17.1: Water Transport - Biology

17.1: Water Transport - Biology

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Water potential, transpiration, and stomatal regulation influence how water and nutrients are transported in plants. Water potential refers to the potential energy in water, and water moves towards the areas with the lowest water potential. Water is ultimately pulled to the top of the plant (cohesion-tension theory), and lost through transpiration through stomata. Complex mechanisms control stomatal opening and closure. Both adaptations that increase absorption of water through the roots (Figure (PageIndex{1})) and those that limit transpiration ensure that plants collect and retain enough water.

Specialized membrane domains for water transport in glial cells: high-resolution immunogold cytochemistry of aquaporin-4 in rat brain

Membrane water transport is critically involved in brain volume homeostasis and in the pathogenesis of brain edema. The cDNA encoding aquaporin-4 (AQP4) water channel protein was recently isolated from rat brain. We used immunocytochemistry and high-resolution immunogold electron microscopy to identify the cells and membrane domains that mediate water flux through AQP4. The AQP4 protein is abundant in glial cells bordering the subarachnoidal space, ventricles, and blood vessels. AQP4 is also abundant in osmosensory areas, including the supraoptic nucleus and subfornical organ. Immunogold analysis demonstrated that AQP4 is restricted to glial membranes and to subpopulations of ependymal cells. AQP4 is particularly strongly expressed in glial membranes that are in direct contact with capillaries and pia. The highly polarized AQP4 expression indicates that these cells are equipped with specific membrane domains that are specialized for water transport, thereby mediating the flow of water between glial cells and the cavities filled with CSF and the intravascular space.


AQP4 immunoblots of membrane fractions…

AQP4 immunoblots of membrane fractions from rat brain. A , Membrane fraction (10…

Immunocytochemical localization of AQP4 in…

Immunocytochemical localization of AQP4 in rat brain. A , Cryosection of cerebellar cortex.…

AQP4 in glial processes. Low-magnification…

AQP4 in glial processes. Low-magnification electron micrograph showing the distribution of AQP4 immunoreactivity…

Polarized expression and membrane topology…

Polarized expression and membrane topology of AQP4 in glial cells. Many immunogold particles…

AQP4 in glial lamellae but…

AQP4 in glial lamellae but not in neurons of the supraoptic nucleus. A…

Glial and ependymal expression of…

Glial and ependymal expression of AQP4 in the subfornical organ. A , Immunogold…

Water Transport, the Role in Plant Diversification of

J. Pittermann , . T.J. Brodribb , in Encyclopedia of Evolutionary Biology , 2016


Efficient water transport is paramount to the success of land plants. The evolution of xylem tissue was driven in part by the requirement to move water efficiently from roots to shoots, and by the need to withstand drought and freezing stress, which may block water transport by filling conduits with air. This review explores the links between xylem traits and the diversification of select plant groups, highlighting Devonian experiments in water transport, the Cretaceous evolution of high leaf vein density, and the challenges that Cenozoic climates and angiosperm competition imposed on water transport in conifers and ferns.

REACTIVE OXYGEN SPECIES: Metabolism, Oxidative Stress, and Signal Transduction

Klaus Apel and Heribert Hirt
Vol. 55, 2004


▪ Abstract Several reactive oxygen species (ROS) are continuously produced in plants as byproducts of aerobic metabolism. Depending on the nature of the ROS species, some are highly toxic and rapidly detoxified by various cellular enzymatic and . Read More

Figure 1: Generation of different ROS by energy transfer or sequential univalent reduction of ground state triplet oxygen.

Figure 2: The principal features of photosynthetic electron transport under high light stress that lead to the production of ROS in chloroplasts and peroxisomes. Two electron sinks can be used to alle.

Figure 3: The principal modes of enzymatic ROS scavenging by superoxide dismutase (SOD), catalase (CAT), the ascorbate-glutathione cycle, and the glutathione peroxidase (GPX) cycle. SOD converts hydro.

Figure 4: Schematic depiction of cellular ROS sensing and signaling mechanisms. ROS sensors such as membrane-localized histidine kinases can sense extracellular and intracellular ROS. Intracellular RO.

