Few Membrane Water Transport Studies In Plants example essay topic

Plants are complex organisms that are essential to the biological processes that occur on a regular basis on Earth. The growth and differentiation of the various plant tissue and organ systems are controlled by various internal and external factors. Internal factors in a plant are essential and comprehending these processes will allow us to understand the workings of a plant. Of these internal factors, there are numerous biochemical and physical internal movements to comprehend, which continually occur in all plants. Studies of light-sensing mechanisms are revealing important roles for ion channels. Ion channels perform signal-transducing functions in the complex array of mechanisms.

Permeation of water across biological membranes is facilitated by aquaporins, an ubiquitous membrane channel protein. This and similar proteins play a significant role in the movement of water and other materials around the interior of a plant. The combined actions of diffusion and specific auxin influx and efflux carriers transport the vital plant hormone called auxin. These carriers are quite significant to the speedy growth of plants.

Ion channels are integral membrane proteins that facilitate the movement of their ionic substrates across the lipid bilayer, a structure that would otherwise present a formidable kinetic barrier to their movement (Figure 1). Because channels are passive transporters, the direction of transport is dictated by the difference in electrochemical potential of the substrate. Zygotes of the brown algae Fucus and Pelvetia provide an example of single-celled systems with a light-sensing mechanism that appear to involve channels. Asymmetric illumination with blue light causes inward ionic currents on the region in which the rhizoid will later initiate. This change in current distribution could be produced by a clustering of channels at the rhizoid initiation site, by the local activation of channels that are uniformly distributed, or a combination of both. The preponderance of evidence indicates that "Ca^2+ carries at least a share of the current entering the site of rhizoid initiation" (Robinson, 1996), resulting in locally "higher cytoplasmic Ca^2+ concentrations detectable with fluorescent dyes and confocal microscopy" (Pu & Robinson, 1998).

Because the fucoid zygote and its rhizoid can be studied with electrophysiological techniques and is a well-established cell biological system, studies of its light responses could reveal important and generally acceptable details in plant systems. Volume changes in motor cells are responsible for the reversible movements of pul vini and stomata, and occur as an osmotic consequence of altered solute fluxes. They are demonstrably affected by light. Channels conduct the K+ influx that helps drive the volume changes but it is not strictly correct to assign these K+ channels a signal transducing role in the movement response to blue light. The evidence to date indicates that "blue light activates the plasma membrane H+-ATPase, producing and outward current that hyperpolarizes the membrane" (Kinoshita & Shima zaki, 1999). This tips the K+ electrochemical potential gradient more steeply inward, which could increase the rate of passive K+ uptake through voltage-dependant channels that are more likely to open as the membrane hyperpolarizes.

To reverse the process and have K+ leave the cell, the direction of the gradient in K+ electrochemical potential must switch from inward to outward. For this to happen, the membrane potential must change. The reigning idea is that the opening of Cl- channels and inhibition of the H+-ATPase depolarizes the membrane to an extent that makes K+ efflux a passive process meditated by outward-rectifying channels. The guard cell has proven to be a powerful system for studying the control of transport processes at the plasma membrane as well as hormone signal transduction involving ion transport. For a long time, it was assumed that water crosses biological membranes solely by simple diffusion through the lipid bilayer.

Nevertheless, membrane water permeability of some tissues is extremely high and can not be due to simple diffusion only. Koefoed-Johnsen and Using suggested for the first time that pores or channels could exist in the membranes (Figure 2). The observation that the mercurial compound pC MBS considerably decreased the water permeability in red cells and other biophysical investigations reinforced the idea that the pore could be a protein. Similarly, a few membrane water transport studies in plants also led to the discussion of water-filled pores. However, these were heavily disputed due to technical problems associated with unstirred layer effects, which underestimate the diffusional contribution to water permeability. Aquaporins have an important impact on the studies of water permeation across membranes.

Since then, "large numbers of MIP proteins", which aquaporins belong to, "have been identified in all kinds of organisms" (Johanson et al., 2001). However, the function of all these proteins is not yet known. "Most of the MIP family members have 250-300 amino acids" (Park and Saver, 1996) and a molecular mass between 27 and 31 kDa. The comparison of the primary structure of MIP proteins among different organisms and kingdoms shows that some residues are highly conserved.

