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Mineral salt uptake
5B.1 and 2B.2
By CSE
Mineral culture solutions
• Each lack a certain mineral
• Enables deficiency symptoms to be seen
easily
• Chlorosis – poor growth and yellowing of
leaves due to lack of nitrate or Mg
• Phosphate deficiency leads to poor root
growth
Nitrates
• Nitrogen needed for protein synthesis, nucleic
acids, coenzymes, vitamins, chlorophyll
• Absorbed as nitrates from soil or from
surrounding water if aquatic or as ammonium ions
• Macronutrient
• Deficiency – poor growth, yellowing of leaves =
chlorosis
Nitrate is reduced to nitrite and then to
ammonia, using either NADH produced in
mitochondria or NADPH produced in
chloroplasts:
NO3NO2NH3
Nitrate reductase
Nitrite reductase
Ammonia then combines with the keto acid
∂ ketoglutaric acid to form the amino acid, glutamic
acid.
This then donates its amino group to other keto acids
in transamination reactions to form the corresponding
amino acid.
Eg it can donate its amino group to oxaloacetic acid
( a keto acid) to form aspartic acid (an amino acid)
Phosphates
• Absorbed in large amounts from soil solution
• Absorbed as hydrogen phosphate (HPO42-) in
normal soils or dihydrogen phosphate (H2PO4-) in
acid soil
• Assimilated into organic ATP during respiration
then into nucleic acids, sugar phosphates and
phospholipids
• Deficiency means poor growth particularly of
roots
Magnesium
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Absorbed as Mg2+
Needed in small amounts, micronutrient
Is a component of chlorophyll
Is an activator of some enzymes such as
ATPase
• Deficiency = chlorosis of leaves
Other minerals
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Eg Copper, molybdenum, manganese, zinc
Many act as enzyme activators
Called micronutrients or trace elements
Molybdenum is a component of nitrate reductase
which catalyses the reduction of nitrate to nitrite.
• Iron is a constituent of some of the electron
carriers in respiration and photosynthesis.
• Some micronutrients are only needed by particular
plants eg silicon is needed by grasses as silica is a
constituent of cell walls. Sodium is needed for C4
plants.
Mineral uptake difficulties
• Some minerals are even more dilute in soil
than CO2 is in the atmosphere eg phosphate
concentrations are usually only a few parts
per million.
• Minerals move slowly through the soil. Air
currents take CO2 to the leaf but the only
movement of soil water is by very slow
capillary flow brought about by water uptake
through root.
• Minerals in solution diffuse more slowly than
CO2 diffuses in air
Uptake
• Root hairs – long hair like extensions of the outer
epidermal cells of the root
• Penetrate between soil particles
• Increase surface area of root. The root system of a
50g rye plant has a surface area / mass ratio over
2000 times greater than that of a 70kg human.
• Main entry point for water and mineral ions
• Ions enter by diffusion or active transport,
depending on its electrochemical gradient
• The cells of outer part of the root have a
membrane potential, the outside being 100mV
positive to the inside, as all cells
• Uncharged solutes uptake depends on concentration
gradient of the solute
• Movement of a charged ion depends on its concentration
inside and outside the cell and the electric potential
difference across the cell membrane = electrochemical
gradient is combination of these 2.
• All anions (nitrate, sulphate, phosphate) enter root hair
cells against their electrochemical and concentration
gradient by active transport, so needs energy
• Cations eg sodium, calcium, magnesium have an
electrochemical gradient which favours their entry by
passive diffusion (need to be moved out of cells by active
transport) as they are positive so electric attraction greater
than concentration gradient difference as they are already
inside root. It depends on active processes as the
generation of the membrane potential is due to the
pumping out of positively charged H+ . The energy comes
from respiration.
• On entering root, ions may stay in
cytoplasm or vacuole or pass out to the
cellulose walls
• Then pass through outer parts of the root by
the apoplast or symplast pathway
• Apoplast = movement through adjacent cell
walls
• Symplast = water moves along same water
potential gradient across interconnected
cytoplasm of the cells of the root system
Apoplast route
• Cellulose is loose, open network and can be
half full of water
• As water/ salts drawn into xylem, more
water/salts attracted from adjacent cell wall
• Move across root until reach endodermis
where casparian strip of suberin stops it
• Water/salts have to enter cytoplasm by
active transport or diffusion
Symplast route
• Cytoplasm of adjacent plant cells connected
by strands called plasmodesmata which go
through pores in the cellulose walls.
• Water moves from the soil across the
cytoplasm of adjacent cells along the water
potential gradient from the root hair cells to
the xylem