Osmosis in Plants

12 Key excerpts on "Osmosis in Plants"

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  eBook - PDF

  Plant Physiology

  Theory and Applications

  • S. L. Kochhar, Sukhbir Kaur Gujral(Authors)
  • 2020(Publication Date)
  • Cambridge University Press

    (Publisher)

  The term for water movement in this manner is called ‘osmosis’, which may be thought of as a special type of diffusion. Osmosis is defined as the movement of water across a selectively permeable membrane from an area where its concentration is higher to an area where its concentration is lower. The phenomenon of osmosis in non-living systems can be demonstrated by a U-tube glass apparatus having pure water and 5 per cent glucose solution separated by semi-permeable membrane. The water molecules will move from pure water to the 5 per cent glucose solution, till the concentration of water becomes equal on both the sides (Figure 1.14). In such a physical system, there is only one force involved, i.e., difference between concentrations at two ends. When an actively metabolizing plant cell is immersed in pure water or hypotonic solution, 4 water molecules tend to enter into the cell sap as a result of endosmosis. If the direction of movement is reverse, i.e., from cell sap to external medium as in hypertonic solutions, it is 4 External solution is said to be hypotonic, hypertonic or isotonic with reference to cell sap if its osmotic pressure is less than, higher than or equal to osmotic pressure of the cell sap, respectively. Plant–Water Relations 35 termed exosmosis. In an isotonic solution, the cell will not lose or absorb any water, thus maintaining the net volume of water constant. 5% glucose solution Pure water 30 min U-Tube Semi-permeable membrane Fig. 1.14 Demonstration of osmosis. Osmotic Pressure (OP): It is the maximum amount of pressure that can be developed in a solution separated from pure water by a semipermeable membrane. OP = CRT C = concentration of the solute R = gas constant (0.082 L atm K -1 mol -1 ) T = absolute temperature (273.18 +°C). This equation is applicable only in the case of non-electrolytes such as sucrose. In the case of electrolytes, such as NaCl, OP obtained is multiplied by its degree of dissociation.

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  Principles of Horticulture: Level 3

  • Charles Adams, Mike Early, Jane Brook, Katherine Bamford(Authors)
  • 2015(Publication Date)
  • Routledge

    (Publisher)

  Diffusion can occur with gases, e.g. oxygen and carbon dioxide exchange in the leaf, or water vapour in transpiration or with dissolved substances, e.g. movement of minerals in the soil. Osmosis The movement of water from a high water (low solute) concentration to a low water (high solute concentration) across a selectively permeable membrane. Solutes are substances dissolved in the water, e.g. sugars, ions. A selectively permeable membrane allows water to pass freely but excludes some solutes. The rate of osmosis depends on the concentration gradient of water across the membrane (the greater the difference in water concentration the faster the rate). Movement of water into and out of cells across the cell membrane, that is, in the symplast (p. 90 ). Mass flow (Bulk flow) The movement of water in response to a pressure gradient. The rate of flow depends on the pressure difference at each end of the system, the radius of the vessel, the distance travelled and the viscosity (thickness) of the liquid. Movement is fastest when the gradient is steep, the viscosity is low, the vessel is wide and the distance is short. The pressure which causes water to move is called hydrostatic pressure. Its values can be positive (a pushing pressure) or negative (a pulling or sucking pressure). Water movement: in the xylem (transpiration pull) (p. 91 ), in the phloem (mass flow) (p. 100 ) and between cells in the apoplast (p. 90 ). Active transport The uptake of a substance across a membrane against a concentration gradient. Active transport uses carrier proteins embedded in the membrane which move substances selectively across it. It is an energy requiring process. Uptake of mineral nutrients by cells (see p. 99 ).

  The solute concentration inside plant cells is largely regulated by the concentration of potassium ions (K+) in the cell vacuole hence the plant's requirement for potassium (see p. 170 ).

  Water is unusual

  Water is one of the most common substances on Earth; it covers 70 per cent of the Earth's surface and is vital for all known forms of life. Pure water is tasteless, it does not conduct electricity (but conductivity increases with salt impurities, see salt concentration measurement p. 178 ) and it has a neutral pH (neither acid or basic) (see p. 178 ). Each water molecule contains two atoms of hydrogen and one atom of oxygen (see Basic chemistry p. 173 ), its chemical formula is therefore H2

