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Thursday, November 8, 2018
Transpiration

Author: Nur Alwani Binti Aminorasid, 2018438306 (Transpiration)
     All plants from the smallest weeds to the tallest tree need water.
The roots of plants absorb water and mineral salts from the soil through the root hairs by process of osmosis. But the most of water absorbed is lost, in the form of water vapour, into the air. Only small percentage of water is used by the plant for growth and metabolism. The rest gets evaporated from the surface of leaves. This evaporation of water from the plant is called transpiration.
  How transpiration takes place in plants? 
     In plants the water required is absorbed by the roots from the soil with the help of root hairs by a process called osmosis. Osmosis is movement of water molecules from an area of low solute concentration to an area of high solute concentration across a semi-permeable membrane until the concentrations are equal (equilibrium).

     The water is further transported upward through the stems to the leaves through xylem, which acts as a pipeline. Xylem vessel has thick walls. Water movement in xylem is one-way only. There are two properties of water molecules that helps water move upward which are cohesion and adhesion force. Cohesion force is water molecule attracted to each other meanwhile adhesion force is water molecules attracted to wall of xylem. Plant forced water up from the root to the stem and the rest of the plant in the continuous stream.

     Water then enter mesophyll cell through stomata. The stomata are surrounded by guard cells which help the stomata to open and close. 
     During day time, chloroplasts produce sugar this lowers water potential of guard cell and they take in water by osmosis. This makes guard cells turgid because the guard cells have a different cell wall thickness. When turgid, guard cells bend more on the outside, this opens the stomata and water can be lost. Transpiration takes place during day time when the stomata are open.
     During the night, all the sugar produced by the chloroplast get used up so the water potential of the guard cells increases, more water needs sugar, the guard cells lose water by osmosis and become flaccid and stomata starts to close. This reduces water loss. Translocation takes place during night when the stomata are close.

    Transpiration is also done through lenticels which are tiny opening on the stem and a small amount of water is evaporated through them. This is called lenticular transpiration. Lenticels are less in number compared to stomata. Transpiration also takes place from the surface of leaves. Leaves are covered by a waxy layer of cuticle which is called cuticular transpiration.  The cuticle actually helps the leaf to reduce the amount of transpiration taking place.

     In conclusion, there are three types of transpiration which are stomatal transpiration, lenticular transpiration and cuticular transpiration. In fact, transpiration is important to plants as the transpiration pulls helps in water rising to the leaves against the force of gravity. In addition, transpiration is more on the under surface of leaf than on the upper surface of the leaf. The external factor that affect the rate of transpiration: temperature, air movement, humidity and light intensity.

References:

-Benjamin Cummins (2007), Biological Science (3 ed.), Freeman, Scott, p. 215
-Jane B., Lisa A., Michael L., Steven A., et al. (2014). Campbell Biology (10th ed.). USA: Pearson Education, Inc.
-Sinha, Rajiv Kumar (2004-01-01). Modern Plant Physiology. CRC Press.
-Taiz, Lincoln (2015). Plant Physiology and Development. Sunderland, MA: Sinauer Associates, Inc. p. 101. 
-Transpiration: The Water Cycle. Retrieved November 5, 2018, from https://water.usgs.gov/
edu/watercycletranspiration.html

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