Figure 5: Different roles of ROS under conditions of (a) pathogen attack or (b) abiotic stress. Upon pathogen attack, receptor-induced signaling activates plasma membrane or apoplast-localized oxidase.

Water Is an Excellent Solvent

Because water is polar, with slight positive and negative charges, ionic compounds and polar molecules can readily dissolve in it. Water is, therefore, what is referred to as a solvent—a substance capable of dissolving another substance. The charged particles will form hydrogen bonds with a surrounding layer of water molecules. This is referred to as a sphere of hydration and serves to keep the particles separated or dispersed in the water. In the case of table salt (NaCl) mixed in water (Figure 3), the sodium and chloride ions separate, or dissociate, in the water, and spheres of hydration are formed around the ions. A positively charged sodium ion is surrounded by the partially negative charges of oxygen atoms in water molecules. A negatively charged chloride ion is surrounded by the partially positive charges of hydrogen atoms in water molecules. These spheres of hydration are also referred to as hydration shells. The polarity of the water molecule makes it an effective solvent and is important in its many roles in living systems.

Figure 3. When table salt (NaCl) is mixed in water, spheres of hydration form around the ions.


Osmosis is the movement of water through a semipermeable membrane according to the concentration gradient of water across the membrane, which is inversely proportional to the concentration of solutes. While diffusion transports material across membranes and within cells, osmosis transports only water across a membrane and the membrane limits the diffusion of solutes in the water. Not surprisingly, the aquaporins that facilitate water movement play a large role in osmosis, most prominently in red blood cells and the membranes of kidney tubules.


Figure 6. In osmosis, water always moves from an area of higher water concentration to one of lower concentration. In the diagram shown, the solute cannot pass through the selectively permeable membrane, but the water can.

Osmosis is a special case of diffusion. Water, like other substances, moves from an area of high concentration to one of low concentration. An obvious question is what makes water move at all? Imagine a beaker with a semipermeable membrane separating the two sides or halves (Figure 6). On both sides of the membrane the water level is the same, but there are different concentrations of a dissolved substance, or solute, that cannot cross the membrane (otherwise the concentrations on each side would be balanced by the solute crossing the membrane). If the volume of the solution on both sides of the membrane is the same, but the concentrations of solute are different, then there are different amounts of water, the solvent, on either side of the membrane.

To illustrate this, imagine two full glasses of water. One has a single teaspoon of sugar in it, whereas the second one contains one-quarter cup of sugar. If the total volume of the solutions in both cups is the same, which cup contains more water? Because the large amount of sugar in the second cup takes up much more space than the teaspoon of sugar in the first cup, the first cup has more water in it.

Returning to the beaker example, recall that it has a mixture of solutes on either side of the membrane. A principle of diffusion is that the molecules move around and will spread evenly throughout the medium if they can. However, only the material capable of getting through the membrane will diffuse through it. In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system. Thus, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane—osmosis—will continue until the concentration gradient of water goes to zero or until the hydrostatic pressure of the water balances the osmotic pressure. Osmosis proceeds constantly in living systems.

Monday, June 19, 2017

Chiral water in DNA's hydration shell

In a clever study of DNA hydration using SFG spectroscopy, Poul Petersen and his coworkers have found that the chiral spine of hydration in the minor groove, inferred from oxygen locations for hydrated crystalline DNA by Dickerson and collaborators in the 1980s, exists also in aqueous solution under ambient conditions, and entails orientational ordering of the hydrogen bonds in the single-file water chain that fits into this narrow groove (M. L. McDermott et al., ACS Centr. Sci. 10.1021/acscentsci.7b00100 2017 – paper here). I wrote a news story for Chemistry World on this work (here).

I applaud the ambition of Modesto Orozco of the Barcelona Institute of Science and Technology and colleagues in writing a paper called “The multiple roles of waters in protein solvation” (A. Hospital et al., JPCB 121, 3636 2017 – paper here). There’s a title guaranteed to say to me “Read this now!” And the ambition continues in the extent of the systems they investigate with MD: a range of proteins, at a range of temperatures, some denatured, some with crowding agents, some with high concentrations of urea. They say that the results illustrate “the dramatic plasticity of water, and its chameleonic ability to stabilize proteins under a variety of conditions”, which seems a fair way to summarize the matter. I’m not sure I see any surprises here, and the denaturant effects of urea are discussed with something of a “water structure” flavour, but it’s a kind of snapshot of the sorts of things hydration water gets up to.