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  The Living Barrier

  A Primer on Transfer across Biological Membranes

  • Roy J. Levin(Author)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  2. Classical osmosis. Water will move across biological mem-branes by classical osmosis wherever there is a difference in solute concentrations in the fluids bathing the membrane. This difference in concentration could come about by active pumps transferring 150 The Living Barrier either ions or non-electrolytes across a membrane. As we know that cells have active pumps for ions and non-electrolytes, it has often been suggested that osmosis is likely to be the major mechan-ism of transcellular fluid movement, the cell producing a high concentration of solute which then causes water to move from one compartment to another by osmosis. Under these circumstances water transfer is passive or secondary to net solute transfer, without solute transfer there can be no fluid transfer. One of the practical difficulties in accepting this explanation of fluid transfer has been with epithelia that transfer large quantities of fluid across their membranes from one compartment to another. More often than not the fluid transferred has an osmotic pressure or osmolarity identical to that of blood or plasma. In some situations it has been found that these epithelia (the small intestine is the one commonly used) actually appear to transfer water from a low chemical potential to a higher one. That is, the intestine apparently can undertake active transfer of water! This cannot, of course, occur thermodynamically unless the cells have an active water pump able to transfer pure water molecules across the membrane. Most biologists are loath to accept such a pump and prefer to keep the idea that without net solute transfer there can be no net water transfer. But how then to explain this uphill transfer of water by the intestine? A possible way out of the dilemma was found by Jared Diamond in his experiments on the absorption of fluid across the gall-bladder. He showed that there was no need to postulate active pumps for water.

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  The Movement Of Molecules Across Cell Membranes

  • Wes Stein(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  C H A P T E R 7 The Movement of Water Water is, of course, the major constituent of living matter, and the movement of water across cell membranes is quantitatively the major phenomenon in biological transport. W e have considered in Chapter 3 the molecular basis of water movement and especially the view that water movement occurs by diffusion through pores in the cell membrane (Section 3 . 7 ) . In the present chapter we shall emphasize the physio-logical, as opposed to the molecular, aspect of water movement. W e shall consider the factors which determine the rate of movement of water across cell membranes and into tissues, and also the factors which de-termine the steady state distribution of water in both cells and tissues. 7.1 T h e Volume of a Cell at the Steady State If the tonicity, that is, the concentration of osmotically active material of the medium surrounding an animal cell, is maintained constant and if the cell remains metabolically active, the cell volume will remain constant over a long period of time. Yet if red cells are placed in media containing different concentrations of impermeable substances, the cells respond to the change of external osmotic pressure as if they were, to a very good first approximation, perfect osmometers (LeFevre, 1 9 6 4 ) . The cell vol-ume is, therefore, capable of varying. Thus the cell is not enclosed by a rigid framework. Yet its volume is maintained constant under physio-logical conditions, and we can show that this constancy requires the continued performance of metabolic activity. If a tissue is removed from the body into an incubation medium and prevented from metabolizing, it will soon swell through absorbing water (and salts) from the medium ( T a b l e 7.1) (Heckman and Parsons, 1 9 5 9 ) . If metabolism recommences, the accumulated water and salts can be returned to the medium and the initial cell volume regained.

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  Agricultural Physics

  The Commonwealth International Library: Physics Division

  • C. W. Rose, W. Ashhurst, H. T. Flint(Authors)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  This phenomenon is referred to as osmosis. Identical principles will control the movement of solutes, provided the effect of electrical potential gradients across the membrane are considered if the solute is ionic, and provided the solute transfer is passive or non-active. (In active transport, energy released by chemical changes can transfer molecules against the gradient in their total potential if the molecules are uncharged, or against the gradient in electrochemical potential if they are ionic (Dainty< 24 >).) Nevertheless, the term osmosis is usually, though not always, restricted to solvent transfer (Curtis and Clarke)). 212 AGRICULTURAL PHYSICS From Chapter 5 it follows that differences ΛΨ in the total potential may be resolved into: ΛΨ = ΑΦ+ΛΟ, (8.11) where Φ refers to hydraulic and O to osmotic potentials. Consider the simple osmometer of Fig. 52, in which the solvent is taken as -Tube cross sectional area -a FIG. 52. A simple osmometer. water. The difference in total potential on a unit volume basis, zdîfVoi, between water on the solution and water-reservoir side of the semipermeable membrane is the work required to transfer unit volume of water across the membrane from the reservoir to the solution side. From eqn. (5.10) and Fig. 52 it is given by: ΛΨ νο ι =pgh-n, (8.12) where is the osmotic suction of the solution, defined in section 5J2 ? and p the solution density, assumed constant over height h. PLANT-WATER RELATIONSHIPS 213 Thus it is seen that the effect of the addition of solutes is to lower the total potential of water in the solution. Initially, A Ψ ν0 is negative, and water will flow from the region of higher total potential (the water reservoir) into the solution, thus increasing h and so ΑΨ ν0 (eqn. (8.12)). The inflow of water will continue until equilibrium is achieved when Δ Ψ νο ι is zero, and pgh is equal to . The achievement of equilibrium in this way results in some decrease in it due to dilution of the solution by the incoming water.

