IB Biology Topic B2.1.13: Membrane Fluidity and the Fusion and Formation of Vesicles Notes

Vesicles Are Small but Powerful Tools for Cellular Transport

Membrane Fluidity Lets Vesicles Form and Fuse

1. Their spherical structure lets them constantly form, move, and fuse.
2. This depends on the fluidity of membranes, which lets them change shape as needed.
3. Phospholipids and proteins can move laterally within the membrane.
   1. This fluidity lets membranes bend, pinch, and fuse.
   2. Without fluidity, vesicles could not form or merge with target membranes.
4. Vesicle formation: the membrane pinches off to create a vesicle.
5. Vesicle fusion: the vesicle membrane merges with another membrane, such as an organelle or the plasma membrane.

How Vesicle Formation Works

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

Endocytosis: Bringing Materials Into the Cell

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.

Endocytosis in Action

Exocytosis: Releasing Materials From the Cell

How Exocytosis Works

1. A vesicle moves toward the plasma membrane.
2. The vesicle membrane fuses with the plasma membrane, which membrane fluidity makes possible.
3. The vesicle contents are expelled outside the cell.
4. The vesicle membrane becomes part of the plasma membrane.

Exocytosis in Action

Vesicles Also Transport Materials Inside the Cell

Vesicles carry materials within the cell, moving between organelles.

1. Protein transport from ER to Golgi
   1. Proteins are made in the rough endoplasmic reticulum (rER).
   2. Proteins collect 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