Movement of Large Molecules: Exocytosis & Endocytosis
(a) Large molecules (proteins, nucleic acids, polypeptides larger than a few amino acids, polysaccharides larger than a few sugars) are not carried by transport proteins.
(b) There are mechanisms for moving larger molecules, but they don’t enter into
cytoplasm.
(i) Exocytosis
Membrane vesicle fuses with cell membrane, releases enclosed material to extracellular space. Ex: release of digestive enzymes from pancreatic cells; mucus, milk, hormones, etc.(ii) Endocytosis
Cell membrane invaginates, pinches in, creates vesicle enclosing contents.Three common situations
1. Phagocytosis
Typically works on debris, bacteria, other particulate matter. Contents of the “phagosome” are usually fused with lysosome to create “phagolysosome”, where material is broken down. Especially common in white blood cells such as macrophages and other leukocytes.2. Pinocytosis
Similar to phagocytosis, but ingests fluid rather than particulate matter. “Cell drinking”. Ex. cells lining blood capillariestake fluid from blood (but not red cells), move fluid across their cytoplasm, release into extracellular space surrounding cells outside the capillary.
3. Carrier-mediated endocytosis (CME)/ receptor-mediated endocytosis:
Very specialized system. Certain important molecules or ions are not brought into cell by transport processes, but by CME. The body uses hormones to regulate membrane transport in this way Eg. 1. iron is carried through blood tightly bound to transferrin protein carrier. To get iron into cells, cell membrane contains special receptorproteins that bind transferrin, move towards special regions of membrane under which lie clathrin proteins.
Endocytosis occurs inside clathrin “cage”, moves inside cell. Cage eventually recycles back to cell surface, returning transferrin proteins to cell exterior. However, iron is released inside cell, exits from vesicles, becomes bound to ferritin.
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