Mass Transport- Phloem Flashcards
What does phloem tissue transport?
Solutes- dissolved substances
What are sieve tube elements?
Living cells that form the tube for transporting solutes
What is the role of companion cells?
Carry out living functions for sieve cells
What is translocation?
The movement of solutes to where they’re needed in a plant
What kind of process is translocation?
Energy-requiring
What way does translocation move solutes?
From sources (where solutes are made) to sinks (wher solutes are used up)
What is the role of enzymes in translocation?
Maintain a concentration gradient from source to sink by changing solutes at the sink
What happens at the source? (step 1 of mass flow hypothesis)
- Active transport is used to actively load solutes from companion cells into sieve tubes of phloem at the source
- Lowers water potential inside sieve tubes, so water enters tubes by osmosis from xylem and companion cells
- Creates high pressure inside sieve tubes at source end
What happens at the sink? (stage 2 of mass flow hypothesis)
- Solutes removed from the phloem to be used up
- Increases water potential inside sieve tubes, so water also leaves tubes by osmosis
- Lowers pressure inside sieve tubes
What is the result of translocation? (stage 3 of mass flow hypothesis)
- Pressure gradient from source to sink
- Gradient pushes solutes along sieve tubes towards sink
- Solutes will be used or stored at sink
How does ringing provide evidence for mass flow?
If a ring of bark is removed from a tree then a bulge forms above the ring that has a higher concentration of sugars than fluid below the ring which shows there’s downwards flow of sugars
How do radioactive tracers provide evidence for mass flow?
Can track movement of organic substances in a plant
How do aphids provide evidence for mass flow?
Pierce the phloem then bodys are removed leaving mouthparts behind, allows sap to flow out and sap flows quicker nearer the leaves so evidence of a pressure gradient
