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– 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– 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 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

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.

10 20

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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

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

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 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

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 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