BIO 125 Lecture Notes - Water Structure & Properties - PDF

BIO 125 LECTURE / FIRST SEMESTER AY '24-'25 / Professor Rosario Rubite WATER STRUCTURE & PROPERTIES PROPERTIES OF WATER o Air-water in...

BIO 125 LECTURE / FIRST SEMESTER AY '24-'25 / Professor Rosario Rubite WATER STRUCTURE & PROPERTIES PROPERTIES OF WATER o Air-water interface PLANT PHYSIOLOGY Cohesive - Cohesion minimizes the surface - Greek words o Sticking together of like molecules area of water o Physis: nature - Water that evaporates from a leaf ▪ To increase: H-bonds o Logos: discourse o Replaced by water from vessels in the leaf must be broken - Discourse about nature of plants - Hydrogen bonds cause water molecules Energy(surface - Restricted meaning leaving the veins to tug on molecules further tension) is required o Study of how plants function down o Plants are viewed as biochemical machines o Upward pull: transmitted to the roots - Interaction of water molecules is greater WATER - Cohesion among water molecules plays a than intermolecular attraction between - Substance that makes life possible key role in the transport of water against water & air - Life on Earth began in water gravity in plants o Surface tends to contract & behaves like an o Evolved there for 3 billion years before - Xylem tissues extend from leaf down to the elastic membrane spreading onto land roots - Reason why water drops tend to be spherical - Terrestrial organisms are tied to water o Pull: relayed molecule by molecule up to or the water surface can support the weight o Cells are about 70-95% water the root tip of small insects - Polar o Water molecule in root is replaced by water - Water molecules are hydrogen bonded with o Overall charge: uneven in soil one another & the ones below o V-shaped - Important in increasing life span of cut o Makes the water behave as though it is o 2 hydrogen atoms form single polar covalent flowers coated with invisible film bonds with an oxygen atom o Cut the lower end of peduncle to remove ▪ Region around oxygen: partial negative charge air spaces Oxygen is more electronegative o Continuous column of water - Influences the shape of the surface in the ▪ Region near the 2 hydrogen atoms: partial ▪ Prevent early wilt mesophyll especially in spongy layer positive charge Exhibits adhesion or - Adhesion o Composed of parenchyma cells with - Slightly negative region of one molecules attraction to a solid o Attraction between unlike molecules numerous intercellular spaces o Attracted to the slightly positive region of a nearby molecule phase o Also due to hydrogen bonding ▪ Saturated with water vapor ▪ Forming a hydrogen bond - Adhesion of water to the walls of the vessels When it transpires, the water vapor Weak bond: 1/20th as strong as covalent bonds o Helps counter the downward pull of gravity exists the stomata Continually forms, break ups, & reforms during water transport o Film evaporates on a hot day Responsible for many of the unusual - Hydrogen bonding ▪ Remaining will retreat in the pores of the physical properties of water o Responsible for adherence of water cell wall, attracted by adhesion to the - Each water molecule can form hydrogen bonds molecules to cellulose on vessels/tracheid hydrophilic walls with up to four neighbors walls ▪ Cellulose microfibril are attracted by - Substantial percentage of all water molecules ▪ Repeating glucose units adhesion to the hydrophilic walls are bonded to their neighbors ▪ Exposed oxygen atoms on its surface can o Adhesion & surface tension cause the o Making water more structured than most other form H-bonds with water as water moves surface water to form a liquids through xylem meniscus/concave shape High surface tension - Surface tension ▪ More concave, the greater the pulling o Force exerted by water molecules at the force air-water interface - Create a pressure in the rest ▪ Resulting from the cohesion properties of of the liquid water o At the evaporation o Measure of how difficult it is to surfaces of the leaves stretch/break the surface of the liquid generates the physical o Greatest among together with Mercury: forces that pull water water through the plant’s - Result of unequal attraction vascular system gonzales | 林青霞 1  BIO 125 LECTURE / FIRST SEMESTER AY '24-'25 / Professor Rosario Rubite High specific heat - Ability of the water to stabilized temperature liquid to the gaseous state at constant o Makes water a good solvent depends on its high specific heat temperature o Polar water molecules surround cations & - Specific heat - Water anions o o Amount of heat