3.4 - Movement of Substances Across a Plasma Membrane and Its Application in Daily Life

Experiment: Concentration of Cell Sap in Plant Tissue

• Problem: What concentration of extracellular solution is isotonic to potato cell sap?
• Hypothesis: An extracellular solution isotonic to potato cell sap causes no change in the mass of a potato strip.
• Manipulated variable: sucrose-solution concentration. Responding variable: percentage change in mass of potato strips. Fixed variables: soaking period, surrounding temperature and type of potato.
• Prepare equal potato strips and record their initial masses.
• Immerse them for 30 minutes in distilled water and 0.1–0.6 M sucrose solutions; blot dry and record final masses.
• Plot percentage change in mass against sucrose-solution concentration.
• The concentration at 0% change in mass is isotonic to the potato cell sap.

Phenomenon of Plant Wilting

• Excessive fertiliser dissolves and makes soil water hypertonic to the cell sap of roots.
• Water diffuses by osmosis from root cell sap into the soil; cells become plasmolysed and the plant wilts.
• Cells recover after watering if plasmolysis has not continued too long; prolonged plasmolysis causes death.

Food Production and Preservation

• Osmosis and diffusion are applied in preparing preserved eggs, smoked bananas, salted fish, pickles and preserved cabbage.
• Sugar, salt and vinegar are materials used to produce preserved food.
• A concentrated sugar or salt solution is hypertonic; water moves out of cells by osmosis.

Applications in Daily Life

Saline Solution

• Medical saline is isotonic to blood plasma.
• It contains 0.85–0.90 g sodium chloride per 100 ml.

Rehydration and Isotonic Drinks

• Oral rehydration salts replace water and electrolytes lost by individuals with diarrhoea.
• Isotonic drinks replace water and electrolytes such as potassium and sodium lost by athletes through perspiration.

Liposomes

• A liposome is a vesicle containing an aqueous solution surrounded by a phospholipid bilayer membrane.
• Water-soluble drugs are transported in the hydrophilic region; water-insoluble drugs are transported in the hydrophobic region.
• Liposomes protect orally taken drugs or active substances from gastric juices so they can reach target cells.

Reverse Osmosis in Water Purification

• Reverse osmosis obtains fresh water from seawater by desalination.
• Pressure pushes seawater through a selectively permeable membrane.
• Water molecules pass through; foreign particles, salts and microorganisms are blocked.
• The water released is pure fresh water.