Resource Acquisition and Transport in Vascular Plants
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| Adaptations for ___ were key steps in the evolution of vascular plants. | acquiring resources
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| ___ drives the transport of water and minerals from roots to shoots via the xylem. | Transpiration
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| Sugars are transported from ___ to ___ via the phloem. | - sources
- sinks
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| How do water and minerals get into the root cells in the first place? | - the epidermal cells of root tips are permeable to water, and many are differentiated into root hairs
- root hairs absorb soil solution
- soil solution is drawn into the hydrophilic walls of the epidermal cells
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| Once in, how do water and minerals get into the xylem (which is in the center of the root)? | via the apoplastic or symplastic route
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| Explain the apoplastic route. | - soil solution makes its way thru the apoplast of a root hair cell and epidermal and cortical cells
- passes thru casparian strip of endodermis (filtered at this point)
- enters vessels in vascular cylinder and is transported up the plant
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| Explain the symplastic route. | - soil solution passes thru plasma membrane of root hair (filtered at this point)
- travels thru symplast of epidermal and cortical cells
- passes thru casparian strip of endodermis
- enters vessels in vascular cylinder and is transported up the plant
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| What is the function of the endodermis? | - it is the last checkpoint for the selective passage of minerals from the cortex into the vascular cylinder
- just like plasma membrane, it functions as a filter, however, both routes MUST pass through it
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| Casparian strip | a water impermeable ring of wax in the endodermal cells of plants that blocks the passive flow of water and solutes into the stele by way of cell walls
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| stele | the central core of the stem and root of a vascular plant, consisting of the vascular tissue (xylem and phloem) and associated supporting tissue
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| xylem sap | the water and dissolved minerals in the xylem
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| transpiration | the evaporative loss of water from a plant through stomata
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| The xylem sap is mainly ___ upward. | pulled
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| What is root pressure and how does root pressure develop? | - pressure exerted in the roots of plants as the result of osmosis
- root cells actively pump minerals into xylem
- casparian strip prevents them from leaving
- this makes the rel. conc. of water inside the plant lower than the outside, so it comes in
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| guttation | - root pressure sometimes causes more water to enter the leaves than is transpired -> guttation
- the exudation of water on the tips or edges of some plant leaves
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| Why is root pressure alone not sufficient for getting xylem sap to the top of most plants? | - the positive pressures produced are too weak to overcome the gravitational force of the water column in the xylem (particularly in tall plants)
- some plants don't even generate root pressure
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| cohesion-tension hypothesis | - the leading explanation of the ascent of xylem sap
- states that transpiration exerts a pull on xylem sap, putting it under negative pressure
- the cohesion of H2O molecules transmit this pull along the entire length of the xylem (shoots to roots)
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| cohesion | - the linking together of like molecules, often by hydrogen bonds
- or, the attractive force between molecules of the same substance (ex. water)
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| Explain why H2O molecules stick together. | - polar covalent bond between O and H2 creates a partially neg. charge around the O and a partially pos. charge at the H (b/c O is more electroneg.)
- partially neg. O's are attracted to partially pos. H's of other molecules of water -> hydrogen bonds
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| transpiration | - evaporation of water from stomata in plant leaves
- a pulling process
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| adhesion | - the attractive force between water molecules and other polar substances (ex. cellulose molecules in the xylem cell walls)
- offsets the downward force of gravity
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| Why does water evaporate from leaves? | - the concentration of water is highest in the air spaces of leaves and lowest in the dry air outside
- so, when stomata are open, water automatically diffuses = transpiration
- high concentration -> low concentration
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| How does loss of water vapor from the leaf translate into a pulling force for upward movement of water through a plant? | - cohesion of water makes it possible to pull up a column of xylem sap w/o the molecules separating
- H2O exiting the xylem in the leaf tug on adjacent H2O, and this pull is relayed, molecule by molecule, down the entire column of water in the xylem
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| translocation | the transport of organic nutrients (mostly sucrose) in the phloem of vascular plants
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| phloem sap | - the sugar-rich solution carried through a plant's sieve tubes (can also contain amino acids, hormones, and minerals)
- like maple syrup
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| What is the direction of translocation compared to the direction of water/mineral transport? | - moves from sites of sugar production to sites of sugar use or storage
- SOURCE TO SINK
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| How does sucrose get from photosynthetic cells in the leaves to the phloem? (A mesophyll cell is a type of parenchyma cell.) | - sucrose must be transported into sieve tube elements before being exported to sugar sinks
- apoplastic and symplastic routes
- active transport can be used
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| How does sucrose get from a sugar source to a sugar sink? What is bulk flow by the creation of positive pressure? | - the phloem sap moves through sieve tubes by bulk flow driven by positive pressure
- the building of pressure at the source and reduction of that pressure at the sink cause sap to flow from source to sink
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| What are the four basic steps of the bulk flow of phloem? | - loading of sugar (active transport)
- uptake of water (osmosis, xylem -> phloem)
- unloading of sugar (pressure relief)
- recycling of water (osmosis, phloem -> xylem)
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