IB Biology Topic B2.2.9: Structure and Function of Vesicles in Cells Notes

Vesicles: The Cell's Internal Delivery System

1. Vesicles are small, membrane-bound sacs enclosed by a single phospholipid bilayer.
   1. Because their membrane has the same basic structure as the plasma membrane and organelle membranes, vesicles can fuse with these membranes, merging their contents and lipid bilayers.
   2. This makes vesicles the cell's main tool for moving materials between compartments.
2. Vesicles are constantly formed, transported along the cytoskeleton, and fused with target membranes, so they are not permanent structures.
   1. A single vesicle might exist for only seconds before delivering its cargo and merging with its destination.
   2. This constant turnover lets cells respond quickly, scaling secretion or uptake up or down as needed.

How Vesicles Form: Budding and Pinching Off

1. Vesicle formation starts when a region of membrane begins to curve.
   1. The membrane bulges inward (for endocytosis) or outward (for secretion from an organelle), enclosing the material to be transported.
   2. As the bulge deepens, the neck narrows until the membrane pinches off, forming a sealed sac with cargo inside.
2. This process needs ATP and specialised coat proteins that help curve the membrane into shape.
   1. Without these proteins, the membrane would not bend sharply enough to form a vesicle.

Clathrin: Scaffolding for Vesicle Formation

1. The best-studied coat protein is clathrin.
   1. Clathrin molecules assemble on the cytoplasmic face of the membrane, forming a lattice-like cage that forces the membrane to curve inward.
   2. As the lattice grows, it shapes the membrane into a deep pit, which pinches off to become a clathrin-coated vesicle.
2. Once the vesicle separates from the donor membrane, the clathrin coat disassembles and its subunits are recycled.
   1. The now-uncoated vesicle can fuse with its target membrane.
   2. Clathrin-mediated endocytosis is the main mechanism for receptor-mediated endocytosis, where specific molecules (like cholesterol bound to LDL) are selectively taken up.

Types of Vesicles and What They Do

1. Transport vesicles
   1. They shuttle proteins and lipids between organelles, most commonly from the rER to the Golgi and from the Golgi to other destinations.
   2. Each transport vesicle carries specific cargo selected during budding.
2. Secretory vesicles
   1. They store substances that will be released from the cell by exocytosis.
   2. The vesicle moves to the plasma membrane, fuses with it, and releases its contents outside; neurotransmitters and insulin are released this way.
3. Endocytic vesicles
   1. They form when the plasma membrane buds inward to bring extracellular material into the cell, a process called endocytosis.
   2. This can be non-selective, engulfing surrounding fluid, or highly selective, using membrane receptors that recognise specific molecules.
4. Lysosomes
   1. These are specialised vesicles filled with hydrolytic enzymes that work at an acidic pH (about 4.5 to 5).
   2. They break down waste, pathogens, and damaged organelles; digesting the cell's own worn-out organelles is called autophagy.

Vesicles Maintain and Grow Membranes

1. Vesicles do not just carry soluble cargo.
   1. When a vesicle fuses with the plasma membrane during exocytosis, its own lipid bilayer becomes part of the plasma membrane.
   2. This is how cells add new phospholipids and membrane proteins (made in the rER) to their surface.
2. Endocytosis works in reverse, removing membrane from the cell surface.
   1. The balance between exocytosis (adding membrane) and endocytosis (removing it) determines whether the plasma membrane grows, shrinks, or stays the same size.
   2. Growing cells tip the balance toward exocytosis, while cells that need to shrink tip it toward endocytosis.