IB DP Biology- A1.1 Water - IB Style Questions For SL Paper 2 - FA 2025

(a)
Cohesion is the attraction of water molecules to each other, caused by hydrogen bonding between their positive and negative ends. This property is crucial for:
• Transport in plants: It allows water to be pulled up in a continuous column through the xylem during transpiration .
• Surface tension: It creates high surface tension, allowing some insects to walk on the water’s surface .
• Thermal properties: Cohesion contributes to water’s high specific heat capacity, which helps maintain a stable temperature in aquatic environments and organisms .

(b)
• Humidity: An increase in humidity decreases the transpiration rate because the concentration gradient of water vapor between the leaf and the air is reduced .
• Temperature: An increase in temperature increases the transpiration rate because it increases the rate of evaporation .
• Wind: An increase in wind increases the transpiration rate by removing the layer of humid air (boundary layer) from the leaf surface .
• Light Intensity: An increase in light intensity generally increases the transpiration rate because it causes the stomata to open .

(c)
Osmosis is the passive movement of water across a semipermeable membrane from an area of low solute concentration to an area of high solute concentration (down its water potential gradient) .
• In a hypotonic solution (lower solute concentration outside): Water will move into the plant cell by osmosis. The cell will swell, but the rigid cell wall prevents it from bursting. The cell is described as turgid as the contents push against the wall (turgor pressure) .
• In a hypertonic solution (higher solute concentration outside): Water will move out of the plant cell by osmosis. This causes the cell to become flaccid, and the cell membrane pulls away from the cell wall in a process called plasmolysis .
• In an isotonic solution (same solute concentration): There is no net movement of water, and the cell is in a state of dynamic equilibrium.