9.2 Transport in the Phloem of plants

Question 1

Pressure flow hypothesis

Answer 1

1. Glucose is created and converted into sucrose (sugar) in source
2. Sucrose moves from source to companion cell and into plhoem via active transport using ATP
3. High pressure in the phloem is created, water in the xylem diffuses into the phloem via osmosis
4. Water and sugar in phloem travel from a high pressure to low pressure gradient near the sink through the sieve tube elements.
5. Sugar is pumped from the phloem into the sink through cotransport using H+ and ATP
6. Water diffuses back into the xylem via osmosis

Question 2

Source and Sink

Answer 2

The source is where the organic compounds are synthesised

The sink is where the compounds are delivered to for use or storage

Question 3

Phloem sap

Answer 3

Organic molecules dissolved in water

Question 4

Sieve Element Cells

Answer 4

• long and narrow cells that are connected together to form the sieve tube
• connected by sieve plates at their transverse ends
• no nuclei
• thick and rigid cell walls to withstand the hydrostatic pressures

Question 5

Companion Cells

Answer 5

Provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at source and sink
- infolding plasma membrane which increases SA:Vol ratio to allow for more material exchange
- high in mitochondria to fuel the active transport

Question 6

Identification of xylem and phloem in microscope images of stem

Answer 6

Xylem inside

Question 7

Identification of xylem and phloem in microscope images of the root

Answer 7

Xylem inside

Question 8

Sink examples

Answer 8

Roots, bulbs, seeds, stems, fruits

Question 9

Source examples

Answer 9

Leaves, tubers, bulb

Question 10

phloem loading

Answer 10

Sucrose is moved into the phloem from souce by cotransport using ATP

Question 11

Hydrostatic pressure gradient

Answer 11

pressure difference between source and sink

Question 12

2 main cell types of phloem

Answer 12

(Both living)
- Companion cell
- Sieve tube members

Question 13

Translocation Rate

Answer 13

Patometer:
- Measure transpiration

Aphids can be used to collect sap at various sites along a plant’s length and thus provide a measure of phloem transport rates

Question 14

Use of Aphids to measure Translocation Rate

Answer 14

• A plant is grown within a lab with the leaves sealed within a glass chamber containing radioactively-labelled carbon dioxide
• The leaves will convert the CO2 into radioactively-labelled sugars (via photosynthesis), which are transported by the phloem
• Aphids are positioned along the plant’s length and encouraged to feed on the phloem sap with its stylet
• the aphid stylet is severed and sap continues to flow from the plant at the selected positions
• the aphid stylet is severed and sap continues to flow from the plant at the selected positions
• calculated based on the time taken for the radioisotope to be detected at different positions along the plant’s length