A more specific study of protein hydration dynamics is described by Dongping Zhong and colleagues at Ohio State University, who use tryptophan as the reporter group to characterize the dynamics at 17 sites on the surface of the β-barrel protein rat liver fatty acid binding protein (J. Yang et al., JACS 139, 4399 2017 – paper here). They observe three quite distinct dynamical timescales. The water in the outer hydration layer is bulk-like, relaxing quickly (hundreds of fs). For the inner layer, reorientational motion happens on a few-ps timescale, while larger-scale network restructuring takes many tens of ps. The last of these seem to drive protein fluctuations on comparable timescales.

The dynamics of the protein hydration layer are examined by Biman Bagchi and colleagues of the Indian Institute of Science in Bangalore by calculating those around residues (Trp, Tyr, His) previously used as natural probes in spectroscopic studies (S. Mondal et al., arxiv preprint 1701.04861). They find a range of different timescales, including accelerated as well as retarded rotations. Since NMR measurements give average values, these findings might explain the apparently discrepancy between such studies and those (such as Zewail’s) that focus on specific residues. The protein side-chain dynamics seem particularly to influence the slow solvation component.

The role of hydration in the protein dynamical transition around 230 K has been widely debated. Prithwish Nandi and Niall English at University College Dublin find in MD simulations of lysozyme that the protein and hydration water dynamics seem to be correlated up to about 285 K, at which point the protein-water hydrogen-bond network becomes too disrupted to sustain the coupling (JPCB 120, 12031 2016 – paper here).

However, the whole notion of coupling between the protein and hydration dynamics in the vicinity of the

200-220 K dynamical transition is challenged by Antonio Benedetto of University College Dublin on the basis of elastic neutron-scattering from lysozyme (arxiv preprint 1705.03128). Specifically, the water begins to relax at 179 K, while the protein doesn’t do so until 195 K. It seems puzzling, and no explanation is advanced here for the discrepancy with a considerable body of earlier results.

I missed previously this nice paper from H. F. M. C. Martiniano and Nuno Galamba in Lisbon on the structure and dynamics of water around a hydrophobic amino acid (PCCP 18, 27639 2016 – paper here). It reports MD simulations of the hydration of valine, and distinguishes between two populations of water molecules in the hydration shell: those that have have four and less than four neighbours. The latter, they say, have faster librational dynamics than bulk water and faster orientational dynamics than four-coordinated “tetrahedral” water. Meanwhile, four-coordinate water in the hydration shell are “more tetrahedral” than bulk water at all temperatures. It would seem, then, that this work argues the case for “tetrahedrality” as a useful concept for characterizing water structure, while advising caution about how it is used and interpreted for the bulk.

Guanidinium is a complicated osmolyte. It can act as both a protein denaturant and stabilizer, depending on the counteranion. Jan Heyda at the Institut für Weiche Materie und Funktionale Materialien in Berlin and colleagues have setout to understand why, using MD simulations and FTIR (J. Heyda et al., JACS 139, 863 2017 – paper here). Their test peptide, an elastin-like polypeptide, was stabilized in the collapsed state by Gnd sulphate by an excluded volume effect (Gnd being depleted at the peptide/water interface). GndSCN was stabilizing at low concentrations thanks to Gnd+’s ability to crosslink the polymer chains, but at higher concentration it became a denaturant. GndCl, meanwhile, was a denaturant at all concentrations, since in this case partitioning of the chloride to the polymer surface enables recruitment of Gnd+ to the surface too, where it stabilizes the unfolded state. A very graphic example of how the details of direct interactions between polymer, anion, cation (and potentially water) all matter in figuring out what is going on.

