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 systems 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 fungus. 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 flow 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 nutrients 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 mechanisms. 3. Restriction of sodium is common to most angiosperms and gymnosperms. 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 unidirectional. 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 concentrations. 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 proteins. 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 passively. 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 actively. 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 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. 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 regulators. 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.