Plant Physiology - M.M. Musiienko 2001

Root nutrition of plants
Active ion transport

Active Transport is the key regulator of transmembrane potential. Since all ions are charged, their diffusion rate is determined not only by membrane permeability but also by the concentration difference across the membrane (chemical potential), as well as the electrical potential generated between the inner and outer sides of the membrane. This is why we say that ion movement is driven by the Electrochemical Potential gradient. This voltage across the membrane (the inner side of the membrane typically has a negative charge, which is why cations are absorbed) is called the transmembrane potential.

One of the primary ions involved in generating the transmembrane potential is H⁺. Theoretically, the following MAIN MECHANISMS OF pH gradient generation in plant Cells are possible:

·electrogenic H⁺-transporting ATPase;

·neutral H⁺-ATPase performing K⁺/H⁺ exchange;

·neutral exchange of Ca⁺⁺ and protons (Ca⁺⁺-ATPase);

·redox chain;

·passive H⁺ influx or OH efflux;

·endocytosis.

The energy of the proton motive force is expended in various ways or, in other words, through different mechanisms: this can be electrical or electrophoretic coupling of H⁺ transport with diffusive ion transfer. In this case, the electrical potential acts as the driving force for non-specific cation transport into The Cell (electrogenic antiport) or anion transport in the direction of active transport (electrogenic symport).

It is important to note that the transport pathways of H⁺, cations, and anions are structurally isolated, i.e., localized in different (possibly adjacent) Regions of the membrane. On the other hand, H⁺ transport can be chemically coupled with cation antiport (for example, K⁺/H⁺ exchange involving K⁺-stimulated H⁺-ATPase or 2H⁺⁺/Ca⁺⁺ exchange). There can be a similar symport of anions, such as chloride-stimulated ATPase on the tonoplast. In addition, the proton motive force can be used for active transport (chemical coupling via carriers) — H⁺/cation antiport, H⁺/anion symport, or H⁺/nonelectrolytes (sugars, Amino Acids).

Finally, the pH gradient affects the distribution of weak acid growth regulators and various bases between the extracellular and intracellular environments.

In addition to ion diffusion through certain membrane regions driven by the electrochemical potential gradient, ions also move through specific membrane zones — ion pumps — at the ex-

pense of ATP energy. Active transport, unlike diffusion, including that along the transmembrane potential gradient, consists in transporting ions against this gradient.

Ion pumps. These are enzyme Proteins, but unusual ones. While most Enzymes catalyze The conversion of substrates into reaction products, in this case, we are talking about converting the chemical bond energy of a substrate molecule (ATP, in this case) into the kinetic energy of ion movement.

In a simplified form, this can be illustrated by the example of the sodium pump, which constantly pumps out ballast Na⁺ ions (Fig. 123).

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Fig. 123. Model of the sodium-potassium pump operation: a — a Na⁺ ion in the Cytoplasm binds to a transport protein molecule; b — a reaction involving ATP, resulting in the phosphate group (P) attaching to the protein while ADP (adenosine diphosphate) is released; c — phosphorylation induces a conformational change in the protein, causing the release of Na⁺ outside the cell; d — a K⁺ ion in the extracellular space binds to the transport protein (e), which in this form is better adapted for binding K⁺ than Na⁺; f — the phosphate group is cleaved from the protein, restoring its original shape, and the K* ion is released into the cytoplasm. The transport protein is now ready to export another Na⁺ ion from the cell

An enzyme protein, sodium ATPase, is embedded in The Plasma Membrane. The name itself suggests that it hydrolyzes ATP in the presence of sodium. The Active Site of the enzyme is directed inward toward the cytoplasm, and it contains a Na⁺-binding group; thus, it selectively binds sodium ions.

Upon interacting with cytoplasmic ATP, the enzyme hydrolyzes it into ADP and P_i. Moreover, The Fate of the high-energy bond energy is unusual—it is converted from chemical to kinetic energy. Having acquired this energy, the protein macromolecule undergoes conformational changes. It seemingly flips in the membrane so that its Na⁺-binding group faces outward (flipping, relatively speaking, as The Nature of this phenomenon is not yet fully understood). In this flipped state, the ATPase acquires new properties. First, it loses its ability to bind ions, so sodium, once on the outer side of the membrane, dissociates from the active site into the external environment. Second, this flipped state is unstable, so the protein returns to its original initial position, but spontaneously, like a stretched spring contracting. As a result, the active site, along with the Na⁺-binding group, turns back toward the cytoplasm and can bind a new sodium ion.

It should be noted that membrane ion pumps resemble Muscle contractile proteins—Actomyosin. Indeed, actomyosin is also an ATPase (but unlike plant ATPases, it is activated by Ca²⁺) and converts the chemical energy of ATP into the mechanical energy of muscle fiber contraction. Therefore, it is no exaggeration to say that membrane ATPases are, in a way, The Muscular System of the cell.

But let us return to the Na⁺-dependent ATPase. As it turns out, along with sodium ions, it is also activated by magnesium and potassium, the latter being particularly important. Indeed, while pumping ballast sodium out of the cell, this membrane ATPase simultaneously actively pumps (against the gradient) potassium, which the cell needs. Therefore, it was named the sodium-potassium ATPase. This enzyme system has been well studied in animal membranes, which cannot be said for plant membranes. Indirect evidence suggests that similar types of pumps also operate in plant membranes. Thus, the presence of ATPases in the Plasmalemma of ROOT cells has been proven; they require magnesium, are activated by sodium and potassium, and, similarly to the former, are inhibited by ouabain. It has been shown that in halophytes, The activity of such ATPases is quite high because they need to intensively pump out sodium ions. An enzyme that promotes the Hydrolysis of ATP to ADP and P_i in the presence of K⁺ was detected on the plasmalemma of oat root cells. Obviously, this enzyme acts as a carrier that uses the energy of ATP hydrolysis to transport potassium ions across the plasmalemma.

