Vesicles Are Small But Powerful Tools for Cellular Transport
Definition
Vesicles
Vesicles are small, membrane-bound sacs composed of a phospholipid bilayer. They enclose a fluid-filled space and can carry various substances.
- Their spherical structure and dynamic nature allow continuous cycles of formation, movement, and fusion.
- This capability arises from the fluidity of membranes, enabling them to change shape as needed.
Membrane Fluidity Enables Vesicle Formation and Fusion
- The fluid nature of membranes is essential for vesicle function:
- Phospholipids and proteins can move laterally within the membrane.
- This fluidity allows membranes to change shape—bending, pinching, and fusing.
- Without fluidity, vesicles could not form or merge with target membranes.
- Key processes enabled by membrane fluidity:
- Vesicle formation: Membrane pinches off to create a vesicle.
- Vesicle fusion: Vesicle membrane merges with another membrane (organelle or plasma membrane).
How Vesicle Formation Works
- Step 1: Membrane Bending
- A section of membrane curves inward or outward.
- Specific proteins help shape the membrane and select cargo (substances to be transported).
- Step 2: Pinching Off
- The curved section pinches off completely, forming a closed vesicle.
- This process requires energy (ATP) and proteins that facilitate membrane fission.
- Step 3: Vesicle Release
- The vesicle separates from the original membrane.
- It now contains enclosed materials and can move through the cytoplasm.
Endocytosis: Bringing Materials Into the Cell
Definition
Endocytosis
The process by which the plasma membrane pinches inward to form a vesicle that brings external materials into the cell.
