5.15
Phagocytosis
Cells pull particles inward and engulf them in spherical vesicles in an energy-requiring process called endocytosis. Phagocytosis (“cellular eating”)…
Phagocytosis is a type of endocytosis, often described as cell eating. It begins when receptors on the cell surface bind to a particle that the cell will engulf.
After the cell recognizes the particle, it extends a projection called a pseudopod.
In mammals, the particle is often an invading microorganism or cellular debris from damaged tissue.
The pseudopod surrounds the particle, and the membranes fuse to form a membrane-bound vesicle called a phagosome. This vesicle contains the engulfed material.
The phagosome then moves through the cytoplasm and fuses with a lysosome. This fusion forms a phagolysosome, where enzymes digest the engulfed material.
In mammals, specialized immune cells such as macrophages, neutrophils, and dendritic cells carry out phagocytosis.
Q1: What is phagocytosis and how does it work?
Phagocytosis is a type of endocytosis where cells engulf large particles like bacteria, viruses, or cellular debris. The process begins when cell surface receptors bind to a particle, triggering the cell to extend projections called pseudopods that surround and envelop the material into a membrane-bound vesicle called a phagosome.
Q2: Which cells perform phagocytosis in mammals?
Specialized immune cells carry out phagocytosis in mammals, including macrophages, neutrophils, and dendritic cells. These cells use phagocytosis to eliminate unwelcome pathogens like viruses and bacteria, as well as infected host cells and cellular debris from damaged tissue.
Q3: What happens after a phagosome forms?
After the phagosome forms, it moves through the cytoplasm and fuses with a lysosome to create a phagolysosome. Inside this structure, degradative enzymes digest the engulfed material. Eventually, the phagolysosome forms a waste-containing residual body that is released from the cell.
Q4: How do phagocytes recognize particles to engulf?
Phagocytes recognize particles through surface receptors that bind to the particle when contact occurs. Sometimes this is a chance encounter, but other times the phagocyte follows chemical signals to the particle in a process called chemotaxis. Different phagocytes use distinct receptors that may be general or specific.
Q5: What role do pseudopods play in phagocytosis?
Pseudopods are projections of cytoplasm that the phagocyte extends around a bound particle. These projections continue to surround the particle until it is completely enveloped and pinched off into the cytoplasm, forming the phagosome that contains the engulfed material.
Q6: Why is phagocytosis considered an energy-requiring process?
Phagocytosis is energy-requiring because it involves active cellular processes including receptor binding, pseudopod extension, membrane fusion, and vesicle movement through the cytoplasm. These dynamic steps require ATP to power the molecular machinery that recognizes, surrounds, and processes engulfed particles.
Q7: How does phagocytosis differ from other types of endocytosis?
Phagocytosis is one of three major types of endocytosis and is specialized for engulfing large objects like cells, bacteria, and viruses. Unlike receptor-mediated endocytosis, which targets specific molecules, phagocytosis uses general or specific surface receptors to recognize and engulf large particles through pseudopod extension.