Final answer:
Engulfment, part of endocytosis, involves the cell wrapping external particles in vesicles, facilitating nutrient uptake and waste removal. Direct fusion is the merging of lipid bilayers without vesicle formation, crucial in processes like neurotransmitter release and viral entry.
Step-by-step explanation:
Endocytosis is a type of active transport in which particles are taken into a cell by the invagination of the cell membrane. There are different types of endocytosis:
- Phagocytosis: This is when a cell engulfs a large particle, such as a microorganism, by extending pseudopodia and forming a phagosome.
- Pinocytosis: This is when a cell takes in fluid and dissolved substances by forming small vesicles.
- Receptor-mediated endocytosis: This is a more specific form of endocytosis in which particles are taken in through receptor-ligand interactions.
Direct fusion refers to the merging of lipid bilayers of two membranes without the involvement of vesicle formation. This process is essential in various cellular activities, such as the fusion of neurotransmitter vesicles with the cell membrane during exocytosis or the fusion of viral membranes with host cell membranes during viral entry.