During transpiration plants move water from the roots to their leaves for photosynthesis in xylem vessels. Glucose made in photosynthesis is then moved to all cells in phloem vessels for respiration.

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The uptake of water can be measured using a potometer. Under normal circumstances, the rate of water uptake gives a measure of the rate of transpiration.

A simple **potometer** is a piece of capillary tubing to which a plant has been connected. The water uptake is measured by recording the time taken for a bubble in the tube to move a set distance.

**Aim of the experiment**

To find the rate of water uptake of a plant.

- Independent variable - time
- Dependent variable - the distance moved by the bubble along the capillary tube
- Control variables - temperature, air flow or draughts, adequate supply of water

Care must be taken when inserting the plant shoot into the rubber tubing attached to the capillary tube and with the scissors used to cut the plant shoot.

Time in minutes | Distance moved by bubble in mm |
---|---|

0 | 0 |

5 | 16 |

10 | 32 |

15 | 47 |

20 | 64 |

25 | 80 |

30 | 95 |

Plot a line graph of the data.

The rate of water uptake is best found using the gradient of the graph.

You need to calculate the gradient of the graph - and not take values from the table of results.

Choose any two points on straight line:

- choose as wide an interval along the x-axis as is possible
- try to make sure that these correspond with values of the y axis so that you can read accurately

The most accurate results will be obtained if these points are as far apart as possible. Here, the two points on the line, labelled P and Q, are at either end of the line.

**Calculate the gradient:**

**P Value of x = 0 minutes**

**P Value of y = 0 millimetres**

**Q Value of x = 30 minutes**

**Q Value of y = 96 millimetres**

The distance moved by the bubble is 3.2 mm/minute.

Note that rate is always a compound measurement It is made up of two - or more - measurements. In this case, millimetres per minute.

Note also that we calculate the gradient/rate from the line of best fit and not from the table of data.

- Question
When will the water uptake of a plant not equal the rate of transpiration?

When it is short of water, eg it has wilted.