WATER AND PLANT CELLS

Objective of General Instruction

Studentsare expected to knowandbe able toexplainthe propertiesandfunctions ofwaterfor the plant.

Students are able tounderstandandexplain theconcept ofthe movement ofwaterthat affectsvarious physiological processesin plants.

Objective of Specific Instruction

Studentsare expected tounderstandand be able to explainthe natureandfunction ofwateras anessentialcomponentin the life which isnecessarytounderstandall ofthe physiological processesinplants.

Students are able tounderstandandexplain themass flow, diffusionandosmosis. Understanding of themovement ofwaterwillfacilitate the understanding ofthe process ofmineralnutrientsabsorption fromthe soil, transpirationandtranslocation.

The waterin theplant body

Water is essential component for all living things including plants. Why do plants need water? Water maintains turgor pressure of plant cells. Water facilitates the transport of nutrients absorbed by the roots from the soil to be distributed to all parts of the plant. Water is also a major component in the process of photosynthesis to produce carbohydrates that determine the productivity of agricultural crops. On the other hand, the water is also involved in the regulation of temperature in the plants body through the process of transpiration. In this condition water contributes in cooling system of plant body.

Some plants have a water content reaches about 90-95 % (Taiz and Zeiger, 2013). When the water content is very low, for example in seeds caused them to enter a dormancy phase in which the metabolic processes occur very slow or even stopped. High water content in plant cells shows that water has very important role in the biochemical reactions that occur in the plant body. Plant physiology is the study of plant processes and functions at the level of cell, tissue, organ or individual plants. Therefore, in studying the physiology of the plant and for understanding plant function it should begin with an understanding of the properties of water that support metabolic processes in plant cells.

Polarandhydrogenbondingproperties of water

Water, H2O, isa moleculecomposedofoneoxygenatomwithtwohydrogenatoms. Position ofthe Hatomsinwatermoleculesforms an angle of104.45ºCwithOatomsto formstrongpolar covalent bonds (Figure 1.1). Oxygenhas8protons, while hydrogenonlyhas1proton. Electron in water moleculeis notevenly distributed between thenegatively charged (-)oxygen and thepositively charged(+) hydrogenatom.Thismeans thatoxygenhas higherelectronaffinity(more electronegative) to attract electronsthanhydrogen. As a result, wateris apolarmolecule.Polarity ofthe watermoleculescreatesallthe unique propertiesof watersuch assurface tension, freezing point, andsolubility.

(Lodish et al., 2007)

Figure1.1Polarcovalent bondto water molecules

In addition covalent bonds, liquid water also has hydrogen bonds. Hydrogen bonding occurs when an atom of hydrogen is attracted by rather strong forces to two atoms instead of only one (Pauling, 1948).

Ahydrogenbondinwater moleculesis theattractive forcesbetweenpolarmoleculesin whichthe hydrogen(H) bound to thehighelectronegativeatom, oxygen(O) (Figure 1.2). Hydrogen bondscanoccurbetweenmolecules(intermolecular) orin adifferent part ofthemolecule(intramolecular). Hydrogen bonds between water moleculs result cohesion while hydrogen bonds between water molecules and other molecules such as cellulose result adhesion. Intermolecularhydrogen bondslead tothe highboiling pointof water(100 °C).

(Hayley Biology, 2013)

Figure1.2. Model ofthe hydrogen bondsbetweenwater molecules

Properties ofwater in plant physiology

As mentioned in the former that many biochemical processes in plants involve water which is supported by its specific properties. The shape of water molecules, its polarity, and hydrogen/covalent bonding make them as unique molecules. Some properties of water are relatively high cohesive and adhesive forces, high latent heat of vaporization, high heat capacity and incompressible.

A. The thermal properties of water

Waterisliquidatphysiologicaltemperature(i.e between0-100ºC). It means that water involves in physiological processes only at liquid phase. In other words, inanyplacelifecanonlyoccurbetweena temperature of 0and100 ºC. Temperatures below 0 °C will inhibit a significantchemicalmetabolism, while temperatures above100ºCtend to damage/break thechemical bonds.

Waterhas ahighheat of vaporization. Large quantitiesof energy (about44kJmol-1) is requiredtochangewaterfromliquidphase togas phase at constant temperature. Those high energy is required to break hydrogen bonds between water molecules. When large quantities of heat from radiant of the sun captured by the leaf surfaces are used to form gas phase consequently the plant temperature will decrease. These propertiesare responsible forthe use ofwateras anevaporativecooling system. This mayexplainwhy theleavestranspiration has an important role in plants temperature regulation. In is noted thatthe waterin liquid phase seemed to'postpone' evaporation.

