Describe Transpiration

Did you know that plants only use about 5-6% of all the water they absorb for photosynthesis? So what happens to all that water anyway?  Let's find out more below.

Plants can’t help but lose water continually to the air. This is called transpiration. 

Water is constantly lost from the leaves of a plant through pores called stomata. When a plant opens its stomata to allow carbon dioxide in for photosynthesis, water will evaporate and diffuse out of the stomata. More water is drawn up from the stem and the roots to replace the lost water. As water moves from the roots to the leaves, more water is drawn up from the soil into the root hair cells. This process is known as the transpiration stream.

Although transpiration is inevitable, it's also quite useful! It helps the plant remain cool and allows minerals to be drawn up the plant along with the water.

Factors affecting transpiration

Transpiration is affected by many factors:

1)  Temperature - increasing the temperature makes transpiration happen faster, the plant loses more water from its leaves.
2)  Humidity - if it's really humid it means there's a lot of moisture in the air, the plant doesn't transpire as much so doesn't lose as much water.
3)  Wind - if it's really windy water vapour is blown away from the leaf. This causes the leaf to transpire faster so the plant loses more water from its leaves.
4)  Light intensity - if its really sunny, the stomata will open to let in more carbon dioxide for photosynthesis, this causes the plant to lose water.

Plant adaptations

Plants have adaptations that allow it to do a particular job.

Root hair cells are specialised cells found at the roots of a plant (see image above). These cells are thin and long making them useful to manoeuvre between soil particles in search of water. The large surface area of the root hair cell allows a greater chance of contact with water and minerals.​ 

Another adaptation of the plant is found in the leaf of a plant​. The lower epidermis layer contains the stomata (stoma for one pore). These stomata allow gases in and out of the underside of the leaf. The stomata are found between guard cells which open or close the stomata. The stomata will open to allow in carbon dioxide for photosynthesis during the day but will close during the night when there's no sunlight. 

Water moves through xylem vessels.  Xylem vessels are a hollow continuous tube that transports water in one direction and minerals from a plants roots to the plant's leaves via the stem.

Another vessel the plant has is called the phloem. The phloem vessels move food substances that the plant has made by photosynthesis to where they are needed (for example in growing parts of the plant and storage). Food travels up and down the stem. This is known as translocation. Phloem vessels are made up of living cells and have no nuclei, allowing food substances to take up maximum space.

In the following activity, you will describe how transpiration occurs in plants.