transpiration is one of those topics which is often hidden in other chapters so this is all to do with plant structure and transpor in Plants it's particularly important you could get a question on it in your exams so this video is just highlighting areas to revise all part of leaving Sur biology transpiration is the loss of water vapor from the surface of plants mostly the leaves so it's really the evaporation of water and again this is mostly taking place from the leaves or the aerial parts of the plant this evaporation of water takes place through little tiny pores in the leaf known as the stomata stomata are literally these tiny holes on the leaves and the opening and closing of the stomata is controlled by particular cells known as guard cells so how do these guard cells function what makes the guard cells close the stomata osmosis is the key to understanding how the guard cells open and close the stomata so when the stomata are open the guard cells are tured and buckle outwards when they lose water by osmosis they become flaccid and this closes the stomata so osmosis is the process by which they become tured or flacid but what makes this happen what drives the osmosis well one factor is the internal concentration of carbon dioxide inside the leaf so when there is high CO2 inside the leaf this is mostly at night the stomata are closed and when it's low they open another factor is when there's very high temperatures the stomata clows and this is all down to obic acid this plant growth regulator a chemical produced by plants abscissic acid is known as the plant stress hormone it's produced in many areas of the plant for example it's produced in the roots and it can also be produced in the leaves essic acid is produced in times of drought this would ensure that the stomata are closed and so therefore water loss is minimized sometimes the exam questions connected with transpiration are to do with the rate of transpiration and environmental conditions so transpiration rate increases when there is dry or low humidity in the at atmosphere so dry weather low humidity when it's Breezy weather you'll get an increase in the rate of transpiration and when it's warm and sunny the rate of transpiration will increase so just be mindful that you might get a graph and be able to link this information to the graph the structure of the leaf is a very important diagram and you're often asked how is the leaf adapted to ensure that water loss due to transpiration is controlled well first of all there is a waxy cuticle layer a transparent layer on the top of the leaf which is quite thick there is also a waxy cuticle lay on the under surface but it's not as thick it's also important to note that most of the stomata they're great as in number on the under surface of the leaf so this is really important there are stomata on the top of the leaf but not as many as there is on the unders surface and remember the unders surface will not get direct sunlight ensuring that there's always a continuous flow of water from root to Leaf is very important this is known as the transpiration stream a continuous column of of water in the plant from root to leaves this is important for photosynthesis water enters plants at the roots where it's absorbed by the process of osmosis and it moves from cell to cell until it enters xylm that vascular tissue at the center of the root and it's in xylm that water is transported there are xylm tracheids and xylm vessels and these form Long Hollow continuous tubes from root to Leaf remember that xylm is dead and it's also reinforced with lignan so make sure that you can draw and label diagrams of xylm and flum learn all the plant structure diagrams very important for the exams and the root is another important example so water enters the plant through the root and the root hairs or the presence of these root hairs will greatly increase the surface area for the absorption of water so it's really important you can label this diagram there are also root zones which are often asked to Mark in on diagrams the first one is the zone of protection look for where the root cap is second you have the zone of cell production look for where the meristem is then you have the zone of cell elongation just above it and then you have the zone of differentiation look for the root airs everything we've discussed so far is all part of the broader topic of water transporting plants and this is really important to be able to discuss first of all you must state that Dixon and Jolie two scientists from Trinity College in Dublin put forward the cohesion tension model this is a very important model it explains how water is transported upwards in a plant against the force of gravity but before you discuss that in an answer you must give an an account of root pressure just State what it is and then go on to the cohesion tension model and use the letters TCT to help you structure your answer you must mention transpiration adhesion cohesion and tension but before you start mentioning those always have an account of root pressure this is a very important topic so I would spend time revising this particularly for your exams so what are the exam questions like what could I be asked that's a good question to consider when you're getting ready to revise well make sure at minimum you know the adaptations of the leaf a good diagram of the leaf stomatal opening and closing water transport particularly vascular tissues make sure you know about the root and how to draw it and label it and the root zones and the cohesion tension model so the best of luck with all of that revision all you can do is begin use the videos use your textbook use your notes do pass papers good luck