| Criteria | Marks |
|---|
| Describes in detail two mechanisms that maintain plant water balance. | 3 |
| Briefly describes two mechanisms that maintain plant water balance. | 2 |
| Outlines one mechanism. | 1 |
Sample answer:
Control of water movement in plants occurs via 2 main mechanisms: 1. Water moving into plants at root hairs: • Plants can change the osmotic rate by altering dissolved solute concentration in root hairs. ie. Increased solute concentration = increased water uptake by osmosis 2. Water moving out of plant from leaf stomata: • Plants can change their transpiration rate by altering turgor of stomatal guard cells • Decreased plant water levels stimulate release of stress hormone, abscisic acid, which initiates stomata to close • When the plant has sufficient water, the guard cells become turgid. This causes cells to bend and open, allowing transpiration to commence once more. Other factors/adaptations in plants may also assist in facilitating this balance of WATER INPUT <-> WATER OUTPUT • Eg. Sunken stomata, reduced leaf size