Phloem Flashcards
What is the function of the phloem?
transports organic solutes (dissolved substances) like sugars e.g. sucrose to all cells in a plant → all cells need sugar to respire
What is the structure of the phloem?
- sieve tube elements are living cells that form the tube (have no nucleus and few organelles)
- there is a companion cell for each sieve tube element for each sieve tube element and they carry out living functions for sieve cells such as providing ATP for active transport of solutes
What is translocation?
the movement of solutes/organic substances to where they are needed in a plant → transports substances from the source (Where they are produced) to t he sinks (where they are needed)
What is needed for translocation?
- energy
-water provides medium in which these substances dissolve for transport in the phloem
What maintains the concentration gradient in translocation?
enzymes maintain conc gradient from source to sink by changing the solutes at the sink (by breaking them down or making them into something else) → makes sure there is always a lower conc at the sink than the source
What are the three main stages of the mass flow hypothesis?
1.) how sucrose transports from the source to the sieve tube element
2.) mass flow of sucrose through sieve tube elements
3.) transport of sucrose to the sink(respiring cells)
How is sucrose transported from the source to the sieve tube element?
- photosynthesis occurring in the chloroplasts creates organic substance which creates high conc of sucrose at the source so sucrose diffuses down its conc gradient into the companion cell via facilitated diffusion
- hydrogen ion are actively transported out of the companion cell through the protein pump into the spaces withing the cell walls using ATP
- this creates a conc gradient so the hydrogen ions move down their conc gradient via carrier protein into the sieve tube elements
- sucrose is transported with the hydrogen ion (co-transport)
How does mass flow of sucrose occur through the sieve tube element?
- the increase of sucrose in the sieve tube element lowers the water potential (of STE)
- water enters the STE from surrounding xylem vessels and companion cells via osmosis → the increase in water volume in the sieve tube element increases the hydrostatic pressure causing liquid to be forced to the sink
How is the sucrose transported to the sink?
- sucrose is used or stored as insoluble starch at the sink
- more sucrose is actively transported into the sink cell which causes water potential to decrease
- results in osmosis of water from the sieve tube elements so the hydrostatic pressure decreases in the sieve tube element into the sink cells →decreases volume in the STE so hydrostatic pressure decreases in the STE
- results in hydrostatic pressure gradient from the source to the sink end so the gradient pushes solutes along the sieve tubes to the sink
- the higher the conc of sucrose as the source the higher rate of translocation
What is supporting evidence for the mass flow hypothesis?
- Ringing experiment → ring of bark removed (including phloem not xylem) which results in trunk swelling above the removed section - analysis shows liquid contains sugar so when phloem is removed sugar can’t be transported
- Radioactive traces →isotopes of carbon in from of CO2 is supplied to leaf which is used in photosynthesis to create sugars that can be traced
- Metabolic inhibitors →translocation stops when inhibitors that prevent ATP synthesis are added into phloem = shows ATP needed for active transport
- Aphids →sap fed on by aphids flows out of the stem quicker at the top shows pressure gradient
What is the evidence against the mass flow hypothesis?
- sugars travel to different sink not just ones with highest water potential
- pressure gradient would not be high enough for sufficient rate of flow through sieve plates
