Which component must substances pass through to enter a typical animal cell?

Substances must pass through the cell membrane to enter a typical animal cell. The cell membrane serves as a selective barrier that regulates the movement of materials into and out of the cell. It is primarily composed of a phospholipid bilayer with embedded proteins that facilitate transport and signaling.

The cell membrane's selective permeability allows it to control the internal environment of the cell, ensuring that essential nutrients can enter while waste products and harmful substances can be kept out. The membrane employs various mechanisms, such as passive diffusion, facilitated diffusion, and active transport, to enable specific substances to traverse it.

In contrast, the cell wall is present in plant cells and provides structural support but is not found in animal cells. The nucleus is responsible for housing genetic material and does not serve as a passageway for substances entering the cytoplasm. The endoplasmic reticulum functions in protein and lipid synthesis and is not involved in the direct entry of substances into the cell from the external environment.

Thus, the cell membrane is the critical component through which all substances must pass to enter a typical animal cell.