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? Biological membranes are electrically polarized, like a battery.

3.  G for ion transport  G = zF  E m z is charge on the ion (unitless): K + = +1, NO 3 - = -1, Ca +2 = +2, SO 4 -2 = -2 other molecules have not net charge F is Faraday’s constant = 9.65 x 10 4 J vol -1 mol -1 E m is membrane potential, volts

NO 3 - Example: uptake of NO 3 - against volt potential  G = zF  E m  G = (-1)x9.65x10 4 J Volt -1 mol -1 x (-0.15Volt) = 1.45 x 10 4 J mol -1 = 14.5 kJ mol -1

4. Movement along electrical and concentration gradients  G = zF  E m RT log(C 2 /C 1 ) note rearrangment as  E m = -2.3 RT/zF log(C 2 /C 1 ) R = J mol -1 K -1 z is charge of solute F is Faraday’s constant = 9.65 x 10 4 J vol -1 mol -1 E m is membrane potential, volts

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?

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

V S (substrate concentration) V max 1/2 V max KmKm V max x S K m + S V = Michaelis-Menten equation

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”

Competitive inhibitor increases K m and does not affect V max, but a higher [S] is required to reach V max. Mechanisms 1.Competitive inhibitor binds at same active site as substrate, making less enzyme available to catalyze E+S reaction. 2.Competitive inhibitor binds at another site on enzyme, causing a conformational change in active site that reduces affinity for the primary substrate. “allosteric inhibitor”. Substrate concentration With competitive inhibitor No inhibitor

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

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.

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

I.Water Plant cells are mostly water; % 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.

Terrestrial primary productivity is strongly dependent on water availability.

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

Mangroves are rooted in sea water Are they water stressed? of  S

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

Stomata

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

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)

Properties of water, continued Cohesion is the attraction of like molecules (H 2 O 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.

Water’s thermal properties High specific heat = 4.18 kJ kg -1 0 C -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?

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.

What factors determine the direction of water movement? 1.Gravity 2. Pressure 3. Concentration

Gravity Water flows downward if it can.

Height, meters but it flows upward in trees. How does this work? How do we relate the energetic status of water to height?

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

Water moves from higher to lower 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.

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

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?

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

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?

The concept of water potential, , 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:  W  S  P  g  W = total water potential  S = solute potential  P = pressure potential  g = gravitational potential All units will be pressure, pascals, Pa. MPa is megapascal, 10 6 Pa

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 m 3 = joules pressure = energy/volume

The reference condition for water potential thinking: Pure water (  S = 0), at ground level (  g = 0) and atmospheric pressure (  P = 0) has a total water potential,  W, of 0 MPa.

Water tends to move spontaneously from regions of higher to lower values of  Because all of the components of  W have units of pressure (Pa), this is the same as saying water tends to move from regions of higher to lower total pressure.  W 1  W > 2 MPa > -2 MPa > -4 MPa > -1 MPa NO!

Water tends to move spontaneously from regions of higher to lower values of  Because all of the components of  W have units of pressure (Pa), this is the same as saying water tends to move from regions of higher to lower total pressure.  W 1  W > 2 MPa > -2 MPa > -4 MPa > -1 MPa NO!

 W  S  P  g How do we express  S,  P, &  g in units of pressure?  S, the solute pressure or solute potential.  S = -RTC S Where R is the gas constant, T is Kelvin temp., and C S is the solute concentration. R = MPa liters o K -1 mol -1 C s = mol liter -1 Bottom line: adding solutes to water decreases the solute potential.

 S = -RTC S What is the solute (osmotic) potential of sea water? assume 25 o C or 298 o K C S = 1.15 mole liter -1 of Na + + Cl - + other ions  S = ( MPa liter o K -1 mol -1 )(298 o K)(1.15 mol liter -1 )  S = MPa

 W  S  P  g The pressure potential  P is just what we would measure with a pressure gauge.

 W  S  P  g How do we calculate the gravitational potential?  g =  gh  g = density x g x height

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?  g = 1000 kg m -3 x 9.8 m s -2 x 100m = 9.8 x 10 5 Pa or 0.98 MPa So, to hold water at that height, there must be a counteracting negative pressure of at least MPa in the xylem

What do various values of  W mean for plant function?