Essentially the same team – which includes Paul Cremer, Joachim Dzubiella and Pavel Jungwirth – have put together a review of such ion-specific effects that, it seems to me, will be the go-to resource for this field for some time to come (H. I. Okur et al., JPCB 121, 1997 2017 – paper here). I need say no more if you want to understand how the thinking on Hofmeister has developed over the past several years, this is where to come.

Does water play the role of reactant in O-O bond formation in photosystem II? That idea has been suggested, water acting as a nucleophile that attacks a terminal oxo group. But Per Siegbahn of Stockholm University uses DFT calculations to determine the free-energy barriers for the six most plausible modes of attack and finds that these barriers are all too high (PNAS 114, 4966 2017 – paper here) – a notion put forward previously but here refined using improved structural data and computational methods.

I didn’t even know that lipid bilayers, like proteins, show a dynamical transition around 200 K or so. But it seems they do. V. N. Syryamina and S. A. Dzuba of the Russian Academy of Sciences in Novosibirsk have studied thus for two types of phosphocholine bilayers in water using a technique (also new to me) called electron spin echo envelope modulation spectroscopy to follow hydrogen (deuterium) motions (JPCB 121, 1026 2017 – paper here). They find that the dynamical transition in the bilayer interior at 188 K is accompanied by the onset of water motion in the first hydration layer, and that another transition around 100 K is accompanied by restricted reorientational motions of water. What I can’t tell from these results is whether there is any sign of slaving of water to lipid dynamics or vice versa.

I’m not going to pretend to understand the Bayseian model used by Nathan Baker of PNNL in Washington and colleagues to estimte small-molecule solvation free energies (L. J. Gosink et al., JPCB 121, 3458 2016 – paper here). But it’s basically a method for aggregating many other calculational procedures, and seems to work better than any such techniques in isolation.

Mihail Barbiou of the European Institute of Membranes in Montpellier and colleagues have used artificial water channels in liposomes, made from stacked imidazoles, to investigate water transport along water wires, analogous to those that thread through aquaporins (E. Licsandru et al., JACS 138, 5403 2016 – paper here). The channels can conduct around a million water molecules per second, a rate two orders of magnitude greater than AQPs, and also conduct protons (but not other ions) efficiently. The chirality of the channels seems to be important for producing strong dipolar orientation in the water wire. Let me also draw attention to Mihail’s nice review of artificial water channels, which includes this example, in Chem. Commun. 52, 5657 (2016) (paper here).

The water channel in stacked imidazoles.

More on water confined in pores: in MD simulations, Xiao Cheng Zeng at the University of Nebraska and colleagues see low- and high-density liquid states of water within single-walled carbon nanotubes of 1.25 nm diameter at ambient temperature (K. Nomura et al., PNAS 114, 4066 2017 – paper here). The two phases are, however, separated by a hexagonal “tubular ice” phase (which has already been observed experimentally).

How does water freeze at liquid-vapour interfaces? Specifically, does the interface itself nucleate or suppress freezing? That’s a question relevant to a host of real-world phenomena such as ice nucleation in clouds and other atmospheric processes, but it’s been hard to study experimentally, but Amir Haji-Akbari and Pablo Debenedetti in Princeton study it computationally in a free-standing 4-nm-thick water nanofilm (PNAS 114, 3316 2017 – paper here). Although the rate of ice nucleation in this confined geometry is seven orders of magnitude greater than that in the bulk, nucleation doesn’t start in the surface layers but rather in the (non-bulk-like) interior of the film, where the conditions favour the formation of “double-diamond” water cages that serve as the seeds for the nucleation and growth of cubic ice.

And here’s a truly surprising thing, discovered by Pablo and Amir in another paper working with Elia Altabet: making hydrophobic plates confining water to a space just over 1 nm wide more flexible by just an order of magnitude decrease in the modulus increases the evaporation rate by nine orders of magnitude, and decreases the condensation rate from the vapour by no less than 24 orders of magnitude, changing the timescale of the process from nanoseconds to tens of millions of years (Y. E. Altabet et al., PNAS 114, E2548 2017 – paper here). This, at any rate, is what is implied by simulations for plates 3 nm square. Evaporation proceeds via the formation of bubbles at the surfaces that then grow and coalesce to form a gap-spanning cavity. For stiff plates this coalescence is rare, and so is the subsequent growth of the cavity above the critical size for nucleation of the vapour phase. For softer, more flexible plates these configurations occur much more frequently. Such a sensitivity of a drying transition to subtle changes in the mechanical properties may well have implications for processes involving hydration changes at or close to membrane proteins, and could presumably have ramifications for materials design of surfaces on which protein adhesion needs to be controlled.

