A scientist investigates the movement of substances across the cell membranes of two cell samples.
Sample L contains liver cells surrounded by a solution containing urea. Sample R contains barley root hair cells in a solution containing nitrate ions.
The concentrations remain constant while each rate is measured.
| Sample | Substance | Concentration inside the cells in mmol/dm3\text{mmol/dm}^3mmol/dm3 | Concentration in the surrounding solution in mmol/dm3\text{mmol/dm}^3mmol/dm3 | Direction of movement | Rate when respiration occurs in μmol g−1 h−1\mu\text{mol g}^{-1}\text{ h}^{-1}μmol g−1 h−1 | Rate when respiration is stopped in μmol g−1 h−1\mu\text{mol g}^{-1}\text{ h}^{-1}μmol g−1 h−1 |
|---|---|---|---|---|---|---|
| L | urea | 6.4 | 1.2 | out of the cells | 5.1 | 5.0 |
| R | nitrate ions | 7.5 | 0.5 | into the cells | 3.2 | 0.0 |
Identify the sample in which the substance moves against its concentration gradient.
Explain why stopping respiration has a different effect on the rate of movement in sample L and sample R.
Use data from the table in your answer.
A second sample of barley root hair cells has the same nitrate ion concentrations and rate of respiration as sample R. Its cell membranes contain fewer carrier proteins for nitrate ions.
Suggest why this sample has a lower rate of nitrate ion uptake.
194 exam-style questions on OCR GCSE Biology 2.1 Supplying the cell, covering 2.1.1 Transport across cell membranes, 2.1.2 Mitosis and the cell cycle, 2.1.3 Importance of cell differentiation, and 2.1.4 Stem cells. Each one has a worked solution and a mark scheme showing where the marks go.