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
1515
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
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