Endocytosis and exocytosis

Vesicular transport: moving material in bulk

Vesicular transport moves substances inside small membrane-bound sacs called vesicles. Unlike a channel or a pump, which handles single molecules, a vesicle carries a volume of material and part of the membrane with it. The process costs ATP because the membrane must be reshaped and the vesicles moved along the cytoskeleton. There are two directions. Endocytosis brings material into the cell by folding the membrane inward and pinching off a vesicle. Exocytosis sends material out by fusing a vesicle with the cell membrane and opening it to the outside. Because membrane is removed from the surface during endocytosis and added during exocytosis, the two processes are linked, and the cell balances them to keep its surface area stable.

For example: A channel protein lets one ion pass at a time, while a single vesicle can carry thousands of molecules in one move. The two mechanisms solve different problems.

Endocytosis and its three forms

Endocytosis is the uptake of material into a cell by a vesicle formed from the cell membrane. It has three main forms. Phagocytosis, literally “cell eating”, takes in large particles such as bacteria, dead cells and debris; the membrane extends around the particle and encloses it in a large vesicle, which then fuses with a lysosome so the contents can be digested. The cells that do this are called phagocytes, and white blood cells such as macrophages and neutrophils are the main examples. Pinocytosis, “cell drinking”, takes in droplets of extracellular fluid and whatever is dissolved in them; it is non-selective and happens in most cells. Receptor-mediated endocytosis is selective: molecules in the fluid bind to specific receptors on the membrane, which gather into a pit that pinches off. This is how cells take up cholesterol carried in LDL particles and how some viruses and toxins enter cells.

For example: A macrophage extends its membrane around a bacterium, encloses it in a vesicle, and fuses that vesicle with a lysosome whose enzymes destroy the bacterium.

Common misconception: Pinocytosis is not selective. It takes in whatever is dissolved in the droplet, whereas receptor-mediated endocytosis selects specific molecules.

Exocytosis and secretion

Exocytosis is the release of material from a cell when a vesicle fuses with the cell membrane and opens to the outside. The vesicle’s membrane becomes part of the cell membrane, and its contents are discharged into the extracellular fluid. Exocytosis is the final step of the secretory pathway, so it is how cells release proteins that were built on the rough ER and packaged by the Golgi: hormones such as insulin, digestive enzymes, antibodies and mucus. It also releases signalling molecules such as neurotransmitters, which are stored in vesicles in nerve endings and released when a signal arrives. Some vesicles wait for a specific trigger; for example, a rise in blood glucose triggers insulin vesicles in a pancreatic cell to fuse with the membrane. Exocytosis is therefore both a delivery system and a way of adding membrane to the cell surface.

For example: A nerve cell stores neurotransmitters in vesicles; when a signal arrives, the vesicles fuse with the membrane and release their contents into the synapse.

Recycling the membrane

Endocytosis removes membrane from the cell surface and exocytosis adds it, so the two must be balanced to keep the cell’s size and surface area stable. In cells that secrete heavily, such as a pancreatic cell releasing insulin, the amount of membrane added by exocytosis is large, and an equal amount is reclaimed by endocytosis to prevent the cell from growing. In cells that take in a lot, endocytosis dominates for a time, and membrane is returned by exocytosis or delivered to lysosomes for recycling. The membrane itself is therefore not a fixed structure but a dynamic surface that is continuously renewed. This recycling also explains how a cell can change which receptors are present on its surface, adjusting its sensitivity to hormones and other signals.

For example: After a cell takes up cholesterol through LDL receptors, the receptors are often returned to the surface so they can be used again.

Why vesicular transport matters clinically

Vesicular transport appears in many clinical situations. Phagocytosis is central to defence against infection: when it is impaired, infections become more frequent and more severe. Receptor-mediated endocytosis explains inherited high cholesterol: if the receptors that take up LDL are faulty or too few, LDL remains in the blood and is deposited in arteries. Exocytosis explains how hormones and enzymes reach the blood, so a defect in the secretory pathway can cause a deficiency disease. Some bacterial toxins and viruses exploit endocytosis to enter cells, and some medicines are designed to block the receptors they use. Because vesicles move along the cytoskeleton and need ATP, anything that disrupts the cytoskeleton or the energy supply also disrupts secretion and uptake. Understanding these routes connects cell biology directly to infection, immunity and treatment.

For example: In familial hypercholesterolaemia, LDL receptors are defective, so receptor-mediated endocytosis of LDL is reduced and cholesterol accumulates in the blood.