Phloem

• Phloem is a bidirectional vessel

  
• Contains sieve elements which allow sugars to pass from one cell to next downwards

  
• Contains companion cells which provide energy for active transport of sugars all over plant.

  
• Translocation moves organic molecules (sugars, amino acids) from source to sink.

  
• Phloem vessels still have cross walls called sieve plates that contain pores.

  
• Companion cells actively load sucrose into the phloem.

  
• Water follows high solute in phloem by osmosis. A positive pressure potential develops moving mass of phloem sap forward.

  
• Phloem still contains small amount of cytoplasm along the walls but the organelle content is greatly reduced.

  
• Companion cells actively unload (ATP used) the organic molecules