Practical: Factors Affecting Membrane Structure & Permeability
Low Temperatures (below ): Ice crystals form within the cell and the extracellular environment. These sharp crystals can physically pierce the phospholipid bilayer and the tonoplast, leading to massive leakage upon thawing.
Moderate Temperatures ( to ): As temperature increases, phospholipids gain kinetic energy and move faster. This increases the fluidity of the membrane and creates small gaps between the fatty acid tails, slightly increasing permeability.
High Temperatures (above ): The bilayer begins to break down as the kinetic energy overcomes the intermolecular forces holding the phospholipids together. Crucially, membrane proteins denature, losing their tertiary structure and creating permanent holes in the membrane.
| Factor | Primary Target | Mechanism of Disruption |
|---|---|---|
| Temperature | Proteins & Lipids | Kinetic energy increases fluidity; high heat denatures proteins. |
| Organic Solvents | Phospholipids | Solvents like ethanol dissolve the lipid components of the membrane. |
| pH Extremes | Proteins | Changes in concentration disrupt ionic and hydrogen bonds in membrane proteins. |
Identify the Filter: Always explain why a specific colorimeter filter is used. For a red pigment like betalain, a green filter is used because it is the complementary color and will be absorbed most efficiently.
Control Variables: In exam questions, always check for the 'Big Three' controls: volume of water, surface area of beetroot, and incubation time. If any of these vary, the experiment is not a fair test.
Zeroing the Colorimeter: Mention that the device must be 'blanked' or zeroed using a cuvette of pure distilled water before taking readings to ensure that the absorbance measured is solely due to the pigment.
Confusing Fluidity and Permeability: While related, fluidity refers to the movement of components within the membrane, while permeability refers to the movement of substances across it. High fluidity often leads to high permeability.
Ignoring the Tonoplast: Students often forget that for pigment to leak out of a beetroot cell, it must cross two membranes: the tonoplast (around the vacuole) and the plasma membrane (around the cytoplasm).
Cuvette Handling: Scratches or fingerprints on the clear sides of a cuvette will scatter light and falsely increase the absorbance reading. Always handle cuvettes by the frosted sides.