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

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.

Phloem

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.