energy required to raise the o At 25 C, heat of vaporization is 44 kJ/mole - Solvents dissolve solutes temperature of a substance by a specific ▪ Highest volume for any liquid o Creating solutions o amount (1 C) - Hydrogen bonds must be broken before a ▪ Ionic compounds like sodium chloride - Due to hydrogen bonding water molecule can evaporate from the (NaCl) can dissolve in water o Heat must be observed to break hydrogen liquid Sodium being surrounded by an oxygen bonds & is released when hydrogen bonds - Evaporation from the moist surface cools the region in water which is negatively form surface charged - Investment of heat o Most energetic molecules escape the Chloride is attracted to positively o Causes relatively little change to the surface charged hydrogen regions of water temperature of water ▪ Leaving behind the lower energy hence ▪ Because much of the energy is used to cooler molecules disrupt hydrogen bonds, NOT move molecules faster - Water resists changes in temperature o High specific heat High tensile strength - Tensile strength ▪ Takes a lot to heat it up & cool it down o Maximum force per unit area that a o When it changes temperature continuous column of water can withstand - Evaporative cooling moderates temperature before breaking ▪ It absorbs/loses a relatively large quantity in lakes & ponds of heat o Prevents terrestrial organisms from o High thermal conductivity overheating ▪ Rapidly conducts heat away from point of - Plants may undergo application substantial heat Localized overheating in a cell due to loss as water - Cohesion gives water high tensile strength the heat of a biochemical reaction evaporates from - Must exist for water to be pulled up in a tall o Largely prevented because the heat is leaf cell surfaces tree quickly dissipated throughout the cell o Important - Makes Earth habitable mechanism for - Pushing the plunger, compresses the liquid o Stabilizes ocean temperature o Positive pressure builds up temperature creating regulation in o Small air bubble is trapped favorable environment leaves of terrestrial plants that are often ▪ Shrinks as pressure increases for marine life exposed to intense sunlight o Keeps temperature Solid water floats - Water is unusual - Pulling the plunger, causes the fluid to fluctuations within limits o It is less dense as a solid than as a liquid develop tension that permit life - Allows life to exist under the frozen surface o Negative pressure builds up o Any air bubbles will expand as the pressure - Water that dominates is reduced the composition of Solvent of life - Many of the solutes of importance to plants ▪ Presence reduces the tensile strength of biological organisms like mineral nutrients are charged the water column moderates changes in - Water Expansion upon - Water expands when crystallizes temperature better than o Medium for movement of molecules within freezing o Reaches highest destiny at 4oC if composed of a liquid & between cells ▪ Ice floats because it is less dense than with a lower specific heat - Forms the environment liquid water High heat of - Heat of vaporization o Which most of the biochemical reactions of - Most substances shrink vaporization o Quantity of heat that a liquid must absorb the cell occur o When they change from liquid to solid for 1 gram of it to be convertexd from the - Cells are made up of 70-95% water - Important - Polarity o Allows life to exist under the frozen surface gonzales | 林青霞 2  BIO 125 LECTURE / FIRST SEMESTER AY '24-'25 / Professor Rosario Rubite TRANSPORT PROCESSES AT THE CELL LEVEL & BULK FLOW External forces - Greater the force, faster the rate of diffusion TYPES DIFFUSION Size of molecules - Larger the molecule, slower rate of diffusion Hypertonic - Higher concentration of solutes - Movement of molecules down/along a concentration gradient by - For gasses Hypotonic - Lower concentration of solutes random thermal agitation o Graham’s law of diffusion Isotonic - Equal concentration of solutes - No need for the cell to spend its energy ▪ Rates of diffusion are inversely - Direction of osmosis - Over short distances is rapid proportional to the square roots of their o Determined only by a difference in total solute o 2.5s in a cell size of 50 um densities concentration - Over long distances is far too slow for mass transport ▪ Kinds of solutes in the solution do not matter o Average time for a particle to diffuse: L2/Ds - Water molecules move at equal rates from one to the o L2: distance other o Ds: diffusion coefficient o With no net osmosis ▪ Depends on identity of particle & diffusion medium - In animal cell - Fick’s equation o Water balance between cell & its environment is crucial to organisms o Cells without walls cannot tolerate too much uptake/loss of water o Fares best in an isotonic environment ▪ Unless has a special adaptation to offset the osmotic uptake/loss of Solubility in diffusion - More soluble a substance is in the diffusion water medium medium, faster it will diffuse Hypertonic - Environment will lose water o Unless diffusion medium is concentrated - Shrivels & probably die o Rate of diffusion is directly proportional to the concentration gradient Ex: oxygen is more soluble in air than in water o Flux (Js) ▪ Amount of substance (s) crossing a unit area per unit time Presence of other - Decreases the rate of diffusion Ex: Moles/m2/s molecules o Because of additional collisions that occur o Diffusion coefficient (D) - Direction of net diffusion is not influenced by the presence of other types of molecules Hypotonic - Environment will gain water ▪ Proportionality constant - Swell & burst ▪ Measures how easily a substance moves in a particular medium ▪ Ex: diffusion is faster in air than liquid; larger molecules have smaller diffusion coefficients ▪ Negative sign: flux moves down a concentration gradient - Net movement of molecules from regions of higher concentration to OSMOSIS Isotonic - Experiences no net movement of water across its low concentration - Diffusion of water across a semipermeable membrane plasma membrane o Through random thermal motion of individual molecules - Movement of water through a differentially permeable membrane from - Concentration of water going in & out is equal - Movement of molecules from one location to another that continues a region of high water concentration to a region of low water even at equilibrium concentration o At dynamic equilibrium o Area of high free energy to an area of low free energy of water ▪ Movement is still taking place from one area to another - In plant cell ▪ Free energy ▪ Concentration in the 2 areas are equal o Has walls that contribute to the cell’s water balance Useful/available energy - Net diffusion Hypertonic - Cell has no advantages Capacity to do work o Direction of greatest number of molecules o As the plant loses water, volume (protoplast) shrinks - Special example of net diffusion o At dynamic equilibrium, net diffusion = 0 ▪ Eventually plasma membrane pulls away from the o Concentrates on movement of water through a differentially FACTORS AFFECTING RATE OF DIFFUSION wall becoming plasmolyzed permeable membrane Temperature - High temperature - Plasmolysis is usually lethal - Tap water is hypertonic compared to distilled water o Increase in the kinetic activity of molecules, o Common examples o But hypotonic when compared to seawater increase speed in diffusion ▪ “Burning” of plants after spraying with insecticides - Movement of water in osmosis cannot be accurately explained in terms Brown color indication of dead cells Concentration - Steeper the gradient, faster the rate of of differences in concentration ▪ Excessive addition of chemical fertilizers gradient diffusion May lead to death of cells & even entire plant - Influenced by distance between 2 regions ▪ Salting of meat & fish o (a) & (b) have the same concentration Excess salt prevents growth of decay organisms ▪ (a) have less distance than (b) by plasmolyzing cells of such mold/bacteria (a) is faster gonzales | 林青霞 3  BIO 125 LECTURE / FIRST SEMESTER AY '24-'25 / Professor Rosario Rubite ▪ Jams & jellies o Which means free energy of water is higher in beaker than - Reference standard: pure water Sugar preservation osmometer o Ψ𝑈 = 0 MPa o Prevents growth of molds o Net diffusion of water will be towards osmometer & liquid rises in the - Osmotic potential (Ψ𝑠) ▪ Undesirable plants capillary tube o Amount by which water potential is reduced as a result of the Can be eliminated by applying salt to soil at the - Rise of solution level does not continue indefinitely but eventually presence of solutes base of the plant reaches a definite height o Negative value Hypotonic - Swell until the elastic wall opposes further uptake o When the level remains constant, the system is in equilibrium - Hydrostatic pressure/pressure potential (Ψ𝑝) - Cells expands as water gets inside the cell o if not equal, magkaka-net diffusion o 0 or positive o Protoplast pushes against the wall & applies turgor - Osmosis in non living systems can be demonstrated quite easily o Positive pressure operating in plant cells is the wall pressure/turgor pressure