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level: 9.2 Phloem Transport

Questions and Answers List

level questions: 9.2 Phloem Transport

QuestionAnswer
How do plants transport organic compounds?source to sink
What are the properties of the phloem? (6)transport water and food 2 directional made of sieve element cells which forms a tube sieve cells are connected with a holey sieve plate supported by companion cells for loading/un movement is controlled w hydrostatic P from X
What is active transport used for in phloem?to load organic compounds into pl=hloem sieve tubes at source
How does the phloem cause water uptake?high solute concentrations in source cause water to be take up through osmosis
What does the hydrostatic pressure gradient in the phloem do?causes the material in the phloem to flow towards sinks
In what form is sugar transported? (2)sucrose (disaccharide) soluble and metabolically inert (still)
What makes up the phloem tube? (2)sieve element cells companion cells
How are sieve tubes specialized? (5)long and narrow connected by sieve plates porous for flow no nuclei to maximize space for translocation thick rigid cell wall for hydrostatic pressures
What are companion cells used for?metabolic support to help with loading and unloading materials at source and sink
How are companion cells specialized? (3)infolding plasma membrane = + SA: Vol ratio for more material exchange many mitochondria = active transport of materials from source to sink transport proteins in membrane: move materials in / out of sieve tube
How can phloem and xylem cells be distinguished?xylems have a larger diameter
Why do sieve element cells need companion cells? (3)bec sieve cells have no nuclei and few organelles plamodesmata is a lot bw sieve elements and companion cells = connects the cytoplasm of the 2 cells
How does the arrangement of x + p differ in roots for monos and dis? (5,3)Monocotyledons large stele (middle part) vessels form a radiating circle around it xylems are placed more internally phloem places more externally O = outside Dicotyledons small stele xylem internally placed = forms an X phloem surrounding it = surrounding gaps
How does the arrangement of x + p differ in stems for monos and dis? (2,3)Monocotyledons vascular bundles are scattered through the stem phloem is placed externally o= out Dicotyledons vascular bundles arranged in a circle around the stem's centre (pith) x+p is separated w/ cambium xylem inside, phloem outside
How is ATP used for apoplastic loading of sucrose? (4)1) H+ ions are AT out of P cells by proton pumps (hydrolysis of ATP) 2) conc of H+ ions + out of cell = proton gradient 3) H+ ions passively diffuse back into phloem w/ co-transport protein = sucrose movement 4) loads sucrose into sieve tube
What does the active transport of solutes cause the sap solution to be..hypertonic (more solutes than water)
Why does the sap solution being hypertonic cause water to be drawn up? what does this cause?(4)water is drawn up by the xylem bec the water moves to the higher solute concentrations bec water is incompressible = hydrostatic p in phloem increases forces phloem sap to move to areas with lower pressure (mass flow) = solutes are transported away from source to the sink
What happens once the solutes are unloaded in sinks? (3)sap solution = hypotonic water is drawn out of phloem = xylem by osmosis = hydrostatic p at sink is always lower than hydrostatic p at source
What happens to the solutes when they are transported to the sink? (2)metabolised stores within the tonoplast of vacuoles
What are aphids?insects that feed on the sap in phloem
How are aphids specialized to drink phloem? (3)stylet which pierces plant sieve tube = sap extraction aided w/ digestive enzymes to soften tissue layers if stylet is severed = saps continue to flow bec of hydrostatic P
How can aphids be used to measure translocation rate? (6)1) used to collect sap at various sites of the plant's length 2) plant grown w' leaves in radioactively labelled CO2 3) CO2 becomes radioactively labelled sugars 4) once stylet is severed the sap continues to flow 5) sap is analysed for radioactively labelled sugars translocation rate is calculated based on time taken for the radioisotope to be detected in diff positions of the plant's length
How is the rate of phloem transport determined?by the conc of dissolved sugars in the phloem
What are the factors affecting translocation rate? (4)photosynthesis rate (light intensity, CO2 conc, temperature) respiration rate (affected by factors that stress the plant) transpiration rate (determines how much water enters phloem) sieve tube's diameter (affects hydrostatic P + differs bw plant species)