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 ➡ seeps across root towards centre ➡ enter xylem vessels ➡ transported to stem & leaves (other parts of plants)
Transpiration
Actions of water being evaporated out from plants cells via stomata.
Water diffusion: osmosis
Water ➡ root hairs ➡ centre (xylem vessels) ➡ stem (transported through vascular bundles) ➡ leaves ➡ water evaporated from cells (in air spaces) ➡ water vapour diffuses into the air through the stomata ➡ 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 ➡ across cells in cortex ➡ 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 ➡ glucose ➡ used in respiration
OR - sucrose ➡ starch ➡ stored
In flowers:
- sucrose ➡ sweet nectar (to attract pollinators)
In fruits:
- sucrose ➡ sweet flesh of fruit (to attract pollinators)
In growing part of plants:
- sucrose ➡ 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