fungal hyphae are concentrated 
         and somewhat less rapidly in older regions.
   C. Apoplastic Pathway:
          1. Diffusion and bulk flow of water from cell to cell through non-
              living spaces between cell-wall polysaccharides.  
          2. Apoplastic pathway ends at the endodermis with the waterproof 
              Casparian strip.  Final mineral absorption control point for 
              many species.
              a. Angiosperms have another Casparian Strip in the 
                  hypodermis, also called the exodermis.  This Casparian 
                  Strip develops and matures farther from the root tip (up to 
                  12 cm) than does the comparable strip in the endodermis.
              b. Thus, the exodermis can occur in older regions of primary 
                  roots that have not lost their external cells.
              c. The exodermis restricts mineral nutrient movement and is 
                  an important control point that forces external solutes to be 
                  absorbed by the selective plasma membrane of exodermal 
                  cells.  Ions can move to the xylem from cell to cell via a 
                  symplastic pathway once inside the cytosol of the 
   D. Plasmodesmata: tubular structures that extend through adjacent cell 
        walls and middle lamella of all living plant cells.  Plasmodesma 
        are made of a tube of plasma membrane that is continuous
        between the adjacent cells.  Inside the plasma membrane tube is 
        another tube called a desmotubule.  The desmotubules are 
        compressed endoplasmic reticulum that extend from one cell to 
        the other.
   E. Metabolic energy and ATP are required for transfer of mineral 
        nutrients into the xylem.  The pericycle or immature living xylem 
        cells absorb ions from other living cells on one side and secrete 
        them into dead mature xylem cells on the other side.
   F. In mycorrhizae solutes first enter the fungal cytoplasm and travel
        toward the root via the symplastic pathway.  There are 
        no plasmodesmata or other cytoplasmic connections between the 
        fungal hyphae in the Hartig net and the root cells.  The fungus 
        releases the solute into apoplastic space of the root.  Suberin 
        layers on exodermal cells of the root force solutes into the 
        cytoplasm of the exodermal cells (symplastic pathway).  
        Mineral nutrients are then transported via the symplastic pathway 
        across the root cortex cells to the xylem.  Solutes (photosynthate)
        transferred from the plant root to the fungal partner enter the same 
        restricted apoplast space that is sealed from other organisms.  
        This prevents other soil microbes form absorbing nutients being 
        transferred between the plant roots and fungus.

IV. Membranes (review from Bgy 11 and Bgy 33)
   A.  Fluid Mosaic Model:  there are three major components
      1. Lipids
      2. Proteins
      3. Sterols
   B. Lipids
      1. Four abundant phosphlipids:  phosphatidyl choline, phosphatidyl 
          ethanolamine, phosphatidyl glycerol, phosphatidyl inositol
      2. Two abundant glycolipids: monogalactosyldiglyceride, 
          digalactosyldiglyceride (mainly in chloroplasts)
   C.  Sterols:  Main function of sterols in membranes is to stabilize the 
          hydrophobic interior and prevent it from becoming too fluid as the 
          temperature rises.
   D. Proteins: three known types: catalytic proteins, proteins that make 
          up solute channels, proteinaceous carriers.
   E. Ca+: bonds hydrophilic portions of phospholipids to each other and 
          to negatively charged parts of proteins within the membrane.
   F. Membranes have sidedness
      1. Creates binding sites for growth regulators and pathogens

V. Principles of Solute Absorption
   A. Cells that are not alive and metabolizing have membranes that are 
       permeable to solutes
      1. Water molecules and dissolved gases (H2, O2, and CO2) 
         diffuse passively through membranes.
      2. Hydrophobic solutes move across membranes at rates related to 
         their lipid solubility
      3. Hydrophilic molecules and ions with similar lipid solubility's 
         penetrate at rates inversely related to their size.
   B. Many Solutes Are Accumulated Inside Cells
      1. ACCUMULATION:  cells can absorb essential solutes fast and over 
           long periods of time so that solute concentrations become 
           much higher within the cells than in the external solution.  
      2.  Plant cells use energy (ATP) for accumulation of mineral nutrients
      3.  Restriction of sodium is common to most angiosperms and 
   C. Absorption of Solutes Is Specific and Selective
      1. Uptake mechanisms can sometimes be "fooled".
      2. Selectivity of ion transport by roots applies to organic 
           compounds (amino acids and sugars) and occurs in all 
           parts of the plant.  Selectivity supports the theory that 
           proteinaceous carriers in membranes help move solutes into 
           cells.  Protein carriers selectively recognize nutirents which 
           activate or inactivate the transport.
   D. Absorbed solutes can leak out of the cell slowly
      1. EFFLUX:  outward movement of nutrients is often slow
      2. Slow leakage shows that absorption (INFLUX) is primarily 
      3. Membrane carriers or unidirectional channels speed only 
            inward absorption.  Na+, Ca2+, and Mg2+ diffuse inward down 
            a concentration gradient and are transported outward with the 
            aid of ATP-dependent pumps.
      4. Respiration and solute absorption are probably strongly related 
            because respiration provides the ATP needed for solute 
    E. The rate of solute absorption varies with solute concentration
      1. Diffusion to the root surface is the limiting factor.  Absorption 
            properties of the root is of limited importance for plant nutrition
      2. CARRIERS:  membrane proteins that specifically recognize 
            certain solutes, combine with them, and speed their transport 
      3. The rate of absorption increases rapidly as the solute 
            concentration increases in low concentration ranges similar to 
            those found in soils.  At higher solute concentrations the 
            absorption rate starts to level off.  Carriers in the membrane 
            transport solutes quickly until it becomes saturated by excess 
            solutes at high solute concentrations.

