Exocytosis - Online Tutor, Practice Problems & Exam Prep

Exocytosis is the process of secreting molecules outside cells, occurring via two main pathways: the constitutive secretory pathway, which operates continuously, and the regulated secretory pathway, which is activated by signals like calcium in specialized cells. This process involves vesicles carrying cargo that fuse with the plasma membrane, releasing their contents. The fusion increases membrane size, balanced by endocytosis, which retrieves excess membrane. Key in neurotransmitter release, exocytosis is essential for cellular communication and maintaining membrane integrity.

Exocytosis

Hi. In this video, we're going to be talking about Exocytosis. So, Exocytosis is the process of secreting molecules outside the cells. It's a process of getting things out. So, there are 2 main pathways. The first is the constitutive secretory pathway, and this is going to be a pathway that operates all the time, just continuously, and it's going to be the default pathway. So, most things that need to get out of the cell are just going to go through this pathway, and it's just constantly running, constantly getting rid of things. This is the pathway also that supplies the plasma membrane with newly synthesized lipids. So, the plasma membrane just doesn't exist for all eternity. It constantly needs to be replaced. And so this pathway brings those new lipids to the plasma membrane in the process of secreting something else. Now there's also a separate pathway, this is the regulated secretory pathway, and so this occurs only in specialized cells that produce and release a lot of molecules. And so, how this pathway works is it's got this continuous pathway. Instead, special secretory vesicles actually store whatever molecule needs to be released. And they bud from the Golgi and they dock, so they just kind of sit at the plasma membrane waiting for their release. When they get some type of signal, usually, the signal is calcium, but it can be other things, the vesicles will fuse and release all that material into the extracellular environment. So this is really common in nerve cells. And so both of these vesicles that are, you know, taking up material somewhere in the Golgi or somewhere in the cell and transporting them out, travel on microtubules, which guide vesicles to the cell surface. So here's an area really close to the plasma membrane, and when a chemical signal comes in, so for, say, calcium, this results in fusion and release of these molecules extracellularly, which can then go on and do other things, like in nerve or neuron transmission neurotransmitter transmission. But exocytosis really occurs in 4 steps. So the first thing is that there's a vesicle, it's carrying something, it's carrying cargo, and it moves towards the plasma membrane. Now, it doesn't necessarily move everywhere in the plasma membrane. Right? There are specific domains of the plasma membrane that exist. So for instance, polarized membranes have different domains, which means one section of the plasma membrane is different chemically than another section. And so, vesicles are usually targeted to one domain only and not all of them. And this is through various sorting signals, where the membrane fusion causes the membrane to become larger because there's more membrane there. Luckily, the cell isn't just entirely growing, just larger and larger every time something needs to be exocytosed, because the opposite, endocytosis, molecules entering the cell, will actually take that extra membrane and bring it into the cell. So here we have, vesicle fusion. We have a vesicle. It's headed towards the plasma membrane. It can dock really close here. It releases it fuses, releases its content, and eventually, this entire membrane will be added to the, membrane fused with, in this case, the plasma membrane. But that is okay, because endocytosis will take that extra membrane and bring it back into the cell. So that is the process of exocytosis. So now let's turn the page.

Which of the following is not a pathway of exocytosis?

• A. Constitutive secretory pathway
• B. Regulated secretory pathway
• C. Continuing secretory pathway

When a vesicle fuses to the plasma membrane what happens to the lipids in the vesicles?

• A. They are released with the contents of the vesicle
• B. They are degraded
• C. They are incorporated into the plasma membrane

Here’s what students ask on this topic:

What is exocytosis and why is it important?

Exocytosis is the process by which cells secrete molecules outside their plasma membrane. It involves vesicles carrying cargo that fuse with the plasma membrane, releasing their contents into the extracellular space. This process is crucial for various cellular functions, including neurotransmitter release in nerve cells, hormone secretion, and the delivery of newly synthesized lipids to the plasma membrane. Exocytosis helps maintain cellular communication, membrane integrity, and overall cellular homeostasis.

What are the main pathways of exocytosis?

Exocytosis occurs via two main pathways: the constitutive secretory pathway and the regulated secretory pathway. The constitutive pathway operates continuously and is the default route for most molecules that need to be secreted. It also supplies the plasma membrane with newly synthesized lipids. The regulated secretory pathway, on the other hand, is activated by specific signals, such as calcium, and is used by specialized cells to release large amounts of molecules stored in secretory vesicles.

How does the regulated secretory pathway work?

In the regulated secretory pathway, specialized cells store molecules in secretory vesicles that bud from the Golgi apparatus. These vesicles dock near the plasma membrane and wait for a signal, usually calcium, to trigger their fusion with the membrane. Upon receiving the signal, the vesicles fuse with the plasma membrane, releasing their contents into the extracellular space. This pathway is common in nerve cells for neurotransmitter release and in other cells for hormone secretion.

What role do microtubules play in exocytosis?

Microtubules play a crucial role in exocytosis by guiding vesicles carrying cargo to the cell surface. These cytoskeletal structures provide tracks along which vesicles can travel from their origin, such as the Golgi apparatus, to the plasma membrane. This ensures that vesicles reach their target destination efficiently, facilitating the timely release of their contents during exocytosis.

How is membrane size regulated during exocytosis?

During exocytosis, the fusion of vesicles with the plasma membrane increases the membrane's size. However, this is balanced by endocytosis, the process of taking molecules into the cell, which retrieves excess membrane. This dynamic balance ensures that the cell does not grow uncontrollably in size and maintains membrane integrity and homeostasis.