Which type of cells are characterized by a plasma membrane, lack of nucleus, and a disc shape with a concave center?

• A. Erythrocytes
• B. Fibroblasts
• C. Monocytes
• D. Neurons

Answer: (A)

Erythrocytes, commonly known as red blood cells, are distinguished by their unique structure and composition. They possess a plasma membrane that encloses their contents, but they lack a nucleus and most organelles. This absence of a nucleus allows more space for hemoglobin, the protein responsible for oxygen transport.

Their disc shape with a concave center is an important adaptation that enhances their functionality. This shape increases the surface area-to-volume ratio, facilitating efficient gas exchange by allowing a greater amount of oxygen to bind to hemoglobin as well as improving flexibility, which is crucial for navigating through small capillaries.

In contrast, fibroblasts, monocytes, and neurons have distinct characteristics. Fibroblasts are connective tissue cells that are nucleated and play a role in tissue repair and synthesis. Monocytes are a type of white blood cell that also contain a nucleus and are involved in immune responses. Neurons, the fundamental units of the nervous system, have complex structures including a cell body, dendrites, and axons, and they also contain a nucleus.

Thus, the defining features of erythrocytes—namely, a plasma membrane, lack of a nucleus, and a specific disc shape with a concave center