Phloem, the plant's sugar highway, moves vital nutrients from leaves to hungry cells. Its structure, with sieve tubes and companion cells, allows for efficient transport. Understanding phloem is key to grasping how plants distribute energy throughout their bodies.

The pressure flow hypothesis explains how phloem sap moves from high to low pressure areas. This process, driven by osmosis, doesn't need extra energy along the way. Phloem's ability to transport sugars is crucial for plant growth and survival.

Phloem Structure

Sieve Elements and Companion Cells

• Sieve elements are the main conducting cells of the phloem
  • Lack a nucleus, ribosomes, and a vacuole at maturity to allow for efficient transport of sugars and other organic compounds
  • Connected end-to-end to form sieve tubes for long-distance transport
• Companion cells are specialized parenchyma cells that are closely associated with sieve elements
  • Contain a nucleus, ribosomes, and other organelles to support the functioning of sieve elements
  • Provide energy and assist in loading and unloading of sugars and other compounds into and out of sieve elements (sucrose, amino acids, hormones)

Sieve Plates

• Sieve plates are the end walls of sieve elements that contain numerous pores
  • Allow for the passage of phloem sap between adjacent sieve elements
  • Pores are lined with callose, a polysaccharide that can regulate the flow of phloem sap by opening or closing the pores (wound response, seasonal changes)
• Sieve plates are an important structural feature that facilitates efficient long-distance transport in the phloem
  • Pores are large enough to allow for the passage of macromolecules (proteins, RNA) in addition to sugars and other small organic compounds

Phloem Transport

Pressure Flow Hypothesis

• The pressure flow hypothesis explains the mechanism of phloem transport
  • Suggests that the movement of phloem sap is driven by a pressure gradient between the source and sink tissues
  • High osmotic potential in the source tissues (leaves) drives the loading of sugars into the phloem, creating a high hydrostatic pressure
  • Low osmotic potential in the sink tissues (roots, fruits) leads to the unloading of sugars, creating a low hydrostatic pressure
• The pressure gradient generated by the difference in osmotic potential causes the phloem sap to flow from source to sink
  • Flow is passive and does not require energy input along the transport pathway
  • Rate of flow is determined by the magnitude of the pressure gradient and the resistance of the phloem pathway (sieve plate pores, viscosity of phloem sap)

Phloem Sap and Translocation

• Phloem sap is the fluid that is transported through the phloem
  • Composed primarily of water, sugars (sucrose), amino acids, hormones, and other organic compounds
  • Concentrations of solutes in the phloem sap are much higher than in the xylem sap (10-25% vs. <1%)
• Translocation is the long-distance transport of phloem sap from source to sink tissues
  • Occurs in both directions, from leaves to roots and from roots to leaves (bidirectional)
  • Speed of translocation can range from 1-10 cm/hour, depending on the plant species and environmental conditions (temperature, light intensity)
  • Translocation is essential for the distribution of photosynthates and other organic compounds throughout the plant body

Phloem Pathways

Symplastic Pathway

• The symplastic pathway involves the movement of solutes through the cytoplasm of interconnected cells via plasmodesmata
  • Plasmodesmata are channels that connect the cytoplasm of adjacent cells, allowing for the direct exchange of small molecules and macromolecules
  • Sugars and other organic compounds can move from the mesophyll cells to the phloem through the symplastic pathway (cell-to-cell transport)
• The symplastic pathway is important for the loading of sugars into the phloem in source tissues
  • Sugars are actively transported from the mesophyll cells into the companion cells and then into the sieve elements via plasmodesmata
  • This process is mediated by sugar transporters and requires energy input (ATP)

Apoplastic Pathway

• The apoplastic pathway involves the movement of solutes through the cell walls and intercellular spaces, outside of the cytoplasm
  • Solutes can diffuse freely through the cell walls and intercellular spaces, but must cross the plasma membrane to enter the cytoplasm
  • The apoplastic pathway is important for the unloading of sugars from the phloem in sink tissues (roots, fruits)
• The apoplastic pathway is also involved in the retrieval of solutes from the apoplast back into the symplast
  • Sugars and other organic compounds that leak out of the phloem can be actively transported back into the phloem via the apoplastic pathway
  • This process helps to maintain the pressure gradient and ensures efficient transport of solutes in the phloem