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CHAPTER 8: TRANSPORT SYSTEM IN PLANTS (Transport of Water (Transpiration,…
CHAPTER 8: TRANSPORT SYSTEM IN PLANTS
Transport of Water
Introduction
Plants need water to:
keeping cells turgid - leaves and petals in proper position
solvent for metabolic reaction
reactant in photosynthesis
Water taken up via
root hair
:arrow_right: seeps across root towards centre :arrow_right: enter xylem vessels :arrow_right: transported to stem & leaves (other parts of plants)
Transpiration
Actions of water being evaporated out from plants cells via stomata.
Water diffusion: osmosis
Water :arrow_right: root hairs :arrow_right: centre
(xylem vessels)
:arrow_right: stem
(transported through vascular bundles)
:arrow_right: leaves :arrow_right: water evaporated from cells
(in air spaces)
:arrow_right: water vapour diffuses into the air through the stomata :arrow_right: into environment
Produces tension:
transpiration pull
- enable water to be pulled upwards (against gravitational pull)
water cohesion
: enable water to be pulled in a continuous stream - water molecules tend to stick to each other, preventing the column from breaking
air lock
: when water in column breaks, air will fill in the gap - water stops flowing upwards
How does it works?
Water loss at the top portion of the plants (leaves & shoots) - reduces water pressure at the top
Water at the bottom parts (roots) having higher water pressure
Water at the bottom parts (higher water potential) 'push' upwards to the top (lower water potential) - down water potential gradient
Deliver water throughout the plants
Water potential drives transpiration:
Water potential in air < roots
: water goes into the roots
Water potential in air (depends on the humidity) < leaves
: water transpire out
Water potential in cell walls (of mesophyll cells) < xylem vessels
: water being evaporated out
Water potential in upper part of vessel system (leaves) < lower part of vessel system (roots)
: movement of water upwards through xylem vessels
Water moves into the roots:
Via root hairs
(constantly being produced due to short lifespan)
short lifespan
(always get destroyed)
grow in between soil particles
increase surface area for water absorption - speeds up absorption
also takes up mineral salts (inorganic ions)
KEYPOINTS:
Water moves into roots:
via osmosis into root hairs
soil has higher water potential than cytoplasm in root hair cells
Water moves across the roots:
Water moves from root hairs :arrow_right: across cells in cortex :arrow_right: into xylem system
Water seeps through the cells in cortex, without going into the cells
Water moves up xylem vessels:
Xylem vessels:
long hollow dead cells
placed end to end - form a long continuous tube
having pits
(small holes)
at the sides facilitate water movement into & out from xylem vessels
No cytoplasm in the cells
Water & mineral ions - free to pass
Effects of Environmental Conditions on Transpiration Rate
To fasten transpiration rate:
High temperature
increase rate of evaporation in leaves
Dry Air
increase rate of evaporation in leaves
lower water potential in the air than in the leaves
Wind Speed
take away humid air around the leaves
lower water potential in the air than in the leaves
Light
stomata opens up during photosynthesis - letting out CO2
water vapour will diffuse out from the leaves via the opened stomata
wihtout light, the diffusion of water vapour out from stomata is significantly reduced
Adaptations to different environment
In hot temperate regions (deserts)
- cactus:
reduced area-to-volume ratio
- less area for transpiration (almost spherical in shape & no broad leaves)
photosynthesis in stem
instead of leaves
water reservoir
in stem
extensive rooting system
In water-logged environment (ponds)
- water lily:
leaves lies on top of water
(to allow transpiration & to get plenty of sunlight)
stomata on upper leaves surface
(instead of lower surface)
- to obtain CO2 from air
air spaces
in roots - absorbing oxygen
Translocation
Introduction
Action of photosynthesized products
(e.g. glucose)
being transported to the other parts of the plant
Using phloem tubes
(next to xylem tubes)
Components involved in translocation:
photosynthesized products: glucose
complex sugar: sucrose (changed from glucose in leaves)
amino acids
What happens to sucrose after after arriving at site:
In roots:
sucrose :arrow_right: glucose :arrow_right: used in respiration
OR
sucrose :arrow_right: starch :arrow_right: stored
In flowers:
sucrose :arrow_right: sweet nectar (to attract pollinators)
In fruits:
sucrose :arrow_right: sweet flesh of fruit
(to attract pollinators)
In growing part of plants:
sucrose :arrow_right: polysaccharide cellulose
(cell walls of new cells)
Source and sinks
Source: site of production of glucose or sucrose
e.g.: leaves, tuber
Sinks: site of product being used
e.g.: fruits, growing shoots
Translocation of chemicals
Contact pesticides
touch pests to kill
easy to be missed - pests hide underside leaves or between crevices
Systemic pesticides
plants absorb the pesticide and distribute the pesticide evenly throughout the plant
pest feeds on the plant will ingest pesticides too