Which of the following describes the plasma membrane?

A. A double layer of proteins enclosing the plasma: Proteins are embedded within the plasma membrane and serve as channels, receptors, or structural support, but the membrane itself is not composed of a double layer of proteins.

B. A membrane composed of tiny shelves or cristae: Cristae are folds of the inner mitochondrial membrane that increase surface area for energy production. They are specific to mitochondria and do not describe the plasma membrane.

C. A single-layered membrane that surrounds the nucleus of the cell: The nuclear envelope surrounds the nucleus and consists of a double lipid bilayer, not a single layer, and is distinct from the plasma membrane.

D. A phospholipid bilayer surrounding the cell: The plasma membrane consists of a phospholipid bilayer with hydrophilic heads facing outward and hydrophobic tails inward. Embedded proteins and carbohydrates allow selective permeability, communication, and protection of the cell.

A. Pinocytosis: Pinocytosis is the process by which cells engulf extracellular fluid and dissolved solutes. It is a form of “cell drinking” and occurs in many cell types but is not specialized for attacking pathogens.

B. Phagocytosis: Phagocytosis is the process by which certain cells, such as macrophages and neutrophils, engulf large particles, bacteria, or debris. It is a key mechanism in the immune response, allowing white blood cells to destroy pathogens.

C. Exocytosis: Exocytosis is the process of vesicles fusing with the plasma membrane to release substances outside the cell. While important for secretion, it is not the mechanism by which pathogens are engulfed.

D. Intracellular vesicular trafficking: This refers to the movement of vesicles within a cell to transport molecules between organelles. It is involved in normal cellular logistics but does not directly involve engulfing pathogens.