(ii) practical investigations into the effects of solutions of different…

The idea

Water potential (Ψ) is the tendency of water to move; pure water at atmospheric pressure is 0 kPa, and solutions have negative values. In a practical investigation you prepare a series of solutions of known water potential, for example sucrose solutions, and place samples of plant tissue (such as potato cylinders or onion epidermis) and animal cells (such as red blood cells or cheek epithelial cells) into them. You record mass change in plant tissue or observe lysis and crenation in animal cells. Plant cells with a cellulose wall resist bursting: in a solution of higher (less negative) water potential they take up water and become turgid, while in a solution of lower (more negative) water potential they lose water and become plasmolysed. Animal cells lack a wall, so in pure water or very dilute solution they swell and lyse, and in concentrated solution they shrink and crenate.

What earns the marks

• Water potential is measured in kPa; pure water is 0 kPa and solutions have negative water potential values.
• Water moves by osmosis from a region of higher (less negative) water potential to a region of lower (more negative) water potential across a partially permeable membrane.
• Plant cells placed in a solution of higher water potential gain water, become turgid, and the cellulose wall prevents lysis; in a solution of lower water potential they lose water and become plasmolysed.
• Animal cells placed in a solution of higher water potential swell and may lyse because they lack a cell wall; in a solution of lower water potential they lose water and crenate.
• Percentage change in mass is calculated as (final mass − initial mass) ÷ initial mass × 100, and controlled variables such as temperature, time and tissue size must be kept constant.

Common mistakes and how to put them right

• Thinking pure water has a high positive water potential.

  Put it right by stating pure water is 0 kPa and solutions are negative.

• Assuming plant cells burst in pure water.

  Put it right by explaining the cellulose wall resists expansion so cells become turgid, not lysed.

• Confusing the direction of osmosis.

  Put it right by stating water moves from higher (less negative) to lower (more negative) water potential.

Exam tips

• Always state the direction of water movement in terms of higher and lower water potential, not just 'high' and 'low' concentration.
• Link the presence or absence of a cell wall to the observed outcome (turgid versus lysed).
• When describing a method, name the controlled variables and explain why each is kept constant.