A. Why does soap dissolve the membrane easily?
B. What is selective permeability? How does the cell achieve this?
C. What are the major molecules in the cell membrane?
D. Define the characteristic of being amphipathic? Explain how this affects transport across the membrane
The Correct Answer and Explanation is:
A. Why does soap dissolve the membrane easily?
Soap molecules are amphipathic, meaning they have both hydrophobic (water-repelling) and hydrophilic (water-attracting) regions. Cell membranes are primarily made of phospholipid bilayers, which also have hydrophobic tails and hydrophilic heads. Soap molecules disrupt the organization of these lipid bilayers by inserting themselves into the membrane, breaking apart the lipid structure and causing it to dissolve. This is why soap is effective at destroying membranes, including those of bacteria and viruses.
B. What is selective permeability? How does the cell achieve this?
Selective permeability is a property of the cell membrane that allows certain molecules to pass through while blocking others. The cell achieves this through the structure of the lipid bilayer and the presence of membrane proteins. Small, nonpolar molecules can diffuse freely through the lipid bilayer, but ions and large polar molecules require specific transport proteins. Channels, carriers, and pumps regulate entry and exit, allowing the cell to control its internal environment.
C. What are the major molecules in the cell membrane?
The major molecules in the cell membrane include:
- Phospholipids: Form the basic bilayer structure.
- Proteins: Integral and peripheral proteins involved in transport, signaling, and structural support.
- Cholesterol: Regulates membrane fluidity and stability.
- Carbohydrates: Attached to proteins and lipids on the extracellular surface, important for cell recognition and communication.
D. Define the characteristic of being amphipathic. Explain how this affects transport across the membrane (300 words):
An amphipathic molecule has both a hydrophilic (water-attracting) region and a hydrophobic (water-repelling) region. This dual nature is essential for the formation and function of the cell membrane. The primary example of an amphipathic molecule in the membrane is the phospholipid. Each phospholipid has a polar, hydrophilic head and two nonpolar, hydrophobic tails. When placed in water, phospholipids spontaneously arrange themselves into a bilayer: the hydrophilic heads face outward toward the water on both sides of the membrane, while the hydrophobic tails face inward, away from the water.
This amphipathic arrangement creates a semi-permeable membrane, where the hydrophobic core acts as a barrier to most water-soluble (polar) substances, while allowing small, nonpolar molecules (like oxygen and carbon dioxide) to diffuse through easily. Larger or charged molecules, such as glucose or ions (Na⁺, K⁺, Cl⁻), cannot pass freely through this hydrophobic core. Instead, they require transport proteins—which are also amphipathic—to help them cross the membrane.
Transport proteins like channel proteins and carrier proteins span the bilayer and provide specific pathways for substances to enter or exit the cell. Some proteins create pores that allow ions or water molecules to move through by facilitated diffusion, while others use energy (ATP) to actively transport molecules against a concentration gradient.
In summary, the amphipathic nature of membrane components enables the membrane to maintain structural integrity while also allowing selective transport. This ensures the cell can control its internal environment, take in nutrients, expel waste, and respond to external signals effectively.
