IB Biology Exam Prep

Category - Biology

Which of the following situations best demonstrates phagocytosis?
  1. A bacterium is surrounded by a plasma membrane, engulfed, and enclosed by a vacuole.
  2. An antigen binds to a specific receptor on the surface of a plasma membrane, which invaginates around the antigen.
  3. A food particle is surrounded by a plasma membrane, engulfed, and diffuses through the lipid bilayer.
  4. An antibody within the cell is enclosed in a vesicle that fuses with the plasma membrane and releases the antibody from the cell.
Explanation
Correct Response: A. Phagocytosis is the type of endocytosis that facilitates bringing large particles such as bacteria into the cell. Endocytosis is used when the particles are too large to enter through the plasma membrane. During phagocytosis, the plasma membrane surrounds the particle, enclosing it in a vacuole so that it can be engulfed by the cell. Although a cell is able to deactivate an antigen, such as a pathogen, when it binds to a specific receptor (B) on the membrane, phagocytosis is not necessary for this process to occur. Phagocytosis occurs when particles, including food, are too large to diffuse through the plasma membrane, which is composed of a lipid bilayer (C). Instead the cell surrounds the particle with its membrane and forms a vacuole around the particle to allow digestion. The process of antibody-containing vesicles fusing with the plasma membrane to release the antibodies from the cell (D) is an example of exocytosis, rather than phagocytosis.
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