Absorption of Mineral Salts

I.   Absorbing Surfaces of Roots
   A.  Plants are able to solve the problem of absorbing scarce 
         amounts of water and mineral elements from soils by 
         producing extensive root stems.
   B.  The overall shape of a plant root system is controlled by 
         the genetic make-up of the plant rather than 
         environmental mechanisms.
      1.  Grasses (monocots) have fibrous root systems
      2.  Perennial herbaceous (nonwoody) dicots have taproot 
      3.  Other herbaceous dicots have root systems made-up of a 
            taproot that is often difficult to distinguish from 
            the branch roots.
      4.  Root systems generally extend outward from the stem much 
            farther than the spread of the above-ground branches.
   C.  The soil environment will also influences the shape of a 
         plant root system
      1.  Dry soils:  plants will put more biomass in roots than 
            in shoots.  This will result in a greater root-to-
            shoot ratio.  Thus there will be a large root system 
            to absorb more water to support the shoot system.
      2.  Roots will grow wherever they can (nature abhors a 
            vacuum).  Important soil factors influencing the 
            growth of roots are:
           (a) mechanical impedance of the soil
           (b) temperature of the soil
           (c) aeration of the soil
           (d) availability of water in the soil
           (e) availability of mineral salts in the soil
      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 (plants that 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.  The ability of roots to grow toward water and ions 
            allows them to 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 (the permanent wilting point) 
            diffusion of water and dissolved ions can decrease 
            almost 1,000-fold.
      3.  At the soil permanent wilting point (1.5 MPa), 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.
   D.  In order to increase the absorption of ions and water, 
         roots must increase their surface are.  This is 
         accomplished by ROOT HAIRS.  Root hairs are modified 
         epidermal cells in the region of root elongation (a 
         region distal to the root cap that extends along 
         the root  about 1 cm).  In soils that are moderately dry 
         the root hairs are more frequent and extend over a 
         greater region of the root than in wet soils.  Root hairs 
         are common in Angiosperms but are not found in Conifers.  
         Conifers depend on mycorrhizae for increased water and 
         nutrient absorption as do some angiosperms.

II.  Mycorrhizae (fungus-root)
  A. Mycorrhizae are a symbiotic and mutualistic association 
       between nonpathogenic fungus and living root cells of a 
       host plant.  The plant cells affected by the symbiotic 
       fungi are usually the cortical and epidermal cells.  In 
       this association the fungi receive organic nutrients from 
       the plant, while the fungi are able to improve the water 
       and mineral absorption by the plant roots.  This 
       association starts with the infection of young roots by the 
  B. There are two main groups of mycorrhizae, which are 
        determined by the amount of fungal penetration into the 
        plant root cells:
     1. Ectomycorrhizae:  the fungal hyphae form a mantel AROUND 
           the outside of the root.  The fungal hyphae also extend 
           within the root in the intercellular spaces of the 
           epidermis and cortex.  However, there is no 
           intracellular penetration of the host cells by the 
           fungal hyphae.
         Hartig net: an extensive intercellular 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:  the fungal hyphae develop extensively 
           WITHIN the cortical cells of the host plant root.  
           There are three subgroups of the endomycorrhizae.  Most 
           common group are the vesicular-arbuscular mycorrhizae 
           (VAM).  Fungi are members of the Endogonacae, which 
           form an internal network of hyphae between host root 
           cortical cells.  The fungal hyphae form highly branched 
           structures called ARBUSCLES ("dwarf tree") which are 
           surrounded by the host plant cell's plasma membrane.  
           Thus the hyphae do penetrate through the cell wall, but 
           do not penetrate into the cytoplasm of the cell.  The 
           hyphae also extend out into the soil, where they 
           increase absorption of  mineral salts and water.  
           Endomycorrhizae are found in most species of herbaceous 
           angiosperms, gymnosperms (Cupressus, Thuja, Taxodium, 
           Juniperus, Sequoia), ferns., lycopods, and bryophytes.
      3. Ectendotrophic mycorrhizae:  intermediate properties of 
           both the ectomycorrhizae and endomycorrhizae.
   C. The fungal partner in the mycorrhizal relationship receives 
        sugars from the host plant.  Thus, one can predict that 
        what ever effects the host plant host will affect the 
        fungal partner.
      1. Plants that are deficient in sugars like those plants 
           grown in shade have poor mycorrhizal development.
      2. Plants growing on fertile soils will have less developed 
           mycorrhizae than plants grown on nonfertile soils.  
           This occurs because the plant does not require the 
           added absorptional benefit of the fungi.
      3. Mycorrhizae greatly increase host plant absorption of:
          (a) phosphate
          (b) ions that diffuse slowly 
          (c) ions that are in high demand (PO4-, NH4+, K+, NO3-).
      4. Mychorrhizal associations have been found to be most 
           advantageous to trees growing on nonfertile soils such 
           as mine-waste areas, landfills, and roadsides.

