D2.3: Water Potential

D2.3 Water Potential

D2.3.1: A solution is a homogenous mixture of a solvent (dissolves the thing), and a solute (what gets dissolved)

• Solvation = technical word for dissolving; how the process of dissolving goes about

• Immiscible: Not forming a homogeneous mixture when mixed

• Heterogenous: Things aren’t smoothly proportioned

• A solute dissolves in a solvent to form a solution. The solute is present in a smaller quantity than the solvent. 

• A solvent is a substance in which a solute is dissolved to form a solution

• Solvation describes the interaction between a solvent and a solute. The solute particles interact with the solvent through various intermolecular forces such as hydrogen bonding

• Water is a polar molecule with weak positive charges on hydrogen atoms and weak negative charged on the oxygen atom

• Water molecules form hydrogen bonds with other charged particles

• When a solute dissolves in water, the solute particles interact with water particles through hydrogen bonding

• When polar molecules dissolve in water, hydrogen bonds form between water molecules and the other polar molecules

• Positive attracts negative -> because of that is splits up the ions of the solute

• When ionic compounds dissolve in water, water molecules surround the ions of the compound

• The partially negatively-charged oxygen atoms of water are attracted to and surround, positive ions (cations). The partially negative hydrogen atoms of water are attracted to and surround negative ions (anions). The attraction between water molecules and ions helps separate and disperse ions within a solution

D2.3.2: 

• Concentration is the quantity of a solute in a given quantity of solution

• A hypertonic solution has a higher concentration of solutes compared to another solution

• An isotonic solution has the same concentration of solutes compared to another solution

• A hypotonic solution has a lower concentration compared to another solution

• Water molecules move from a hypotonic solution to a hypertonic solution

D2.3.3:

• Osmosis is the passive movement of water molecules from a hypotonic solution to a hypertonic solution through a partially permeable membrane

• Cells are placed into a hypertonic solution relative to the cytoplasm of the cells

• Water will move out of the cytoplasm which is hypotonic to the hypertonic solution surrounding the cells

• Cells are placed into a hypotonic solution relative to the cytoplasm of the cells

• Water will move from the hypotonic solution surrounding the cells into the hypertonic cytoplasm

• Cells are placed into an isotonic solution relative to the cytoplasm of the cells

• There will be no net movement of water as the cytoplasm and solution are isotonic

• There is dynamic equilibrium, as the number of water particles entering and exiting the cell are equal

D2.3.4: Make sure you know how to do standard deviation and error bars

D2.3.5: Animal cells in a hypertonic solution

• There is a net movement of water out of a cell surrounded by a hypertonic solution. 

• If the cell has no cell wall, the cell shrinks, and this can lead to death. 

• The loss of water from an animal cell is known as crenation.

• Animal cells in a hypotonic solution

•  There is a net movement of water into the cell surrounded by a hypotonic solution

• If the cell has no cell wall, the cell swells, and may lead to the bursting of the plasma membrane

• The bursting of an animal cell is known as cytolysis.

• Animal cells in an isotonic solution: There is a dynamic equilibrium where there is no net change of water in the cell

• Multicellular organisms, like humans, need to maintain an isotonic tissue fluid around their cells

• If the tissue fluid is hypertonic to the cytoplasm of cells, crenation chorus

• If the tissue fluid is hypotonic to the cytoplasm of cells, cytolysis occurs

• A freshwater habitat is hypotonic compared to the cytoplasm of cells

• Water moves into the unicellular organisms living in a freshwater habitat, by osmosis

• Many unicellular organisms without cell walls remove excess water from the cell using a contractile vacuole to prevent cytolysis

• This is osmoregulation, a form of homeostasis

• Osmoregulation is maintenance of constant osmotic pressure in the fluids of an organism by the control of water and salt concentrations

D2.3.6: Plant cells in a hypertonic solution

• There is a net movement of water out of a cell surrounded by a hypertonic solution

• If the cell has a cell wall, the plasma membrane and cytoplasm shrink and detach from the cell wall

• This is known as plasmolysis and leads to cell death.

