Phloem Flashcards
What do phloem vessels transport?
They transport sugars (sucrose) and amino acids made in leaves to all other parts of the plant
How are the phloem and xylem arranged ?
In vascular bundles
What is the structure of the Phloem?
Phloem is made up of living cells called sieve tube elements, which have perforated cell walls, called sieve plates, between them and companion cells beside them.
Is phloem made up of the living or dead cells?
Living cells
What are the companion cells of the phloem?
Companion cells regulate the movement of solutes and provide ATP for active transport. Strands of cytoplasm called plasmodesmata connect the sieve tube element and companion cell.
What is translocation of the phloem?
Plants transport sucrose and other substances from sources (e.g., leaf cells) to sinks (e.g., roots, meristem).
How does the loading of sucrose occur?
- At a source - usually a photosynthesising leaf - sucrose is loaded into the phloem.
- The companion cells use proton pumps to pump out hydrogen ions, using ATP, creating a higher concentration of hydrogen ions outside the companion cell.
- A co-transporter protein in the companion cell membrane then transports these hydrogen ions back into the cell in conjunction with sucrose molecules.
- Sucrose then diffuses from the companion cell into the sieve tube elements via plasmodesmata.
- Water moves via osmosis from the companion cell to the sieve tube elements, due to the water potential gradient set up by the movement of sucrose into the sieve tube elements previously.
How does the unloading of sucrose occur?
At a sink - for example, a root - the sucrose moves out of the sieve tube elements by diffusion.
* The concentration gradient is maintained by converting sucrose into glucose and fructose.
* This loading and unloading causes mass flow through the phloem via hydrostatic pressure and a pressure gradient.