III.  Movement of Ions Into the Plant Root
  A. Mineral elements can reach the plant root in three ways:
     1. diffusion through the soil solution
     2. passively carried into the roots as water moves by bulk 
     3. growing of roots toward the nutrient elements
  B. Mineral salts are absorbed and transported upward from the 
       regions of the root which contain root hairs or by older 
       root regions farther back from the root tip.  Mycorrhizae 
       near root tips (where fungal hyphae are concentrated) 
       absorb nutrients rapidly while mycorrhizae in older regions 
       of the root absorb nutrients more slowly.
  C. Apoplastic Pathway of mineral nutrient absorption and 
      movement in the plant root:
     1. The minerals move with the diffusion and bulk flow of 
          water from cell to cell through the non-living spaces 
          between cell-wall polysaccharides.  
     2. The apoplastic pathway ends at the endodermis were the
          waterproof Casparian Strip blocks mineral movement.  
          The Casparian Strip is the final mineral absorption 
          control point for many species.
        a. Angiosperms have a second Casparian Strip in the 
           hypodermis, also called the exodermis.  The exodermis 
           lies just inside the epidermis within the cortex of the 
           root.  The exodermis Casparian Strip develops and 
           matures farther from the root tip (up to 12 cm) than 
           does the comparable strip in the endodermis.
        b. The exodermis can occur in older regions of primary 
           roots that have not lost their external cortical and 
           epidermal cells.
        c. The exodermis, like the endodermis, restricts mineral 
            nutrient movement and is thus an important control 
            point for mineral absorption.  The exodermis forces 
            external solutes to be absorbed by the selectively 
            permeable plasma membrane of exodermal cells.  Once 
            mineral nutrients are moved into the cytoplasm of the 
            exodermis, they can move to the xylem from cell to 
            cell via the symplastic pathway.
  D. Movement of mineral nutrients from cell to cell in the 
       symplastic pathway is facilitated by Plasmodesmata. 
       Plasmodesmata are tubular structures that extend through 
       the 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 plant 
       cells.  Inside the plasma membrane tube is another tube 
       called a desmotubule.  The desmotubules are compressed 
       endoplasmic reticulum that extend from one cell to another.
  E. In order to transfer mineral nutrient into the xylem, 
       metabolic energy and ATP are required.  Either the 
       pericycle or immature living xylem cells absorb mineral 
       nutrients from other living cells on one side and secrete 
       them into dead mature xylem cells on the other side.
  F. In situations where mycorrhizae are involved, solutes are 
       first enter absorbed by the fungal cytoplasm and then 
       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 fungi release the mineral nutrients into the 
       apoplastic space of the root.  The suberinized Casparian 
       strip on the exodermal cells of the root force solutes into 
       the cytoplasm of the exodermal cells (symplastic pathway).  
       The mineral nutrients are then transported via the 
       symplastic pathway across the root cortex cells to the 
       xylem.  The solutes (photosynthate) from the plant root are 
       transferred to the fungal partner via the same restricted 
       apoplast space that is sealed from other organisms.  This 
       prevents other soil microbes form absorbing nutrients being 
       transferred between the plant roots and fungus.

IV. Membranes (review)
  A. Fluid Mosaic Model:  there are three major components
     1. Lipids
     2. Proteins
     3. Sterols
  B. Lipids
     1. Four abundant phospholipids:  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 in which essential nutrients are 
         absorbed fast and over long periods of time so that 
         the nutrient concentration becomes much higher within the 
         cell than in the external solution.
     2. Plant cells use energy (ATP) for accumulation of mineral 
  C. Absorption of Solutes Is Specific and Selective
     1. Uptake mechanisms can sometimes be "fooled".
     2. Ion transport selectivity by roots applies to organic 
          compounds (amino acids and sugars) as well as mineral 
          nutrients and occurs in all parts of the plant.  
          Proteinaceous carriers in membranes help move solutes 
          into cells.  Protein carriers selectively recognize 
          nutrients which activate or inactivate the transport 
     3. Restriction of sodium is common to most angiosperms and 
  D.   Absorbed solutes can slowly leak out of the cells
     1. EFFLUX:  outward movement of nutrients from a cell 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 bit are transported 
          outward with the aid of ATP-dependent pumps.
     4. Cellular respiration and solute absorption are strongly 
          related because respiration provides the ATP needed for 
          use by the protein pumps in solute absorption.
  E. The rate of solute absorption varies with solute 
       concentration (similar to, but not like enzyme kinetics)
     1. The major limiting factor is the diffusion of the mineral 
          nutrient to the root surface.  Therefore the actual 
          absorption properties of the root is of limited 
          importance for plant nutrition.
     2. CARRIERS:  membrane proteins that specifically recognize 
          specific solutes, combine with them, and speed their 
          inward transport.
     3. The rate of absorption increases with the increase in 
          solute concentration under low nutrient concentration 
          ranges similar to those found in soils.  At higher 
          solute concentrations the absorption rate levels off.  
          Carriers in the membrane transport solutes until they 
          becomes saturated by excess solutes at high solute 
     4. MULTIPHASIC KINETICS:  similar to, but not exactly like 
          enzyme kinetics.  It has been found that kinetic curves 
          are multiphasic when it comes to membrane transport 

