Shannon teaches Microbiology and has a Master's and a PhD in Biomedical Science. She also researches cancer and neurodegenerative diseases.
What Is Exocytosis?
Exocytosis is a process by which a cell transports secretory products through the cytoplasm to the plasma membrane. Secretory products are packaged into transport vesicles (membrane-bound spheres). Let's look at some examples of cellular secretory products:
- Secreted protein - enzymes, peptide hormones, and antibodies
- Neurotransmitters from nerve cells
- Plasma membrane proteins
- Antigens - pieces of bacteria or other invaders which stimulate the immune response
Exocytosis can be either calcium-dependent or calcium-independent. In calcium-dependent exocytosis, an influx of calcium into the cell will stimulate secretion. This happens when a protein is only secreted when needed. Transport vesicles with their cargo will travel from the Golgi apparatus (the protein-packaging organelle) to the plasma membrane. The vesicle then docks at the plasma membrane and waits for a secretion signal. This is also called regulated exocytosis because secretion from the vesicles is controlled.
In contrast, calcium-independent exocytosis occurs constantly. Just like with regulated secretion, the transport vesicle and its cellular product will travel through the cell to the plasma membrane. Unlike regulated secretion, the vesicle does not wait at the plasma membrane. Instead, fusion with the plasma membrane and secretion of vesicle contents into the extracellular environment happens automatically. This constant flow of secretory product is also called constitutive exocytosis because secretion from the vesicles is not controlled. The following figure illustrates the differences:
An error occurred trying to load this video.
Try refreshing the page, or contact customer support.
You must cCreate an account to continue watching
Register to view this lesson
As a member, you'll also get unlimited access to over 84,000 lessons in math, English, science, history, and more. Plus, get practice tests, quizzes, and personalized coaching to help you succeed.
Get unlimited access to over 84,000 lessons.Try it now
Already registered? Log in here for accessBack
Example: Regulated Exocytosis
Cells will use the products they secrete to communicate with each other. An example of this is a neuron, or nerve cell. How does this work? Say you want to move your big toe. Neurons in your brain are in contact with neurons in your spine. The neurons in your spine are in contact with neurons that lead to your big toe. In your big toe, neurons are in contact with the muscles in your big toe.
For you to move your big toe, your thought has to be turned into an electrical impulse. This impulse will travel down your nerves to your toe muscles. This stimulates your toe muscles to contract, and your toe moves! So, how does exocytosis come into play? Well, there is a gap between each neuron. The gap is very tiny, but large enough to stop the electrical impulse.
Luckily, your neurons store neurotransmitters (signal molecules) at the plasma membrane in this gap. The electrical impulse travels down the neuron to the gap. At the gap, the electrical impulse stimulates neurotransmitter release. The cell on the other side of the gap will bind to the neurotransmitters, and an electrical impulse begins in the recipient cell. This continues until the impulse reaches its destination: the muscles of your big toe. The following figure illustrates this point:
Example: Constitutive Exocytosis
Adrenocorticotropic hormone (ACTH) is the protein in charge of causing cortisol release from your adrenal glands. Cortisol is the natural steroid in your body. It's needed when you get stressed out, say during an exam (psychological stress) or when you are sick (physical stress). These are examples of when you need a big dose of cortisol. However, a low, constant level of cortisol is needed all the time. Since ACTH stimulates cortisol release, ACTH is released at a low, constitutive level.
Exocytosis is the process by which cargo-laden transport vesicles move through the cytoplasm to the plasma membrane. Once at the plasma membrane, two different things can happen. Sometimes, the vesicle can dock and wait for a secretion signal. This happens in calcium-dependent (regulated) exocytosis. Other times the vesicle does not dock. Instead, it fuses with the plasma membrane and secretes its contents into the extracellular environment. This happens with calcium-independent (constitutive) exocytosis. Whether a vesicle docks and waits or fuses depends on the cargo within the vesicle.
You'll have the opportunity to pursue these goals after viewing the lesson on exocytosis:
- Provide a description of exocytosis
- Present examples cellular secretory products
- Discuss the occurrence of regulated exocytosis and constitutive exocytosis
To unlock this lesson you must be a Study.com Member.
Create your account
Register to view this lesson
Unlock Your Education
See for yourself why 30 million people use Study.com
Become a Study.com member and start learning now.Become a Member
Already a member? Log InBack
Exocytosis: Definition & Examples
Related Study Materials
Explore our library of over 84,000 lessons
- College Courses
- High School Courses
- Other Courses
- Create a Goal
- Create custom courses
- Get your questions answered