Translocation

The process:

1. Active loading of sucrose increases the solute concentratin, thus decreasing the water potential gradient. Water moves into the sieve tube via osmosis.
2. High hydrostatic pressure forces the sap to move along the tube (mass flow)
3. The gradient of pressure in the sieve tube is reinforced by the unloading of sucrose and consequent loss of water by osmosis at the sink (e.g root cell)
4. Xylem recycles the water from sink to source

Active loading:

1. The companion cell loads sucrose into the phloem by an active process.
2. ATP is used by the companion cells to actively pump H+ ions out of the cell. 
3. This sets up a steep concentration gradient and the H+ ions diffuse back into the companion cells.
4. This diffusion occurs through special co-transporter proteins which enable the H+ ions to bring sucrose molecules into the companion cells.
5. As the concentration of sucrose builds up inside the companion cells, sucrose diffuses into the sieve tube elements through the numerous plasmodesmata.

The effect of active loading on the pH and electrical potential difference:

• Sieve tube element would be alkaline compared to the companion cell which would be acidic. 
• Positive companion cell, negative sieve tube element

The effect of adding a respiratory inhibitor:

• Reduce ATP causing the carrier protein to stop working. Therefore active loading can no longer take place.

Coupled transport:

1. Antiport=travelling opposite ways
2. Symport=travelling the same way

Source: leaf

Sink: roots, seeds, fruits, petals, bud