VII. Energetics of Passive and Active Membrane Transport:
  A. Some solutes 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 from the cell causes the 
       cytosol to become negatively charged.  100-150 mV
  C. K+ is near equilibrium within the cell and is absorbed 
  D. Na+, Ca2+, and Mg2+ are transported actively out of the cell 
       while their inward movement is passive.
  E. Mn2+, Fe2+, Zn2+, Cu2+.  Absorption is passive however the 
       inward movement depends on the energy-dependent production 
       of ATP and its hydrolysis which causes a negative charge 
       inside the cytosol.  For all anions, internal 
       concentrations are high, showing that anions were absorbed 
  F. There is a repulsion factor caused by the negative charge 
       inside the cell, which helps explain why CO2 and H2CO3 are 
       absorbed faster than both HCO3- and CO32-, and why H2PO4- 
       is absorbed faster than HPO4 CO32-.

VIII.  ATPase Pumps: Transport Of Calcium and Protons
  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 
  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. Channels and Carriers Speed Passive Transport
  A. Carriers are integral membrane proteins that span the 
       membrane.  They pick up solutes and move them across the 
       membrane.  Undergo a reversible conformational change that 
       facilitates solute transfer.
  B. Channel proteins are integral membrane proteins 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. Cell membranes have two main types of channels that 
         respond to different cellular stimulators:  
         a. Gateing system that responds to the voltage gradient
              across the membrane
         b. External modulator stimuli such as light or growth 

X. Proton Pumps for Ion Transport Across Membranes
  A. UNIPORT or FACILITATED DIFFUSION (Simplest cation absorption)
       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:  When plant cells need to absorb 
       sugars or anions, the cells take advantage of the pH 
       gradient between the cell wall and cytosol.  Typically the 
       electropotential gradients of the uniport will not allow 
       absorption of neutral molecules such as sugars, and will 
       actually repel anions (bicarbonate, nitrate, chloride, 
       phosphate, and sulfate).  In these cases cotransport 
       occurs.  In cotransport, H+ moves inward down its  
       electrochemical-potential gradient, while carrying the 
       anion or  neutral molecule actively.  An energy-releasing 
       process drives is coupled to an energy-requiring one.
  C. COUTERTRANSPORT or ANTIPORT:  In the case of coutertransport, 
       passive H+ absorption is used to transport cations out of 
       cells.  Here the carrier protein combines with H+ on the 
       outside of the cell and, for example, Na+ on the inside of 
       the cell.  The carrier protein then transports them in 
       opposite directions.
  D. Transport of nutrients across the vacuole membrane also 
       occurs.  Transport of solutes across the tonoplast into the 
       central vacuole uses energy from either the ATPase or the 
       pyrophosphatease pump.

XI. Absorption of Large Molecules by Organelles
  A. ATP is usually or always required
  B. Great specificity with respect to which organelle absorbs 
       with protein
  C. Specificity in protein absorption is confined to only a 
       relatively small portion of the protein, often some 20-50 
       amino acids connected at one end of the molecule.  
     Chloroplast: TRANSIT SEQUENCE
     Mitochondria: LEADER SEQUENCE
     Endoplasmic Reticulum and Nucleus: SIGNAL SEQUENCE.
  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.  Correlation 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 absorption.
  B. Shoot probably supplies the roots certain growth regulators 
       that affect absorption by roots.
  C. Interdependence between activities of roots and shoots:
     1. Correlation between the rate of shoot growth and the rate 
          of absorption of nitrogen, phosphorus, and potassium 
          have been obtained
     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 roots.
     4. Maximum absorption of nitrate and ammonium ions correlate 
          with maximum photosynthetic rates, except absorption 
          lags by about 5 hours.  This suggests a need for 
          carbohydrate translocation and root respiration during 
          the lag period.