Osmosis and Plant Transport

Osmosis and Plant Transport

Section: Osmosis

  • Osmosis is a specific type of diffusion that involves the movement of water molecules across a semi-permeable membrane.
  • This movement always goes from an area of higher water concentration (dilute solution) to an area of lower water concentration (concentrated solution).
  • The semi-permeable membrane allows the passage of certain molecules but not others. It is selective in its permeability.

Section: Plant Transport

  • The xylem vessels and phloem tubes are the primary transportation systems within a plant.


  • Xylem vessels transport water and dissolved minerals from the roots to all other parts of the plant.
  • They are made up of dead cells, providing a continuous, hollow tube for movement.
  • They also provide structural support to the plant.


  • The phloem tubes comfortably transport food in the form of sugar (sucrose) from the leaves where it’s produced (during photosynthesis) to the rest of the plant.
  • Phloem is made up of living cells. The movement of food is termed translocation.

Section: Osmosis in Plant Cells

  • Osmosis plays a crucial role in the transport of water from the roots to the rest of the plant.
  • Water is taken up by the roots from the soil through osmosis. This occurs as the soil is usually a higher concentration of water compared to the root hair cells.
  • If a plant is well watered, its cells will be turgid – firm and full of water. This helps to support the plant.
  • If a plant cell loses too much water, it becomes flaccid and the plant may wilt.
  • In extreme cases, a plant cell may lose so much water that it becomes plasmolysed – the cell membrane detaches from the cell wall.

Section: Adaptations for Transport in Plants

  • Root hair cells have a large surface area to absorb more water from the soil.
  • The stomata on the bottoms of leaves allow for gas exchange whilst also limiting water loss.
  • Guard cells surrounding the stomata can control their opening and closing to manage water loss.
  • Most plants have a wax cuticle to reduce water loss.
  • The transpiration stream - The evaporation of water from the leaves creates a suction to pull more water up the plant, that is replaced by osmosis in the roots.