Waterhas ahighspecific heat(heat capacity). Heat capacity is the capacity of water to raise the temperature of a substance. Heat capacity shows the capability of substance to absorb heat energy. While specific heat is the specific amount of heat in calories needed to raise the temperature of1gramof water by1degree celsius. The raise of temperature is required to break the hudrogen bonds between water molecules. Waterhas a quitelargespecific heat. This means thatliquidwatercanabsorba relativelylargeamount ofheatbeforeboilingandevaporating. A lot of energy(4.184 Jg-1 C-1, or theunitnon-SIis thecaloriesthat1cal=4.184J) is required toraise the temperature ofwatertobreak thehydrogen bonds.

Both of high specificheat capacity andhighheat of vaporization of water resultof thestrong hydrogen bondsbetweenthe molecules. Thus, wateris slowtoheat upandcool down, orin other wordsthe water'slow' changes in temperature. Thistraitis importantin the role ofwateras athermalbuffer. Soit is not surprisingthat thedesertplant is asucculentthatcantolerate againsttemperaturefluctuations.

B. Cohesion, adhesion and surface tension supporting water capillarity

Large amountsof energyare neededtobreak throughthe surface of thewater, because thewatermoleculesat thesurfacearemore strongly attractedbyotherwatermoleculesin the liquid(cohesion) more thanattracted by thewatermoleculesin the air. Cohesion cause the water has high tensile strength known as the maximum force per unit area. Water hasa veryhighsurface tension(Figure 1.3) – the energy required to increase the surface area. Due to the high tension of water thedrops ofwater are spheric (molecules tend to stick together)(Figure 1.4) and the existence of meniscus shape at leaf surface during transpiration processes.

When water is transported through the xylem there is an attraction between water molecules and solid phase of cell walls known as adhesion. The surface tension together with cohesion and adhesion canmove upthe water through thexylemvessels. Capillary actionrefers to the tendencyof watertomove upa narrowtubeagainst the force ofgravity(Figure 1.3, 1.4). Thistraitis veryimportant forgrowth of allvascular plants, such astrees.

(Hyperphysic, 2013)

Figure1.3. Surface tension and capillary actionof water

(Anonym, 2013)

Figure1.4. Thesurface tensionof waterthatcauses the water toformdrop

If thewateris loadedintothetube and at both endsputpistonthen at the certain time the pistonwill not beable topushtogether. This showsthat thewateris a goodhydraulicsystembecause whenpresseddoes notcompressandgeneratepositivepressure(hydrostatic pressure). Thispressureprovidesthe driving forceforcell growthandmovementin plants. Pressureis measured inPascals(or actuallymegapascals, MPa). OneMPais roughly equal totenatmospheresor10bar. Conversely, when the water resists the pull it gives negative hydrostatic pressure.

C. Water in biochemical reactions

Wateris theuniversalsolvent. Water is a good polarsolventandis oftenreferred as theuniversalsolvent. Water dissolvesmoredifferentkinds ofmoleculesthanother solvents. Substances thatdissolve inwater, e.g, salts, sugars, acids, alkalis, andsomegases-especiallyoxygen, carbondioxide(carbonation) are knownashydrophilicsubstances(like water), whilethosewhodo notmix withwater(eg, fatsandoils), knownashydrophobicsubstance(not like water). Most of themajor componentsin cells(proteins, DNAandpolysaccharides) are also inthe aquatic environment.

Watertransparentto light. Wateris transparentinthevisibleelectromagneticspectrum. This is importantbecause thechloroplastsinthecellobviouslysurroundedbytransparent water so that photosynthesis process can occur. From an ecological perspective, light penetration in the depth ofwaterdeterminethe distribution ofaquatic plants. Aquatic plantscan liveinwaterbecausesunlightcanreach them.

Water isinert/chemicallyinert. This meansthat thewaterdoes notreactunlessthe water reactsenzymatically.

Waterdissociates intoprotonsandhydroxideions. This character is important to determine thesystempH.Biologicalsystemsare verysensitive topH. Most of thebiologically activesystemrequiresbuffer. Watercanionizeto formahydrogenion(proton /H+) and hydroxide ions(OH-). Acidic compoundsactas aprotondonor, potentially increasing [H+]. For exampleHClH++Cl-.

In contrast, an alkalinecompoundwhichacts as aprotonacceptor, potentially increasing [OH-]. Thiscompoundhas the potential toreducethe concentration ofprotons.

