Translocation

Where does translocation occur between ?

The sieve tube and the companion cell

Describe the sieve tube

Made from sieve tube elements joined end to end and at the end of the sieve cell = sieve plate with perforated cross walls to block callose and allow movement of assimilates from one tube to another. Also sieve tubes have little cytoplasm and no nucleus to give space for movement of sap by mass flow.

Describe active loading

1. Hydrogen ions are actively pumped out of companion cell into tissue, lowering the concentration in the cytoplasm

2. The ions move back in but via facilitated diffusion as they bring sucrose with them by co transport

3. The addition of sucrose lowers the water potential of the companion cell into tissue

4. This leads to water moving in via osmosis and forming cell sap

5. The sucrose then diffuses out of the companion cell into the sieve tube down a concentration gradient through the plasmodesmata

6. Water follows, moving into the sieve tube by osmosis

7. The addition of water to the tube increases the hydrostatic pressure so the tube contents move to an area of low pressure (sink) via mass flow

8. At the sink, sucrose moves out by diffusion into tissues by diffusion or active transport

What are plasmodesmata

Gaps where substances can pass between sieve tubes and companion cells

What is mass flow ?

Movement via pressure, it is not membrane controlled

Why can a plant leaf be described as an organ ?

Contain multiple tissues which as xylem phloem cambium spongy mesophyll etc

They are adapted to carry out their role of photosynthesis