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
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
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
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. 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
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.
4. MULTIPHASIC KINETICS.
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
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
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
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.