transpiration Flashcards
Define the term transpiration
Transpiration is the loss of water vapour from the leaves through the stomata.
Transpiration is a consequence of…
gaseous exchange in plants
why is transpiration a consequence of gaseous exchange in plants?
Transpiration will always occur when the stomata are open. When photosynthesis takes place, more stomata will open (to allow more gaseous exchange) and thus, transpiration is a consequence of gaseous exchange in plants
what are the three forces to bring water up the xylem to the leaves, and which of the three is the main force?
Root pressure
Capillary action
Transpiration pull - main force
how does root pressure work?
The respiring cells around the xylem vessels in the roots use active transport to pump mineral salts into the vessels. This lowers the water potential in the xylem vessels, which causes water to move into the xylem vessels by osmosis. This pushes water into the xylem vessels and upwards and it is called root pressure.
how does capillary action work?
Water molecules are polar, and the partial negative charge on the oxygen atom in one water molecule attracts the hydrogen atom in the neighbouring molecule. This is cohesion (forces of attraction among water molecules). Water is also attracted to the hydrophilic parts of the walls of xylem. This is adhesion (forces of attraction between water molecules and the inner walls of the tube). As a result of the cohesion between water molecules and adhesion between water and the walls of the xylem, water can be pulled up in a continuous stream.
what happens in a leaf, that results in transpiration pull?
Water evaporates from the film of moisture surrounding the mesophyll cells into the intercellular air space.
The water vapour then diffuses out of the leaf via the stomata by a process known as transpiration.
As water leaves the mesophyll cells to replenish the film of moisture, the water potential of the cell (surrounding the intercellular air spaces) decreases, and water moves from the neighbouring cells to these mesophyll cells by osmosis. The neighbouring cells then draw water from cells deeper inside the leaf. These cells then draw water from the xylem.
This results in transpiration pull, a suction force which pulls the whole column of water up the xylem.
importance of transpiration
- Creates transpiration pull, which draws water and mineral salts up from the roots to the stem and leaves. Transpiration pull:
• Supply water to the mesophyll cells in the leaves for photosynthesis.
• Supply water to all cells for metabolic processes.
• Keep cells turgid so that the leaves can spread out widely to trap sunlight for photosynthes and keeping the plant upright - Removes latent heat which helps cool the plant, preventing it from being scorched.
factors affecting the rate of transpiration
Humidity of the air surrounding the leaf
Wind
Temperature of the air surrounding the leaf
Light intensity (does not affect evaporation)
how humidity of the air surrounding the leaf affects the rate of transpiration (higher vs lower)
Higher humidity decreases the rate of transpiration.
Lower humidity increases the rate of transpiration.
explain how humidity of the air surrounding the leaf affects the rate of transpiration (higher vs lower)
Higher humidity indicates the presence of a lot of water vapour in the air surrounding the leaf. This decreases the water vapour concentration gradient. As such, water vapour diffuses out at a lower rate, leading to lower transpiration rate.
Lower humidity indicates drier air, and lesser water vapour in the air surrounding the leaf. This increases the water vapour concentration gradient. As such, water vapour diffuses out at a higher rate, leading to higher transpiration rate.
how does wind affect the rate of transpiration? (high vs low/no)
High wind speed increases the rate of transpiration.
Low wind speed/no wind decreases the rate of transpiration.
explain how wind affects the rate of transpiration (higher vs lower)
Higher wind speed indicates that the water vapour in the air surrounding the leaf is blown away. This increases the water vapour concentration gradient. As such, water vapour diffuses out at a higher rate, leading to higher transpiration rate.
Lower wind speed may result in water vapour accumulating in the air surrounding the leaf. This decreases the water vapour concentration
gradient. As such, water vapour diffuses out at a lower rate, leading to lower transpiration rate,
how temperature of the air surrounding the leaf affects the rate of transpiration (higher vs lower)
Higher temperature increases the rate of transpiration.
Lower temperature decreases the rate of transpiration.
explain how temperature of the air surrounding the leaf affects the rate of transpiration (higher vs lower)
Higher temperature increases the rate of
evaporation, increasing the concentration of water vapour in the intercellular air spaces. This increases the water vapour concentration gradient. As such, water vapour diffuses out at a higher rate, leading to higher transpiration rate.
However, at very high temperatures, the stomata will close to prevent excessive transpiration.
Lower temperature decreases the rate of evaporation, decreasing the concentration of water vapour in the intercellular air spaces. This decreases the water vapour concentration gradient. As such, water vapour diffuses out at a lower rate, leading to lower transpiration rate.
