Transport in the xylem of plants Flashcards
1
Q
What is transpiration?
A
- The loss of water vapor through the leaves, stem and other aerial (above ground) structures, water moves against gravity because of this passive process
- 95% of the water absorbed by the roots is released into the atmosphere by transpiration
2
Q
Why does transpiration occur?
A
- It happens due to the adaptations of leaves: the stomata, tiny pores on the underside of the leaf that control the gas exchange as well as the spongy mesophyll which provides a large and moist surface for gases to exchange
- Transpiration happens because of the gas exchange, when CO2 and O2 diffuses for the purpose of photosynthesis
3
Q
Why do the concentrations of CO2 and O2 decrease?
A
- The concentration of both gases must decrease within the leaves for diffusion to occur (higher to lower concentration) down the concentration gradient
- Oxygen is a competitive inhibitor of the enzyme rubisco and therefore must leave the plant
4
Q
What effect does transpiration have on the plant?
A
- Has a cooling effect on the leaf
- Exerts a pull to move water from the roots into the leaves
5
Q
How can transpiration be limited?
A
- Closing the stomata, this also means CO2 cannot diffuse in and photosynthesis cannot occur
- Most plants close their stomata at night to conserve water and open them during the day
- When they are open, the water that is lost must constantly be replaced
6
Q
Why is transpiration inevitable?
A
- When the stomata are open to allow CO2 to diffuse into the leaf, water and oxygen will diffuse out. All gases must diffuse down their concentration gradients
- When water vapour diffuses from inside of the leaf (high humid concentration) to the outside (where the humid concentration is less) this is transpiration
- Hence the loss of water vapour is the inevitable consequence of gas exchange
7
Q
What is the structure and function of xylem vessels? What is lignin?
A
- Transports water and dissolved minerals
- Narrow tubes, from roots through the stems of the plants
- Make of dead cells
- Hollow for water efficiency, continuous tube
- Wall strengthened by lignin (complex polymer that binds with cellulose to provide great strength and rigidity)
8
Q
Be able to draw and label the structure of primary xylem vessels in stems.
A
- Check book Five structures: - Cellulose - Remains of horizontal cell wall - Lignin - Pits - Lumen
9
Q
What two forces are involved in transpiration?
A
- Adhesion: the polarity of water also interacts with the hydrophilic cellulose in the cell walls of the leaf. The water is pulled out of the xylem into the leaf cells
- When water leaves the plant, water evaporates from the mesophyll cell wall into the air spaces to replace the lost water
- Cohesion: water molecules from weak hydrogen bonds with each other due to their polarity. The transpiration pull extends into the xylem vessels.
- When water evaporates from the mesophyll cell wall, this pulling force (cohesion) is exerted. This tension draws water from the xylem into the leaf cells
10
Q
Path way of water?
A
root xylem, stem xylem, mesophyll cell wall, leaf air spaces, water vapour lost to the atmosphere
11
Q
Where is active transport involved during transpiration?
A
- Mineral ions like nitrate and potassium are found dissolved in low concentrations in water
- The root hairs in the roots have protein pumps that actively transport mineral ions from the surrounding water into the cytoplasm against the concentration gradient
- ATP energy is required, hence root hair cells have a high rate of cellular respiration - With a high mineral ion concentration, water can move in through osmosis and the dissolved minerals in water passively moves through the xylem
12
Q
What is water potential?
A
- Measure of water’s tendency to move in a particular direction which brings the transpiration pull, concentration gradient, pressure of cell walls and gravity together
- Pure water has water potential 0.0MPa (all other values are negative)
- Water flows passively by osmosis through protein channels called aquaporins in the cell membrane
13
Q
Summary of water transport in plants:
A
- Water moves from the soil into the root cells by osmosis due to high solute concentrations in the cytoplasm (active transport of mineral ions)
- Water moves up through xylem vessels to the leaves by transpiration pull in which cohesive properties and lignin are important
- Water moves into the leaf cells and adhesive properties with the cell wall is important which allows gases to diffuse in and out
- Water evaporates from the mesophyll cell walls into the air spaces of the leaf
- Water vapour diffuses out of the leaf into the atmosphere which is a consequence of gas exchange for photosynthesis
14
Q
What are adaptations of xerophytes?
A
- Live in conditions where water is difficult to obtain and conserve the water they acquire and reduce water loss
Adaptations:
- Thick waxy cuticle on the leaf (hydrophobic) acts as a barrier to prevent water loss
- Few stomata which reduces transpiration rate
- Reduced air spaces in leaf mesophyll, reduced surface area for evaporation
- Curled or rolled leaves for a reduced surface area
- Deep branched roots for increased ability to take up water
- CAM physiology which reduces transpiration rate because stomata close during the day and only open at night to store CO2 at cooler temperatures and during the day photosynthesis can occur without water loss
15
Q
What are adaptations of halophytes?
A
- Plants that live in areas with high salinity (salt) such as ocean shorelines
- They have a higher concentration of ions in the roots than in the surroundings to acquire water, which is difficult as there is a high ions concentration too
- Adaptations to excrete excess salt or to conserve fresh water
Adaptations:
- Selectively permeable membrane in root cells to exclude salt by having no ion channels
- Xerophytic adaptations by having few stomata, thick cuticle
- High concentration of organic solutes for increased osmolarity
- Salt storage in vacuoles to protect cellular organelles and enzymes from damage by high salt concentrations
