Fill in the blanks
Organic molecules are transported by the phloem through the process of ________.
Active translocation
Which cells are alive but lack organelles?
Sieve tube cells
Hydrostatic pressure gradient
The stress that develops when solutions containing different concentrations of solute in a common solvent are separated by a membrane that is permeable to the solvent, but not the solute.
How is the structure of phloem sieve tubes related to its function?
- The ends of the sieve element cells are connected with other sieve elements to form a sieve tube.
- Sieve plates are found at the interface between two sieve cell element cells; they contain large pores in the cell wall speeding the transport of subtances between sieve element cells.
Xylem is ___ directional.
Unidirectional
What is the name for the structures that allow the flow of material between phloem cells?
Sieve Plate
Sieve tubes
An element of phloem tissue consisting of a longitudinal row of thin-walled elongated cells with perforations in their connecting walls through which food materials can pass.
How does sucrose travel through active transport into the phloem?
- H+ ions are actively transported using ATP out of the phloem cell. High H+ ion concentration gradient builds up outside the cell.
- H+ ions flow back into the cell, and the energy released is used to co-transport sucrose into the phloem cell.
- Consequently, the concentration of sucrose is relatively high in the phloem and the water concentration is relatively low.
- Water moves down the concentration gradient in the xylem by osmosis into the phloem cells.
Companion cells move materials by actively pumping compounds from within the _______ space of companion cells.
Cell wall
What do aphids possess that is inserted into the plant sieve element to allow the sap to be extracted?
Stylet
Difference between apoplast and symplast.
- A significant amount of sugar travels through cell walls from mesophyll cells to the cell walls of companion cells, and sometimes sieve cells, where a sucrose transport protein then actively transports the sugar in. This is referred to as the apoplast route.
- In other species, much of the sucrose travels between cells through connections between cells called plasmodesmata. This is referred to as the symplast route.
How does phloem transport water and solutes along a hydrostatic pressure gradient?
- Water is incompressible, it occupies a fixed volume. The walls of the sieve tube are rigid. These two factors cause a build-up of hydrostatic pressure at the source.
- Water and the solutes (sucrose and amino acids) flow down the hydrostatic gradient to the sink where pressure is relatively low.
- This is due to the active unloading of sucrose and hence loss of water by osmosis at the sink.