on the wall o Differentially permeable membrane-dialysis tube or cellophane pressure - Turgid ▪ Weight of the column of water exerts a hydrostatic pressure which o In the osmometer: hydrostatic pressure o Healthy state for most plant cells increases the free energy of water inside the osmometer by an - Matrix potential (Ψ𝑚) amount equal to the external force Eventually the increased free energy, caused by the buildup of o Component of water potential influenced by the presence of a matrix hydrostatic pressure reaches a point where it is equal to the free ▪ Surfaces to which water molecules are adhered energy of water in the solution & the number of molecules moving - Gravity (Ψ𝑔) in the 2 directions is equal o Causes water to move downward unless opposed by an equal & b. If no capillary tube, but only enlarged portion opposite force is present - Hydrostatic pressure would still develop but would not be visible as a column of liquid - Turgor pressure = wall pressure - In a mature cell o Equal & opposite o There is a thin layer of cytoplasm o Reaches turgid state surrounding a vacuole most of the water o Contributes to mechanical support of plants entering the cell passes through the especially herbaceous plants Problems: cytoplasm & then vacuole Isotonic - No net movement of water into the cell 1. If a limp cell with osmotic pressure of -10 bars were placed in distilled - As water concentration increases within the vacuole, developing - Cell is flaccid water, what will be the cell’s water potential, osmotic potential, & hydrostatic pressure forces the cytoplasm against the cell walls - Plant may wilt pressure potential after it reaches equilibrium with the water outside? - Since walls are rigid, the resistance to expansion is in effect the wall pressure against cytoplasm & vacuolar content WATER POTENTIAL (Ψ𝑈) - Osmometer - Free energy of water is affected by o Equilibrium was reached even though the concentrations on opposite o Presence of solutes sides of the membrane were not equa o External force ▪ Hydrostatic pressure, wall pressure/turgor pressure - Combined effect of these factors are included in a single measurement - Potential in water potential o Refers to the capacity to do work when water moves an area of higher Ψ to an area of lower Ψ - Measure of the free energy of water/unit of volume (Jm-3) - Cell is fully turgid so solving at equilibrium o Water potential of cell equilibrates & becomes 0 o Osmotic potential is still at -10 bars even if net movement is toward the cell a. Osmometer filled with 50% sugar solution separated from a beaker of - Simplifying the mathematics pure water by a differentially permeable membrane like a dialysis tube o Assume water enters is not sufficient to dilute the solutes & change or cellophane that allows water to penetrate but not sugar, dyed with the cell volume so osmotic potential remains at -10 congo red - Computing for pressure potential, arrive at +10 bars - Water concentration is greater in the beaker than in osmometer gonzales | 林青霞 4  BIO 125 LECTURE / FIRST SEMESTER AY '24-'25 / Professor Rosario Rubite 2. If we consider the same cell in the previous example, with an osmotic - Passive movement of molecules down its concentration gradient ▪ Artificial phospholipid bilayer vs biological membrane pressure of -10 bars & placed in an external solution with osmotic (uncharged)/electrochemical gradient (ions) via a transport protein Similarities potential of -2 bars, what will be the water potential, osmotic potential, & TYPES OF TRANSPORT PROTEIN o Similar permeability for non-polar molecules pressure potential at equilibrium? Channel protein - Provide corridors allowing water/specific ion to ▪ Oxygen & carbon dioxide cross the membrane ▪ CO2 permeability: 10-2 - Some are gated channels o Similar permeability for uncharged molecules o + K gates in guard cell membrane ▪ Glycerol permeability: 10-6 o Cl- channels Differences o + Ca channels o Biological membranes are more permeable to ions (K+) - Involved when large quantities of solutes must ▪ K+ permeability cross the membrane rapidly Bio: 10-6 - Very rapid process Artificial: 10-10 8 o ~10 ions/sec through each channel protein ▪ Contain transport proteins - Cell has no turgor pressure Transfer/carrier - Selectively binds to a solute on one side of the Facilitate the passage of selected ions & other polar - Osmotic & water potential are equal with -10 bars protein membrane molecules - Not enclosed so pressure potential is 0 o Releases the solute on the other side AQUAPORINS - Water potential & osmotic