Ion transport

Ion transport

Movement of salts and other electrolytes in the form of ions from place to place within living systems.

Ion transport may occur by any of several different mechanisms: electrochemical diffusion, active-transport requiring energy, or bulk flow as in the flow of blood in the circulatory system of animals or the transpiration stream in the xylem tissue of plants. The best-known system for transporting ions actively is the sodium/potassium (Na/K) exchange pump, which occurs in plasma membranes of virtually all cells.

Experimental studies revealed that many transport processes, such as in bacterial cells and in the mitochondria of eukaryotic cells, are associated with a transport of protons (hydrogen ions, H⁺). This fact led to the concept of proton pumps, in which the coupling or transfer of energy between oxidation processes and synthesis of adenosine triphosphate (ATP) and between hydrolysis of ATP and transport or other cellular work is explained in terms of a flow of protons as the means of energy transfer.

The processes of oxidation in the citric acid cycle of reactions in mitochondria are known to be coupled with the synthesis of ATP, which is formed from adenosine diphosphate (ADP) and inorganic orthophosphate (P_i), through the system of enzymes and cytochromes known as the electron transfer chain or electron transport system. This system transports electrons, removed in dehydrogenation from the organic molecules of the citric acid cycle on one side of the mitochondrial membrane, to the site of their incorporation into water, formed from two hydrogen ions and an atom of oxygen on the other side of the membrane. The flow of electrons from a relatively high potential level in the organic substrate to a level of lower potential in water constitutes, in effect, a current of negative electricity, and it was proposed that the flow drives a flow of protons in the opposite direction, as a current of positive electricity. This proton flow in turn is proposed as the force that drives the synthesis of three molecules of ATP for every two electrons flowing through the electron transport system. In effect, this is the machinery of the cellular power plant.

The Na/K ATPase pump then provides an example of a way in which a proton pump may transfer energy between the hydrolysis of ATP and a process of cellular work. The enzyme which is the basis of the pump is known to be bound to the lipid bilayer of the plasma membrane through phosphatides and to function only when so bound. The binding of Na⁺, K⁺, H⁺, and ATP to active sites on the enzyme presumably has an allosteric effect, changing the shape of the enzyme molecule, activating the hydrolysis of ATP, and opening pathways of exchange of Na⁺ and K⁺.

Transport processes are involved in uptake and release of inorganic ions by plants and in distribution of ions within plants, and thus determine ionic relations of plants. The cell wall and the external lipid-protein membrane (plasmalemma) have to be passed by the ions. Intracellular distribution and compartmentation are determined by transport across other membranes within the cells. The most important one is the tonoplast separating the cell vacuole from the cytoplasm.

Within tissues the continuous cell walls of adjacent cells form an apoplastic pathway for ion transport. A symplastic pathway is constituted by the cytoplasm extending from cell to cell via small channels of about 40 nanometers diameter (plasmodesmata) crossing the cell walls. Transport over longer distances is important in organs (roots, shoots, leaves, fruits), which are composed of different kinds of tissues, and in the whole plant. Xylem and phloem serve as pathways for long-distance transport. Roots take up ions from the soil and must supply other plant organs. The nutritional status of roots and shoots regarding both inorganic anions and organic substrates plays a large role in regulation of ionic relations of whole plants. Phytohormones affect transport mechanisms; they are produced in particular tissues, are distributed via the transport pathways, and thus exert a signaling function. See Phloem, Plant hormones, Plant tissue systems, Xylem

The pipe system of the xylem in its mature transporting state is composed of rows of dead cells (tracheids, tracheary elements) whose cross-walls are perforated or removed entirely. The driving force for long-distance transport in the xylem is very largely passive. Transport is caused by transpiration, the loss of water from the aerial parts of the plant, driven by the water potential gradient directed from soil to roots, leaves, and atmosphere. A normally much smaller component driving the ascent of sap in the xylem is osmotic root pressure due to the pumping mechanisms concentrating ions in the root xylem, with water following passively. In a simplifying way the xylem can be considered as pathway for long-distance transport of ions from root to shoot, and the phloem for metabolite transport from photosynthesizing source leaves to various sinks in the plant. The long-distance transport pathways of the phloem are the sieve tubes, pipe systems with porous structures in the cross-walls (sieve plates) but, in contrast to vessels of the xylem, having living cytoplasm. Concentration and pressure gradients built up by active loading and unloading of sieve tubes in the source and sink regions, respectively, are the driving forces for transport.