Indirect evidence also includes the ionic ratios in The plant cell system: thus, potassium, sodium, calcium, and H⁺ ions are concentrated in The Cell wall and vacuoles. The accumulation of Ca²⁺ and H⁺ protons in the cell wall is a particularly important fact for regulating the operation of H⁺ pumps and K⁺ channels of the plasmalemma, as well as the associated cotransport of protons and anions. Low pH values of the vacuolar sap, as well as the concentration of cations and non-metabolic chloride ions, indicate high activity of the tonoplast H⁺-ATPase, which is likely coupled with the cotransport of chloride ions. In addition to the Na⁺, H⁺-ATPase, the function of an ion pump is performed by Ca⁺⁺-ATPase, anion ATPase, and H⁺-ATPase.

In plant cells, a particularly important role is played by the hydrogen pump—H⁺-ATPase, which Functions in the plasmalemma and, possibly, the tonoplast. It generates an electrical (Δψ) and chemical (pH) gradient of hydrogen ions across the membrane. The proton ATPase (hydrogen pump) functions in the membranes of Mitochondria and METABOLISM/14.html">Chloroplasts. Indeed, it is with its help that the processes of Oxidative Phosphorylation AND Photophosphorylation take place in these Organelles.

The existence of such pumps has also been established in the Plasma Membranes of bacterial cells (since Bacteria lack the aforementioned organelles), and therefore ATP Synthesis in them occurs with the participation of the plasmalemma.

The 1970s was a period when various lines of evidence emerged for the existence of electrogenic H⁺ pumps and their functional role as generators of the proton motive force—the primary driver of transmembrane exchange of ions, sugars, and amino acids. The Emergence of a new chemical tool—the fungal toxin fusicoccin, currently the most potent stimulator of H⁺ pumps—significantly contributed to the expansion of research on ion transport.

The 1980s can be called a triumph for proponents of the H⁺-pump hypothesis, which was successfully demonstrated in membrane vesicles of the plasmalemma and tonoplast. The final step was also taken: H⁺-transport proteins were isolated and reconstituted into an artificial Lipid Bilayer Membrane. It took almost 25 years from the initial concepts in the early 1960s to the recognition of the fact that H⁺ pumps function in plant cell membranes. This path was difficult not only in terms of methodological search, but also in the clash of ideas and harsh criticism of the new step (V.V. Polyovoy's original hypothesis on the involvement of H⁺ pumps in growth processes was hard to accept).

Model systems in the form of sealed plasmalemma vesicles from maize coleoptile cells have been demonstrated, which allow The Study of passive and active ion transport using fluorimetric Methods. Using potential-dependent probes, as well as a pH-sensitive intravesicular probe, the generation of both Components of the electrochemical potential of hydrogen ions—the transmembrane potential and the proton gradient—was detected on the vesicle membranes (Inge-Vechtomova, 1993).

These data essentially limit our current knowledge of Active transport MECHANISMS to The sodium and hydrogen pumps. Yet, how many other substances—mineral nutrient ions, organic acids, amino acids, and sugars—are actively absorbed and released by plants? And here it turned out that nature is extremely economical, because these active transport mechanisms are quite sufficient for all this diversity. This became possible due to cotransport, that is, The transport of two different substances using the same mechanism. As we have already observed, the pump, while pumping out sodium, simultaneously facilitates the transport of potassium (antiport).

This raises the question: why did nature favor these two specific mechanisms during evolutionary development? The reason is that these two ions (sodium and hydrogen) are always available to the cell in unlimited quantities.

Sodium is in excess in the environment. Its concentration inside cells is always several times higher than that of potassium, so the cell is forced to constantly pump out the excess. Since such a pump had to operate continuously, why not use it as a driving force to absorb potassium ions and other substances essential for the cell?

As for hydrogen, its cellular reserves are also inexhaustible, but for a different reason: these ions are continuously generated within the cell during Respiration and redox reactions. The accumulation of such ions is harmful because it alters the pH, with all the ensuing consequences. One way to get rid of them is active extrusion. Therefore, the proton pump can also be used as a driving force for the symport and antiport of various substances.

Attention should be paid to the phenomena of endocytosis and pinocytosis in plant organisms. In endocytosis, compounds enter The Cell as a result of plasma membrane invagination; in the case of soluble substances, this phenomenon is called pinocytosis. Although they differ from membrane transport systems, their operating principle is still based on the ability of membranes to "selectively recognize" certain molecules. Therefore, the controlled movement of substances from Cell to Cell is also possible through Endocytosis and Exocytosis, which involves the Transport of substances in vesicles.

Of course, other transport mechanisms, as yet unknown to us, are also possible.

Thus, a new field of research has emerged today—the energetics of Introduction/36.html">Biological Membranes, which, as defined by V.P. Skulachev, is becoming the theoretical basis of Water-Salt Metabolism in plants and A number of biotechnological developments, including the targeted transport of metabolites, genetic material, and physiological plant breeding.