B2.2.9 Structure and function of vesicles in cells (HL) Notes

Vesicles Facilitate Transport, Storage, and Digestion in Cells

1. Dynamic Nature: They are constantly forming, transporting, and fusing with other membranes, allowing the cell to rapidly adapt to changing needs.
2. Membrane Composition: Their bilayer structure is similar to the plasma membrane, enabling fusion and exchange with organelles or the cell surface.
3. Specialized Functions: Different vesicles store, transport, or digest cellular materials, helping maintain orderwithin the cell.

Key Characteristics of Vesicles

1. Membrane Composition
   1. The vesicle membrane is composed of a phospholipid bilayer, similar to the plasma membrane.
   2. This feature allows vesicles to fuse with other membranes, such as the plasma membrane or organelle membranes, facilitating material exchange.
2. Dynamic Nature: Vesicles are continuously formed and recycled, enabling cells to respond quickly to changing needs.
3. Specialized Functions: Vesicles transport materials, store substances, and even digest cellular debris.

Functions of Vesicles

1. Transport of Substances

1. Exocytosis: Vesicles carry materials (e.g., neurotransmitters, hormones) to the plasma membrane for secretion.
2. Endocytosis: Vesicles form by budding in from the plasma membrane to bring external materials (e.g., nutrients, pathogens) into the cell.
3. Intracellular Transport: Vesicles shuttle proteins, lipids, and waste products between organelles (e.g., rER → Golgi → lysosomes).

2. Membrane Growth and Maintenance

1. Delivery of Membrane Components: Vesicles transport phospholipids and membrane proteins synthesized in the rER to the plasma membrane, aiding cell growth and repair.
2. Organelle Function: Vesicles deliver enzymes and cargo to organelles like lysosomes, ensuring they functionproperly (e.g., in digestion or recycling cell components).

How Are Vesicles Formed?

1. Budding Process: A membrane (e.g., plasma membrane or organelle membrane) bulges inward or outward, enclosing the material to be transported.
2. Pinching Off: The membrane then pinches off, forming a sealed vesicle that carries its cargo.
3. Energy Requirement: This process requires ATP and the action of specialized proteins that help curve the membrane.

The Role of Clathrin in Vesicle Formation

1. Clathrin Assembly: Clathrin proteins form a lattice-like network on the inner surface of the budding membrane, causing it to indent.
2. Vesicle Budding: As the clathrin coat forms, the membrane pinches off, creating a clathrin-coated vesicle.
3. Coat Removal: Once the vesicle is free, clathrin disassembles, allowing the uncoated vesicle to fuse with its target membrane.

Types of Vesicles and Their Functions

1. Transport Vesicles: Move proteins and lipids between the ER and Golgi, or from the Golgi to other organelles.
2. Secretory Vesicles: Store substances (e.g., insulin) for exocytosis, releasing them outside the cell.
3. Lysosomes: Contain digestive enzymes that break down waste, pathogens, or damaged organelles (autophagy).
4. Endocytic Vesicles: Form during endocytosis, bringing extracellular materials into the cell, including receptor-mediated uptake of specific molecules.

Why Are Vesicles Important?

1. Compartmentalization: Vesicles isolate chemical reactions (e.g., digestive enzymes in lysosomes) to prevent damage to other cellular components.
2. Efficient Transport: They enable rapid and targeted movement of molecules across the cell, vital for processes like hormone secretion or nutrient uptake.
3. Adaptive Responses: Cells can quickly increase or decrease secretion and endocytosis, helping them respond to environmental changes.