[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe how transpiration takes place in Plants you should then be able to describe the cohesion tension theory of water movement and the evidence which supports this Theory in this topic we've been looking at how water moves in a plant remember that water is a reactant in photosynthesis which takes place in the leaves I'm showing you the structure of a leaf here the main site of photosynthesis is the Palisade mesophyll which is found in the top half of the leaf although I should point out that photosynthesis also takes place in other parts of the leaf water passes into the leaf through the xylem vessels in the vascular bundle now the surface of the leaf is covered in a waterproof layer called the waxy cuticle and the job of the waxy cuticle is to reduce water loss from the surface of the leaf by evaporation so as we've seen water is an essential reactant for photosynthesis however photosynthesis also requires the gas carbon dioxide and this carbon dioxide diffuses into the leaf from the external air photosynthesis produces the gas oxygen which diffuses out of the leaf now on the lower surface of the leaf we find thousands of tiny pores called stomata and when the plant photosynthesizes these stomata are open allowing carbon dioxide to diffuse into the leaf and oxygen to diffuse out I'm showing you here a drawing of the internal structure of a leaf now a key idea you need to understand is that the surface of the cells in the leaf are covered with a thin layer of water this water evaporates from the surface of the cells and because of this the internal Leaf spaces contain a high concentration of water vapor now generally the level of water vapor in the external air is relatively low so when this tomato are open the water vapor diffuses out of the leaf to the external air and this evaporation of water followed by the diffusion of water vapor is called transpiration now because of the continuous evaporation from their surface the water potential of the cells in the leaf decreases this causes water to move by osmosis from adjacent cells like this now lowers the water potential of these cells causing water to move into them at some point this reaches the xylem with water passing out of this xylem to adjacent cells like this so when transpiration is taking place water is continuously being pulled out of the xylem vessels and scientists call this pulling effect tension the movement of water from the roots up the xylem and out of the leaf is called the transpiration stream and you need to be able to describe how this takes place now we've already seen that water molecules form hydrogen bonds to each other this type of Attraction is called cohesion water can also form hydrogen bonds to molecules in the xylem vessel walls for example carbohydrates and this attraction is called adhesion now one effect of cohesion and adhesion is that water can move a very thin tubes against the force of gravity and scientists refer to this as capillary action so when water is removed from the top of the xylem vessels due to transpiration more water moves up the xylem vessels by capillary action to take its place and this whole process is referred to as transpiration pull so the combined effect of transpiration pull coupled with cohesion and adhesion is that water is drawn into the roots moves up the stem and passes out of the leaves and scientists call this whole process the cohesion tension Theory now even though this theory is over 100 years old it is still accepted by scientists there are two pieces of evidence which support the cohesion tension Theory firstly if a plant stem is cut then air is sucked into the xylem suggesting that the xylem vessels are under tension however the air prevents cohesion between the water molecules so water movement stops secondly if we measure the diameter of a tree trunk we can see that this reduces when transpiration is at its maximum this supports the idea that transpiration pull is generating a negative pressure or tension in the xylem in the next video we look at how to measure the rate of water uptake using a potometer [Music]