1. Cells in isotonic, hypotonic, and Hypertonic solutions

2. 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?

3. 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

4. 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 .

5. 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 it is passing through

6. Water across plant membranes
There is some diffusion of water directly across the bi-lipid 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

7. 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

8. 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

9. Pressure-driven bulk flow drives long-distance water transport
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

10. 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.

11. Osmosis and Tonicity
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.
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.

12. 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.

13. 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

14. 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

15. 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

16. 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

17. 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