VII. The Energetics of Passive and Active Transport:
   A. Some solutes that are absorbed rapidly by cells never reach higher 
        concentrations inside the cell than out outside the cell (N2, Na+)
   B. Cells use energy to pump protons, Na+ , and Ca2+ out into the cell 
        wall.  Loss of cations causes their cytosol to become slightly 
        negatively charged.  100-150 mV
   C. K+, predicted and measured concentrations were similar, in 
        dictating that the ion was near equilibrium and was probably 
        absorbed passively.
   D. Na+, Ca2+, and Mg2+  measured tissue concentration was always 
        less than predicted.  Have even been transported out actively.  
        Their inward movement was passive.
   E. Mn2+, Fe2+, Zn2+, Cu2+.  Absorption is passive.  Depends on 
        energy-dependent production of ATP and its hydrolysis to cause 
        a negative charge inside the cytosol.  For all anions, measured 
        internal concentrations were far higher than those predicted, 
        showing that anions were absorbed actively.  
   F. This repulsion factor (negative charge inside cells) helps explain 
        why CO2 and H2CO3 are absorbed faster than both HCO3- and 
        CO32-, and why H2PO4- is absorbed faster than HPO42-.

VIII. ATPase Pumps:  The Transport Protons and Calcium
   A. ATP phosphohydrolase:  ATPase
   B. In membranes most ATPase instead ensure that much of this 
         energy is used to transport protons from one side of the 
         membrane to the other against an electrochemical gradient.
      1. Ca2+/H+ ATPase:  pump calcium out of the cytosol, with outward 
           to the cell wall via the plasma membrane or inward to the 
           vacuole via the tonoplast.
      2. (Ca + Mg) - ATPase:  does not move H+ in as Ca2+ moves out, 
           and it probably depend s on calmodulin for its actively
   C. Plasma membrane H+-ATPase transports H+ out cytosol and into 
         the cell wall only one proton for each ATP hydrolyzed.  
      1. Causes the pH of the cytosol to increase
      2. Causes the pH of the cell wall to decrease
      3. Causes the cytosol to become electronegative relative to the cell 
           wall as the cytosol loses H+ but retain OH-

IX. Carriers and Channels Speed Passive Transport
   A. Carriers are integral proteins that span the membrane.  
        Pick up solute and move them across the membrane.  Undergo a 
        reversible conformational change that facilitates solute transfer.
   B. Channel proteins are integral protein that exist in one
        conformational structure of lowest free energy specified by the 
        cell environment.  This conformation varies when the cell 
        environment varies.  Specific ions can move through open 
        channels as fast as 108 ions per second (3X to 4X faster than 
        movement via carriers).
      1. Two main types of channels:  
         a. One has a gateing system that responds to the voltage gradient 
              across the membrane
         b. One responds to external modulator stimuli such as light or 
              growth regulators.

X.  Membranes Use Proton Pumps for Ion Transport
   A. Simplest cation absorption:  UNIPORT or FACILITATED 
        DIFFUSION.   Cation absorption is favored by the electropotential 
        gradient.  For K+, NH4+, Mg2+, and Ca2+ this absorption occurs 
        with the help of either a carrier or a channel.
   B. COTRANSPORT or SYMPORT:  For absorbtion of sugars and 
        anions, cells take advantage of the pH gradient between the cell 
        wall and cytosol.  Electropotential gradients will not favor 
        absorption of neutral molecules such as sugars, and it will repel 
        anions (bicarbonate, nitrate, chloride, phosphate, and sulfate).  In
        these cotransport examples, H+ moves inward down its 
        electrochemical-potential gradient, while carrying the anion or 
        neutral molecule actively.  An energy-releasing process drives an 
        energy-requiring one.
   C. COUTERTRANPSORT or ANTIPORT:   passive H+ absorption can 
        be used to transport cations out of cells.  Here a carrier combines 
        with H+ on the outside of the cell and, for example, Na+ on the 
        inside of the cell,  then the carrier transports them in opposite 
   D. Transport of solutes across the tonoplast into the central vacuole 
        uses energy from either the ATPase or the pyrophosphatease 

XI. Absorption of Large Molecules by Organelles
   A. ATP is usually or always required
   B. Great specificity with respect to which organelle absorbs with 
   C. Specificity in the protein is confined to only a relatively small part of 
       it, often some 20-50 amino acids connected at one end of the 
       molecule.  Chloroplast: TRANSIT SEQUENCE;  mitochondria:  
       LEADER SEQUENCE; endoplasmic reticulum and nucleus: 
   D. After the protein is absorbed by the proper organelle, the 
        recognition sequence is split (hydrolyzed) off and the mature 
        functional protein is released.

XII. Correlations Between Root and Shoot Functions in Mineral Absorption.
   A. Absorption of mineral salts should be controlled in part by activities 
       of the shoot
      1. Demand Sense:  shoot might increase root absorption of mineral 
           salts by rapidly using them in growth products
      2. Supply Sense: shoot supplies carbohydrates via the phloem that 
           the root must respire to produce ATP that drives mineral slat 
   B. Shoot probably supplies the roots certain growth regulators that 
       affect absorption by roots.
   C. Interdependence between activities of roots and shoots:
      1. correlations between the rate of shoot growth and the rate of 
           absorption of nitrogen, phosphorus, and potassium have been 
      2. Respiration rates of roots over time are sometime highly 
           correlated with rates of photosynthesis.
      3. Root respiration correlated with rate of sugar translocation to the 
      4. Maximum absorption of nitrate and ammonium ions correlate with 
           maximum photosynthetic rates, except that absorption lags by 
           about 5 hrs, suggesting the need for carbohydrate 
           translocation and root respiration during the lag period.