Transpiration & Translocation Flashcards
What is the definition of transpiration?
The loss of water vapour by evaporation and diffusion from the surface of the leaves and stems of a plant.
Around 99% of water absorbed is lost by transpiration.
How is transpiration a consequence of gas exchange?
The stomata must be open to allow exchange of CO2 and O2 - when the stomata is open, water vapour moves out of the leaf by diffusion.
How are leaves adapted to control the rate of transpiration?
- waxy cuticle (waterproof layer)
- guard cells to open / close stomata
- very few stomata on upper leaf surface
How is the transpiration stream different to transpiration?
The transpiration stream refers to the flow of water up the xylem from roots to leaves.
How does water potential in leaves increase?
Large air spaces between the mesophyll cells allow water vapour to collect - this causes the water potential to rise.
How does water move from the xylem to outside of the leaf?
- moves into the leaf by passing through pits (non-lignified areas)
- moves along mesophyll cells via apoplast or symplast pathway
- evaporates from mesophyll cell wall to form water vapour
What is adhesion?
- adhesion is the force of attraction between two particles of different substances (water molecule / xylem wall)
- xylem wall is also polar and can form intermolecular associations with water molecules
- as water molecules move up the xylem via capillary action, they pull inward on the xylem walls to generate further tension
What is cohesion?
- cohesion is the force of attraction between two particles of the same substance (between 2 water molecules)
- water molecules are polar and form hydrogen bonds
- cohesion causes water molecules to be dragged up the xylem towards the leaves
How does the cohesion-tension theory work?
Water is drawn into the xylem from the root where there is a higher hydrostatic pressure - creating a pressure gradient.
This creates a tension in the xylem which pulls up water in a continuous column - water molecules also held by cohesion and adhesion.
What evidence is there for the cohesion-tension theory?
- changes in tree diameter - at high transpiration rates (during the day) diameter decreases due to tension. Diameter increases at night.
- cut flowers - often draw air in rather than leaking water out as air moves up the stem.
- broken xylems - broken / cut xylems stop drawing up water as the air drawn in breaks the transpiration stream and cohesion between water molecules.
How do guard cells open and close the stomata?
flaccid guard cells = stomata closed
- water moves out of the vacuoles by osmosis, outer wall is more flexible than the inner wall so cell bends back and closes the stomata
turgid guard cells = stomata open
- water moves into vacuoles by osmosis, outer wall is more flexible than the inner wall so the guard cell bends and opens the stomata
When are the stomata open and closed?
- open in low CO2 concentration inside the leaf and high light intensity
- closed in high CO2 concentration inside the leaf and low light intensity
How does light intensity affect rate of transpiration?
Stoma close in the dark - reducing rate of transpiration.
Once the stoma are open any increase in light intensity has no effect on the rate of transpiration - they will remain open at relatively low light intensities.
How does temperature affect rate of transpiration?
Increase in temperature results in an increase in the molecules’ kinetic energy - increase in temperature will increase rate of transpiration as water molecules move out of the leaf at a faster rate.
If the temperature gets too high, the stomata close to prevent excess water loss.
How does air movement affect rate of transpiration?
When the air is relatively still, water molecules can accumulate near the leaf surface, creating a local area of high humidity, lowering the water potential gradient.
Air currents can sweep water molecules away from the leaf surface, maintaining the water potential gradient.
How does humidity affect rate of transpiration?
High humidity means there is a large concentration of water molecules in the air surrounding the leaf surface, reducing the water potential gradient.
At a certain level of humidity, an equilibrium is reached, meaning there is no net loss of water vapour from the leaves.
What is translocation?
Translocation is the movement of assimilates within phloem sieve tubes from where they are made (source) to where they are required (sink).
It is an active process.
What are the key features of translocation?
- requires ATP energy to create a pressure difference
- movement is bidirectional
- liquid being transported is called phloem sap
- glucose is transported as the disaccharide sucrose
What is meant by a source?
The site where sucrose / assimilates are made and loaded into the phloem (high concentration).
What are some examples of sources in plants?
- green / photosynthetic leaves
- storage organs
- food stores in seeds
What is meant by a sink?
The shite where sucrose / assimilates are unloaded from the phloem for use or storage.
What are some examples of sinks in plants?
- meristems that are actively dividing
- roots that are growing / absorbing mineral ions
- storage compounds
How are compounds loaded into the phloem?
Organic compounds produced at the source are actively loaded into phloem sieve tubes by companion cells.
Loading of sucrose is an active transport process requiring ATP and co-transporter proteins.
Describe the process of active loading into phloem sieve tubes.
- H+ ions are actively pumped out of the cytoplasm of the companion cells via a proton pump into their cell wall - active process involving hydrolysis of ATP.
- Concentration of H+ ions in the cell walls of the companion cells increases compared to inside the cells creating a concentration gradient.
- Hydrogen ions re-enter the cytoplasm down their concentration gradient via a co-transporter protein.
- While transporting the H+ the co-transporter protein also carries sucrose molecules into the companion cell against the concentration gradient via facilitated diffusion.
- Sucrose molecules then diffuse into the phloem sieve tubes via the plasmodesmata of the companion cells.
