How can freeze-fracture techniques be used to determine the orientation of a protein within a biological membrane?

• A. By measuring the electrical conductivity changes in the membrane
• B. By analyzing the lipid composition surrounding the protein
• C. By observing the protein's interaction with water molecules
• D. By visualizing the protein's position in the membrane using electron microscopy after fracturing the membrane

Answer: (D)

Verified step by step guidance

1. Understand the freeze-fracture technique: This method involves freezing a biological membrane and then fracturing it to reveal its internal structures. The fracture typically occurs along the lines of weakness, such as the hydrophobic interior of the lipid bilayer.
2. Prepare the sample: The biological membrane is rapidly frozen, often using liquid nitrogen, to preserve its structure. This freezing process helps to immobilize the components of the membrane, including proteins.
3. Fracture the membrane: The frozen membrane is fractured, usually by striking it with a sharp object. This causes the membrane to split along the plane of the lipid bilayer, exposing the internal surfaces of the membrane.
4. Visualize using electron microscopy: After fracturing, the exposed surfaces are coated with a thin layer of metal, such as platinum or gold, to create a replica of the membrane's surface. This replica is then examined using electron microscopy, which provides high-resolution images of the membrane's structure.
5. Analyze the images: The electron microscopy images allow researchers to visualize the orientation and position of proteins within the membrane. Proteins appear as particles embedded in the lipid bilayer, and their distribution and orientation can be studied to understand their role and function within the membrane.