Contoh : NaOH  Na+ + OH- (if accept proton will form wate)

NH3 (ammonia ) + H  NH4+ (ion ammonium)

The pH scalerangesfrom0 to14. Water is neutralwith a pH of7. CompoundshavingacidicpH rangeof 0 to7,whileapproaching thebasic compoundhavinga pHrangeof7-14. Enzymereactivityis verysensitive topHandbuffersolutionsneededto function.

Wateraffects theshape, stabilityandproperties ofbiological molecules. For example, manyions(such as sodium) andmolecules(such as DNAandwall components) are usuallyhydrated. Thismeansthatwaterishydrogen bonded tothemandinsomecases(e.g, sodium) form ahydrationshellaround it.

Based onsomepropertiesabovethefunction of waterisas follows: the maincomponent ofthe cell, solventforabsorptionandtransportof minerals, a good mediumforchemical reactions, reactantsinsomebiochemicalreactions(photosynthesis), supportstructurethroughturgorpressures, gametetransfermediumforplantsandspread ofpropagules(coconut). The wateralso servesin the'movement' of plantsdue tothe movement ofwaterin and outatcertainparts(the opening of the stomata, flowersbloomingand thediurnalmovement), elongationandcell growthandthermalbuffer.

Cell water potential

Free energy of water is an inevitable topic when we discuss about water potential. Thermodinamycally, free energy is defined as potential for doing works. Water potential is usefull to know the energy status of water to do work and the direction of water movement. Firstly, we should know that the chemical potential of water is quantitative measurement of water free energy.

= = =

Water potential is chemical water potential of water divided by the partial volume of water. In other words, water potential is a measure of water free energy per unit volume (J m-3 in MPa) causing water movement in plants. With its potential the water move from high to low water potential (Figure 1.5).

Figure 1.5. The important concept of water potential

Water potential is a measure of water energy compare to free energy of pure water. The water potential of pure water is zero. There are four major components affecting plant water potential (), namely solute/osmotic (s), pressure (p), matrix (m) and gravity (g) as the equation below:

= solute or osmotic potential is a measure of solute potential in solution which reduce free water energy; Solutes decrease of water free energy then the free energy of water potential in plants usually less than that of pure water. Therefore, solut potential values arealways negative.This is important for measuring water movement into and out of cells such as guard cells function and nutrient uptake.

= pressure potential is positive (due to turgor pressure) or negative (during transpiration) hydrostatic pressure in the system. Pressure potential is usually positive in living cells. While in death cells like xylem pressure potential is negative because tension which develop water column is more dominant.

= matrix potential is reduction of water energy due to the attraction of water molecules to surfaces such as cell walls, soil particles surfaces or oher particles in the system/water; it always negative. This potential based on tendency of water to adhere to the surfaces. This potential is usually ignored because the effect of the surface isnteraction is too small to change the water potential in the system. In saturated soil, water free to flow,and Ψm is not a factor and value is 0. Conversely, in unsaturated soil matrix potenial results from capillary water and adhesion force. The more negative matrix potential the more difficult to remove water from the surface. Consequently, plants have difficulties to extract water from the soil.

= gravity potential is a measure of water energy which depends on the height of water. This potential component is frequently ommited related to water transport at the cell level.

Therefore, the preview equation can be simplified as follows:

Water potential can change by changes in pressure potential or solute potential. Water potential gradient between soil and root hair cells is important to move the water into the cells.

Water transport

To fulfill the water needs of plants the water must flow from soil at the out side into plant cells and finally released out to atmosphere through transpiration processes involving stomata. In plant body water movementoccursin two ways due to the differences ofconcentration andpressure. Water movement in plants is driven by three processes:diffusion, mass flow and osmosis.

  1. Diffusion

Diffusionis therandommovement ofindividualmoleculescaused bythe difference inconcentration. Diffusionof moleculeswillmovefromhighconcentrationto thelowconcentration. Dyedrippedinto thewaterin theareahas ahighconcentration ofthe firstdroplet(Figure 1.6). Gradually,the dye willspreadmovingtowardareas that do notcontain dye. The movementwillstopwhen thedyeconcentrationisthe sameinallareas sothatthewateris initially colorlesschanges intocorrespondingcoloreddyedripped.

(University of California, 2013)

Figure 1.6. Diffusionprocess

Fick's lawstatesthe relationshipbetween therate of diffusion ofthegradient concentration(C1-C2) and theresistance/resistance(r). The rate ofdiffusionis also calledfluxdensity(with units Jmolm-2s-1) andis expressedby the following formula:

Based ontheformula, there are three things thatneed to be observed, namely:

  1. Diffusion rateis directly proportional tothe concentrationgradient. The greater thedifference inconcentrationbetweentwoareasthe greaterthe rate ofdiffusion. When thegradientreaches0indicatesthatdiffusion doesnot occureanymore.
  2. The rate ofdiffusionis inversely proportionalto theresistance. The bigger the barriersthe lower thediffusion rate. Therefore thebarrieris anything thatreducesthe rate ofdiffusion. Membraneisinhibitingthe movement ofthe chargedionsandcompoundsboth inside andoutsidethe cell.
  3. Diffusion rateis alsoinversely proportional to thedistance ofdisplacementormovementas well asthe function ofthe barriers. In other word, the longer the distance the slower the water diffusion.