• Plant cells in a hypotonic solution: There is a net movement of water into the cell surrounded by a hypotonic solution

• The water moves into the cell vacuole

•  The vacuole pushes the cytoplasm and plasma membrane against the cell wall, creating turgor pressure

• Water continues to move into the cell until turgor pressure equals the pressure exerted by the cell wall. The cell is then fully turgid

• Plants maintain a high turgor pressure by having a high solute concentration in their vacuoles

• Turgor pressure provides structural support for plants

• The cell wall prevents cells bursting as water moves into the cell

• Plant cells in an isotonic solution: There is no net movement of water into or out of a cell surrounded by an isotonic solution

• A plant cell will become flaccid in an isotonic solution

• There is not enough turgor pressure to make the cell turgid

• D.2.3.7: Organs harvested for transplants needed to be transported surrounded by an isotonic solution 

• The cell of an organ surrounded by a hypertonic solution would lose water. The cells would crenate and die

• The cell of an organ surrounded by a hypotonic solution would gain water. The cells would undergo cytolysis and burst

• If an organ was not surrounded by an isotonic solution, the tissue, and then the organ, would die

• Intravenous fluids are fluids directly administered into a person’s vein

• Isotonic intravenous fluids are used to support hydration, nutrition, replace lost fluids, and administer medications

• Isotonic fluids are used, as they have the same concentration as blood plasma and tissue fluid

• Using isotonic fluids prevents excessive movement of water in or out of cells

• HL Content: 2.3.8: Water potential is the potential energy of water (with something dissolved in it) per unit volume, relative to pure water. Water potential determines how freely water particles can move in a solution. Water potential is the pressure exerted by water particles on a membrane. It is impossible to measure the absolute quantity of the potential energy of water, so values relative to pure water at atmospheric pressure and 20˚C are used. The units are kiloPascals (kPa). Pure water at 20˚C has a water potential of 0 kPa

HL Content: 2.3.9: Osmosis can also be defined using water potential

• Osmosis is the passive movement of water particles from a region of high water potential to a region of low water potential through a partially permeable membrane. 

• Water will have a high water potential, if there is low solute concentration

• Water will have a low water potential, if there is high solute concentration

• There will be a net movement of water until the water potential is equal in both systems

HL Content: 2.3.10: Water potential is determined by the pressure potential and solute potential in cells with walls

• Water potential (🔱) = Solute potential  (🔱_P)  + pressure potential  (🔱_S) 

• The solute potential of pure water is 0 kPa. As solute is added to water, the solute potential decreases. Increasing the solute concentration of a solution, decreases the solution’s solute potential. Pressure potential: Pressure potential results from the turgor pressure exerted by a cell wall on water in a cell. Pressure potentials are usually positive, but may be be negative in xylem vessels with the sap being transported under pressure

• More solute = less water potential  = lower solute potential 

• Pressure potential only really takes effect in plant cells

HL Content: 2.3.11: Tissue bathed in a hypotonic solution

• If the solution outside the cell is hypotonic, then it will have a relatively high solute potential due to have a low solute concentration 

• The hypertonic cytoplasm of cells in the tissue has a relatively low solute potential 

• Water will move into the cell by osmosis moving from the high water potential outside the cell to a low water potential inside the cell

• As the water moves into the cell, the pressure potential increases, because water is pushed against the cell wall which increases turgor pressure

• There will be continue to be a net movement of water into the cell until the water potential inside the cell equals the water potential outside the cell

• Tissue bathed in a hypertonic solution: If the solution outside the cell is hypertonic then it will have relatively low solute potential due to having a high solute concentration

• The hypotonic cytoplasm of cells in the tissue has a relatively high solute potential

• Water will move out of the cells by osmosis, moving from the high water potential in the cytoplasm of the cells, to the low water potential of the fluid surrounding the cells

• As water leaves the cell by osmosis, the pressure potential decreases. The plasma membrane and cytoplasm no longer push against the cell wall

• Osmosis will continue until the water potential inside the plant tissues’ cells and the solution surrounding them have the same water potential 

Extra Notes

• Hypotonic: Less solute on the outside than the inside of the cell

• Isotonic: Equal concentration between the inside and outside of the cell -> system is in dynamic equilibrium

• Hypertonic: Less solute on the inside than the outside of the cell -> causes plasmolysis (plant cells) of cells can become crenated (animal cells) (plasma membrane shrinks in) (all the water is going to leave)

• Cytolysis -> the cell swells up to the point where it can pop

• High water potential = lots of water molecules (symbol is a trident)

• Isotonic = balanced amount of water going in or out (dynamic equilibrium where there is no net change of water in the cell) -> multicellular organisms like humans need to maintain an isotonic tissue fluid around their cells. If the tissue fluid is hypertonic to the cytoplasm of cells, crenation occurs. If the tissue fluid is hypotonic to the cytoplasm of cells, cytolysis occurs 

• Flaccid = floppy (the cell has lost its support)