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. 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 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 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.

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