Category: Cellular

Introduction

As life on Earth is primarily composed of water it is vital for the quick and accurate control of water at the cellular level. Although water and other molecules are capable of passing through the plasma membrane their movement is restricted by the non-polar area of the lipid bilayer (Diamond, 2012; Kozono et al., 2002). So life has developed a way, the aquaporin, to expedite the flow of water in both directions through their membrane while being able to exercise greater control of flow rate and restrict access to other molecules (Kozono et al., 2002). There are many different kinds of aquaporin channels, there is 13 different kinds in mammals (Aquaporin, n.d.) and at least 30 different kinds in plants (Cushman, 2001), and they vary in their permeability to other molecules (Kozono et al., 2002). Most aquaporins are not able to open and close to restrict the flow of water (Kozono et al., 2002), however, a cells aquaporin could vary in density and location thereby influencing flow potential.

Selective Permeability

There are several factors that allow the aquaporins to restrict access of non-water molecules through aquaporin channels, see figure 1. The first being the physical size restriction. Water molecules passing through an aquaporin are physically restricted to pass through the pore one molecule at a time (Kozono et al., 2002). This physical restriction prevents any molecule greater than the size of restriction to pass through, and the size of the restriction varies between the different types of aquaporins (Kozono et al., 2002). Aquaporins with the smallest restrictions allow only water and molecules smaller than water with a neutral charge (because they are not affected by the electrostatic factor) through the channel (Aquaporin, n.d.); whereas aquaporins with larger openings allow in molecules greater than the size of water molecules, such as glycerol C₃H₈O₃, urea CH₄N₂O, ammonia NH₃, oxygen O₂, and carbon dioxide CO₂, these are called aquaglyceroporins (Aquaporin, n.d.).

The second factor is electrostatic repulsion which prevents the penetration of ions smaller than water (Diamond, 2012). Ions such as potassium K⁺, ammonium NH₄⁺, sodium Na⁺, and calcium Ca⁺², and hydronium H₃O⁺ are prevented by the electrostatic center from passing through the channel (Aquaporin, n.d.). Because of the electrostatic attraction the water molecules will rotate and face its partially negative charged oxygen molecule towards the positive center of the channel (Kozono et al., 2002). This spin is believed to prevent H⁺ that may be bonded to H₂O from passing through the channel (Kozono et al., 2002). By preventing the flow of ions through the aquaporins the cell is better able to actively control its electrochemical potential by the use of other passive and active channels through the plasma membrane (Diamond, 2012). That is a cell can actively pump in/out ions with other channels to change its internal ion concentration thereby promoting an in/efflux of water through passive aquaporin channels. Or a cell could open or close other passive ion channels to influence its permeability to ions.

Other Functions

The aquaporin channels are passive in that the water flows in and out without the input of cellular energy, however, cells are able to exercise some active control such as gating which physically opens or closes aquaporin channels (Cushman, 2001; Aquaporin, n.d.). Control is also possible by actively changing the cells internal ion concentration with the use of ion channels to influence rates of water diffusion though passive aquaporins (Diamond, 2012).

Aquaporins are most abundant in areas of the plant where large fluxes of water occur such as roots, xylem, and phloem (Cushman, 2001). In plants during times of drought stress the creation of some types of aquaporin channels are increased while others are reduced (Cushman, 2001). Also in response to drought or flood some aquaporins in plants can be gated to prevent an influx or efflux of water (Diamond, 2012); there is two different gating responses depending on drought or flood, but both cause a restriction in the channel which prevents the flow of water and other molecules through aquaporin channels (Aquaporin, n.d.). The soluble chemical mercuric chloride HgCl₂ is known to inhibit aquaporin flow when added to plants (Aquaporin, n.d.).