the transport of substances in plants
DESCRIPTION
biology form 5TRANSCRIPT
TRANSPORTTRANSPORTTransport of Substances in PlantsTransport of Substances in Plants
Necessity• Transport substances to all parts of plant
Water – for photosynthesis and cellular metabolism Mineral ions – for healthy growth and development Organic food materials – for cellular metabolism or
storage
• Problem Small TSA/V ratio Long distances
• Overcome Internal system of tubes = VASCULAR SYSTEM
The Vascular System
• The vascular system involved in transporting substances in plants– except movement of gases (diffusion)
• Consist of 2 types of tissues: xylem and phloem– which function as two separate
transport system.
Monocot plant
leaf:
stem: root:
Dicot plant
Xylem
• Transports water and dissolved mineral ions from the roots to the upper parts of the plant– Against gravitational pull
• Gives mechanical support to woody plants
Xylem adaptation • Consists of 4 types of cells:
Xylem vessels Long, hollow, continuous tube Dead cell, no protoplasm Thickened by lignin (patterns: pits, spiral, ring) Numerous pits (holes), end walls open
Tracheids Dead cells, hollow Longer and smaller than xylem vessels Pointed end and pitted, end walls breaks
down in pits Parenchyma – food storage Fibres - support
Phloem
• Transports organic substances (e.g. sucrose, amino acids) synthesised mainly in the leaves to other parts of plant– Used in cellular metabolism
• Growth (shoot, root, buds, fruits)• Respiration
– Stored in in storage organ • e.g. tuber
Phloem adaptation • Consists of
Sieve tubes• Living cells (nucleus disintegrates during
maturation, thin layer of cytoplasm, small mitochondria)
• Cylindrical, elongated, end-to-end• End walls perforated to form sieve plates
Companion cells (transport food into tubes)
• Shorter than sieve tubes• Living cells (with nucleus, many mitochondria)• Adjacent to and support the function of sieve tube
Parenchyma – food storage Fibres - support
Transport of substances:Food (Translocation)
• The movement of sucrose and other organic materials within the plant body
• Primarily through phloem (sieve tube)• Drives by concentration (pressure)
gradients• Importance:
– Plant’s survival– Store and convert sucrose to other sugars
Bark Ringing
• Removal of a ring of tissue external to the xylem from around the trunk of a woody plant.– Removal of phloem
Observation Inference
Tissue above the ring swells up Tissue below the ring withers
•Tissue removed is phloem•Disrupt flow of food to root•Accumulation causes
swelling at the upper part• Insufficient food cause the
lower part to wither
The upper part of plants lives normally
•Xylem is not removed •Flow of water is not
disrupted•The upper part do not wilt
Transport of substances:Water and minerals
• Water by osmosis • Minerals by active transport• Aided by three factors:
– Root pressure – Capillary action
• Cohesion and adhesion
– Transpiration pull
1) Root Pressure
• The process that forces the water absorbed from the soil to move through the roots and up the stem of a plant
• Caused by osmosis of water and active transport of mineral ions into the root xylem
The mechanism: • Cell sap of root hair cells is hypertonic to soil
water• Water enter by osmosis• Cell sap is diluted, hypotonic to adjacent cells• Water moves to the next cells until crosses
cortex and endodermis• Creates root pressure to push water into xylem• Helped by the active transport of minerals into
xylem that increase osmotic pressure
Guttation
• In small plants, water may be pushed out of special pores at leaf (hydathodes)
• Usually seen early morning
2) Capillary action
• Chemical forces that move water as a continuous column rather than as individual molecules.
• Combination of 2 forces due to hydrogen bonding: cohesion & adhesion
• The forces can be overcome by gravity.
Cohesion and Adhesion• Cohesion:
attraction between water molecules, prevents break of water column
• Adhesion: attraction between water molecules and xylem walls, prevents slipping of molecules
3) Transpiration pull
• When water evaporates, it creates an empty space which is filled continuously.
• As the water moves, more water is absorbed to replace the evaporated water (transpiration pull)
Con
cep
tualise!
Transpiration • The process of water loss (as water
vapour) from plants to the atmosphere due to evaporation (99% of water intake)– Through the stomata in leaves (90%)– Through cuticle (5%) or lenticels (5%)
• Importance – Creates transpiration pull – Provide cooling effect and turgor pressure– Maintain osmotic pressure
The process:
• Water absorbed by roots to mesophyl cells
• Heat from Sun cause water to evaporate to air spaces
• Water vapour diffuse through stomata (traspiration)
Regulation by stomata:Open
• During day– Photosynthesis
produce glucose– K+ pumped into
guard cells – Accumulation of
glucose and K+
increase osmotic pressure
– Water enters by osmosis
– Guard cells become turgid, curved outwards, stoma opens
– Transpiration occurs
Regulation by stomata:Close
• At night/dark– No
photosynthesis, no glucose produced
– K+ moves out to epidermal cells
– Osmotic pressure decrease
– Water diffuses out by osmosis
– Guard cells become flaccid, stoma closes
– Transpiration stops
Factors affecting transpiration rate:Air Movement
• More air movement, easier evaporation Tra
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Air movement
Factors affecting transpiration rate: Temperature
• Higher temperature, higher rate of transpiration Tra
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Temperature
Factors affecting transpiration rate: Light intensity
• Higher intensity, higher rate of transpiration
Lig
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Time
Tra
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Light intensity
Transpiration rate
Factors affecting transpiration rate: Relative humidity
• Higher surrounding humidity, lower rate of transpirationTra
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Relative humidity