Water Potential Notes

Water Potential

Lesson Starter

• Go to www.socrative.com, click Student Login.
• Room name: Amouzandeh11B

Fertile Question

• How does water potential regulate the movement of water in and out of cells and tissues, and how does this contribute to the structure and function of living organisms?

Theme: Level of Organisation

• Molecules (1)
• Cells (2)
• Organisms (3)
• Ecosystems (4)

Unity & Diversity (A)

• Common ancestry has given living organisms many shared features.
• Evolution has resulted in the rich biodiversity of life on Earth.

Form & Function (B)

• Adaptations are forms that correspond to function.
• These adaptations persist from generation to generation because they increase the chances of survival.

Interaction & Interdependence (C)

• Systems are based on interactions, interdependence, and integration of components.
• Systems result in the emergence of new properties at each level of biological organization.

Continuity & Change (D)

• Living things have mechanisms for maintaining equilibrium and for bringing about transformation.
• Environmental change is a driver of evolution by natural selection.
• 1. 9 Membrane Transport

Success Criteria

• D2.3.8:
  • Define the term water potential.
  • State the symbol and unit for water potential.
  • State that pure 20°C water at standard atmospheric pressure has a water potential of 0kPa.
• D2.3.2:
  • Define osmolarity, isotonic, hypotonic, and hypertonic.
  • State the unit for concentration of a solute in a volume of solution.
  • Outline the net movement of water between hypotonic, hypertonic, and isotonic solutions.
• D2.3.3:
  • Predict the net movement of water based on the environment of a cell.
• D2.3.5:
  • State the effects of hypertonic and hypotonic solutions on cells without a cell wall.
  • Explain why tissue fluid in multicellular organisms must be isotonic to the cells of the tissue.
  • Outline the role of the contractile vacuole in freshwater unicellular organisms.
• D2.3.6:
  • Describe the strength and permeability of a cell wall.
  • Explain the effects of hypertonic and hypotonic solutions on cells with a cell wall with specific reference to turgor pressure and plasmolysis.
• D2.3.9:
  • Explain the movement of water from higher to lower water potential.
• D2.3.10:
  • Describe the impact of solute potential and pressure potential on the total water potential of cells with walls.
  • Explain why solute potentials can only range from 0kPa downwards.
  • State that pressure potentials are generally positive inside cells.
  • State a cell type in which the pressure potential is negative.
• D2.3.11:
  • Explain the movement of water in plant cells bathed in a hypotonic solution in terms of solute and pressure potentials.
  • Explain the movement of water in plant cells bathed in a hypertonic solution in terms of solute and pressure potentials.
• D2.3.7:
  • State the effects of isotonic, hypertonic, and hypotonic solutions on human cells.
  • Outline the use of “normal saline” in medical procedures.
• D2.3.4:
  • Explain the change in mass and/or volume of plant tissues placed in either hypotonic or hypertonic solutions.
  • Determine the concentration of solutes in a plant tissue given changes in plant tissue mass and/or length when placed in solutions of various tonicities.

The Effect of Drinking Seawater

• Seawater is a hypertonic solution with a high solute concentration and low water potential.

Water Potential

• Water potential is a measure of the potential energy of water per unit of volume of water.
• The units are usually kilopascals (kPa).

Water Potential of Pure Water

• Pure water in standard conditions (20 °C) has a water potential of 0 kPa, which is the highest possible water potential.
• The calculated value for water potential is always relative to pure water at atmospheric pressure at 20°C because it is difficult to measure the potential energy contained in water molecules.

Effect of Solutes on Water Potential

• The addition of a solute reduces the water potential (i.e., makes the water potential more negative).
• As water potential becomes more negative, it becomes harder for water to move.

The Effect of Saltwater Fish in Freshwater

• Freshwater is a hypotonic solution. It has a low solute concentration and high water potential.

