vesicles
Summary
Vesicles are small membrane-bound sacs that carry cargo within a cell, into it or out of it. They bud from one membrane and fuse with another, so the bilayer is conserved and the vesicle interior stays equivalent to the extracellular space or organelle lumen, not the cytosol.
Detail
Meaning: A vesicle is a small, spherical, membrane-bound compartment made of a phospholipid bilayer. Vesicles move proteins and lipids between organelles and the plasma membrane. Examples include transport vesicles (rough ER to Golgi and onward), secretory vesicles (exocytosis), endocytic vesicles (phagocytosis, receptor-mediated endocytosis) and synaptic vesicles (neurotransmitter release triggered by Ca²⁺ entry). Lysosomes are Golgi-derived organelles that contain hydrolytic enzymes. Budding and fusion depend on coat proteins (such as clathrin) and on membrane-protein pairs that recognize the target membrane. Because the bilayer is pinched off and merged, the vesicle interior corresponds to the extracellular space or organelle lumen. Exocytosis therefore adds membrane to the cell surface and endocytosis removes it.
On the MCAT: Questions trace a secreted protein's path: rough ER → vesicle → Golgi → secretory vesicle → plasma membrane. Passages may describe a mutation or toxin that blocks vesicle fusion and ask what accumulates or which membrane shrinks or grows. Neuron questions link Ca²⁺ influx to synaptic vesicle fusion.
Common confusion: Vesicle contents are not in the cytosol, and exocytosis releases them without crossing the bilayer. Vesicle transport needs energy (ATP or GTP). Peroxisomes are not Golgi products; they import their proteins after translation.
Sources
- Molecular Biology of the Cell
- Campbell Biology
- Lehninger Principles of Biochemistry
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.