18.2
Phagocytosis
Cells pull particles inward and engulf them in spherical vesicles in an energy-requiring process called endocytosis. Phagocytosis ("cellular eating")…
One type of endocytosis, phagocytosis, or cell eating, occurs when receptors on the cell's surface encounter a particle to be ingested.
The cell then extends a projection called a pseudopod. In mammals, the object is often an invading microorganism or cellular debris from damaged tissue. The pseudopod surrounds the item, and the membranes fuse together creating a vacuole with the particle trapped inside.
Eventually, the vacuole fuses again, this time with a lysosome which is full of enzymes ready to digest the invading particle.
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Q1: What is phagocytosis and how does it work in cells?
Phagocytosis, or cell eating, is a type of endocytosis where cells engulf large particles like bacteria, viruses, or cellular debris. When surface receptors encounter a particle, the cell extends projections called pseudopods that surround and trap the object inside a vesicle called a phagosome. This energy-requiring process allows cells to internalize materials for destruction or nutrient absorption.
Q2: How do immune cells use phagocytosis to protect the body?
Immune cells including neutrophils, macrophages, and monocytes use phagocytosis to destroy pathogens like viruses and bacteria, as well as infected host cells. These specialized phagocytes recognize unwelcome invaders through surface receptors, engulf them, and eliminate them through enzymatic degradation. This process is essential for defending the body against infections and clearing cellular debris.
Q3: What role do lysosomes play after a phagosome forms?
After a phagosome forms around an engulfed particle, it fuses with a lysosome, a spherical organelle containing degradative enzymes. This fusion creates a phagolysosome where the acidic environment and enzymes neutralize or eliminate the trapped particle. Eventually, the remaining waste forms a residual body that is released from the cell.
Q4: What are pseudopods and why are they important in phagocytosis?
Pseudopods are temporary projections of cytoplasm that extend from the cell surface during phagocytosis. These structures surround and engulf particles bound to cell surface receptors, gradually enveloping the target until it is completely enclosed and pinched off into the cytoplasm. Pseudopods are essential for the cell's ability to capture and internalize large objects.
Q5: How do phagocytes find and recognize particles to engulf?
Phagocytes locate particles through direct contact or by following chemical signals in a process called chemotaxis. Once near a particle, the phagocyte binds to it via surface receptors that may be general, responding to various stimuli, or specific to particular targets. Different types of phagocytes use distinct receptors, allowing them to recognize diverse pathogens and debris.
Q6: What organisms besides immune cells use phagocytosis?
In addition to immune system cells, amoebae, algae, and other single-celled organisms use phagocytosis to absorb nutrients from their environment. These organisms engulf food particles and other cells through the same mechanism of pseudopod extension and vesicle formation. For these organisms, phagocytosis serves primarily as a feeding mechanism rather than a defense strategy.
Q7: What is the difference between phagocytosis and receptor-mediated endocytosis?
Phagocytosis is a type of endocytosis specialized for engulfing large particles like cells and bacteria through pseudopod extension. Receptor-mediated endocytosis, by contrast, is a more selective process where specific receptors bind to particular molecules, forming smaller vesicles. While phagocytosis handles bulk particle uptake, receptor-mediated endocytosis targets specific cargo molecules with high precision.