Xylem and Phloem

There are two types of transport vessels in plants:

  • Xylem
  • Phloem
Illustration depicting the internal structure of a plant stem. On the left, a cross-sectional view showcases vascular bundles composed of xylem (blue segments) and phloem (orange segments) tissues. On the right, a cylindrical green stem gives an external view, with the top showing the vascular bundle arrangement from a bird's-eye perspective. The term 'Internal stem structure' is labelled at the bottom right."

Xylem

The main function of the xylem is to transport water and minerals from the roots to the stem and leaves. They are made up of elongated dead cells lined end to end, forming a continuous tube. Through this tube, water is transported in a transpiration stream.

Movement in the xylem is unidirectional (one way), flowing from the roots to the leaves, with no end walls between cells.

Diagram illustrating the structure of a xylem vessel. The vessel, shown in vertical section, has a central blue region representing the flow of water and minerals, indicated by upward-pointing arrows. The movement is labelled as 'one-way only'. The sides of the vessel are green, and a note points out 'No end walls between cells'. The title 'Xylem vessel' is displayed prominently at the top in a green banner.

Xylem vessels are well adapted to carry out their functions in plants. For example, they have the following features:

  • No organelles or cytoplasm, so there is more space to transport water
  • Tough walls strengthened by lignin, which prevents it from collapsing inwards
  • Walls waterproofed with lignin to aid the upward movement of water.

Phloem

The phloem transport dissolved sugars and amino acids both up and down the plant through translocation.

Diagram illustrating the structure of a phloem vessel. The vessel, shown in vertical section, displays a series of green cells with a central region showing orange arrows indicating a 'two-way flow' of water and food. The end walls of the cells are dotted, and a label mentions 'Cells have end walls with perforations'. The title 'Phloem vessel' is displayed at the top in a green banner.

The phloem tubes are made up of living cells. They have end walls with pores that allow dissolved substances to be transported from one cell to another. These pores are called sieve plates, which allow substances to flow through easily.

  • Next to sieve tubes are companion cells, which provide the energy required for the transport of substances in the phloem

Comparing Xylem and Phloem

XylemPhloem
Type of transportationTranspiration streamTranslocation
Substances transportedWater and mineral ionsSucrose and amino acids
Movement of substancesUpwardsUpwards and downwards
Nature of tissueDead cells without cytoplasm or organelles.Living tissue
Type of processPassive processActive process

Xylem – Passive Process

The movement of water in the xylem is a passive process powered by the evaporation of water from the stomata in the leaves, which creates a negative pressure or suction that pulls water up from the roots.

Phloem – Active Process

In contrast, the movement of sugars, amino acids and mineral ions in the phloem requires energy. The loading of sugars into the phloem at source locations (like leaves) usually requires active transport, which involves energy.

This creates a high concentration of solutes in the phloem, causing water to move in from the xylem, creating pressure that pushes the sugars towards sink locations (like roots or fruits). This whole process, involving active transport and the pressure-driven flow of water, makes it an active process.

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