Question

a biological membrane is a bilayer that contains lipids with
○ hydrophobic head groups oriented toward the solvent (extracellular fluid and cytosol) and hydrophilic tail groups pointing inward.
○ hydrophilic head groups pointing inward and hydrophobic tail groups oriented toward the solvent (extracellular fluid and cytosol).
● hydrophilic head groups oriented toward the solvent (extracellular fluid and cytosol) and hydrophobic tail groups pointing inward.

which of these are components of biological membranes?
☐ nucleic acids
☐ lipids
☑ proteins

Explanation:

First Question (Multiple Choice about Membrane Lipid Orientation)

Biological membranes are lipid bilayers. Lipids have hydrophilic (water - attracting) heads and hydrophobic (water - repelling) tails. The aqueous environment is outside (extracellular fluid) and inside the cell (cytosol). So hydrophilic heads face the solvent (water - based), and hydrophobic tails face inward (away from water). The first option has hydrophobic heads facing solvent (wrong, as heads are hydrophilic). The second option has hydrophilic heads inward and hydrophobic tails outward (wrong, opposite of correct orientation). The third option (with the filled circle) has hydrophilic heads toward solvent and hydrophobic tails inward, which is correct.

• Nucleic acids (like DNA, RNA) are not components of biological membranes. They are involved in genetic information storage/transfer.
• Lipids: Biological membranes are mainly composed of a lipid bilayer (phospholipids etc.), so lipids are components.
• Proteins: Membranes have integral and peripheral proteins that perform functions like transport, signaling, etc., so proteins are components.

Answer:

The correct option is: hydrophilic head groups oriented toward the solvent (extracellular fluid and cytosol) and hydrophobic tail groups pointing inward.

Second Question (Components of Biological Membranes)