4-2-movement-into-and-out-of-cells AS investigating-transport-processes-in-plants

Investigating diffusion & osmosis: plant tissue

Investigating diffusion

• The permeability of cell membranes can be affected by environmental factors such as chemicals or temperature

• This can be investigated using beetroot:

  • Pieces of beetroot (obtained using a cork borer) are placed into water at different temperatures or into different alcohol concentrations

  • Increases in cell membrane permeability result in the red pigment (normally contained within the large central vacuole) leaking out of the beetroot cells by diffusion

  • Qualitative or quantitative measurements of the changes in the colour of the surrounding solution can be taken e.g., using a colorimeter or a set of colour standards

• The red pigment leaks out via diffusion from regions of high concentration in the large central vacuoles of the beetroot cells to a region of low concentration in the solution outside the beetroot pieces

Investigating osmosis

• Evidence of osmosis occurring in plant cells can be shown when plant cells undergo plasmolysis:

  • If a plant cell is placed in a solution with a lower water potential than the plant cell (such as a concentrated sucrose solution), water will leave the plant cell through its selectively permeable cell surface membrane by osmosis

  • As water leaves the vacuole of the plant cell, the volume of the plant cell decreases

  • The protoplast (living part of the cell inside the cell wall) gradually shrinks and no longer exerts pressure on the cell wall

  • As the protoplast continues to shrink, it begins to pull away from the cell wall

  • This process is known as plasmolysis – the plant cell is plasmolysed

• This process can be observed using epidermal strips (sections of the very thin outer layer of tissue in plants)

• Plants with coloured sap (such as red onion bulbs, rhubarb petioles and red cabbage) make observations easier

• The epidermal strips are placed in a range of concentrations of sucrose solution or sodium chloride solution

• The strips are then viewed under a light microscope

• Plasmolysis may take several minutes to occur