BIOL 2021 Lecture Notes - Lecture 10: Cell Membrane, Pinocytosis, Clathrin
12 May 2018
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• Phagocytosis
o Ingest microorganisms, dead cells.
o Protozoa: for feeding.
o Animal cells: specialized phagocytes.
▪ Scavenge dead cells, defend against invaders.
o Cells of immune system = macrophages and neutrophils.
o Phagocytosis is a special form of endocytosis in which a cell uses large
endocytic vesicles called phagosomes to ingest large particles such as
microorganisms and dead cells. In protozoa, phagocytosis is a form of
feeding. Phagocytosis is important in most animals for purposes other
than nutrition, and it is carried out mainly by specialized cells—so-called
professional phagocytes. In mammals, two important classes of white
blood cells that act as professional phagocytes are macrophages and
neutrophils.
• Video 13.5 phagocytosis
• Fig 13-61A neutrophil during phagocytosis
o Process require a trigger by cargo.
o Receptors on phagocyte bind to particle to be ingested.
o Activates signals inside cells.
o Phagocytosis is a cargo-triggered process. That is, it requires the
activation of cell-surface receptors that transmit signals to the cell interior.
Thus, to be phagocytosed, particles must first bind to the surface of the
phagocyte (although not all particles that bind are ingested).
• Question: phagocytic cells such as macrophages engulf dead cells and even
insert particles like plastic beads. Why don’t they accidentally engulf live cells?
o Answer: Live cells have “don’t eat me” protein signals on their plasma
membranes that bind to inhibiting receptors on the macrophages. Dead
cells lose these signals.
• Pinocytosis
o Constitutive.
o No trigger needed.
o Most (not all) pinocytosis uses clathrin-coated vesicles.
• Fig 13-48 clathrin-coated vesicles in plasma membrane.
o Pinocytosis in all eukaryotic cells.
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Document Summary
In protozoa, phagocytosis is a form of feeding. Phagocytosis is important in most animals for purposes other than nutrition, and it is carried out mainly by specialized cells so-called professional phagocytes. That is, it requires the activation of cell-surface receptors that transmit signals to the cell interior. Thus, to be phagocytosed, particles must first bind to the surface of the phagocyte (although not all particles that bind are ingested): question: phagocytic cells such as macrophages engulf dead cells and even insert particles like plastic beads. Why don"t they accidentally engulf live cells: answer: live cells have don"t eat me protein signals on their plasma membranes that bind to inhibiting receptors on the macrophages. Since a cell"s surface area and volume remain unchanged during this process, it is clear that the same amount of membrane being removed by endocytosis is being added to the cell surface by the converse process of exocytosis.