potential are equal at -2 bars - Involves conformational change of the transport - Selective protein channels that facilitate - Net movement of water into the cell protein water diffusion - When there is a large amount of external solution - Much slower - Integral membrane proteins that form o Concentration will not change noticeably o 100-1000 ions/sec water-selective channels across o Similarly the very small amount of water that develops turgor ▪ Since selective membranes pressure will not significantly change the concentration within the cell - Have much in common with enzymes - Water diffuses faster through these - Osmotic potential within & without is considered the same so it remains o Specific binding sites for solutes channels than through the lipid bilayer at -10 bars o Do not catalyze a chemical reaction o Facilitate water movement into plant cells o Net uptake is not sufficient to dilute ▪ Do catalyze a physical process ACTIVE TRANSPORT Transport molecule across a membrane that would otherwise be relatively impermeable - Pumping of solutes against their own gradients to the substrate - Requires cell to expend its own metabolic energy - Can become saturated o Usually but not always through hydrolyzing ATP - Critical for a cell to maintain internal concentrations of small molecules - How do water molecules cross the membranes? that would otherwise diffuse across membranes o Lipid bilayer - Performed by specific proteins embedded in the membranes ▪ Water molecules are so small o Integral proteins Move relatively freely across the lipid bilayer of membranes even 1. Primary active transport - When osmotic potential increase in the cell as shown in the table though the middle zone of that bilayer is hydrophobic - Membrane proteins (pump) couple directly to a source of energy (ATP o Result of food production/loss of water by evaporation ▪ Was not sufficient to account for the observed rates of water hydrolysis, oxidation-reduction reaction) o Osmotic & water potential becomes more negative movement across the membranes until the discovery of aquaporins o Membrane permeability MEMBRANE TRANSPORT ▪ Artificial membranes made of pure phospholipids have been used Non-mediated - Without the aid of transporters extensively to study membrane permeability - Diffusion o Osmosis o Dialysis Mediated - With the aid of protein transporters - Passive transport/facilitated diffusion - Active transport - Most pumps transport ions o Hydrogen or calcium ions FACILITATED DIFFUSION TYPES - Type of membrane transport process gonzales | 林青霞 5  BIO 125 LECTURE / FIRST SEMESTER AY '24-'25 / Professor Rosario Rubite Electroneutral - No net movement of charge 2. Secondary active transport BULK FLOW transport - H+/K+-ATPase in the gastric - Uses the energy stored in electrochemical-potential gradients to drive - Pressure-driven bulk flow drives long-distance water transport mucosa of animals the transport of other substances against their - Concerted movement of groups of molecules en masse o Pumps one H+ out for every gradient/electrochemical potentials - Most often in response to a pressure gradient one potassium in - Independent of solute concentration gradient - Main method of water movement in xylem, cell walls, & soil o Xylem Electrogenic - Ion transport involving a net movement of charge ▪ Water movement is continuous from root to stem to leaves transport across the membrane Utilized for long distance transport of water in plants - Hydrolyzes ATP o Cell walls o Uses the released energy to pump hydrogen ions ▪ Apoplastic movement out of the cell o Creates a proton gradient ▪ Because the H+ concentration is higher outside the cell than inside o Also creates a membrane potential/voltage - Proton gradient also functions in cotransport - Doubling the radius, flow rate increases ▪ Charge difference across the membrane o Downhill passage of one solute (H+) is coupled with the uphill passage o Proving why vessel elements is more efficient than tracheids with Due to differential distribution of ions of another such as NO3- or sucrose regards to water transport - Energy driving: provided by H+ ion, concentration gradient, & the Ex: water moving through a hose, flowing river, rain falling membrane potential o Rather than ATP hydrolysis TYPES Symport - Protein involved: symporter ▪ Proton pump moves positive charges outside the - 2 substances are moving in the same direction cell through the membrane Making the inside negative relative to the Antiport - Protein involved: antiporter outside - Coupled transport - Both