phloem transport

Question 1

phloem function

Answer 1

Function: transport organic molecules throughout the plant from a source to a sink

Question 2

phloem sink

Answer 2

plant organ that uses or stores sugar e.g. roots, buds, stems, seeds, fruits

Question 3

phloem source

Answer 3

plant organ that is the net producer of sugar, either by photosynthesis or hydrolysis of starch e.g. leaves

Question 4

phloem structure

Answer 4

made up of living cells (sieve plate with pores)
a companion cell with a nucleus and dense cytoplasm connected to the sieve tube by plasmodesmata
and a sieve tube that lacks a nucleus or cytoplasm

Question 5

How can aphids be used to calculate phloem transport rates?

Answer 5

Aphids are insects that use their stylet to pierce the phloem and eat the sap.
Plant grown in lab and is exposed to radioactive CO2
Incorporated in photosynthesis which produces radioactive sucrose
Apid put on a plant
Their sylhet is cut off so phloem fluid flows out
The time taken for the sucrose to come out of the cut through leafe measure the rate of phloem sap movement

Question 6

pressure-flow hypothesis (TRANSLOCATION)

Answer 6

Loading of sugar into the sieve tube at the source reduces the relative water concentration in the sieve tube members, causing osmosis from surrounding cells.
This is achieved by active transport, energy provided by the companion cells

The uptake of water at the source causes a positive hydrostatic pressure in the sieve tube, resulting the the bulk flow of phloem sap

Hydrostatic pressure is reduced by sugar removal at the sieve tube at the sink. The sugars are changed at the sink to starch, which is insoluble and has no osmotic effect

Xylem recycles the pure water by carrying it from the sink back to the source.