Which term is the outermost layer of the cell primarily composed of lipids and proteins?

• A. Nucleus
• B. Organ
• C. Cytoplasm
• D. Cell Membrane

Answer: (D)

The outer boundary of a cell is the cell membrane, also called the plasma membrane. It is built mainly from a phospholipid bilayer with proteins embedded in or attached to it. The lipid bilayer forms a flexible, semi-permeable barrier: the hydrophilic heads face the watery exterior and interior, while the hydrophobic tails hide inside, creating a stable boundary. Proteins in the membrane perform essential roles such as transporting substances, signaling, and acting as enzymes or receptors. The nucleus sits inside the cell as the control center containing DNA, the cytoplasm is the internal fluid where organelles reside, and an organ is a larger structure within an organism, not the boundary. So the outermost layer composed predominantly of lipids and proteins is the cell membrane.