Cells in
isotonic,
hypotonic, and
Hypertonic
solutions
Aim: How do we predict the effect of a
hypotonic, hypertonic, and isotonic
solution on a cell?
• Do Now: What do you think will happen to
cells placed in a strong salt solution?
• How do you explain the process of
osmosis in this condition?
Water and plant cells
• 80-90% of a growing plant cell is water
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–
–
–
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
Water Transpiration
• The evaporation of water into the atmosphere from
the leaves and stems of plants.
• It occurs chiefly at the leaves while their stomata are
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 .
Water transport processes
• Moves from soil, through plant, and to atmosphere
by a variety of mediums
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–
–
–
Cell wall
Cytoplasm
Plasma membranes
Air spaces
• How water moves depends on what it is passing
through
Water across plant membranes
• There is some diffusion of
water directly across the bilipid membrane.
• Auqaporins: Integral
membrane proteins that form
water selective channels –
allows water to diffuse faster
– Facilitates water movement in
plants
• Alters the rate of water flow
across the plant cell
membrane – NOT direction
Permeability and Diffusion
• The plasma membrane is
selectively permeable.
• This means that only some
molecules can cross.
• Small uncharged molecules
like O2, CO2 and H2O pass.
• 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.
20
10
15
15
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
Pressure-driven bulk flow drives
long-distance water transport
• Bulk flow:
– Concerted movement of groups of molecules en masse, most often
in response to a pressure gradient.
• Dependant on the radius of the tube that water is traveling
in.
– Double radius – flow rate increases 16 times!!!!!!!!!!
• This is the main method for water movement in Xylem, Cell
Walls and in the soil.
• Independent of solute concentration gradients – to a point
– So different from diffusion
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.
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 the
osmolarity of a
solution--the amount of
solute in a solution.
• Solute--dissolved
substances like sugars
and salts.
• Tonicity is always in
comparison to a cell.
• The cell has a specific
amount of sugar and
salt.
Tonic Solutions
• A Hypertonic solution has more solute than
the cell. A cell placed in this solution will give
up water (osmosis) and shrink.
• A Hypotonic solution has less solute than the
cell. A cell placed in this solution will take up
water (osmosis) and blow up.
• An Isotonic solution has just the right amount
of solute for the cell. A cell placed in this
solution will stay the same.
Plant cell in hypotonic solution
• Flaccid cell in 0.1M sucrose
solution.
• Water moves from sucrose
solution to cell – swells up –
becomes turgid
• This is a Hypotonic solution has less solute than the cell.
So higher water conc.
• Pressure increases on the
cell wall as cell expands to
equilibrium
Plant cell in hypertonic solution
• Turgid cell in 0.3M sucrose
solution
• Water movers from cell to
sucrose solution
• A Hypertonic solution has
more solute than the cell. So
lower water conc
• Turgor pressure reduced and
protoplast pulls away from
the cell wall
Yw 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 status of plant.
• Because of water loss to the atmosphere plants are
seldom fully hydrated.
• They suffer from water deficits
– Leads to inhibition of
Plant growth – most likely to be affected
Photosynthesis
Yw 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 this
hormone in desert plants
Summary
• Water is important to plants
– Makes up the media in which all biochemical processes
occur 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 a
plant