B2.1.13 Membrane fluidity and the fusion and formation of vesicles (HL) Notes

Vesicles Are Small But Powerful Tools for Cellular Transport

1. Their spherical structure and dynamic nature allow continuous cycles of formation, movement, and fusion.
2. This capability arises from the fluidity of membranes, enabling them to change shape as needed.

Membrane Fluidity Enables Vesicle Formation and Fusion

1. The fluid nature of membranes is essential for vesicle function:
   1. Phospholipids and proteins can move laterally within the membrane.
   2. This fluidity allows membranes to change shape—bending, pinching, and fusing.
   3. Without fluidity, vesicles could not form or merge with target membranes.
2. Key processes enabled by membrane fluidity:
   1. Vesicle formation: Membrane pinches off to create a vesicle.
   2. Vesicle fusion: Vesicle membrane merges with another membrane (organelle or plasma membrane).

How Vesicle Formation Works

1. Step 1: Membrane Bending
   1. A section of membrane curves inward or outward.
   2. Specific proteins help shape the membrane and select cargo (substances to be transported).
2. Step 2: Pinching Off
   1. The curved section pinches off completely, forming a closed vesicle.
   2. This process requires energy (ATP) and proteins that facilitate membrane fission.
3. Step 3: Vesicle Release
   1. The vesicle separates from the original membrane.
   2. It now contains enclosed materials and can move through the cytoplasm.

Endocytosis: Bringing Materials Into the Cell

1. How Endocytosis Works
   1. The plasma membrane curves inward around external material.
   2. The membrane pinches off, forming a vesicle inside the cell.
   3. The vesicle carries the enclosed material into the cytoplasm.

Exocytosis: Releasing Materials From the Cell

1. How Exocytosis Works
   1. A vesicle moves toward the plasma membrane.
   2. The vesicle membrane fuses with the plasma membrane (enabled by membrane fluidity).
   3. The vesicle's contents are expelled outside the cell.
   4. The vesicle membrane becomes part of the plasma membrane.

Internal Transport Using Vesicles

Vesicles also transport materials within the cell between organelles.

1. Example: Protein Transport from ER to Golgi
   1. Proteins are synthesized in the rough endoplasmic reticulum (rER).
   2. Proteins accumulate in vesicles that pinch off from the rER.
   3. Vesicles move through the cytoplasm to the Golgi apparatus.
   4. Vesicles fuse with the Golgi membrane, delivering proteins for processing and packaging.

Comparing Endocytosis and Exocytosis