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Water :
Diffusion and Osmosis
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Most organisms are comprised of at least 70% ormore water. Some
plants, like a head of lettuce, are made up of nearly95% water;
When organisms go dormant, they loose most oftheir water. For
example, seeds and buds are typically less than 10%water
Water is the limiting resource for crop productivityin most
agricultural systems
Earth is the water planet (that's why astronomers get so
excitedabout finding water in space
).
I. Water is absolutely essential for all
living organisms
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II. Water is polar and readily forms
hydrogen bonds
Cohesion, adhesion capillary action
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III. The Properties of Water-1
a l iquid at physio log ical temperatures(0100C)
high heat of vapor ization cooling system high specif ic heat
(heat capacity). thermal
buffer
high sur face tension capillary action, a
meniscus forms. (negative presssuretension)
a un iversal so lvent.
Plant Physiology (Biology 327) - Dr. Stephen G. Saupe; College
of St. Benedict/ St. John's University; Biology Department;
Collegeville, MN 56321
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III. The Properties of Water-2
high tensi le strength and incom pressib i l itywater is good
for hydraulic systems and
produces positive pressures (hydrostaticpressures). This
pressure provides the drivingforce for cell growth and other plant
movements
t ransparent to l ightphotosynthesize
chem ically inert.
disso ciates into protons and hydrox ide ions.
affects the shape, stabi l i ty & pro pert ies ofbio logica
l m olecu les.
Plant Physiology (Biology 327) - Dr. Stephen G. Saupe; College
of St. Benedict/ St. John's University; Biology Department;
Collegeville, MN 56321
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IV. Functions of Water
a major component of cells
a solvent for the uptake and transport of materials
a good medium for biochemical reactions
a reactant in many biochemical reactions (i.e.,
photosynthesis) provides structural support via turgor pressure
(i.e., leaves)
the medium for the transfer of plant gametes (sperms swimto eggs
in water, some aquatic plants shed pollenunderwater)
offspring (propagule) dispersal (think "coconut") plant
movements are the result of water moving into and out
of those parts (i.e., diurnal movements, stomatal opening,flower
opening)
cell elongation and growth
thermal buffer
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V. Acids and Bases
Water ionizes to form a hydrogen ion (or proton/H+) andhydroxide
ion (OH-)
An acid is a substance that increases the [H+], or as a
proton
donor.eg. HCl H+ + Cl-
A base is a substance that increases the [OH-]; or from
theperspective of a proton, a base is a substance that decreasesthe
proton concentration; it is a proton acceptor.
e.g. NaOH Na+ + OH- (accepts protons to make water)e.g. NH3
(ammonia) + H
+ NH4+ (ammonium ion)
pH increases, the [H+] decreases and the [OH-] increasespH
decreases, the [H+] increases and the [OH-] decrease
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VI. Living systems are very sensitive
to pH
Organisms must maintain pH withintolerable ranges. This is a
good example
of homeostasis. A buffer is a solution that resists
fluctuations in pH when additional OH- or
H
+
are added. A buffer maintain a constant pH and
usually consist of a proton donor and aproton acceptor.
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VII. Water MovementThere are two major ways to move
molecules:
A. Bulk (or Mass) Flow. : mass movement ofmolecules in response
to a pressure gradient, fromhigh to low pressure, following a
pressure gradient.
B. Diffusion : spontaneous, random movement ofmolecules from an
area of high free energy (higherconcentration) to one of low free
energy (lowerconcentration) concentration gradient.
Does not require energy (exergonic). Net diffusion stopswhen
concentration on both sides equal (if crossing amembrane) or when
there is a uniform distribution ofparticles. When equilibrium is
reached : Molecules continueto move, but no net change in
concentration.
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Factors influencing the rate of
diffusion
1. Concentrat ion Gradient. directly proportional to
theconcentration gradient, indirectly proportional to
resistance,inversely proportional to distance traveled
2. Molecu lar Speed: directly proportional to temperature
andindirectly related to molecular weight
3. Temperature
4. Pressure
5. Solute effect on the chem ical potent ia l of the solvent.
Soluteparticles decrease the free energy of a solvent. The critical
factoris the number of particles, not charge or particle size.
Ficks Law Jv = (C1 - C2)
r
Diffusion rate (Jv), concentration gradient (C1 C2) and
resistance (r)
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Diffusion
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Osmosis : the diffusion of a solvent (typically water)
across a membrane
Osmosis
Dialysis : the diffusion of solute across a semi-
permeable membrane
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- Osmosis - diffusion of water.
- Osmosis affects the turgidity of
cells, different solution can affect
the cells internal water amounts
Contractiles Vacuoles are found infreshwater microorganisms -
they
pump out excess water
Turgor pressure occurs in plants
cells as their central vacuoles fill with
water.
Osmosis
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