Final answer:
Pathogens can enter host cells through endocytosis or membrane fusion, using effector proteins or surface proteins. Viruses can enter cells through endocytosis or membrane fusion, with the viral capsid degrading and releasing the nucleic acid for replication. Receptor-mediated endocytosis and channel formation are common entry mechanisms for animal viruses.
Step-by-step explanation:
Entry to a cell can occur by endocytosis. For most kinds of host cells, pathogens use one of two different mechanisms for endocytosis and entry. One mechanism relies on effector proteins secreted by the pathogen; these effector proteins trigger entry into the host cell. The second mechanism relies on surface proteins expressed on the pathogen that bind to receptors on the host cell, resulting in entry.
Viruses can enter host cells in various ways. Some viruses can enter through endocytosis, where the cell membrane surrounds and engulfs the entire virus. Others enter through membrane fusion, where the viral envelope fuses directly with the cell membrane. Once inside the cell, the viral capsid degrades and the viral nucleic acid is released for replication and transcription.
Animal viruses may enter cells through receptor-mediated endocytosis, where the virus binds to its receptor on the host cell and is taken inside the cell via a vesicle. Alternatively, non-enveloped viruses may penetrate cells by undergoing shape changes after binding to the receptor, creating channels in the host cell membrane through which the viral genome is injected into the host cell.