5.2.2 Symplasmatic/aktive
Both the plasma membrane and the tonoplast act as barriers to ion transport due to their structure. This structure leads to specific characteristics of nutrient uptake:
  • Selectivity – Preferential uptake of certain elements and reduced uptake or exclusion of others.
  • Accumulation – Ion concentration in the cell sap can be higher than in the external solution.
  • Genetic Variability – Species and cultivars of the same species differ in their uptake characteristics.

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Membrane structures differ from cell walls:
  • Plasma membranes and the tonoplast are similar in that they consist of a continuous lipid bilayer with globular proteins embedded within it.
  • The polar lipids in the membrane orient their charged regions toward the surface, making them generally impermeable to inorganic ions.
  • Protein molecules are termed extrinsic if they are bound to the surface by electrostatic forces. They are called intrinsic if they are integrated into the membrane. These intrinsic proteins function as protein channels to facilitate the passage of charged ions across the membrane.
  • Uncharged molecules cross the membrane inversely proportional to their molecular size.
  • Lipophilic molecules (highly soluble in organic solvents) cross the membrane more easily than expected based on their molecular size.

Active ion transport across membranes:
Evidence for the existence of active transport:
  • Accumulation against a concentration gradient.
  • Inhibition of uptake by inhibitors, e.g., cyanide salts.
  • Competition for uptake between different ions.
  • The uptake rate is not a linear function of external concentration.

There are fundamentally three types of active ion transport:
  • Ion channels
  • Ion (plasma) pumps
  • Cotransport mechanisms
The carrier hypothesis is based on the concept that ion transport across membranes is mediated by transport proteins. Emanuel Epstein and C.E. Hagen were the first, in the 1950s, to apply the principles of enzyme kinetics (Michaelis-Menten kinetics) to nutrient uptake (see below)
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This curve is characterized by the following parameters:
  • In = net influx of a nutrient
  • Imax = maximum influx (the root's capacity for nutrient uptake)
  • Km = the Michaelis constant, describing the affinity of the uptake system for a nutrient
  • Cmin = the minimum concentration to which a plant can deplete a solution

The proton pump is driven by ATP, transporting H⁺ ions across a membrane. This process:
  • Creates a pH gradient and an electrical potential difference
  • Is highly energy-demanding (ATP), with energy produced in mitochondria through respiration
  • Is particularly active in root hairs, consuming 25–50% of cellular ATP
  • Is stimulated by monovalent and divalent cations but is insensitive to anions
  • Requires a magnesium cofactor for activity in both the plasma membrane and tonoplast

The membrane potential consists of two components:
  • The chemical potential gradient, determined by the ion's activity on each side of the membrane
  • The electrical potential gradient, which is the voltage difference across the membrane
Plasma-Membrane
Plasma membranes contain ion channels (macromolecular pores). These are passive transport systems driven by electrochemical potential gradients. Ion channels can transport 10⁶ ions/s, whereas ATPases transport 200–600 ions/s.
These pores are water-filled and exhibit low ion selectivity. Well-known examples include the potassium channels in the plasma membrane of guard cells. Ion channels can also exist for Fe, Mn, Zn, Cu, and Ni.
The primary function of these channels is likely osmoregulation, and they are mostly in a closed state.

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Ionenkanal schematisch. Quelle: http://at22.bphys.uni- linz.ac.at/bioph/teach/biophy/biophy4.htm

Co-Transport (Symport)
All co-transport mechanisms consume energy. Ions are bound to protons that are generated by a proton pump. It is considered symport when protons and ions are transported in the same direction—this is the case for most anions. Antiport occurs when the proton and ion are transported in opposite directions.
Tonoplast
In the tonoplast, ATPase transport pumps operate.
  • Anions can be transported through ion channels if the ATPase generates a slight positive potential on the inner side of the membrane.
  • Anions can also be transported via counterport (exchange) mechanisms, which involve the exchange of anions (e.g., phosphate moving into the cell, bicarbonate moving out).
Loading of the Xylem
Loading of the Xylem
Xylem loading refers to the movement of ions from the external solution (soil solution) into the xylem. This is an energy-dependent symplastic transport; the innermost "barrier" is the plasma membrane of the xylem parenchyma.
When ions are present in the plant cell, they move from cell to cell via plasmodesmata.
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