Optimization of lead compounds for drug discovery is a complicated business, and when this is done by empirical combinatorial screening, the results can sometimes be counterintuitive, with nonpolar groups in the ligand juxtaposed to polar groups in the target for example. Ariel Fernandez at the Argentine Institute of Mathematics and Ridgway Scott of the University of Chicago review a method for understanding some of those apparent conundrums that involves a consideration of the relevant hydration structures, and in particular the role of what Ariel calls dehydrons (water-exposed backbone hydrogen bonds, which lead to frustration in the hydrogen-bonding arrangements of adjacent water molecules) (Trends Biotechnol. 35, 490 2016 – paper here). Their approach uses the WaterMap software to identify “hot” water molecules that might profitably be displaced by a ligand to increase the binding energy and drug specificity.

The hydrogen-bond network of pure water is of course riddled with defects which underpin fluctuations of the network. Because of topological constraints these tend to occur in correlated pairs. Ali Hassanali at the ASICTP in Trieste and colleagues have studied these correlations using ab initio modelling (P. Gasparotto et al., J. Chem. Theor. Comput. 12, 1953 2016 – paper here). They say that the defect pairs have some similarities to those in solid states of water, and are rather insensitive to the details of the water potentials used.

One of water’s well known “anomalies” is the decrease in viscosity with increasing applied pressure, which seems to be a consequence of a collapse of the hydrogen bonding network. This effect is larger at low temperatures, but whether that trend continues into the supercooled region hasn’t been studied previously. Now Frédéric Caupin and colleagues at the University of Lyon have investigated this effect down to 244 K and for pressures of up to 300 MPa, and find that indeed the viscosity reduction can be dramatic – by as much as 42% (L. P. Singh et al., PNAS 114, 4312 2017 – paper here). They argue that the results can be understood by invoking a two-state model under these conditions: a mixture of a high-density “fragile” liquid and a low-density “strong” liquid.

Finally, I have taken what I hope is a somewhat fresh look at the many roles of water in molecular biology in an article for PNAS, for a special issue on water (2017 – paper here), which I hope extends the general message of my 2008 Chem Rev article (paper here) using some more recent examples.

Transport in Plants

Multicellular plants have a small surface area: volume ratio so diffusion would be too slow to provide necessary substances like water, minerals and sugars and to remove waste substances. Also multicellular plants are large so have a greater demand for substances. Therefore plants need transport systems to move substances to and from individual cells quickly.

(b)describe, with the aid of diagrams and photographs, the distribution of xylem and phloem tissue in roots, stems and leaves of dicotyledonous plants

(c) describe, with the aid of diagrams and photographs, the structure and function of xylem vessels, sieve tube elements and companion cells

(d) define the term transpiration

Transpiration is the evaporation of water from a plant’s surface, especially the leaves.

Transpiration involves 3 processes:

  1. Water leaves the xylem and passes to the mesopyll cells by osmosis.
  2. Water evaporates from the surface of the mesophyll cells, to form water vapour, into the air spaces.
  3. The water vapour potential in the leaf is higher than outside, so water molecules will diffuse out of the leaf.

(e) explain why transpiration is a consequence of gaseous exchange

It is a side effect of gas exchange as a plant needs to open its stomata to let in carbon dioxide so that it can produce glucose by photosynthesis, but it also lets water out as there’s a higher concentration of water inside the leaf than in the air outside water moves out of the leaf down the water potential gradient when the stomata is open.

(f) describe the factors that affect transpiration rate

(g) describe, with the aid of diagrams, how a potometer is used to estimate transpiration rates

A potometer is used to estimate the rate of water loss. It is not an exact measure, as it actually measures the rate of water uptake by a cut shoot. It is important to make sure there are no air bubbles inside the apparatus. Water lost by the leaf is replaced from the water in the capillary tube. The movement of the meniscus at the end of water column can be measured.