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  Eco-Hydrology

  • Andrew J. Baird, Robert L. Wilby, Andrew J. Baird, Robert L. Wilby(Authors)
  • 2005(Publication Date)
  • Routledge

    (Publisher)

  2

  WATER RELATIONS OF PLANTS

  Melvin T. Tyree

  INTRODUCTION

  Water relations of plants is a large and diverse subject. This chapter is confined to some basic concepts needed for a better understanding of the role of plants in eco-hydrology, and readers seeking more details should consult Slatyer (1967) and Kramer (1983).

  First and foremost, it must be recognised that water movement in plants is purely passive’. In contrast, plants are frequently involved in ‘active’ transport of substances; for example, membrane-bound proteins (enzymes) actively move K+ from outside cells through the plasmalemma membrane to the inside of cells. Such movement is against the force on K+ tending to move it outwards, and such movement requires the addition of energy to the system to move the K+ . Energy for active K+ transport is derived from ATP (adenosine triphosphate). While there have been claims of active water movement in the past, no claim of active water transport has ever been proved.

  Passive movement of water (like passive movement of other substances or objects) still involves forces, but passive movement is defined as spontaneous movement in a system that is already out of equilibrium in such a way that the system tends towards equilibrium. Active movement, by contrast, requires the input of biological energy and moves the system further away from equilibrium or keeps it out of equilibrium in spite of continuous passive movement in the counter-direction. The basic equation that describes passive movement is Newton’s law of motion on Earth where there is friction:

  where v is velocity of movement (m s−1 ), F is the force causing the movement (N) and f is the coefficient of friction (N s m−1 ).

  In the context of passive water or solute movement in plants, it is more convenient to measure moles moved per s per unit area, which is a unit of measure called a flux density (J). Fortunately, there is a simple relationship between J, v and concentration (C, mol m−3 ) of the substance moving: J = Cv. Also, in a chemical/biological context, it is easier to measure the energy of a substance, and how the energy changes as it moves, than it is to measure the force acting on the substance. Passive movement of water or a substance occurs when it moves from a location where it has high energy to where it has lower energy. The appropriate energy to measure is called the chemical potential, μ, and it has units of energy per mol (J mol−1 ). The force acting on the water or solute is the rate of change of energy with distance, hence F = −(dμ/dx), which has units of J m−1 mol−1 or N mol−1 (because J = N m). So replacing F with −(dμ/dx) and v with J

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  Plant Physiology Theory and Practice

  • Emea, A(Authors)
  • 2018(Publication Date)
  • Agri Horti Press

    (Publisher)

  The water content of the plant as a whole, and hence of its constituent cells, is controlled principally by the relative rates of transpiration and the absorption of water. The latter process is influenced very markedly by the water content and other conditions prevailing in the soil. Individuals of the same species invariably have a higher osmotic pressure when growing under drought conditions than when provided with a favourable water supply. This is at least partially due to the low water content of the leaves which results when the available soil water supply becomes low. Other factors which are also probably involved are a decrease in the growth rate of the plant which often permits an accumulation of mineral salts and soluble foods, and a shift of the starch 1 soluble carbohydrates It should perhaps be emphasized that the term This ebook is exclusively for this university only. Cannot be resold/distributed. The Osmotic Quantities of Plant Cells 155 osmotic pressure of a tissue can possess meaning only in the sense of the average osmotic pressure of the cells composing that tissue. The solute Content of the Cell sap is Controlled by the specific metabolic processes of the plant and by the absorption of mineral salts by the plant from its environment. The rate of photosynthesis is an important factor indetermining the osmotic pressure of plant cells, particularly those of leaf tissues. The influence of the inherent metabolic processes of any species upon the kinds and concentrations of various types of soluble organic compounds produced, such as simple carbohydrates, organic acids, amino acids, etc. has an exceedingly important effect on the magnitude of the osmotic pressure in any species. Metabolic conditions and their effects upon osmotic pressures may also be altered by environmental conditions. A well-known example of this is the difference in the osmotic pressures of sun and shade leaves on the same plant.

