Transpiration 9.3 Flashcards
Why do we need transpiration?
Photosynthesis is the process by which plants make their own food and for that they need both water and Co2. Water has to be transported from the roots to the leaves and Co2 must be taken in through the stomata.
The exchanges that take place during transpiration
The Co2 needed will diffuse down a concentration gradient through the stomata into the air spaces in the leaves and oxygen will diffuse out as it is made as a waste product. Water can also evaporate from the surface of the leaves
What is transpiration
It’s the loss of water as a consequence of gas exchange in the leaf. As the stomata open, Co2 diffuses in, down a concentration gradient as the air outside the leaf has higher conc than inside the leaf. Oxygen will diffuse out by the same principle, and while this is happening, water vapour in the air inside the leaf will diffuse out. This loss of water is called transpiration
What are stomata?
These are pores in the underside of the leaf that are controlled by guard cells and they can open and close to allow gas exchange to occur.
When do the stomata operate?
The function most in the day as this is when photosynthesis occurs and a lot of gas exchange occurs. They open less at night as no photosynthesis is happening, however because of this they need to take in oxygen for respiration, so some still open.
What happens to water after it’s in the xylem?
After it’s been taken up by the root and taken to the xylem, the water will move up the xylem until it reaches the leaves. Here it will move into the cytoplasm of the mesophyll cells in the symplast pathway by osmosis, or will use the apoplast pathway and diffuse across. In the apoplast pathway, the water will evaporate from the freely permeable cellulose walls into the air spaces between the cells inside the leaf. It now exists as water vapour and will move from the internal air, through the stomata to the external air by diffusion.
Transpiration Stream
The movement of water from the roots to out of the leaf is called the transpiration stream.
Cohesion tension theory
The transpiration stream can move water from the roots to the highest leaves as high as 100m. As the water molecules in the mesophyll cells evaporate, they reduce the water potential in the mesophyll cells, so water from a neighbouring cell will move into it by osmosis along the apoplast and symplast pathway and so on through each cell until it reaches the xylem. Water in the xylem will move into the closest mesophyll cell by osmosis which will decrease the water potential in the xylem. Water molecules will form hydrogen bonds with the lignified cell walls of the xylem which is adhesion and it will also form hydrogen bonds with itself which is adhesion. This will cause capillary action to occur where the water will move up the xylem against gravity. This means water will be drawn up into the xylem continuously and is called the transpiration pull.
Evidence for the cohesion tension theory
- the diameter of trees decreases when transpiration is at its highest in the middle of the day and this is because of the increased pressure in the xylem. When the transpiration rate is at its lowest at night, the diameter of trees will increase
- when the xylem vessel is broken air will be pulled in instead of water leaking out so no water will be in the stems
Measuring transpiration
You can do this by using a potometer and doing the amount of water taken up in a certain amount of time ands measuring how certain factors affect it. You can measure the amount of water taken up by looking at how much an air bubble moves in a certain amount of time
Stomata/ Guard Cell Structure
They have a thin outer wall and a thick inner wall and less flexible which helps it open by becoming bean shaped. They also have cellulose hoops which prevent it from swelling outwards, instead they elongate
Stomatal Opening
When the conditions for photosynthesis are good, the guard cells will actively pump out H+ ions and leave them negatively charged. This charge will cause potassium ion channels to open and potassium ion will flow into the guard cells. This will lower the water potential. Water will then move into the guard cells by osmosis and will increase the turgor pressure of the cell. The thick inner wall isn’t as flexible so wont bend as much as the other wall, this causes the guard cells to become a bean shape and will open and make a stomatal pore.
Stomatal Closing
When the gas exchange has occurred, or the conditions are no longer favorable, the potassium ions will move out of the guard cells by facilitated diffusion, which increases water potential inside the guard cells. The turgor pressure is now low and so the guard cells are no longer expanded and will close the stomatal pore.
Factors Affecting Transpiration
Light, Temperature, Air Movement, Humidity, Soil Water Availability
Light
Light is will increase the rate of photosynthesis which means you will need more gas exchange and more water uptake. More stomata will be open so more water will be lost by evaporation and diffusion out of the leaf so the rate of transpiration will increase, the more light there is.
