Water and Plant cells

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• Water and Plant cells
• Water--the most amazing molecule
• A. Its shape and polarity make it a sticky
  molecule
• 1. Water is not linear, but shaped more like
  an open V. The angle between the hydrogen
  atoms is about 1050.
• 2. Of the two atoms, Oxygen is much more
  electronegative than Hydrogen. This means that
  it has a much stronger attraction for electrons
  than hydrogen does.
• a. Oxygen hogs the electrons,
• so it has a partial negative
  Figure3.3 charge.
• b. The two hydrogens have the
• electrons less of the time, so
• each has a partial positive
• charge.

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• Water and Plant cells
• 3. These partial (positive and negative)
  charges are attracted to each other. The
  effect is a weak bond called a hydrogen bond,
  between the hydrogen of one molecule and the
  oxygen of another. This makes water molecules
  tend to stick together.
• a. In cold water, at any instant there
  would be a relatively high Fig.
  3.4a percentage of water molecules that are
  participating in hydrogen bonds. As water
  freezes to ice, the hydrogen bonds lock in
  place, giving solid water its crystalline
  lattice. Locking these bonds at arms
  length increases the volume.

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• Water and Plant cells
• b. In hot water, a lower percentage of water
  molecules would be found participating in
  hydrogen bonds at any instant. In gaseous water
  vapor, hydrogen bonds occur much more rarely.
  Figure 3.4
• B. Hydrogen bonds give water some amazing
  properties
• 1. Solid water (ice)--Ice floats when put in
  liquid water.
• Why?
• Why is this important?

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• Water and Plant cells
• 2. Liquid water
• a. Has relatively high boiling point.
• b. Has a high specific heat. That is to
  say that it resists change in temperature. It
  takes a lot of heat to warm water up, and
  anyone who wants to cool water down had
  better be ready to absorb lots of energy.
  This is why coastal areas are protected
  from major temperature swings.

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• Water and Plant cells
• c. Has a high latent heat of vaporization.
  This means that it takes a lot of heat to
  turn it from liquid to a gas, again because so
  many hydrogen bonds have to be broken. This
  is incredibly important as we sweat. It is
  even more important to the plant to keep its
  highly irradiated leaves from overheating.
• When we get hot in
• the sun, we seek a
• shady place. Plants
• cant do thatthey
• are providing the
• shade!

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• Water and Plant cells
• d. Has high surface
• tension. This
• means that an
• undisturbed
• surface of water
• resists disturbance
• because the water
• molecules have a greater attraction to each
  other (cohesion) than they do to the air at
  the surface.

Is this how Jesus walked on water?
7

• Off the Record
• Science and Miracles
• Many people try to see if they can
• explain Jesus miracles by science.
• Have you heard or read any
• examples?
• Hebrews 13
• Peters first attempts at walking on
• water with Jesus showed inadequate
• faith.

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• Water and Plant cells
• Freestyle ski jump training demonstrates surface
  tension and

Pipes to release air bubbles
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• Water and Plant cells
• e. Water has high tensile strength. This is
  the property of a substance in a column to
  resist collapse.
• It is best seen in a syringe, where water is
  drawn in and then it
• is capped. Positive pressure can be applied as
  you try to expel
• the water. Since the water is under pressure but
  not moving,
• this is called hydrostatic pressure. Even more
  important is
• negative hydrostatic pressure, which can be
  applied by pulling
• on the plunger. It is the ability to resist this
  pressure (forced
• on the water by gravity pulling down as it is
  being moved to
• the top of a
• Douglas fir
• tree) that allows Fig. 3.5
• extremely tall
• plants to exist.

So how/why does water move up the plant?
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• Water and Plant cells
• Water movement processes
• A. Diffusion is movement
• of a substance in a medium in
• the direction from high
• concentration towards lower
• concentration, driven by random movement of
  molecules.
• Can this do work?
• If so, where does it Fig. 3.7
• get the free energy?

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• Water and Plant cells
• 1. Adolf Fick described the physical
  principles that determine how fast one
  molecule will diffuse in another. Ficks
  Law
• JAB -DAB(DcAB/Dx)
• Where JAB is the flow rate (e.g. in moles m-2
  s-1)
• DAB is the diffusion coefficient, dependent
• upon both the solvent and the solute.
• DcAB is the difference between the
• beginning concentration and the
• ending concentration.
• Dx is the distance from the area of
  concentration to the area of least
  concentration.

