Exocytosis facts for kids
Exocytosis is how a cell sends things out, like tiny packages of molecules. Think of it as the cell's way of taking out the trash or sending important messages. This process uses energy, so it's a type of active transport. Cells use exocytosis (and its opposite, endocytosis) because many important substances are too big or too "water-loving" to simply pass through the cell's outer skin, the cell membrane.
During exocytosis, small sacs called vesicles carry these molecules. These vesicles travel to the cell membrane, and then they temporarily join with it. When they join, the contents of the vesicle, like proteins or hormones, are released outside the cell.
Exocytosis also helps cells add new parts to their cell membrane. Vesicles can carry new proteins (like ion channels or cell surface receptors) or lipids. When these vesicles fuse with the cell membrane, their contents become a permanent part of the membrane.
Contents
How Exocytosis Was Discovered
The word "exocytosis" was first suggested by a scientist named Christian de Duve in 1963.
Types of Exocytosis
In living things with many cells, there are two main ways exocytosis happens:
- Regulated Exocytosis: This type needs a special signal, often a rise in calcium inside the cell. It's like a secret handshake that tells the cell to release its packages. A good example is how brain cells (neurons) send signals to each other. They release special chemicals called neurotransmitters only when a signal arrives and calcium levels go up.
- Constitutive Exocytosis: This type happens all the time, without needing a special signal. It's like a constant delivery service. All cells do this to release materials that build up the space around cells (called the extracellular matrix) or to add new proteins to their cell membrane.
Scientists have also found a third type in some tiny living things called prokaryotes, like certain bacteria. These bacteria can pinch off tiny sacs from their outer membrane. These sacs carry signals to other cells, helping the bacteria control their environment or even invade a host. This shows that exocytosis isn't just for complex cells!
Steps of Exocytosis
Exocytosis is a carefully planned process that involves several steps:
Moving the Packages (Vesicle Trafficking)
First, the vesicles, which are like tiny delivery trucks, need to move to the right place. For example, vesicles carrying proteins from the Golgi apparatus (the cell's packing center) to the cell surface often use special "motor proteins" that travel along tracks inside the cell. These tracks are made of cytoskeleton parts, like actin and microtubules. Once the vesicles get close to their target, they meet special "tethering factors" that help hold them in place.
Getting Close (Vesicle Tethering)
After traveling, vesicles get "tethered" or loosely connected to the cell membrane. This is like a boat getting tied to a dock, but still having some room to move. This step helps gather the vesicles in the right spot, especially in brain cells where many signal packages need to be ready.
Attaching to the Membrane (Vesicle Docking)
Next, the vesicles "dock" or attach more firmly to the cell membrane. They get very close, almost touching. This prepares them for the final step of joining with the membrane.
Getting Ready (Vesicle Priming)
In brain cells, there's an extra step called "priming." This is like getting everything perfectly set up so that when a calcium signal arrives, the neurotransmitters can be released almost instantly. It involves many changes to proteins and fats in the vesicle and membrane. Cells that release things constantly (constitutive exocytosis) don't need this priming step.
Merging and Releasing (Vesicle Fusion)
Finally, the vesicle's membrane merges with the cell's outer membrane. This is driven by special proteins called SNARE proteins. When the membranes merge, the contents of the vesicle are released outside the cell.
This merging does three important things:
- It adds new surface area to the cell membrane, which is important for cell growth.
- It releases the substances inside the vesicle, which can be waste products, hormones, or neurotransmitters that send messages.
- Any Proteins that were part of the vesicle's membrane now become part of the cell's outer membrane. This helps the cell control what goes in and out.
Taking Back the Packages (Vesicle Retrieval)
After releasing their contents, vesicles are often taken back into the cell through a process called endocytosis. Some vesicles might just "kiss and run," meaning they briefly open, release some contents, and then quickly detach and go back inside. Other vesicles fully merge and then need to be completely rebuilt from the cell membrane by a special group of proteins.
Both releasing and taking back vesicles use a lot of energy. This energy comes from the cell's powerhouses, the mitochondria. Scientists have seen that after a cell releases its contents, there are often partially empty vesicles left. This suggests that vesicles can be reused many times until they are completely empty.
Images for kids
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This picture shows the tiny parts that make exocytosis happen when brain cells release messages. The SNARE proteins help the vesicle join with the cell membrane, and synaptotagmin acts like a sensor for calcium.
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This diagram shows how two membranes might curve and join together to form a "fusion pore," allowing the contents of the vesicle to be released.
See also
In Spanish: Exocitosis para niños