concentration gradient & membrane potential o Downhill movement of protons drives the active are forms of potential energy transport of a solute in the opposite direction o Can be harnessed to perform cellular work o Often used to drive the transport of many different solutes - Long distance water transport in plants o Described by Poiseuille equation Ex: electrogenic ion pump - Pumps 3 sodium ions out for every 2 potassium ions in - Results to a net outward movement of 1 positive charge gonzales | 林青霞 6  BIO 125 LECTURE / FIRST SEMESTER AY '24-'25 / Professor Rosario Rubite WATER ABSORPTION & CONDUCTION WITHIN THE PLANT o Water retention o Result of cohesion of water molecules to the sides of the soil surface & WATER ABSORPTION & TRANSPORT ▪ Water is not readily held in pores larger than 10 to 60 um cohesion of water molecules - Water moves from the soil through the plant to the outside air ▪ Water holding capacity - Water molecules at the liquid surface o Result of a difference in water potential just like in a cell Soil saturated with water o More attracted to other water molecules at its sides & inwards than it o From a region with higher water potential to a region with lower water Soil is freshly watered is to air potent o Water will percolate down to the pores until it has displaced most - Cohesive forces between molecules cause the surface of the liquid to if not all of the air contract to the smallest possible surface area - surface tension o Not only influences the shape of the surface o Greater attraction between water molecules at the air-water interface also creates pressure/pull in the rest of the liquid ▪ Surface tension generates the physical forces that pull water from locations where hydrostatic pressure is higher ▪ Field capacity Water will drain freely from the large pores due to gravity Water that remains after free (gravity) drainage is completed is held in capillary pores o Aeration ▪ Loam soil is a compromise of water retention & aeration for optimal plant growth SOIL-PLANT-ATMOSPHERE CONTINUUM - Pathway for water moving from the soil through the plant to the - As more water is removed from the soil, more atmosphere acute menisci is formed - Continuum o Results in greater tensions o Highlights the continuous nature of water connection through the o Developed negative pressure pathway ▪ Estimated by the following formula SOIL - Complex medium - Water-filled pore spaces are interconnected o Solid phase o Macropores are interconnected with the T: surface tension of water (7.28 x 10-8 MPa) ▪ Comprised of inorganic rock particles micropores r: radius of the curvature of the air-water & organic materials - Water is absorbed by the roots from the center of interface In various stages of decomposition the largest spaces between particles - Value of the pressure potential in the soil can be o Soil (liquid) - Attractive forces are not very powerful negative ▪ Containing dissolved solutes o Plant roots absorb capillary water o Radius of the air-water interface can become very o Gas phase - As soil dries out small ▪ Generally in equilibrium with o Water recedes into smaller spaces AVAILABLE & NON AVAILABLE WATER IN THE SOIL atmosphere - Decrease in water causes the surface of soil solution to develop Capillary water - Gravitational water leaves the soil o Algae, bacteria, fungi, earthworms, & concave menisci o Leaves moisture in the form of fluid water various other organisms are found in - Brings the solution into tension/negative pressure by surface tension held at soil surfaces & feeding the small soil channels of soil - Determined by sand, silt, & clay - Principal source of water for plants - Soil structure affects - Attractive forces are not very powerful o Porosity o Plant roots readily absorb such water ▪ Interconnected channels between soil particles Hygroscopic water - Occurs as thin films on soil surfaces ▪ Large pores - Cannot be absorbed by the plant ▪ Capillary pores - Meniscus is formed ▪ Occupies 40-60% of soil by volume gonzales | 林青霞 7  BIO 125 LECTURE / FIRST SEMESTER AY '24-'25 / Professor Rosario Rubite Permanent wilting - Percent of water remaining in the soil when a - Water crosses at least 2 membranes for each point plant wilts & will not recover unless water is cell in its path added to the soil o Plasma membrane on entering & exiting - Occurs before all the capillary water has - Transport across the tonoplast may also be been absorbed involved SOIL WATER POTENTIAL o Membrane which bounds the chief vacuole of a plant cell Symplast - Water travels from one cell to the