Molecular speed. The rate ofdiffusion is also determined by speed ​​ofmovingmolecules. It will be a) directly proportional totemperatureandb) inversely proportional to themolecular weight(heavy particlesmove slowerthanlightparticles).

Temperature. Temperature will increase molecular speed. So, increased movement consequently will increase rate of diffusion (Figure 1.7).

(Chaplin, 2013)

Figure 1.7. The effect of temperature on the rate of diffusion

Pressure. Like temperature, the higher the pressure the higher the molecular speed which consequently increase the rate of diffusion.

Effectof solute(dissolved substance) on thechemical potentialof solvent. Particles of solut will decrease free energy of solvent. In this case, the numbers of particles have more influent than charge of particles.

  1. Bulk flow / Mass flow

Mass flowisthe movement ofthe molecular mass(water andsolute) due tothepressure differencefrom thehighpressure areatowarda lowpressure area. Bulk flow of water molecules facilitates the farthest plant parts from soil for obtaining water.

  1. Osmosis

Osmosis is diffusion of solvent especially water through semipermeable membran from high to low concentration. Movement of water by osmosis is passive beacuse the water moves spontaneously. It does not require energy because this movement is combination between diffusion (from high concentration to low concentration) and mass flow (from high pressure to low pressure) (Figure 1.8).

Figure 1.8. Osmosis process

Recent studies show that water movement into the plants cells not only involves phospholipid bilayers through diffusion but also involves aquaporines membrane integral proteins which develop selective pore through bulk flow (Figure 1.9).

Figure 1.9. Aquaporine facilitating bulk flow of water (Taiz and Zeiger, 2013)

Summary

Chemicalandphysicalproperties ofwatersupport continuation ofthe physiologicalprocessesinplant. Therefore understanding theproperties of waterare veryimportantfor studyingotherphysiological processesin plants.

The movement ofwateris due to thedifference in pressureandordifference in concentration. Direction of movement is from high concentration/highpressuretolow concentration/pressure. Transferspeedis affectedbythe pressure, the speed ofthe molecules, andsolutetemperatureinthe system.

Quis

  1. Why iswatersoimportantto living thingsincluding plants?
  2. How thephysical formof wateratphysiologicaltemperature? Explainit.
  3. What are the advantagesof highevaporationin theheatpropertiesof waterfor the plant?
  4. What isthe relationshipbetweenthe surface tension ofwateritnggiownedbyphysiological processesin plants?
  5. What are the advantagesof wateras theuniversalsolvent?
  6. Describethe nature ofthe transparencyof waterto lightforplantsurvival.
  7. Why the water dissociation process isveryimportant for theprocessplant physiology?
  8. Explainwhat ismean bythe polarnature ofwater.
  9. Explain the functionsof waterfor the survival ofplants based on the propertiesof water.
  10. What are twoways of water movement in plants. What is the differencebetween both of them?
  11. ExplainhowFick's lawdescribestherate of diffusion ofthegradient concentration.
  12. What factorsaffect the rate ofdiffusion? Explain it.
  13. Osmometer which has selective membrane containing solution is placed into the beaker containing pure water. Explain how the process of water diffusion. If the pressure is applied to the osmometer then how the possibility of its water potential?
  14. What isosmosis?

References

Anonym. 2013. Nature wallpapers and desktop background.

Chaplin., M. 2013. Water Structure and Science.

Climate Science Investigation (CSI), 2013. Temperature Overtime.

Harvey Lodish ,Arnold Berk ,Chris A. Kaiser ,Monty Krieger ,Matthew P. Scott ,Anthony Bretscher ,Hidde Ploegh ,Paul Matsudaira, 2008. Molecular Cell Biology. Ed. 6th. W.H. Freeman Publ. Los Angeles.

Hayley Biology. 2013. Chemistry of Life.

Hyperphysic. 2013. Surface tension and bubles.

Pauling, L. 1948. The Nature of the Chemical Bond, 2nd ed. Cornell University Press, New York.

Taiz, L. and E. Zeiger. 2010. Plant Physiology. International Edition 5th (fifth). Sinauer Associates Publ.

University of California. 2013. The Science of Solar.

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