Explain what is meant by a limiting factor in physiological processes such as photosynthesis or mineral uptake.
An investigation was conducted to study the effect of soil salinity, measured as sodium chloride (NaCl\text{NaCl}NaCl) concentration, on the survival of young barley seedlings (Hordeum vulgare). Each experimental group initially contained 120 seedlings.
The results of this study are shown in the table below:
| NaCl concentration (mmol dm−3\text{mmol dm}^{-3}mmol dm−3) | Day | Number of seedlings surviving |
|---|---|---|
| 10 | 5 | 120 |
| 10 | 119 | |
| 15 | 118 | |
| 20 | 117 | |
| 25 | 115 | |
| 50 | 5 | 120 |
| 10 | 115 | |
| 15 | 112 | |
| 20 | 108 | |
| 25 | 102 | |
| 200 | 5 | 120 |
| 10 | 95 | |
| 15 | 72 | |
| 20 | 48 | |
| 25 | 15 |
Summarise the conclusions that can be drawn from these data.
Explain how extremely high soil salinity (NaCl\text{NaCl}NaCl concentration) can lead to dehydration and death of the seedlings, referring to water potential and osmosis.
Calcium (Ca2+\text{Ca}^{2+}Ca2+) ions are actively transported into root hair cells to support cell wall synthesis and cell signalling.
Explain why water molecules can form electrostatic interactions with calcium (Ca2+\text{Ca}^{2+}Ca2+) ions.
Describe how the active transport of ions across a plant cell membrane causes water to enter the cell.
