Transpiration in Plants

It is known that all plants require water to survive. However, when water is absorbed into the plant, a large portion of water is released again through evaporation. Why is that? To know more, we have to understand this process, called transpiration.

When water is absorbed into the plant through its root hairs, it travels up the stem via xylem vessels, which are small tubes made of xylem cells. However, how does the water travel up the plant; defying gravity and Earth’s magnetic field? This is due to a force known as transpiration pull.

Lets start with looking at the water evaporation occurring at the leaf of a plant. The bottom of a leaf is where water evaporates leaving the plant, in other words, this is where transpiration occurs. Water first leave the xylem vessel to enter the mesophyll, the main part of a leaf. The water (in liquid form) evaporates while traveling through the mesophyll cells until reaching an air gap, where they evaporate. The water molecules reach the bottom of the leaf as water vapor, reaching an area known as the stoma, which is a small pore between 2 guard cells, who control the opening and closing of it. When this pore is opened by the guard cells, the water vapor can exit the leaf.

Water molecules also behave like chains; when one molecule evaporates from the stoma, the one behind it follows it through the stomata and evaporate after it. This chain effect is due to a difference in water concentration, causing water particles from an area with high water concentration to move to an area with lower concentration of water. This process is called osmosis. For example, if you dip a paper strip into a bowl of water, the water will gradually travel upwards soaking it. This is because the strip is dry while the bowl has a high volume of water. Water particles tend to move into areas with lower water concentrations (the paper strip) from areas with a higher concentration (the bowl of water).  

Because of water molecules constantly evaporating in stomata, a transportation of water within the plant since the water concentration in the stoma is constantly lowered when water inside evaporates, causing the next water molecule to move in. This pulls the entire “chain” of water molecules, allowing them to move from the root hairs, up the xylem vessel, through the mesophyll and into the stoma. This movement of water molecule chains – caused by osmosis – is called transpiration pull, and is the main process allowing the transportation of water in plants.

The process of water transportation through the plant by transpiration pull is important for many processes. In fact, only 5% of the water is absorbed and used for photosynthesis, and the transportation of water is its main purpose. Small amounts of it is also used for plant growth since plant cells divide and expand by taking up water. Firstly, the flow of water is mainly used for the transportation of minerals and other nutrients in the plant, similar to the circulatory system in our own bodies. This is why water leaves the plant in the end since it serves as a medium for transportation. The other major purpose water plays in the role of producing energy for the plant is in the gas exchange of carbon dioxide and oxygen. Carbon dioxide actually enters the plant by first dissolving into the water inside the stoma. This applies for oxygen as well, where oxygen leaves the plant through the water in stomata.

So, a very small percentage of the water absorbed actually stays in the plant for processes such as photosynthesis and plant growth since they do not require a lot of water. However, some processes in the plant require a very large volume of water, such as the transportation of nutrients and the constant gas exchange of oxygen and carbon dioxide. Hence, this is why a large amount of water is transpired in the plant after absorption, and is not considered to be pointless.