Plant Transport Flashcards
B3.1.9; B3.1.10; B3.2.7; B3.2.8 (10 cards)
What is transpiration in plants?
Transpiration is the loss of water vapor from aerial parts of the plant, mainly through the stomata.
It is a passive process driven by evaporation and diffusion.
It occurs as a consequence of gas exchange: when stomata open to allow CO₂ in for photosynthesis, water vapor escapes.
What factors affect the rate of transpiration?
Sunlight – Light stimulates the opening of stomata, as gas exchange is required for photosynthesis to occur
Temperature – High temperatures increase the vaporisation of water, leading to greater diffusion from the leaf
Wind – The flow of air functions to remove the water vapour surrounding the leaf, increasing diffusion rates
Humidity – A higher concentration of water vapour in the air will decrease the rate of diffusion from the leaf
Describe the process of transpiration and how it leads to the movement of water through the xylem.
Transpiration starts when water evaporates from the spongy mesophyll into air spaces within the leaf.
Light energy is absorbed and converted into heat, which drives the evaporation.
Water vapor exits through stomata, creating a negative pressure gradient.
This pulls water from xylem vessels into leaf tissue via capillary action, generating tension.
The tension draws water upward through the xylem in a continuous column, known as mass flow.
What roles do cohesion and adhesion play in the transport of water through the xylem?
Cohesion: Water molecules are polar and form hydrogen bonds, allowing them to stick together and move up the xylem in a continuous column.
Adhesion: Water molecules form intermolecular bonds with the polar xylem walls, helping generate tension and assisting upward movement.
Together, these forces enable water transport against gravity through the xylem.
How do roots absorb water and minerals from the soil, and what adaptations support this process?
Water uptake maintains the pressure gradient caused by transpiration.
Minerals are absorbed via indirect active transport:
→ Protons are pumped out to displace minerals, which diffuse into root cells.
Water follows by osmosis, moving to areas of higher solute concentration.
Aquaporins (water channels) in root membranes help regulate water uptake.
Root hairs increase the surface area, enhancing absorption efficiency.
How does the stomata help to conserve water when a plant begins to wilt from water stress?
Dehydrated mesophyll cells release the plant hormone abscisic acid (ABA).
Abscisic acid triggers the efflux of potassium from guard cells, decreasing water pressure within the cells (lose turgor).
A loss of turgor makes the stomatal pore close, as the guard cells become flaccid and block the opening.
What are the structural features of xylem vessels that enable water transport in plants?
Xylem is made of dead, hollow cells (no protoplasm) for unimpeded water flow.
Water movement is passive and one-way (from roots to shoots).
Pits in cell walls allow lateral water movement between vessels.
Thickened cellulose walls reinforced with lignin provide strength to resist tension.
What is lignin in xylem vessles and what function does it have?
Lignin is a complex, strong polymer deposited in the walls of xylem vessels.
It gives strength and rigidity that reinforces the cell walls, that helps xylem vessels withstand the negative pressures (tension) created during the process of transpiration without collapsing or breaking.
What are pits in the xylem vessles and what function do they have?
Pits are small, unlignified areas in the walls of xylem vessels, that allow water to move sideways between adjacent vessels or into surrounding tissues.
They provide flexibility and redundancy in the water transport system, ensuring efficient flow even when some vessels are compromised.
Explain the adaptations of xylem vessels for transport of water.
- Lack of cell contents and the absence of end walls form a continuous and unobstructed column for water to travel up the plant.
- Lignified walls ensure that the xylem vessels do not collapse under the tension created by the evaporation of water in the leaves.
- Pits allow for intercellular communication and water movement between adjacent xylem vessels, maintaining the continuity of the water column.