Water Movement and Osmosis

• Water moves via osmosis from higher to lower water potential because solutions with high water potential have high potential energy.
• Water moves from higher potential energy to lower potential energy.
• Water flows until both solutions become isotonic, having the same solute concentration.

Osmosis

• Osmosis → Salt Sucks!
• Water moves to hypertonic areas.
  • Hypotonic Solution: Water moves into the cell.
  • Isotonic Solution: There is no net water movement.
  • Hypertonic Solution: Water moves out of the cell.

Cell Diagrams

• Cell in Hypertonic Solution: Water moves out.
• Cell in Isotonic Solution: No net movement.
• Cell in Hypotonic Solution: Water moves in.

Osmolarity

• Osmolarity: measurement of solute concentration of a solution
• Hyperosmotic side: Higher solute concentration, Lower free H2O concentration
• Hypoosmotic side: Lower solute concentration, Higher free H₂O concentration
• Net water flow moves from hypoosmotic side to hyperosmotic side

Relationship between Solute Concentration and Free Water Molecules

• Low solute concentration: Number of free water molecules = High
• High solute concentration: Number of free water molecules = Low
• Net movement of water molecules goes from high free water concentration to low free water molecule concentration

Revising Water Balance

• Drinking a large amount of water in a short time without consuming any salt can result in abnormal functioning of nerve cells in the brain, which can cause confusion, seizures, coma, or even death.
• When a person drinks too much water too quickly, the fluid surrounding a person’s cells changes from isotonic to hypotonic.

Food Preservation and Salting

• Salting foods prevents them from spoiling because it inhibits the growth of bacteria and molds.
• Archaea living in extremely salty water prevent water loss by maintaining internal solute concentrations to match the environment.

Eukaryotic Cell Structure

Cell Wall:

• Secreted by all plant cells
• Consists mainly of cellulose
• Permeable and strong

Eukaryotic Cell Structure: Vacuole

• Single membrane with fluid inside
• Plant cells: very large, permanent vacuoles
• Animal cells: small, temporary vacuoles used for various reasons (e.g., absorb food and digest it)

Cell Walls and Water Entry

• Plant cells have a cell wall that prevents entry of excess water even if placed in a very hypotonic solutions.
• This prevents the cell from bursting!
• Cell wall has lots of internal turgor pressure
• Pressure inside drops to atmospheric levels
• Cell membrane shrinks away from the cell wall

Animal Cells and Water

• Animal cells have no cell wall, so lyse / burst when water enters (hypotonic) and plasmolyse (shrink) when water exits (hypertonic).

Contractile Vacuoles

• Some freshwater eukaryotes that do not have cell walls (e.g., paramecium and amoeba) have adaptations such as contractile vacuoles to help prevent bursting.

Tonicity and Red Blood Cells

Below is a diagram showing three red blood cells that have been placed in solutions of different solute concentrations. What type of solution has each been immersed in?

Root Pressure, Transpiration, and Water Potential

• How does the root pressure in plant root hair cells affect water potential?
• How does suction exerted on water in the xylem as a result of transpiration affect water potential?

Water Potential Equation

• Water potential is equal to solute potential plus pressure potential.
• It can be calculated using the equation: $\Psi<em>w = \Psi</em>s + \Psi_p$
  • Where:
    • $\Psi_w$ is water (hydrostatic) potential
    • $\Psi_s$ is solute potential (can only be negative, pure water = 0)
    • $\Psi_p$ is pressure potential (negative or positive)
  • ↑ Hydrostatic pressure = ↑$\Psi_w$
  • ↑ Solute concentration = ↓$\Psi_w$

Water Potential and Solute Potential

• Water moves from higher water potential to lower water potential (towards negative $\Psi_w$)
• Low solute → high solute
• Pure water: $\Psi = 0 MPa$
• Solution: $\Psi<em>p = 0 MPa, \Psi</em>s = -1.0 MPa, \Psi = -1.0 MPa$
• The right side of the tube has a lower water potential, so there is net movement of water to the right and the volume on this side will increase.

