Phloem

• What is the role of the phloem vessel?

  It transports products of photosynthesis- sucrose
• What is the structure of phloem sieve tubes like?

  They are alive and have very few organelles BUT they require water and substances which they don't have access to

  • have holes in end walls
• What are companion cells?

  Specialised cells that contain all the organelles necessary to keep the sieve tube cells alive ie nucleus, mitochondria

  • they perform respiration and metabolic reactions for the sieve tube cells
• Why are carbohydrates transported by facilitated diffusion?

  They are too large and are polar (contain oxygen)
• How are dissolved sugars transported in the phloem?

  1. Sucrose moves by active transport from the source to a transfer cell in the phloem
  2. Water moves by osmosis from the xylem to the phloem
  3. Sucrose moves by active transport back into another transfer cell and then into the sink.
  4. Water moves back into the xylem by osmosis