D2.3 Water potential

Question 1

Chemical structure of water and polarity

Answer 1

• H20
• Electrons are not shared evenly between oxygen and hydrogen atoms: oxygen is more electronegative

Question 2

Define electronegativity

Answer 2

• The ability of an atom to attract a shared pair of electrons.

Question 3

Factors that affect electronegativity

Answer 3

• Number of protons in the nucleus: More protons = more electronegative
• Number of orbital shells: less shells = more electronegative

Question 4

Explain hydrogen bonds

Answer 4

• Form when a slightly negatively charged part of a molecule comes close to a slight positively charged hydrogen molecule in the same or another molecule.
• Weak, depicted with dash lines.

Question 5

Water as a solvent: Ions

Answer 5

• Ions are atoms that have either gained or lost electrons, therefore have a charge.
• If the force of attraction between ions and water molecules is greater than the force of attraction between oppositely charged ions, the water can dissolve the substance.

Question 6

Water as a solvent: Covalent compounds

Answer 6

• Covalent compounds can also dissolve in water (glucose, oxygen, alcohol…) by forming intermolecular interactions with the dipolar water molecules, even though they may not dissociate into ions like ionic compounds

Question 7

Define osmosis

Answer 7

The diffusion of water: the net movement of water molecules from a region of high water concentration to a region of low water concentration, through a partially-permeable membrane.

Question 8

Factors that affect water potential of a solution

Answer 8

• Solute potential: amount of solute it contains: the greater the amount, the lower the water potential. This is because water molecules bind to solute molecules, reducing the number of water molecules that are free to diffuse
• Pressure potential: pressure applied to it: greater the pressure, the higher the water potential

Question 9

Calculation for water potential

Answer 9

Water potential = solute potential + pressure potential

Question 10

Explain Isotonic solutions

Answer 10

• Solutions that have the same concentration as the cytoplasm: no net osmosis

Question 11

Explain Hypotonic solutions

Answer 11

Solutions that have a lower concentration than the cytoplasm of the cell: net osmosis into the cell: cell bursts.

Question 12

Explain Hypertonic solutions

Answer 12

Solutions that have a higher concentration than the cytoplasm of the cell: net osmosis out of the cell: cell shrinks

Question 13

Explain Plasmolysis

Answer 13

The shrinking of protoplasm away from the cell wall of a plant or bacterium. Often due to water loss via osmosis, when in a hypertonic solution, resulting in gaps between the cell wall and the plasma membrane

Question 14

Describe the water potential and water movements in plant tissue in hypotonic solutions

Answer 14

• Solute potential of the tissue is more negative than the solute potential of the solution
• Water moves into the tissue, from an area of less negative solute potential to an area of more negative solute potential
• The additional water inside the cells in the tissue causes them to apply an outward turgor pressure, which pushes the cell membrane against the wall. As a result the pressure potential is positive.
• The positive pressure potential can offset the negative solute potential, resulting in an equilibrium

Question 15

Describe the water potential and water movements in plant tissue in hypertonic solutions

Answer 15

• The solute potential of the solution is more negative than the solute potential of the tissue
• Water moves out of the tissue, from an area of less negative solute potential to an area of more negative solute potential to try to equalise the solute potentials