Water Potential and Pressure Potential

• Water moves from higher water potential to lower water potential (towards negative $\Psi_w$)
• High pressure → low pressure
• Pure water: $\Psi = 0 MPa$
• Solution: $\Psi<em>p = +1.0 MPa, \Psi</em>s = -1.0 MPa, \Psi = 0 MPa$
• The water potentials of the two sides are equal, so there is no net movement of water.

Negative Pressure Potential

• When would a cell be exposed to negative pressure potential (-$\Psi_p$)?

Plant Tissue in Hypotonic Solution

• Plant tissue bathed in hypotonic solution will eventually reach an equilibrium where the negative solute potential is equal to the positive pressure potential.

Plant Tissue in Hypertonic Solution

• Plant tissue bathed in hypertonic solution lose water due to a negative solute potential and a negative pressure potential.

Turgidity

• Movement of water into plant cells increases pressure potential.
• This causes turgidity, which helps equalize with the water potential in the solution

Water Potential Problem

• A plant cell is placed in a solution. The initial pressure potential acting on the cell is 600 kPa and the solute potential is –900 kPa.
  • Calculate the water potential for the cell. ($\Psi<em>w = \Psi</em>s + \Psi_p$)
  • The water potential of the solution is –300 kPa. Explain whether the solution is hypertonic or hypotonic to the cell.
  • Will water move into or out of the cell? Explain your answer.
  • How will the resulting water movement affect the pressure potential in the cell?
  • After a certain time, the cell will reach a state of equilibrium. Explain equilibrium with reference to water potential.
  • How will the appearance of the cell change over time?

Scenarios

Explain the following…

• Why organs must be stored in an isotonic solution when being transported for transplant surgery.
• Why IV lines given at hospitals must be an isotonic solution to the blood.
• Why a person needs to use a contact lens solution and not just water when storing and cleaning contact lenses.
• Why salt put on roads during winter storms will kill the plants along the roadway.
• Why salting a slug is probably the most unethical way for a slug to die.
• Why your fingers will prune (swell) after a bath or swim.

Ascites

• Ascites is the accumulation of fluid due to electrolytes (e.g. Na+) and serum proteins in the peritoneal cavity
  • Can be up to 6L in volume
  • It indicates chronic liver disease – liver cannot synthesise proteins
• Causes:
  • Low osmotic pressure due to hypoalbuminemia
  • Proteins helps hold the salt and water inside the blood vessels so fluid doesn’t leak out into the tissues
• Treatment:
  • Prescribe a high protein diet
  • Potentially give them IV albumin (protein)

Pitting Edema

• Increased capillary permeability from hypertension
  • This has caused excessive water/solvent in their tissues that needs to get back into their veins and then urinated out
  • Give them a hypertonic solution in their veins to pull water out of the tissues and into the vein

Solution Comparisons

• A solution with higher solute concentration and a lower water concentration is HYPERTONIC
• A solution with lower solute concentration and a higher water concentration is HYPOTONIC
• When two solutions have the same solute concentration, they are ISOTONIC
• Water moves from a HYPOTONIC to a HYPERTONIC solution

What's the recipe for hot chip success?

• Cut potatoes into batons. Soak in cold water, changing it often, for 1 hour. This removes starch, creating crispy chips.
• Drain. Use an oil thermometer to heat vegetable oil to 170°C. Cook potato, in batches, until soft but not colored.
• Heat the oil to 190°C. Return chips to the oil in batches and cook until crisp and golden. Season with salt to serve.

Potato Lab

• Investigation into osmolarity in Solanum tuberosum (potato)

Aim

• To determine the isotonic solution for the potatoes in the investigation.

