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3.1.3 Plant Transport (Water Movement (role of endodermis (movement of…
3.1.3 Plant Transport
Water Movement
cohesion
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water molecules held together in long chain; pull from above creates tension so more water is pulled up
water potential
too much water loss: plasma membrane no longer attached to cell wall: plasmolysis; cytoplasm and vacuole shrink
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adhesion
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attraction between water molecules and walls of xylem vessel, so water is pulled upwards
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role of endodermis
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layer of cells surrounding medulla and xylem; also known as starch sheath; contains granules of starch
Casparian strip blocks apoplast pathway between cortex and medulla; water and ions have to pass through symplast pathway
plasma membrane contains proteins to pump mineral ions from cytoplasm of cortex cells into medulla and xylem; water potential of xylem decreases so water moves into xylem from cortex cells via osmosis
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Translocation
energy requiring process transporting assimilates, especially sucrose, in the phloem dissolved in water as sap between sources (e.g. leaves) and sinks (e.g. roots, meristem)
Active loading at source
Source: a part of the plant that loads materials into the transport system e.g. leaves photosynthesise and produce sugars and sugars are moved to other parts of the plant
Sucrose is loaded into sieve tube via active transport; ATP in companion cells used to actively transport H+ ions out of companion cells; increase concentration outside; decrease concentration inside cells (concentration gradient)
H+ ions diffuse (facilitated diffusion) back into companion cells via carrier proteins, and take sucrose molecules with them: cotransport / secondary active transport
sucrose concentration in companion cells increases and diffuses through plasmodesmata into sieve tube
Removal at sink
Sink: a part of the plant where the materials are removed from the transport system e.g. roots receive sugar and store them as starch
mass flow; caused by difference in hydrostatic pressure between two ends of tube; pressure gradient; high pressure at source, low pressure at sink
Transpiration
loss of water vapour from leaves of plant; consequence of gas exchange; maintains cell turgidity; supplies water and mineral ions for growth; water evaporating keeps plant cool
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Water Pathways
Symplast pathway: water passes into cytoplasm through plasma membrane, and goes through plasmodesmata from one cell to next
Apoplast pathway: water pass though spaces in cell walls and between cells; NOT through plasma membrane in cell; allow for mass movement rather than osmosis
Vacuolar pathway: water passes into cytoplasm and vacuoles through plasma membrane, and goes through plasmodesmata from one cell to next
Adaptations
Xerophytes
e.g. Cacti
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roots are widespread and run deep; water can be obtained from a large area, and from deep underground
e.g. Marram Grass
leaf is rolled longitudinally so air is trapped inside; created humid microenvironment to less transpiration happens as concentration gradient is not steep
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stomata: on inside of leaves; in pits and covered by hair; if water leaves stomata, hairs trap is and created humid environment reducing transpiration
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Hydrophytes
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a plant adapted to living in water, or when ground is very wet
hydathodes: structures at tip of leaves to release water droplets to allow transpiration stream to carry on
Terrestrial plants
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stomata: located on underside of leaves; don't have direct sunlight so less evaporation; closed at night when no light for photosynthesis
deciduous plants: lose leaves in winter when ground is frozen due to lack of water and temperatures too low for photosynthesis
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