In the plant kingdom, a single plant expresses a considerably larger number of MIP homologues: at least "31 in both wheat and maize" (Johanson et al., 2001). The plant MIP family can be subdivided into four subgroups that are distinguished by the presence of highly conserved amino acid sequences and other structural features like different sizes on the N- or C-termini: PIPs, TIPs and two recently established and two recently established subfamilies, NIPs (NOD 26-like MIPs) and SIPs (Small and Basic Intrinsic Proteins) (Johanson et al., 2001 p. 1362). "The PIP group can be further divided into two subfamilies named PIP 1 and PIP 2, which are each highly conserved and share a high degree of homology" (Schaffner 1998). TIP and NIP members can also be classified into several more divergent subgroups. In addition to water, some MIPs are permeable to other molecules such as urea, glycerol or even CO 2.

"MIP proteins that transport both water and glycerol", or other small, neutral molecules, "are named aquaglyceroporins" (Agre et al., 1998). Most plant MIP proteins are aquaporins when assayed in swelling experiments. PIP 1 family members usually exhibit a lower activity in this assay and they are members that obviously lack water channel activity when assayed in heterologous expression systems (Chaumont et al., 2000, p. 1031). A few plant members have been described to transport other small molecules such as formamide, urea, and glycerol in addition to water. "NOD 26 transports glycerol and formamide" (Rivers et al., 1997). "Tobacco Nt-Tips transports glycerol and urea, and NtA QP 1, a PIP 1 member, transports glycerol" (Ger beau et al., 1999).

"Auxins represent a group of naturally occurring indole molecules that act as hormones in plants" (Davies, 1995). The major form of auxin in higher plants, indole-3-acetic acid (IAA), regulates the fundamental processes of cell division, elongation and differentiation. "IAA is first synthesized within young apical tissues" (Bartel, 1997), then conveyed to basal target tissues using a specialized delivery system named polar auxin transport (PAT) (Figure 3). The polarized movement of auxin has been demonstrated to be critical for many developmental processes, including embryo patterning, vascular differentiation, and root gravitropism (Bennett et al., 1998 p. 52). "Several auxin transport mutants have been isolated to date" (Bennett et al., 1998) and, in some cases, "mutated genes were identified" (Utsuno et al., 1998). Genes of the PIN family have homology with bacterial genes coding for membrane transporters.

Proteins encoded by the AtPIN 1 and AtPIN 2 genes, "the latter also termed AGR" (Utsuno et al., 1998) or "EIR 1" (Luschnig et al., 1998), display asymmetrical cellular distribution within the elongation zones of root and shoot tissues. This suggests that "the PIN gene products might support directional auxin efflux in transport-competent cells" (Galweiler et al., 1998). It has been proposed that "AtPIN 1 mediates PAT throughout the stem and root in plants" (Galweiler et al., 1998), whereas "AtPIN 2 contributes to localized auxin redistribution in the root tip" (Muller et al., 1998). There is evidence that the "AUX 1 protein might have a role in the uptake of auxin into the cell" (Marchant et al., 1999). Mutations within the AUX 1 gene confer an auxin-insensitive phenotype. The altered root-growth response is specific to auxins requiring carrier-mediated uptake, and can be bypassed by growing AUX 1 plants in the presence of the auxin 1-naphthalene acetic acid (1-NAA), which enters plant cells in a carrier-independent manner (Marchant et al., 1999).

Mutations in AUX 1 and AtPIN 2 confer an agravitropic root-growth phenotype. Marchant et al. (1999) have proposed that AUX 1 regulates gravitropic curvature in unison with the auxin efflux carrier to coordinate the localized redistribution of auxin in the root apex, providing the first example of a developmental role for the auxin influx carrier in higher plants. Effective biochemical movement of materials in plants is essential to the organism's survival. These reactions and movements inside the plant allow the conduction of vital processes.

Photosynthesis, stomatal movements, movement of water, and growth all require the biochemical input of cell to cell movement. Ion channels, aquaporins and influx and efflux carriers are active examples of devices that move materials in plants for the benefit of the organism. These devices prove that biochemistry and movement in general is essential in the processes of life and is essential to comprehend for any individual who investigates the life sciences.