D2.3 Water potential

Solvation

Process of surrounding solute particles with solvent molecules

What happens in aqueous solutions (water is the solvent)?

1. Hydrogen bond formation betw solute + water molecules

2. Attractions betw both positively + negatively charged ions and polar water molecules

When water is used as a solvent, what are positive ions attracted to?

Partially negative oxygen in water

When water is used as a solvent, what are negative ions attracted to?

Partially positive hydrogen in water

Is water a good or bad solvent + why?

Good solvent (can dissolve solutes)

• Bc it is polar

What happens when water dissolves ionic compounds?

• Water forms shells around ions

  • Partially positive H attracted to negative ions

  • Partially negative O2 attracted to positive ions

• → prevents them from rejoining

How does water dissolve polar molecules?

Water forms hydrogen bonds w solutes

Movement of water

From less concentrated to more concentrated solutions

• Low solute conc → higher solute conc

Osmosis

Passive net movement of water across a partially permeable membrane from an area of low → high solute conc

• until equal concs on either side (but not equal volume)

• Thru chanell protein (aquaporins)

Hypertonic

Solution has higher solute conc than what it is being compared to

• Water moves out of cells

Hypotonic

Solution has lower solute conc than what it is being compared to

• Water moves into cells

Isotonic

Solution has same solute conc to what it is being compared to

• No net movement of water

Summary of

1. Hypotonic

2. Hypertonic

   1. Isotonic

• Hypotonic: Lower solute concentration.

• Hypertonic: Higher solute concentration.

• Isotonic: Equal solute concentration.

How does osmosis occur?

• PPM = partially permeable membrane

1. PPM allows water to pass thru

2. But PPM less permeable to solutes

3. So when there is CG → osmosis occurs

How can cells control osmosis rates?

1. Change their solute conc

   • Eg contractile vacuoles

2. Change their membrane permeability to water

Osmosis of water thru aquaporins vs phospholipid bilayer

Quicker thru aquaporins

• More aquaporins embedded in cell membrane → cell more permeable to water → faster osmosis

In isotonic environments, is there movement of water?

Yes

• There is dynamic equilibrium

Osmolarity

Total solute conc in a cell (solvent)

• Determines the direction of water movement by osmosis

Changes due to water movement in plant tissue bathed in hypotonic and those bathed in hypertonic solutions

APPLICATION OF SKILLS

Students should be able to:

1. Measure (%) changes in tissue length + mass,

2. Analyse data to deduce isotonic solute concentration.

3. Uuse standard deviation + standard error to help analyse data

If the mass of a plant increases, what type of solution was it placed in?

Hypotonic

If the mass of a plant decreases, what type of solution was it placed in?

Hypertonic

• Bc water moves out of the cell

If the mass of a plant doesn’t change, what type of solution was it placed in?

Isotonic

Find the osmolarity of the plant tissue (isotonic solute concentration- no net change in mass)

0.2

• Solution + plant tissue have equal conc

• Find where it crosses x axis (no net gain in mass)

What can be determined if there are repeats for each concentration?

Standard deviation + standard error

1. This would allow the reliability of length and mass measurements to be compared

Standard deviation

Measures the spread of data around the mean

• Shows how consistent the results are

Standard error

Estimates how reliable the mean is

• Smaller error = the mean is more accurate

How is standard error shown graphically?

Error bars

Control variables for osmosis experiment

1. Get plant tissue samples from sample plant

   • Same species

2. Same SA:V

3. Same time for osmosis to occur

4. Don’t include skin

5. Same temp of solution

What do the top + bottom of error bars show?

The range of values based on variation around the mean:

• Top = mean + standard error (or deviation)

• Bottom = mean - standard error (or deviation)

What does the full length of the error bar show?

The uncertainty / variability in the data

Do plant cells have a cell wall?

Yes

Do animal cells have a cell wall?

No

Function of cell wall

Prevents excess water entering, even if placed in a v hypotonic solution

• Can handle lots of pressure

What happens if a animal cell (cell w/o a cell wall) is placed in a hypotonic solution?

Swells then bursts

• Bc water enters

What happens if a animal cell (cell w/o a cell wall) is placed in a hypertonic solution?

Shrinks → then crenation

• Bc water leaves

Why do freshwater eukaryotes need adaptations?

• No cell wall

• Placed in hypotonic environment → water enters → need to prevent from bursting

Adaptations of freshwater unicellular organisms (no cell wall)

Contractile vacuoles

Function of contractile vacuoles

Removes water

• So prevents bursting

• Needs energy

Why do multicellular organisms need to maintain isotonic tissue fluid?

To prevent harmful changes

Turgid

What happens if a plant cell (has cell wall) is placed in a hypotonic solution?

Develops turgor pressure

• Becomes turgid

What happens if a plant cell (has cell wall) is placed in a hypertonic solution?

Plasmolysis

Plasmolysis

Plasma membrane pulls away from the cell wall

What are plant cell walls made of?

Cellulose

Why is it good if a plant cell is turgid?

Lots of pressure → helps keep the plant uprrught

What happens if a plant cell (has cell wall) is placed in an isotonic solution?

Becomes flaccid

• Due to decreased pressure

Net movement of water for cells in isotonic solutions

No net movement of water

What type of solution should animal cells (no cell wall) be kept/bathed in?

Isotonic

Why is saline used in medical procedures?

Bc it’s isotonic to human cells

• Blood plasma needs to be isotonic to blood cells / other tissues

Medical applications of isotonic solutions

1. Intravenous fluids (IV) given as part of medical treatment

2. Bathing organs ready for transplantation

What is saline solution used for?

Rehydration