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  The Physiology of Flowering Plants

  • Helgi Öpik, Stephen A. Rolfe(Authors)
  • 2005(Publication Date)
  • Cambridge University Press

    (Publisher)

  Development and differentia-tion are associated with changes in aquaporin density; e.g. during the cell elongation stage which is accompanied by rapid water uptake, aquaporins are particularly abundant. A diurnal rhythm in expres-sion of aquaporin genes has been correlated with leaf movements which depend on turgor changes in their motor cells, mediated by water movements in and out of the cells. It must be emphasized that all water movement into or out of plant cells is along the C gradient. There is no active pumping of water against its free energy gradient, at the expense of metabolic energy, as occurs with many other metabolites. The permeabilities of the plasma membrane and tonoplast towards water are so high that the water would leak out very fast; the amount of metabolic energy needed to pump it against the leakage would be unrealistic. Box 3.2 The plasma membrane aquaporins are collectively termed the PIPs (plasma membrane intrinsic proteins) and the tonoplast-located ones, TIPs (tonoplast intrinsic proteins). There exist also aquaglyceroporins, which permit passage of small non-electrolyte molecules as well as water. All the above-mentioned are classed together as MIP, major intrinsic protein family, found in organisms of all kingdoms. Each aquaporin molecule has six transmembrane -helices embedded in the membrane, and forms one transmembrane water channel; the molecules are commonly grouped in the membrane as tetramers. WATER POTENTIALS OF PLANT CELLS AND TISSUES 67 3.4 Water relations of whole plants and organs The water relations of a whole plant, or even an organ such as a leaf, are much more complex than those of individual cells. The formulae given in the preceding section, relating water potential to pressure and osmotic (and matric) potentials are applicable only at the cellular level. There is no such thing as the C p of a whole plant; the value will vary between different tissues.

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  Plant Cell Biology

  • William V Dashek, Marcia Harrison(Authors)
  • 2010(Publication Date)
  • CRC Press

    (Publisher)

  CHAPTER 7 Movement of Molecules Across Membranes Susanna Malmstrom INTRODUCTION In the plant cell, molecules and ions are constantly moved in and out and between compartments across membranes in order to sustain regular physiological functions and to respond to environmental stress. These movements are called transport and are usually accomplished by specific trans- port proteins embedded in the membrane. In this chapter, we examine the basic concepts of plant membrane transport and learn about the different transport proteins, how they work and where they are located in the cell. We also look at some of the common research methods for the study of transport proteins and processes. BASIC CONCEPTS OF MEMBRANES, PERMEABILITY, AND TRANSPORT Ion Concentrations and Homeostasis in the Plant Cell For normal growth and development, plants are dependent on the light energy of the sun, and on water and mineral nutrients taken up from the soil by their roots. With these three essential elements present, plants are able to synthesize all other molecules they need in order to grow and to reproduce themselves. An essential element is defined as one that has a clear physiologi- cal role and whose absence prevents a plant from completing a life cycle. Hydrogen, carbon, and oxygen are obtained from water and carbon dioxide (Fig. 7.1). The mineral nutrients obtained from the soil which plants require are traditionally listed as macro- or micronutrients, depend- ing on their relative concentration in plant tissue (Table 7.1). As the name infers, macronutrients are needed in higher amounts than micronutrients. The reason is that macro- and micronutrients have differ- ent functions in the plant. In general, macronutrients are constituents of organic compounds (proteins, nucleic acids) or are important in the regulation of cellular osmosis (e.g. K + ).

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  Osmotic and Ionic Regulation

  Cells and Animals

  • David H. Evans(Author)
  • 2008(Publication Date)
  • CRC Press

    (Publisher)

  22

  II. WATER PERMEATION

  An understanding of the movement of water across cell membranes is essential to an understanding of the physical basis for osmoregulation. The volume of fluid compartments (intracellular or extracellular) is for, practical purposes, equal to the volume of water that resides therein, and the (passive) distribution of water is determined entirely by the distribution of solutes.

  A.  DRIVING FORCES FOR WATER MOVEMENT

  In one sense, water movements are incredibly simple; water permeation is a passive process. There is no evidence for active water transport; water movement is driven entirely by the gradient of the chemical potential of water. For biological membranes separating two aqueous solutions (for which standard chemical potential of water will be the same), the transmembrane difference in the chemical potential of water given by Equation 1.14 is:

  Δ μ w     =   ν w   ( Δ P −   R T   Δ C s )

  (1.15)

  which we can express in the practical units of pressure by dividing by the partial molar volume of water (vw ) to yield:

  Δ μ w ν w     =   Δ P −   R T   Δ C s

  (1.16)

