Overview
This lecture explains vesicular transport, focusing on endocytosis and exocytosis, key processes used by cells to move large particles into and out of the cell.
Vesicular Transport Basics
- Vesicular transport uses membrane-bound sacs (vesicles) to move large or numerous particles across the cell membrane.
- This process differs from simple diffusion or active transport, as it handles bulk movement using ATP energy.
- Large molecules like proteins, macromolecules, water, and hormones are moved using vesicles.
Types of Vesicular Transport
- Two main types: endocytosis (into the cell) and exocytosis (out of the cell).
- Endocytosis includes phagocytosis, pinocytosis, and receptor-mediated endocytosis.
Endocytosis
- Endocytosis involves the cell membrane folding inward to bring materials inside the cell.
Phagocytosis ("Cell Eating")
- Performed by specialized cells (phagocytes, e.g., macrophages, neutrophils) of the immune system.
- Pseudopods (temporary arm-like extensions) surround and engulf large particles like bacteria, forming a phagosome.
- Engulfed material is digested after fusion with lysosomes containing enzymes.
Pinocytosis ("Cell Drinking")
- All cells perform pinocytosis, ingesting extracellular fluid and dissolved solutes non-selectively.
- The cell membrane folds inward, creating a vesicle that brings in fluid and solutes ("sampling" the environment).
Receptor-Mediated Endocytosis
- Cells selectively take in specific molecules that bind to receptors on the cell surface.
- Clathrin proteins help form coated pits that carry receptor-bound substances into the cell.
- Allows uptake of items like enzymes, hormones, LDL cholesterol, iron, and even some viruses.
Regulation of Cell Surface Receptors
- Down regulation reduces the number of surface receptors, decreasing the cell's responsiveness (e.g., to excess cortisol).
- Up regulation increases receptor numbers, restoring cell sensitivity; mechanisms are not fully understood.
Exocytosis
- Exocytosis expels materials from the cell using vesicles, often produced by the Golgi apparatus.
- The vesicle fuses with the cell membrane, releasing its contents to the extracellular environment (e.g., proteins, hormones).
Key Terms & Definitions
- Vesicle — membrane-bound sac that transports substances in/out of cells.
- Endocytosis — process of materials entering a cell via vesicle formation.
- Phagocytosis — "cell eating"; engulfment of large particles by phagocytes.
- Pinocytosis — "cell drinking"; non-selective uptake of extracellular fluid.
- Receptor-mediated endocytosis — selective uptake of molecules bound to cell surface receptors.
- Exocytosis — process of vesicles fusing with the membrane to release contents outside.
- Pseudopod — temporary membrane extensions for engulfing material.
- Clathrin — protein involved in forming coated vesicles during endocytosis.
- Down regulation — decrease in receptor number on the cell surface.
- Up regulation — increase in receptor number on the cell surface.
Action Items / Next Steps
- Review the mechanisms and differences among phagocytosis, pinocytosis, and receptor-mediated endocytosis.
- Understand examples and significance of exocytosis in cellular secretion.
- Optional: Look up General Adaptation Syndrome for more on cortisol regulation.