  • Cut a shoot underwater to prevent air from entering the xylem
  • Cut ashoot at a slant to increase the surface area available for water uptake
  • Dry the leaves
  • Use non-wilting shoots
  • Allow time for equilibrium and for it to acclimatise
  • Note where meniscus is at start and end of time period

(h) explain, in terms of water potential, the movement of water between plant cells, and between plant cells and their environment

(i) describe, with the aid of diagrams, the pathway by which water is transported from the root cortex to the air surrounding the leaves, with reference to the Casparian strip, apoplast pathway, symplast pathway, xylem and stomata

  1. Water enters from the soil to the root hair and epidermis through osmosis – from a higher water potential (soil) tothe most negative water potential (xylem)
  2. Water enters the cortex by the apoplast pathway between cell walls, the symplast pathway through plasmodesmata and the vacuolar pathway
  3. Water enters the endodermis which has a Casparianstrip which blocks the apoplast pathway so water must be transported by the symplast pathway allowing selective mineral uptake
  4. Water enters the xylem and minerals are moved using active transport which reduces the water potential in the xylem creating a water potential gradient. Water can’t pass through to the cortex again as the endodermis is blocked

(j) explain the mechanism by which water is transported from the root cortex to the air surrounding the leaves, with reference to adhesion, cohesion and the transpiration stream

  1. Minerals are actively transported into the xylem vessels. This lowers the water potential in the xylem and waterfollows by osmosis.
  2. Root pressure pushes some of the water upwards.
  3. Water evaporates from the surface of the leaf by transpiration and water is lost.
  4. The water must be replaced as water moves out of the xylem into the leaf, creating a low hydrostatic pressure, and a pressure gradient, and thus tension.
  5. Water molecules are attracted to each other by forces of cohesion creating a continuous column of water so that water can be moved by mass flow, pulled upwards by tension from above.
  6. Water molecules are also attracted to the walls of the xylem by forces of adhesion and causing capillary action.

(k) describe with the aid of diagrams and photographs, how the leaves of some xerophytes are adapted to reduce water loss by transpiration

A xerophyte is a plant that is adapted to reduce water loss so that it can survive in very dry conditions e.g. marram grass, cacti etc.

(l) explain translocation as an energy-requiring process transporting assimilates especially sucrose, between sources (e.g. leaves) and sinks (e.g. roots, meristems)

Translocation is the movement of assimilates (e.g. sugars – sucrose) through the phloem tissue and is an energy-requiring process. It moves from source to sink.

  • Source: where the assimilates are made/came from (e.g. leaves)
  • Sink: where the assimilates are used/stored (e.g. roots, meristems)

(m) describe, with the aid of diagrams, the mechanism of transport in phloem involving active loading at the source and removal at the sink, and evidence for and against this mechanism

8.2) Water uptake

The root hairs are where most water absorption happens. They are long and thin so they can penetrate between soil particles, and they have a large surface area for absorption of water.

Water passes from the soil water to the root hair cell’s cytoplasm by osmosis. This happens because the soil water has a higher water potential than the root hair cell cytoplasm:

Osmosis causes water to pass into the root hair cells, through the root cortex and into the xylem vessels

The large surface area of root hairs increases the rate of the absorption of water by osmosis and ions by active transport

The elongated section of the root hair, basically provides a large surface area for the absorption of water and inorganic ions.

Additionally, the membrane of the root hair cell is semi-permeable. What that means is basically only minerals and water can go through the membrane, but not necessarily go back out.


Phloem tubes carry food substances like sugar and amino acids produced in leaves during photosynthesis to every part of the plant.

The movement of food substances through the plant is called translocation.

Phloem tubes are made up of columns of living cylindrical cells. The cell walls between adjoining cells develop holes like a sieve allowing transport through the tube.

The image below shows the structure of the xylem and phloem

The table below summarise the main points about the xylem and phloem

Watch the video: Xylem and Phloem - Transport in Plants. Biology. FuseSchool (June 2022).


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