Materials

• 1x Solanum tuberosum
• 1x chopping board
• 1x knife
• 1x corer (diameter 10mm+/- 0.5mm)
• 1x ruler (300mm +/- 0.5mm)
• 1x corer (diameter 5mm+/- 0.5mm)
• 5x boiling tubes and rack
• 1x electronic balance (g +/- 0.005g)
• 5x salt solutions (0%, 5%, 10%, 20%, 30%)
• 1x Sharpie pen
• 1x timer
• 1x 500mL beaker
• Paper towel

Methodology

• Label test tubes with the concentrations of salt solution.
• Collect a potato and a corer.
• Bore 5 whole cores of potato.
• Cut off any potato skin
• Cut your potato cores to the same length using a ruler and paring knife.
• Dry the potato cores on a paper towel.
• Measure length using a ruler and weigh the potato cores using a balance. Record measurements.
• Place potato cores in the beakers. The solutions should completely cover the chips.
• Leave for at least 50 minutes.
• Remove potato cores from the beakers.
• Dry the potato cores on a paper towel.
• Re-measure and reweigh potato cores and record results appropriately.
• Process and present all data in an appropriate manner to allow for data analysis and evaluation of results.

Risk Assessment & Ethical Issues

• Salt solution is a foodstuff and therefore low hazard. However, no foodstuffs should be consumed in the laboratory. Take care when cutting the potatoes – cut onto a tile, and be prepared with first aid equipment for cuts.
• There are no ethical issues associated with this procedure.
• Knife has sharp edges – only use to carefully cut potato cylinder to size

Pre-lab: Sketching

• Sketch a potato cell as it would appear:
  • After being placed in a hypertonic solution
  • After being placed in a hypotonic solution
  • After being placed in an isotonic solution.
• Next to each sketch, explain WHY and HOW the cell has changed after being placed in the solution.

Pre-lab activity

• Identify Identify controlled variables
• Predict Predict a hypothesis
• Write Write a research question.
• Define Define solute and osmolarity.

Research Question

• How does [IV] in unit A from X to Y affect [DV] in unit B in [the scope] as measured by [method]?
• Example: How does different concentrations of NaCl solutions (0.5%, 0.15%, 0.25 %, 0.35% and 0.45%) affect peroxidase activity in Vigna radiata (mung bean) as measured by the % oxygen produced when reacted with hydrogen peroxide (H2O2) after 10 minutes?
• Hint: independent variable (IV) is what is changed; dependent variable (DV) is what is measured.
• Include the scientific name (Genus species) in RQ.

Hypothesis

• The dependent variable will (predict the effect) when the independent variable (describe the changes). This is because…..
• When plant cells are placed in a hypertonic solution (like a high salt or sugar solution), there is a higher solute concentration outside the plant cell than in, therefore water will leave the cell and the cell will become plasmolysed. This will result in a decrease in mass. Conversely, in a hypotonic solution (like a low salt or sugar solution), there is a higher solute concentration inside the plant cell than outside the cell, therefore the cell will become swollen (lysed/turgid) due to water being taken in by osmosis. The cell wall prevents the plant cell from bursting, however it will gain mass. When the solute concentration is the same inside and outside the cell there will be not net movement of water or change in mass. It is the concentration of this hypotonic solution that gives us the solute concentration of the solution making up the internal environment of the cell and the estimation of the osmolarity of a cell.
• In this investigation, cut potato chips will be weighed and then placed in the various concentrations of salt solutions. They will be left for osmosis to occur. Then, the potato chips will be removed from the salt solution dried and reweighed.
  Background: Define solute and osmolarity.

Experimental Results

*Hypertonic
*Isotonic
*Hypotonic

What happens to raw chips when they are soaked in water?

What happens to raw chips when soaked in salt water?

Jigsaw Discussion

• Why can chewing too much sugarless gum (containing sorbitol) or sugarless candy cause diarrhea?
• How do laxatives like Milk of Magnesia® work?
• How does penicillin kill bacteria?
• Within your expert groups, compare your explanations and reach a common understanding of how osmosis applies to your question. Create a drawing to illustrate the osmotic effect on a cellular level.
  Then, form new groups. Each expert teaches the osmosis application they researched.