lec 2- water and plant cells

7/21/2019 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.
http://rds.yahoo.com/_ylt=A0S020quf0pLQgIA6syjzbkF/SIG=13697mr13/EXP=1263259950/**http%3A//www.biology.arizona.edu/biochemistry/tutorials/chemistry/graphics/water.gifhttp://rds.yahoo.com/_ylt=A0S020quf0pLQgIA6syjzbkF/SIG=13697mr13/EXP=1263259950/**http%3A//www.biology.arizona.edu/biochemistry/tutorials/chemistry/graphics/water.gif


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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
http://waterontheweb.org/resources/glossary.htmlhttp://waterontheweb.org/resources/glossary.html


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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

lec 2- water and plant cells

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