Phloem

Specification Point 2.53
  • Describe the role of phloem in transporting sucrose and amino acids between the leaves and other parts of the plant
  • The soluble products of photosynthesis are sugars (mainly sucrose) and amino acids
  • These are transported around the plant in the phloem tubes which are made of living cells (as opposed to xylem vessels which are made of dead cells)
  • The cells are joined end to end and contain holes in the end cell walls (called sieve plates) which allow easy flow of substances from one cell to the next
  • The transport of sucrose and amino acids in phloem is called translocation
  • Transport in the phloem goes in many different directions depending on the stage of development of the plant or the time of year; however dissolved food is always transported from source (where it’s made) to sink (where it’s stored or used)
  • During winter, when many plants have no leaves, the phloem tubes may transport dissolved sucrose and amino acids from the storage organs to other parts of the plant so that respiration can continue
  • During a growth period (eg during the spring), the storage organs (eg roots) would be the source and the many growing areas of the plant would be the sinks
  • After the plant has grown (usually during the summer), the leaves are photosynthesizing and producing large quantities of sugars; so they become the source and the roots become the sinks – storing sucrose as starch until it is needed again

Translocation through the phloem, IGCSE & GCSE Biology revision notes

Translocation through the phloem

Xylem

Specification Point 2.54
  • Describe the role of xylem in transporting water and mineral ions from the roots to other parts of the plant
  • Water travels up xylem from the roots into the leaves of the plant to replace the water that has been lost due to transpiration
  • Xylem is adapted in many ways:
    • A substance called lignin is deposited in the cell walls which causes the xylem cells to die 
    • These cells then become hollow (as they lose all their organelles and cytoplasm) and join end-to-end to form a continuous tube for water and mineral ions to travel through from the roots
    • Lignin strengthens the plant to help it withstand the pressure of the water movement
  • Movement in xylem only takes place in one direction – from roots to leaves (unlike phloem where movement takes place in different directions)
  • Water molecules are attracted to each other by hydrogen bonding – creating a continuous column of water up the plant
  • The water evaporates from the leaves of the plant, creating the transpiration stream

Water uptake, transport and transpiration, IGCSE & GCSE Biology revision notes

Water uptake, transport and transpiration

Comparision between xylem and phloem tissue:

xylem and phloem, Edexcel IGCSE Biology

Edexcel IGCSE Biology Notes

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Author: Caroline

Based in the stunning Cayman Islands, our resident environment expert Caroline is Head of Science at a local secondary school. When she’s not teaching or writing our Biology resources, she’s volunteering for the Department of Environment, helping monitor turtle nests and release hatchlings into the wild. A true biologist!