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• Water and Plant cells
• 2. The real kicker here is distance, since
  the flow rate slows down as the difference in
  concentrations gets smaller. The time it
  takes to completely dissolve a substance is
  related to the square of the distance it must
  diffuse. Fig 3.8

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• Water and Plant cells
• B. Bulk flow describes water moving en masse in
  the same direction. As with diffusion, it moves
  down a gradient pressure instead of
  concentration.
• 1. Water flows in this way up the xylem
  tracts.
• 2. The flow rate (expressed as a volume over
  time) is a function of the channel through
  which it is flowing and the pressure gradient
  which pushes it. This is expressed as
  Poiseuilles Law, which can be applied to any
  liquid by taking into account is viscosity.
  This law states that the flow rate is
  correlated to the length of the radius raised
  to the fourth power.

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• Water and Plant cells
• C. Osmosis is the movement of water through
  selectively permeable membranes, and is
  affected by both pressure and concentration
  gradients.
• Water moves relatively easily through
  phospholipid membranes (although this process is
  aided by auqaporins, which are specific channels
  through which water alone may flow). Figure 3.6

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• Water and Plant cells
• If we put a plant cell in distilled water,
  these two gradients come in opposition to each
  other, and an equilibrium is established that
  keeps the cell rigid, complementing the strength
  of the cell wall.

Inside cell there is high solute concentration
(lower water concentration) which would draw
water in, but this is fighting the
physical force of the turgor pressure that has
built up because of the influx of water.
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• Water and Plant cells
• D. Water potential, a measure of the free
  energy of water (e.g. water in a cell).
• In which of these does the water have the most
  free energy?
• In which does it have the least?
• 0.1 M glucose 0.1 M NaCl distilled water

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• Water and Plant cells
• D. Water potential, a measure of the free
  energy of water (e.g. water in a cell).
• 1. It is derived from the chemical potential
  of the aqueous solution, which is the amount
  of potential energy (measured in Joules or
  kcal) per mole of water in the solution (cell).
  
• 2. The actual expression of water potential
  measures energy per milliliter of water in the
  solution (note, I didnt say per milliliter of
  solution). Were dealing with the energy
  of the water in the solution, much the same way
  as we might look at each of the partial
  pressures of the gases that make up air. But
  thats not to say that the solutes arent
  important, they affect water potential, but
  they are not the water.

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• Water and Plant cells
• 3. Water potential (?W) has three main
  components, only two of which we will consider at
  the cellular level.
• ?W ?S ?P ?G
• a. Solute potential (a.k.a. osmotic
  potential)--Solutes decrease the free energy
  the water in a solution, by increasing the
  randomness (pure water, like pure anything, is
  very ordered). Solute potential of water can
  be estimated by the vant Hoff equation
• ?S -RTcS
• where R is the gas constant (8.32 J mol-1 K-1), T
  is the
• absolute temperature (in K), and cS is the
  number of moles of
• all solutes per liter of water in the solution
  (osmolality).
• Will osmotic potential ever be a positive number?
• What are the units for ?S?

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• Water and Plant cells
• So that calculation of osmotic potential becomes
  a calculation of energy per liter of water, as we
  might expect.
• However, to compare the effect of solute
  potential to the effect of physical (hydrostatic)
  pressure, it would be nice to express solute
  potential as a pressure as well.
• Joules (energy) may be converted to MegaPascal
  liters
• (MPa L -- units of pressure and volume,
  respectively), by
• applying a conversion factor. After this is
  done, the gas
• constant, R, is expressed as 8.3143 X 10-3 MPa L
  mol-1 K-1.
• Applying the vant
• Hoff equation now Table 3.2
• gives us osmotic
• potential in MPa, a
• unit of pressure.

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• Water and Plant cells
• b. Hydrostatic pressure (?P)--The physical or
  mechanical pressure applied against the cell
  wall by the cell. Technically, what we
  usually mean is the relative pressure, how
  much greater or less than the outside air
  pressure is that in the cell.
• For most plant cells to function properly, their
  hydrostatic pressure should be positive (if too
  many are at 0 MPa, the plant would be _____.)
• However, some cells, particularly the xylem, are
  designed to function with negative pressures, as
  the water is drawn up the plant.

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• Water and Plant cells
• c. Gravity potential (?G)--Is the free energy
  of water due to gravity. ?G ?Wgh, where ?W
  is the density of water, g is the acceleration
  of gravity, and h is the height above the
  reference point. In reality, there is a 0.01
  MPa change for every meter that is ascended.
• Over the height of a huge tree, this is a very
• powerful pressure that must be overcome by
• transpiration to keep water flowing up the
• xylem.
• However, on the scale of a cell, there is
• effectively no difference in height. So for
• many calculations it can be ignored, reducing
• the water potential equation to
• ?W ?S ?P