- in flowering plants, tissue that supports biological molecules is the phloem.
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PHLOEM VESSELS
- made up of sieve tube elements, long thin structures.
- their end walls= perforated to form sieve plates
- associated with sieve tube elements are cells called companion cells.
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WHAT IS TRANSLOCATION?
- process where organic molecules and some mineral ions are transported from one part of the plant to another
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SOURCES AND SINKS
- plants transport the sugars produced at photosynthesis from the sites of production = SOURCES to where they'll be stored for future use = SINKS
- sinks can be anywhere in the plant, above and below the source, follows that translocation of
molecules =
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WHICH ORGANIC MOLECULES CAN BE TRANSPORTED?
- sucrose and amino acids.
- phloem also transports inorganic ions such as potassium, chloride, phosphate and magnesium ions
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MECHANISMS OF TRANSLOCATION
- rate of movement is too fast for
diffusion; need mass flow
theory:
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(1) TRANSFER OF SUCROSE INTO SIEVE ELEMENTS FROM PHOTOSYNTHESISING TISSUE
- sucrose diffuses down a conc. grad. by facilitated diffusion from photosynthesising cells into companion cells
- H+ ions = actively transported from companion cells into spaces within cell walls using ATP
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(1.2)
- these H+ ions diffuse down conc. grad. through carrier proteins into sieve tube elements
- sucrose molecules are transported along with the H+ ions in CO-TRANSPORT
- protein carriers are therefore known as CO-TRANSPORT PROTEINS
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(2) MASS FLOW OF SUCROSE THROUGH SIEVE TUBE ELEMENTS
- sucrose is produced by photosynthesising cells (source) and is actively transported into sieve tubes
- this causes the sieve tubes to have lower water potential
(more negative)
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(2.2)
- as xylem has much higher (less -) w.p, water moves from xylem into sieve tubes by osmosis creating high hydrostatic pressure
- at respiring cells (sink), sucrose is either used up during respiration or converted to starch for storage
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(2.3)
- these cells= low sucrose content and so sucrose is actively t into them from sieve tubes lowering their w.p
- due to low w.p water moves into respiring cells from sieve tubes by osmosis
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(2.4)
- as result of water entering sieve tube elements at the source and leaving at sink there's a high hydrostatic pressure at the source and a low one at the sink
- therefore there's a mass flow of sucrose solution down this hydrostatic grad. into sieve tube
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(3) TRANSFER SUCROSE FROM THE SIEVE TUBE ELEMENTS INTO STORAGE OR OTHER SINK CELLS
- sucrose is actively t by companion cells out of the sieve tubes and into the sink cells
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Other cards in this set
Card 2
Front
PHLOEM VESSELS
Back
- made up of sieve tube elements, long thin structures.
- their end walls= perforated to form sieve plates
- associated with sieve tube elements are cells called companion cells.
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