next via the pathway plasmodesmata - Consists of the entire network of cell cytoplasm interconnected by plasmodesmata PATH OF WATER MOVEMENT THROUGH ROOT - Through epidermis & cortex - Liquid part of soil: composed of water o Water may travel via the apoplast, symplast, or transmembrane o Contains dissolved mineral nutrients in the form of ions - At endodermis - Water & mineral ions from the soil enter the plant through the epidermis o Apoplast pathway is blocked by casparian strip of roots -> cortex -> endodermis -> stele ▪ Impregnated with wax-like hydrophobic substance - suberin o Move either apoplastic or symplastic - Generally solute concentration of soil is low Barrier to water & solute movement - Casparian strip - Determined principally by the negative pressure potential ▪ Only via symplast o Prevents further apoplastic diffusion through the endodermis & stele - Uptake of water occurs - In stele ▪ Only possible route for ions is to enter the symplast by channel o Because of a water potential gradient between the soil & root o Symplastic route continues & ends in xylem vessels & tracheids mediated transport ▪ Higher water potential to a lower water potential ▪ Apoplast for long distance transport of water to stem & leaves Transport proteins may occur either outer tangential wall of - Decrease in water potential - Water moves from the soil to the interior of the root along an endodermal cells or through any of epidermal or cortical cell o Attributed to decrease in pressure potential increasingly negative osmotic potential gradient - Symplastic connections ▪ Not osmotic potential o Because of proportional increase in solute concentration in the interior o Facilitate passageway from cell to cell until they reach xylem - Water moves through the soil cells (stele) relative to exterior cells (outside stele) parenchyma in stele o As a result of pressure difference (high to low pressure) by bulk flow ▪ Ions are unloaded at xylem vessel elements for long distance WATER TRANSPORT THROUGH THE ROOTS transport to leaves & other organs - Most absorption takes place in the root hair - Transport of ions in xylem requires transition from symplast to apoplast zone by active transport o Water potential of the root cell sap is more o Ion concentration in apoplast of stele is generally higher than in negative than the soil solution surrounding cortex ▪ There is an increase in solute concentration ▪ Suggests that ions are being accumulated in the xylem against the of a cell concentration gradient Osmotic pressure becomes more negative - Water diffuses less negative osmotic potential to more negative ▪ Decrease in turgor pressure osmotic potential - Suberized tissues may take up water through the - Pathway of water from root xylem -> stem xylem -> leaf xylem lenticels ION UPTAKE BY ROOTS o Driven by pressure gradient & NOT osmotic potential gradient LATERAL TRANSPORT ROUTES Apoplast - Water moves exclusively through the cell wall TRANSPORT OF WATER pathway without crossing any membranes - Root pressure - Continuous system of cell walls & intercellular air o Pressure developing in the tracheary spaces in plant tissues elements of the xylem resulting from the metabolic activities of roots o Results from active absorption of salt by the roots Transmembrane - Route followed by water that sequentially enters o Absent in conifers pathway a cell on one side, exits the cell on the other side, o Water potential enters the next ins the series, & so on ▪ Driving force in water absorption gonzales | 林青霞 8  BIO 125 LECTURE / FIRST SEMESTER AY '24-'25 / Professor Rosario Rubite o Not a major force in the rise of water in plants due to o Difference in water potential between atmosphere & intercellular o Lead to bubble formation due to reduced solubility of gasses in ice ▪ Magnitude of pressure is not enough to push water to the heights spaces in leaves is always great enough to cause the movement of ▪ Can potentially create cavitation in thaw phase reached by most trees water molecules from leaf to air - Gas bubble has formed ▪ Root pressure of any magnitude is absent from conifers o Expand as gas cannot resist tensile forces Among the tallest of trees - Cavitation by water stress ▪ Exudation rates is slower than normal transpiration rate o Appears to occur by air being pulled into the water field conduits o May be a significant factor in the movement of water when through pits connecting with already embolized conduits transpiration is poor - Plants minimize the consequences of xylem cavitation ▪ Usually happens during midnight