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K + NO 3 - Ca +2 SO 4 -2

Biological membranes are electrically polarized, like a battery. K + NO 3 - Ca +2 SO 4 -2. Which ion requires the most energy to move across the membrane, assuming the same concentration gradient for all four?. 3.  G for ion transport  G = zF  E m

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K + NO 3 - Ca +2 SO 4 -2

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  1. Biological membranes are electrically polarized, like a battery. K+ NO3- Ca+2 SO4-2 Which ion requires the most energy to move across the membrane, assuming the same concentration gradient for all four?

  2. 3. G for ion transport G = zF Em z is charge on the ion (unitless): K+ = +1, NO3- = -1, Ca+2 = +2, SO4-2 = -2 other molecules have not net charge F is Faraday’s constant = 9.65 x 104 J vol-1 mol-1 Em is membrane potential, volts

  3. Example: uptake of NO3-against -0.15 volt potential G = zF Em G = (-1)x9.65x104 J Volt-1 mol-1 x (-0.15Volt) = 1.45 x 104 J mol-1 = 14.5 kJ mol-1 NO3-

  4. 4. Movement along electrical and concentration gradients • G = zF Em + 2.3 RT log(C2/C1) • note rearrangment as Em = -2.3 RT/zF log(C2/C1) • R = 8.314 J mol-1 K-1 • z is charge of solute • F is Faraday’s constant = 9.65 x 104 J vol-1 mol-1 • Em is membrane potential, volts

  5. Enzyme kinetics What are enzymes? What kinds of molecules are they made of? What do they do to reaction rates? How do they work? What conditions affect the rate of enzyme-catalyzed reactions? What’s the world’s most abundant enzyme?

  6. Vmax reaction rate, V (moles of product per second) Substrate concentration, S (moles/liter)

  7. Vmax x S Km + S V = Vmax V 1/2 Vmax Michaelis-Menten equation Km S (substrate concentration)

  8. Enzyme specificity is not perfect, other molecules can compete for the active site. “competitive inhibition” Enzyme structure can be modified by other molecules, reducing enzyme activity. “non competitive inhibition”

  9. Substrate concentration Competitive inhibitor increases Km and does not affect Vmax, but a higher [S] is required to reach Vmax. Mechanisms Competitive inhibitor binds at same active site as substrate, making less enzyme available to catalyze E+S reaction. Competitive inhibitor binds at another site on enzyme, causing a conformational change in active site that reduces affinity for the primary substrate. “allosteric inhibitor”. No inhibitor With competitive inhibitor

  10. The most important enzyme in the world? “Rubisco” ribulose 1,5 bisphosphate carboxylase-oxygenase Rubisco RuBP + CO2 -----> 3PGA

  11. Conditions affecting enzyme activity. 1. pH Enzymes have an optimum pH at which activity is maximum, with sharp declines in activity at lower and higher pH. pH affects enzyme activity by altering ionization state of active site or by affecting the 3-D conformation of the active site. 2. Temperature Enzyme activity has temp. optimum; sharp declines at lower and higher temp.. Reaction rates initially increase with temperature Enzyme activity decreases at temperatures high enough to cause “denaturation; unfolding of protein structure and loss of proper conformation for catalysis.

  12. Water and plant cells (chapter 3) I. Background on water in plants II. The properties of water III. Understanding the direction of water movement: Water potential

  13. Water • Plant cells are mostly water; 80 - 95% of the mass of growing cells, (less in wood and seeds) • Living cells must maintain a positive water pressure, or “turgor” to grow and function properly. • Plants lose large quantities of water in transpiration, the evaporation from the interior of leaves through the stomata.

  14. Terrestrial primary productivity is strongly dependent on water availability.

  15. Soil particles can bind water tightly, making it difficult for plant roots to absorb it. How does creosote bush survive?

  16. Mangroves are rooted in sea water Are they water stressed? of YS

  17. Water passes easily throughbiological membranes, particularly through aquaporins - low resistance pores.

  18. Stomata

  19. II. The 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

  20. Cohesion of water molecules gives water high tensile strength - it can withstand high tension (negative pressure) without shearing apart. Water in the xylem is under negative pressure (more on this in Chapter 4)

  21. Properties of water, continued • Cohesion is the attraction of like molecules (H2O here) that gives water its tensile strength. • Adhesion is the attraction of unlike molecules. Water adheres to cell walls, soil particles, glass tubes, etc. • Adhesion explains capillarity & surface tension.

