Transpiration Flashcards
(12 cards)
Two transport systems in a plant.
Xylem vessels -> transports water and minerals from root to other parts.
Phloem vessels -> transports sugars (sucrose) and amino acids made in leaves to all other organs plant parts
How do gasses diffuse in and out of leaves?
via stomata
as leaves are thin and flat with a big SA, this can happen fast enough to meet the demands and of leaf cells for oxygen and CO2
Why do plants need specialised transport systems?
- to deliver water, mineral ions and organic solutes to different cells and tissues
- these molecules cannot be supplied at a fast enough rate across surface of plant because plants are large organisms with small SA: vol ratio
What is transpiration and transpiration stream?
transpiration -> evaporation of water vapour from the stomata
transpiration stream -> movement of water up the plant
Water movement across the root.
- Mineral ions move into the root hair cells by facilitated diffusion and active transport, lowering the water potential inside root hair cells.
- Water then moves from soil into root hair cell by osmosis, due to water potential gradient.
[ root hair cell highly adaptive for water and mineral ions move into uptake: have very large SA and thin walls] - water will then move from cell to cell across cortex of the root, by osmosis again, towards xylem at the centre of the root. Two pathways that water move from cortex cell to cortex cell:
Apoplastic pathway -> water moves from cell to cell through cellulose cell wall. Cellulose fully permeable to water, so quick transport route.
Symplastic pathway -> water moves from cell to cell through cytoplasm, then via connections from one cytoplasm to another (plasmodesmata).
What happens water molecules reach endodermis of the root?
- Cells have a strip of waxy waterproof material in their walls, forming a “casparian strip”.
- Water cannot pass through this, so water has too cross endodermal cells using symplast route only (apoplast pathway blocked).
This slows down flow of water slightly, and gives plant some control over which mineral ions passes through.
Adaptation of xylem vessel.
- Water carried to leaves via xylem vessels in stem.
- Xylem tissue made up of many xylem vessel elements, stacked end to end, forming xylem vessel.
- Begin as living cells -> differentiate -> lose all cell and “end walls” -> become continuous, hallow tubes “non-living” no barrier so unrestricted water flow and faster
- vessels are narrow - increases adhesion and maintains pressure
- cellulose cell walls remain and lined with lignin (lignified walls) -> strong and waterproof substance.
- Tiny holes (pits) in walls of vessel -> allow water to move sideways into neighbouring vessels if there are any blockages, air locks.
3 forces that help move water up xylem vessels.
1- cohesion forces : water molecules are polar attract each other and form H-bonds between themselves. As they move up they attract other molecules pulling them up “cohesion tension theory”
2- root pressure : water enters root hair cells from soil as the water potential is lower inside the cells, creates pressure as water keeps entering -> contributes small amount.
3- adhesion forces: water molecules attracted to other molecules attracted to other molecules ( the lignified cellulose on xylem vessels walls) - H- bonds form allowing water to “crawl” up sides of xylem vessels, moving up - xylem vessels narrow so water molecules more interact with walls ( capillary affect)
This collectively is mass flow
Water transport across the leaf.
- Water moves from xylem vessels in leaf into spongy mesophyll cells via osmosis.
- Water evaporates into the air spaces of the spongy mesophyll cells, making these spaces separated with water vapour. However, some water used by mesophyll for photosynthesis and turgor.
- These air spaces are close to the stomata, so water vapour evaporates out of leaf through stomata, into atmosphere by diffusion down a water potential gradient ( more water potential inside leaf than outside).
- As water evaporates from leaves, water potential gradient maintained, so more water molecules move out of xylem to leaf, “pulling” more water up the xylem via adhesion and cohesion.
This all occurs as long as stomata is open.
Adaptations to minimise water loss due to transpiration.
1- waxy cuticle - reduces H2Oloss via epidermis
2- stomata found on undersurface not top surface - reduces evaporation due to direct heat from sun
3- most stomata closed at night - no light for photosynthesis
4- Plants lose leaves in water -> ground frozen (less H2O available) + when temp too low for photosynthesis.
Explain how xylem vessels for their function of transporting water from roots to leaves.
- Xylem vessels lack all cell contents to be able to form continuous hall tubes that are non-living so there is an unrestricted water flow, which will be faster.
- They have no end walls so there is no barrier present to restrict flow of water, so water can flow more quickly.
- Small diameter so xylem vessels are very narrow, this helps to maintain pressure so water keeps moving upwards, it increases capillary action and adhesion forces as there is more contact between water and walls, stronger force of attraction.
- Lignified walls -> strong -> prevents vessels from bursting due to water pressure, lignin is waterproof substances so prevents water from leaking out.
- Pits - tiny holes - allow water to move sideways - to neighbouring vessels.
Factors affecting rate of transpiration.
Light intensity:
Inc light intensity = incr ROT. Light needed for photosynthesis
Humidity:
Inc humidity = dec ROT -> due to reduced water potential gradient between inside plant and outside dry air.
Temperature:
Inc temp = inc kinetic energy of H2O molecules = inc rate of evaporation from spongy mesophyll cells into air spaces of leaves.
Air movement:
Inc air movement = inc diffusion gradient = inc ROT
Inc still air = dec diffusion gradient = dec ROT
