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Lec 2- Water and Plant Cells
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Transcript of Lec 2- Water and Plant Cells
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Water and
plant cells
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Water and plant cells
80-90% of a growing plant cell is water This varies between types of plant cells
Carrot has 85-95% water
Wood has 35-75% water Seeds have 5-15% water
Plant continuously absorb and lose water Lost through the leaves
Called transpiration
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Water Transpiration
The evaporationof water into the atmosphere from
the leaves and stems of plants.
It occurs chiefly at the leaves while their stomataare
open for the passage of CO2 and O2 during
photosynthesis.
Transpiration is not simply a hazard of plant life.
It is the "engine" that pulls water up from the roots
to:
supply photosynthesis (1%-2% of the total)
bring minerals from the roots for biosynthesis within leaf
cool the leaf .
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Water
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Hydrogen Bonds
The Amazing Hydrogen Bond
results from the partial charges
generated by the 105 degree angle
between the two hydrogen atoms on
the water molecule.
Hydrogen bond: attraction of
hydrogen with a partial positive
charge to another molecule with apartial negative charge.
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Water is Polar
Water is a polar molecule with differing
charges on differing sides.
This sets up a critical difference between
molecules:
Hydrophobic--afraid of water
Hydrophilic--love water
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Water
(A) Hydrogenbonds betweenwater molecules
results in local
aggregations of
water molecules
(B) Theses arevery short lived,
break up rapidly to
form more random
configurations
Due to temperature
variations in water
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Properties of Water
1. Water is Liquid at
Room Temperature.
Structure = Charge
Charge = Hbond
2. Water is the Universal
Solvent.
Water dissolves polar
molecules
Water allows chemical
reactions to occurWater establishes
boundaries
Remember Hydrophilic and
Hydrophobic?
Salts such as sodium chloride (NaCl),
dissolve in water by dissociating as each
ionbecomes surrounded by the polar water
molecules
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Properties of Water
3. Water is Cohesive
Water sticks to sides
Water fills vessels
Water molecules cling,
but also rearrange, this
allows water to flow!
4. The temperature of
water rises and falls
slowly
This prevents sudden ordramatic changes!
One calorie will raise 1g
of water 1 degree C.
This is over two times theamount of energy
required for other
covalent molecules!
Protection and Buffer
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Properties of Water
5. Water has a high heat
of Vaporization
Water to steam requires a
lot of energyThis property helps
moderate the earths
temperature
Life as we know it wouldnot exist if we were all
gasses!
6. Frozen water is less
dense than liquid water
Lakes freeze from the top
downAllows for ice activities
Protects organisms in
Lakes
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Water transport processes Moves from soil, through plant, and to atmosphere
by a variety of mediums
Cell wall
Cytoplasm
Plasma membranes
Air spaces
How water moves depends on what i t is passing
through
(Soil , plant and atmosphere continuum) -SPAC
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Water across plant membranes
There is some diffusion ofwater directly across the bi-lipid membrane.
Aquaporins: Integralmembrane proteins that formwater selective channelsallows water to diffuse faster
Facil i tates water movement inplants
Alters the rateof water flowacross the plant cellmembraneNOT direction
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Permeability and Diffusion
The plasma membrane is
selectively permeable.
This means that only some
molecules can cross.
Small uncharged molecules
like O2, CO2 and H2Opass.
Large or charged molecules
like proteins or ions
cannot pass.
Diffusion is the movement
of molecules from an area
of high concentration to an
area of low concentration.
10 20
1515
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Diffusion
Diffusion works down a concentration gradient. Leads to the gradual
mixing of molecules & eventual dissipation of conc. Differences. It is rapid over short distances, but extremely slow over long distances
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Pressure driven bulk flow drives
long distance water transport
Bulk f low:
Concerted movement of groups of molecules, most often in
response to a pressure gradient.
Dependant on the radius of the tube that water is traveling
in.
Double radiusflow rate increases 16 times!!!!!!!!!!
This is the main method for water movement in :
Xylem, Cell Wallsand in the Soil.
I ndependentof solute concentration gradientsto a point
So dif ferent f rom dif fusion
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Cell water potential
All living things need a continuous input of
free energy to maintain and repair structures,
as well as to grow and reproduce
Biochemical reactions, solute accumulation,
and long distance transport are all driven by
the input of free energy into the plant
This is defined as Water Potential.
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Cell water potential -yw The equation yw =ys + yp + yg Affected by three factors:
s : Solute potential or osmotic potential
The effect of dissolved solutes on water and the cell
p : Hydrostatic pressure of the solution.A +ve pressure is known as Turgor pressure
Can beve, as in the xylem and cell wallthis is important inmoving water long distances in plants
g : Gravity- causes water to move downwards unlessopposed by an equal and opposite force
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Osmosis and Tonicity
Osmosis is the diffusion of
water across a plasma
membrane.
Osmosis occurs when there
is an unequal concentration
of water on either side of
the selectively permeable
plasma membrane.
Remember, H2O
CAN cross the plasma
membrane.
Tonicity is theosmolarity of a
solution--the amount of
solute in a solution.
Solute--dissolved
substances like sugars
and salts.
Tonicity is always incomparison to a cell.
The cell has a specific
amount of sugar and
salt.
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Tonic Solutions
A Hypertonicsolution has more solute than
the cell. A cell placed in this solution will give
up water (osmosis) and shrink.
A Hypotonicsolution has less solute than the
cell. A cell placed in this solution will take up
water (osmosis) and blow up.
An Isotonicsolution has just the right amount
of solute for the cell. A cell placed in this
solution will stay the same.
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Plant cell in hypotonic solution
Flaccid cell in 0.1M sucrosesolution.
Water moves from sucrose
solution to cellswells upbecomes turgid.
This is a Hypotonicsolution -
has less solute than the cell.So higher water concentration.
Pressure increases on the
cell wall as cell expands toequilibrium
The equation yw = ys + yp
s : Solu te potenti al or osmotic potentialThe effect of dissolved solutes on water and the cell
p : Hydrostatic pressur e of the solution.A +ve pressure is known as Turgor pressureCan beve, as in the xylem and cell wallthi s isimportant i n moving water long distances in plants
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Plant cell in hypertonic solution
Turgid cell in 0.3M sucrose
solution
Water movers from cell to
sucrose solution
A Hypertonicsolution has
more solute than the cell. So
lower water concentration.
Turgor pressure reduced and
protoplast pulls away fromthe cell wall
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w
and water status of
plants
Water potential has two main uses
1: Governs water transport across membranes.
2: Uses as a measure of the water statusof plant.
Because of water loss to the atmosphere plants are
seldom fully hydrated.
They suffer from water deficits
Leads to inhibition of
1. Plant growthmost l ikely to be affected
2. Photosynthesis
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w
and water status of
plants
Cell division slows down
Reduction of synthesis of:
Cell wall
Proteins
Closure of stomata Due to accumulation of the
plant hormone Abscisic acid
This hormone induces
closure of stomata during
water stress
Naturally more of abscisic
acid hormone in desert
plants
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Summary
Water is important to plantsMakes up the media in which all biochemical processesoccur that are essential to plant life.
Influences the structure and function of proteins, cell
membranes, nucleic acids, & carbohydrates Water movement driven by free energy. Moves
by
Osmosis, bulk flow, diffusion or a combination
Help moves water from soil through plant to atmosphere
Water potential is a measure of water status of aplant.
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Quiz 1