  22. Water’s thermal properties • High specific heat = 4.18 kJ kg-1 0C-1 • Why don’t saguaros overheat? • High latent heat of vaporization • 44 kJ mol-1 or 2.44 kJ g-1 • Leaves are like swamp coolers! • What’s a sling psychrometer?

  23. III. What factors determine the direction of water movement (through the soil, between cells, from roots to leaves, from leaves into air)? How can we describe these factors in a consistent way? We’ll use the concept of water potential. “Potential” indicates the energetic state.

  24. What factors determine the direction of water movement? • Gravity 2. Pressure 3. Concentration

  25. Gravity Water flows downward if it can.

  26. 100 0.0 0.2 0.4 0.60.81.0 90 but it flows upward in trees. How does this work? How do we relate the energetic status of water to height? 80 70 Height, meters 60 50 40 30 20 10 0

  27. Pressure Water moves from regions of higher to lower pressure garden hose straw through xylem of plants

  28. Water moves from higher to lower pressure

  29. 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.

  30. 3) Concentration Water moves by diffusion from regions of higher to lower water concentration. Solutes added to pure water dilute the water concentration.

  31. Osmosis is the diffusion of water across a selectively permeable membrane from a region of higher to lower water concentration. How does reverse osmosis purify water?

  32. Solutes reduce the concentration of water. Think of the effect of solutes in terms of water concentration.

  33. How can we bring together the influences of gravity, pressure, and solutes in understanding the status of water? Is there a consistent set of units?

  34. The concept of water potential, Y, brings together the influences of gravity, pressure, and concentration (solutes) in describing the energy state of water and the direction of water movement. The water potential equation: YW = YS + YP + Yg YW = total water potential YS = solute potential YP = pressure potential Yg = gravitational potential All units will be pressure, pascals, Pa. MPa is megapascal, 106 Pa

  35. We’ve been talking about the “energy state” of water, but now water potential in terms of pressure. What’s the relationship? Recall from before: pressure x volume = energy Pa x m3 = joules pressure = energy/volume

  36. The reference condition for water potential thinking: Pure water (YS= 0), at ground level (Yg = 0) and atmospheric pressure (YP = 0) has a total water potential, YW, of 0 MPa.

  37. Water tends to move spontaneously from regions of higher to lower values ofY. Because all of the components of YWhave units of pressure (Pa), this is the same as sayingwater tends to move from regions of higher to lower total pressure. YW1YW2 10 --------> 2 MPa 0 --------> -2 MPa -1 --------> -4 MPa -2 ---------> -1 MPa NO!

  38. Water tends to move spontaneously from regions of higher to lower values ofY. Because all of the components of YWhave units of pressure (Pa), this is the same as sayingwater tends to move from regions of higher to lower total pressure. YW1YW2 10 --------> 2 MPa 0 --------> -2 MPa -1 --------> -4 MPa -2 ---------> -1 MPa NO!

  39. YW = YS + YP + Yg How do we express YS, YP, & Yg in units of pressure? 1. YS, the solute pressure or solute potential. YS = -RTCS Where R is the gas constant, T is Kelvin temp., and CS is the solute concentration. R = 0.008314 MPa liters oK-1 mol-1 Cs = mol liter-1 Bottom line: adding solutes to water decreases the solute potential.

  40. YS = -RTCS What is the solute (osmotic) potential of sea water? assume 25 oC or 298 oK CS = 1.15 mole liter-1 of Na+ + Cl- + other ions YS = (-0.008314MPa liter oK-1 mol-1)(298oK)(1.15 mol liter-1) YS = -2.84 MPa

  41. YW = YS + YP + Yg The pressure potential YP is just what we would measure with a pressure gauge.

  42. YW = YS + YP + Yg How do we calculate the gravitational potential? Yg = rgh Yg = density x g x height

  43. Dimensional analysis = density x g x height = kg m-3 x m s-2 x m = N m-2 = pressure, Pa Example: what is gravitational potential of water at 100 m in a tree? Yg = 1000 kg m-3 x 9.8 m s-2 x 100m = 9.8 x 105 Pa or 0.98 MPa So, to hold water at that height, there must be a counteracting negative pressure of at least -0.98 MPa in the xylem

  44. What do various values of YW mean for plant function?

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