A1.1 & D2.3 Water and Water Potential

Question 1

What is the structure of water

Answer 1

• Polar covalent bond due to unequal attraction of electrons.
  – Nuclei of O2 is larger hence, stronger pull.
  – Covalent bonds are strong due to shared electrons.
• Hydrogen bonds between multiple molecules of water – weak bonds due to attraction of polar molecules.

Question 2

What are the chemical properties of water?

Answer 2

1. Cohesion:
   - Water sticks to each other due to its polar nature, forming hydrogen bonds.
   - Strong cohesive forces causes surfaces tension in the surface of a body of water –> allows insects to stick to surfaces,
   - Allows transpiration of water through transpiration pull.
2. Adhesion:
   - The polarity of water allows it to attract to other polar molecules.
   - Capillary action – prevents water from dropping when moving up the xylem
   - Water forms a meniscus as it adheres to the polar glass.
3. Solvent:
   - Polar and hydrophilic substances dissolve in water due to polarity.
   - Water dissolves solutes to form aqueous solution
   – Good medium for enzyme-catalysed reactions and transporting dissolved substances.

Question 3

What are the physical properties of water?

Answer 3

1. Buoyancy:
   - An upward force exerted on an object placed in a medium.
   – Objects less dense or as dense as water will float.
2. Viscosity:
   – Water has low viscosity, little resistance to flow
3. Specific heat capacity:
   - High heat capacity due to restriction of hydrogen bonds.
4. Poor conductors, good insulators

Question 4

Explain movement of water (hint: water potential)

Answer 4

• Water moves from a hypotonic (less solute) to a hypertonic solution across a partially permeable membrane via osmosis.

Question 5

Effect of hypotonic and hypertonic solutions

Answer 5

In cells lacking cell walls:
- Hypotonic: burst under extreme turgor pressure
- Hypertonic: plasmolysis (plant cell), crenation (animal cell)

Question 6

Uses of isotonic solutions

Answer 6

• Intravenous fluid – balances solute conc. in the body
• Organ transplant – immersed in the solution before procedure.

Question 7

Define osmosis

Answer 7

Diffusion of water molecules from a region of lower solute concentration to a region of higher solute concentration through a partially permeable membrane.

Question 8

Define isotonic

Answer 8

Has the same water potential and solute concentration as the surrounding environment, water moves in dynamic equilibrium.

Question 9

Why does water act as a solvent?

Answer 9

Due to the formation of hydrogen bonds between water molecules and solute particles.

Question 10

How does osmosis affect plant cell structure in a hypotonic environment?

Answer 10

Water moves into a plant cell by osmosis, vacuole enlarges, pushes cell membrane against the wall –> makes cell turgid

Question 11

What is the function of a contractile vacuole in a freshwater unicellular organism?

Answer 11

Removes excess water to prevent the cell from bursting in a hypotonic environment.