Which structure allows a host cell to absorb endosymbionts?

The plasma membrane is the correct answer because it is the semi-permeable barrier that surrounds the cell and regulates the movement of substances in and out of the cell. This membrane plays a crucial role in the process of endocytosis, where the cell engulfs external materials—including potential endosymbionts—by folding inwards and forming a vesicle that brings those materials into the cell. The fluid nature of the plasma membrane facilitates this process, allowing the host cell to absorb a variety of substances, including other organisms or their components.

In contrast, the cell wall, while providing structural support and protection, is generally present in plant and fungal cells and does not play an active role in the absorption of endosymbionts. The nucleus, being the control center of the cell housing genetic material, does not directly participate in the absorption of external organisms. Ribosomes are responsible for protein synthesis and do not have any role in the uptake of endosymbionts. Thus, the plasma membrane is the essential structure enabling a host cell to absorb entities from its environment effectively.