Factors affecting the rate of transpiration

Authors Avatar

IB Biology Lab Design


There are several factors which can affect the rate transpiration of plants; these could either increase the rate of water uptake or decrease the rate of water uptake. These factors could be internal or external. In this investigation we will be looking at the external factors which affect the rate of respiration, such as temperature, light intensity... etc.

Transpiration is known as the loss of water vapour through the stomata of the leaves. It is needed to keep the cells of the spongy and palisade mesophyll cells moist as this allows carbon dioxide to dissolve before diffusing into the cells for photosynthesis.

A suction force is created on the column of water below it in the xylem when water is evaporated. The upwards force on the column of water created by transpiration and the downwards force due to gravity created a tension in the column of water.

As there is a difference in the upwards pull, which is greater than the downwards pull, the column of water moves up the xylem. As the cohesion tension theory explains it, water molecules are polar, meaning that they stick to one another. If one molecule moves, it pulls adjacent molecules.

The water is able to evaporate out of the leaf through diffusion as the leaf has a high water potential and the air has a low water potential, making it possible for water molecules to pass down the concentration gradient from the spongy and palisade mesophyll cells into the leave’s internal air spaces before diffusing out into the air.

Teachers aim

Investigate rate of water loss of a plant under different environmental conditions.

General aim

To investigate the impact of different factors on the rate of transpiration using a potometer.


I predict that when the leafy shoot will be placed in windy conditions, the rate of transpiration (water uptake by the plant) would increase. When the leafy shoot will be placed in dark conditions there will be a significant decrease in the amount of water uptake by the shoot, thus resulting in a lower transpiration rate. The third hypothesis that will be tested is that when half the leaves are removed, there will be a significant decrease in transpiration thus a reduced rate of water uptake by the leafy shoot.

Join now!


Controlled variables are maintained at a constant


1 x 1 Potometer

1 x 1 fan

2 x 1 retort stand and clamp

1 x 1 stop watch

1 x 1 thermometer

1 x 1 Balsam leafy shoot

1 x 1 beaker

1 x 1 candle

1 x 1 match box

1 x 1 30 cm ruler

1 x 2 syringe with needle

50 ml DCPIP

1 x 1 black polythene bag

1 x 1 rubber bung


  1. The apparatus was set up as shown on the protocol diagram.
  2. The ...

This is a preview of the whole essay