Plant Transport Systems: Xylem, Phloem, and Transpiration

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Adaptation of Xylem to its Function

  • Long, made from cells joined end to end to form tubes/vessels
  • Cell walls thickened with lignin, a hard, strong material
  • Lignin makes cell walls waterproof
  • Cells have no cytoplasm or organelles (no cell contents)
  • No end-to-end walls between cells
  • Pits in the walls for water movement between vessels

Define Transpiration

Loss of water vapor from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapor through the stomata.

Leaves have many more stomata in their lower epidermis than they do in their upper epidermis, and most transpiration happens there.

Factors Affecting Transpiration

  • The large internal surface area provided by the interconnecting air spaces between mesophyll cells and the size and number of stomata

Mechanism of Transpiration - Transpiration Pull

  • Attractive forces between molecules in a liquid cause cohesion, a property in which the molecules tend to stick together.
  • Water vapor in the air spaces of spongy mesophyll cells is lost from the leaf through the stomata by diffusion.
  • This lowers the water potential in the leaf tissue.
  • Water moves from the xylem to the leaf down the water potential gradient (high in xylem, low in the leaf).
  • Water moves up the stem in the xylem in a continuous column due to tension caused by water loss from the leaf and due to cohesion.
  • Water then moves from the soil to the root cell by osmosis from high water potential in the soil to low water potential in the root hair cell.


  • One of the possible consequences of transpiration is wilting.
  • This happens when the rate of transpiration is greater than the rate at which water is absorbed through the root hair cells.
  • The plant cells lose water, reducing their turgor pressure so they become flaccid.
  • The tissues become limp and the plant is no longer supported.

Benefits of Wilting

  • Causes stomata to close, preventing further water loss.
  • Conserves water for other processes/parts of the plant.
  • Decreases surface area exposed to the Sun.

Effects of Temperature and Humidity on Transpiration Rate

  • Light: As light intensity increases, the rate of transpiration increases because light causes stomata to open wider. As light intensity decreases at night, stomata close so that less water is lost by transpiration.
  • Temperature: As temperature increases, the rate of transpiration increases because water molecules gain energy and move faster.
  • Humidity: As humidity increases, the rate of transpiration decreases because the concentration gradient decreases.

Aphids and Phloem Transport

  • Aphids are small insects that feed on the sap in the phloem.
  • They have piercing mouth parts called stylets.
  • They can be used to study transport of sugars in the phloem.
  • If the stylet of the feeding aphid is cut, the sugary sap will pass out of the stylet for some time.
  • Aphids are crop pests and reproduce very rapidly, so farmers control them by spraying insecticide.

Comparison of Transpiration and Translocation

  • Substances transported/released: Water vapor is lost from leaves through stomata. Sucrose and amino acids are transported in plants.

  • Transport tissue: Xylem, Phloem

  • Direction of movement of substances: From the roots to leaves in one direction (up). From source to sink - bidirectional (up or down)

  • Passive process/Active process

  • Active In hot windy daysActive In sunny warm days when plants have a high rate of photosynthesis

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