Transport of organic substances in the phloem

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  • Translocation - the process by which organic molecules and some mineral ions are transported from one part of a plant to another
  • Phloem - in flowering plants, the tissue that transports biological molecules
  • Phloem is made up of sieve tube elements, long thin structures arranged end to end
  • Their end sieve walls are perforated to form sieve plates
  • Having produced sugars during photosynthesis, the plant transports them from the site of production, known as sources, to the places where they will be used directly or stored for further use - known as sinks
  • As sinks can be anywhere in a plant - sometimes above and sometimes below the source - it follows that the translocation of molecules in phloem can be in either direction
  • Organic molecules to be transported include sucrose and amino acids
  • The phloem also transports inorganic ions such as potassium, chloride, phosphate and magnesium ions

Image result for longitudinal view of phloem

Image result for transverse view of phloem

Mechanism of translocation:

  • Materials are transported in the phloem and that the rate of movement is too fast to be explained by diffusion
  • What is in doubt is the precise mechanism by which translocation is achieved
  • Current thinking favours the mass flow theory, a theory that can be divided into 3 phases

1. Transfer of sucrose into sieve elements from photosynthesis tissue:

  • Sucrose is manufactured from the products of photosynthesis in cells with chloroplasts
  • The sucrose diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells into companion cells
  • Hydrogen ions are actively transported from the companion cells into the spaces within cell walls using ATP
  • These hydrogen ions then diffuse down a concentration gradient through carrier proteins into the sieve tube elements
  • Sucrose molecules are transported along with the hydrogen ions in a process known as co-transport
  • The protein carriers are therefore known as co-transport proteins

2. Mass flow of sucrose through sieve tube

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