Transport in the phloem of plants Flashcards
What is the function of the phloem?
- Phloem is a vascular tissue to conduct sugar molecules from a source to a sink (where they are needed or stored), this requires active transport
What are the 7 structures of phloem?
- Sieve elements: elongated living cells that have limited organelles and therefore need ‘life support’ from companion cells. Sieve elements compose sieve tubes
- Sieve tube: reduce cytoplasm and breakdown of the nucleus
- Sieve plates: pores through the horizontal cells that join sieve elements and allow sap to flow freely
- Companion cells: metabolic support cells that provide bio-molecules to maintain life functions in the sieve elements
- Plasmodesmata: passageway between sieve elements and companion cells for communication
- Cell membrane: present in sieve elements that contain specialized protein pumps to control the composition of sap
- Phloem sap: water and dissolved sucrose, carbohydrates, amino acids, minerals and hormones found in sieve elements
Check book
From where to where is the sap moved to distribute sugars and other organic compounds?
- The sap is loaded with organic compounds at the source and then travels to a sink
- Source: a tissue and storage organs with a high concentration of dissolved sugars that export these to other parts of the plant
- Sink: organs that requires sugars for storage or in respiration
What is translocation? Give examples of sources and sinks.
- The movement of sugar produced in photosynthesis to all other parts of the plant for respiration and other processes
Sources: - Mature leaves, stems (where photosynthesis takes place) e.g. germinating seeds
Sinks: - roots, developing flowers, new leaves (respiring tissues) e.g. growing tubers and bulbs, seeds
Some organs may act as sinks and sources at different stages
Explain the first steps of translocation.
- Sucrose is produced by a source and then loaded into phloem tubes using ATP
- Active transport or active loading of organic compounds increases the solute concentration in the sieve tubes at the source
- Water moves into the sieve tubes by osmosis (concentration gradient)
What happens due to the incompressibility of water? Part 2 of translocation
- Water is incompressible, which means pressure does not affect water. When water enters sieve elements and pushes on the phloem cell walls, it causes increased internal pressure this is hydrostatic pressure
- The sap passes through the pores of the sieve plates towards the sink, this movement is called mass flow (water and solutes move together)
How is the sugar unloaded at the sink? Part 3 of translocation
- Companion cells unload the sugars found inside the sieve tube by active or passive transport and the solute concentration decreases which allows water to return to xylem vessels
- The entry of water creates the high hydrostatic pressure so that sap flows towards the sink
- There is a lower hydrostatic pressure near the sink
- Phloem sap flows from source to sink down hydrostatic pressure gradient called pressure-flow mechanism (from high hydrostatic pressure to low hydrostatic pressure)
Sucrose enters companion cells by active transport
How is the rate of translocation measured and what are aphids?
- Small sap-sucking insects which are used to study translocation and can pierce phloem tubes without triggering defensive responses in the plant
- If the phloem tube /stylet) is separated from the aphid, the phloem sap will continue to flow which kills the aphid hence unethical
- The sap can be analyzed and the direction and rate can be determined
- They can be tested for radioactivity
- The carbon dioxide is radioactive
How do you calculate the rate of translocation?
Rate = distance traveled (cm) / time (h)
The distance the sap traveled between two stylets
Time it took to reach from the first stylet to the second one and its distance
Be able to identify the xylem and phloem in a microscope.
Check in book
