3.AguaCélulas.pdf

FISIOLOGÍA VEGETAL ADRIANA SÁNCHEZ [email protected] WATER & PLANT CELLS CHAPTER 3 Water and plant cells I. Background on water in plants II. The properties of water III. Understanding the direction of water movement: Water potential IV. Cell wall and membrane proteins I. Water...

FISIOLOGÍA VEGETAL ADRIANA SÁNCHEZ [email protected] WATER & PLANT CELLS CHAPTER 3 Water and plant cells I. Background on water in plants II. The properties of water III. Understanding the direction of water movement: Water potential IV. Cell wall and membrane proteins I. Water in plants • Why do plants need so much water? I. Water in plants • Why do plants need so much water? – ca. 97% lost through transpiration – Aprox. 2% à growth – 1% à metabolic processes and photosynthesis • Plant cells are mostly water: 80 – 95% – Wood (Sapwood 35-75%) – Seeds 5-15% Productivity of various ecosystems as a function of annual precipitation • Plants lose large quantities of water in transpiration à evaporation from the interior of leaves through the stomata • • • • CO2 H2 O Water loss through leaf surface Heat dissipation by leaves through transpiration Half of the heat by sunlight is dissipated by transpiration II. Properties of water Polar molecule that forms hydrogen bonds 1) good solvent 2) cohesive properties - attraction to like molecules 3) adhesive properties - attraction to unlike molecules Covalent bond The polarity of water molecules results in hydrogen bonding II. Properties of water • Oxygen has two unshared electrons – Creates electrostatic attractions between water molecules II. Properties of water • Excellent solvent for ionic substances and molecules, versatile • High specific heat capacity (?) • High latent heat of vaporization (?) Effect of temperature? Why is high specific heat capacity and latent heat important to plants? High specific heat capacity: Water absorbs large amounts of energy of heat before changing its Temp II. Properties of water • Cohesion • Adhesion – The attraction of unlike molecules. – Water adheres to cell walls, soil particles, glass tubes, jeans, etc. • Capillarity – Cohesion, adhesion and surface tension give rise to it – A narrower tube: higher water column II. Properties of water • How strong is the tensile strength of water? • Water has a theoretical tensile strength of approximately 130 Mpa but most experimental values are below 30 Mpa Pressure is measured in units of megapascals, ppsi (pounds per sq. in), torr, bars, or atmospheres • Megapascals (MPa).1 MPa = 9.9 atmospheres Understanding Pressure • Suction is an example of negative pressure (-). • Pushing and squeezing are examples of positive pressure (+). • Water pressures in plant cells can be positive (turgor), or negative (tension). – Living cells ≥ 0 MPa to ≈ +3 Mpa – Dead xylem cells ≤ 0 MPa, to as low as -12 MPa. Positive pressure pushes matter away from the force When a cell is fully turgid, the cell wall pushes back against the cytoplasm, like a balloon pushing against its contents Living cells must maintain a positive water pressure, or “turgor” to grow and function properly. Turgor pressure • • Build internal Hydrostatic Pressure Cell enlargement, gas exchange in leaves, transport in phloem, transport across membrane, rigidity and mechanical, stability to nonlignified plant tissues Negative pressure pulls matter towards the force Diffusion & Osmosis Fick’s law? III. Water potential • What factors determine the direction of water movement (through the soil, between cells, from roots to leaves, from leaves into air)? 1. Concentration 2. Pressure 3. Gravity • Measure of the potential energy in water as well as the difference between the potential in a given water sample and pure water. Free energy per unit volume (MPa) Ψ total = Ψ s + Ψ p + Ψ g Concentration Pressure Gravity 100 90 Gravity: Ψ g 80 Water flows upward in trees. 70 60 Height, meters 50 40 30 20 10 Gravity causes water to move downward unless it is opposed by an equal and opposite force... Gravity is usually negligible at cellular level 0 The water potential at cellular level: + Ψs = solute Ψp = pressure forces osmotic force Ψw = water potential = tension that the water column is under All units: pascals, Pa. MPa is megapascal, 106 Pa Concentration/Solutes: Ψs Water moves by osmosis from regions of higher to lower water concentration. Solutes added to pure water dilute the water concentration. The more solutes are in a solution, the more powerfully that solution will be pulling water towards it. Osmotic pressure “pulls” water towards the side with more solutes, so it is a negative pressure term also. Ψs of pure water is 0 Ψs grows more negative the more solutes are added to the solution (-) Ψs can never be > 0 This is important because Ψs counter force that keeps water in a cell when stomata are open “Osmotic adjustment” is the accumulation of solutes in plant cells and is a strategy that plants use to conserve water in periods of water stress. Example: the relationship between Ψs and sucrose concentration Ψs = solute potential + Ψp = pressure potential Ψw = water potential positive for cell walls negative for evaporation (atmospheric pull) Pressure: Ψp Water moves from regions of higher to lower pressure A B C The fuller of water a cell becomes, the greater the cell walls will push against the cytoplasm: POSITIVE!! Which atmosphere is exerting the strongest (negative) pulling force on water in the plant? Relative humidity = 35% A Relative humidity = 75% B Which environment would you expect plants to accumulate the most solutes into cells? Relative humidity = 35% A Relative humidity = 75% B Plants have an important dilemma: conserving vs letting water go This illustrates the battle plants are up against - Evaporation: pulling water out of cells - Osmotic pressure: trying to counteract it and keep water in Example: a cell in a leaf in a herb Ψs = - 1 MPa Ψp of air = -1.5 MPa Ψp of cell walls = +1 MPa Ψw of cell = -1.5 MPa The water column of this plant is experiencing -1.5 MPa of tension. Ψw = water potential How do we measure tension of the water column (Ψw)? How do we measure tension of the water column (Ψw)? E. grandiflora C. tessellata 0 ** ** Ψ (MPa) -0.1 -0.2 ** -0.3 ** -0.4 Dry Rainy -0.5 5 7 9 11 13 15 17 5 7 9 Time of day (h) 11 13 15 17 Plant water status • Water potentials are used as a measure of water status of a plant – Water deficit affects cell expansion – May stimulate root growth but decrease shoot growth – Turgor pressure: stretch cell walls, rigidity of cells and tissues Change in water status causes physiological changes Measure of the rate of passage of CO2 entering or water vapors exiting through stomata