Sugar Transport Flashcards
Describe anatomy of the phloem and how it differs from xylem
Phloem is made up of the sieve-tube elements, companion cells, and sieve plates. Sieve-tube elements are mainly hollow and lack most organelles and nuclei, which allows material to easily move through them. Companion cells support the sieve tube elements and have a high concentration of mitochondria (extra energy) to aid in the loading of the phloem through active transport. Both of these cells types are alive at maturity to allow for this active transport of water and material through the phloem. Sieve plates are found at the end of sieve tube elements and connect cytoplasm through the plasmodesmata. Different from xylem because cells are alive and do active transport where xylem cells are dead at maturity and use passive transport
Explain why the companion cells and sieve tube elements are alive at maturity
Companion cells and sieve tube elements are alive at maturity because movement through the phloem takes place through active transport. This means the cells must be alive at maturity in order to contribute to this movement of material (companion cells actively load metabolites into the sieve-tube elements, which act as conducting cells
Translocation
the transport of photosynthates through the phloem
Photosynthate
organic molecules made by plants, include sugars and secondary metabolites
Secondary metabolites
organic molecules made after sugars, include lipids, proteins, pigments, signaling molecules, etc..
Explain the pressure flow hypothesis for the movement of sucrose in the phloem
movement of water in the phloem is down a water potential gradient driven by differences in the turgor pressure between the sources and sinks. Phloem sap moves from high to low potential and high potential is created as the companion cells actively pump sucrose in from the source, which creates turgor pressure allowing sucrose to enter phloem thru plasmodesmata. Phloem sap moves to the sinks (like a root cell) causing water potential to increase in phloem, so the water returns to xylem near the sinks
How is the movement of phloem sap different than the water and minerals of the xylem?
The movement of phloem sap occurs under positive pressure from the active loading of the sieve tube elements by the companion cells. Xylem transport is caused by negative pressure as water is pulled up from roots by cohesion tension.
Explain the role of primary and secondary active transport for sucrose loading into a vacuole
Primary active transport creates an electrochemical gradient through the use of a proton pump, then secondary active transport loads the phloem by using a symporter and the kinetic E from protons to load sucrose into companion cells, which will then move through plasmodesmata into the sieve tube elements, and then sucrose is loaded into the vacuole using an antiporter
What is the role of osmosis in phloem loading?
osmosis allows for the passive transport of water into the phloem from the xylem (high to low potential)
Describe the difference between a symporter and an antiporter. Provide an example of each one
a symporter moves molecules in the same direction across the membrane (such as protons pulling sucrose across phloem membrane) where antiporters move two molecules in opposite directions across the membrane (protons pumped out of vacuole and pushing sucrose molecules into vacuole )
Explain why diffusion requires energy.
Facilitated diffusion does not use an input of energy from the cell, but does require kinetic energy that is already present in the system. Movement requires energy
What is a gradient? provide an example.
a gradient is a change in concentration as you move from one region to another and an example of this is the electrochemical gradient created by the flow of protons and electrons in and out of the cell.