  The driving force for passive water transport, as it is often described, is the difference between the hydrostatic pressure gradient (ΔΡ) and the gradient of “osmotic pressure” (Δπ) where Δπ = RTΔCs . This conventional usage can be confusing because π is not a pressure; rather, π is a so-called colligative property of the solution, a measure of composition. Likewise, Δπ is not a pressure difference; it is an expression of the difference in water concentration across the membrane. The association of Δπ with a pressure arises from an analysis of the equilibrium distribution of water across a membrane that is permeable to water but impermeable to solute and is configured such that a hydrostatic pressure can be applied to one side as indicated in Figure 1.3 . If a single, impermeant solute (s) is present on both sides of the membrane such that Cs (2) > Cs (1), then the resulting concentration gradient of water will drive water from side 1 to side 2—that is, from high water concentration to low water concentration. An equilibrium can be established by applying a pressure to the piston on side 2 such that the water flow, denoted here as the volume flow Jv (see below) is reduced to zero. In this condition, Δμ

  w

  equals zero, and from Equation 1.16 we obtain the classic van’t Hoff equation22

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  Transport of Nutrients in Plants

  • A. J. Peel(Author)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  10 The osmotic properties of sieve elements and water movement in phloem It is an essential feature of certain of the hypotheses described in Chapter 7 (pressure flow and electro-osmosis) that considerable quantities of water must be moved through the sieve tubes in order to transport solutes. Conversely, the surface movement hypothesis does not have such a requirement, since with this mechanism the water could be completely static. The 'protoplasmic' hypotheses present something of an intermediate picture, for though in trans-cellular streaming a mass movement is envisaged through strands, the amount of water transport relative to that of solutes would be less than in pressure flow or electro-osmosis. Solution flow in an electrokinetic mechanism is brought about by electrical forces at the sieve plate, and the evidence for potential gradients across this structure has already been described. Pressure flow, however, is driven by gradients of hydrostatic pressure between the ends of the sieve tube. It is therefore necessary in the present chapter not only to decide whether bulk-flow mechanisms are viable on the basis of the experimental evidence for concomitant water and solute fluxes, but also to consider whether there is sufficient evidence of the presence of conditions necessary for the production and maintenance of turgor gradients in phloem. T h e o s m o t i c properties o f sieve e l e m e n t s Plasmolysis in mature sieve elements There are considerable concentrations of osmotically active solutes in sieve tube saps (Chapter 2) ; and although mature sieve elements 188 The osmotic properties of sieve elements 189 do not seem to possess a tonoplast, most cytologists have no doubt that a plasmalemma is present in these cells. The conditions necessary for the generation of turgor pressure would, therefore, appear to exist.

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  Water Relations in Membrane Transport in Plants and Animals

  • Arthur M. Jungreis, Thomas K. Hodges, Arnost Kleinzeller, Arthur M. Jungreis, Thomas K. Hodges, Arnost Kleinzeller(Authors)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  & Stahl, C.A. (1975) Proc. Nat. Acad. Sci. U.S.A. 72: 1822. 45 This page intentionally left blank THE OSMOTIC MOTOR OF STOMATAL MOVEMENT Klaus Raschke MSU/ERVA R2a v i t Re l eatcch Labat i a~otcy INTRUDUCTION: Function of Stomata Stomata are small pores in the epidermis of plants, surrounded by pairs of specialized cells called guard cells. These pores open when the guard cells swell; swelling is caused by an osmotic intake of water. The osmotic pressure in the guard cells increases when a low partial pressure of CO2 in the leaf signals a demand for CO2; the osmotic pressure decreases when the CO2 concentration rises or when water stress develops in the plant. In the latter case, a plant hormone, (+)-abscisic acid (ABA), appears to act as a messenger. This hormone is rapidly formed in stressed leaf tissue, presumably in response to a loss in turgor, and then trav-els to the guard cells. Within minutes, at lower concentrations of ABA after about half an hour, the guard cells begin to lose solutes and the pores narrow. Stomata are parts of a feedback system whose function it is to admit CO2 to photosynthesizing tissue while preventing loss of turgor. I wish to describe that part of the system which acts as the effector in the regulation of gas exchange; I shall try to summarize how transport and metab-olism of osmotica are used by the plant to adjust stomatal aperture. (Stomatal action has been treated more extensively than is here possible. See recent review - Raschke, 1975a). CHANGES IN STOMATAL VOLUME, PRESSURE AND SOLUTE CONTENT Knowledge of the volume and pressure changes occurring in guard cells is a requisite for an assessment of the solute requirement for stomatal opening. By taking microphotographs of stomata at various levels of focus and at various stages of stomatal opening we found that the lumina of a pair of guard cells of Vicia faba 47

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