Endocytosis and exocytosis are the processes by which cells move materials into or out of the cell that are too large to directly pass through the lipid bilayer of the cell membrane. Large molecules, microorganisms and waste products are some of the substances moved through the cell membrane via exocytosis and endocytosis.
Why is bulk transport important for cells?
Cell membranes are semi-permeable, meaning they allow certain small molecules and ions to passively diffuse through them. Other small molecules are able to make their way into or out of the cell through carrier proteins or channels.
But there are materials that are too large to pass through the cell membrane using these methods. There are times when a cell will need to engulf a bacterium or release a hormone. It is during these instances that bulk transport mechanisms are needed.
Endocytosis and exocytosis are the bulk transport mechanisms used in eukaryotes. As these transport processes require energy, they are known as active transport processes.
Vesicle function in endocytosis and exocytosis
During bulk transport, larger substances or large packages of small molecules are transported through the cell membrane, also known as the plasma membrane, by way of vesicles – think of vesicles as little membrane sacs that can fuse with the cell membrane.
Cell membranes are comprised of a lipid bilayer. The walls of vesicles are also made up of a lipid bilayer, which is why they are capable of fusing with the cell membrane. This fusion between vesicles and the plasma membrane facilitates bulk transport both into and out of the cell.
What is endocytosis? Endocytosis definition and purposes
Endocytosis is the process by which cells take in substances from outside of the cell by engulfing them in a vesicle. These can include things like nutrients to support the cell or pathogens that immune cells engulf and destroy.
Endocytosis occurs when a portion of the cell membrane folds in on itself, encircling extracellular fluid and various molecules or microorganisms. The resulting vesicle breaks off and is transported within the cell.
Endocytosis serves many purposes, including:
- Taking in nutrients for cellular growth, function and repair: Cells need materials like proteins and lipids to function.
- Capturing pathogens or other unknown substances that may endanger the organism: When pathogens like bacteria are identified by the immune system, they are engulfed by immune cells to be destroyed.
- Disposing of old or damaged cells: Cells must be safely disposed of when they stop functioning properly to prevent damage to other cells. These cells are eliminated through endocytosis.
Types of endocytosis
There are two types of endocytosis: phagocytosis and pinocytosis.
Phagocytosis, also known as cell eating, is the process by which cells internalize large particles or cells, like damaged cells and bacteria.
Within the human body, and in other mammals, phagocytosis is how immune cells engulf and destroy dangerous microorganisms or toxic compounds. Macrophages and neutrophils, types of white blood cells, are the two primary phagocytes. These white blood cells are responsible for clearing out aged and damaged cells, as well as disposing of infectious microorganisms.
Pinocytosis, also known as cell drinking, is common in plant and animal cells. During pinocytosis, the cell takes in substances from the extracellular fluid that it needs to function. These include things like water and nutrients.
Receptor-mediated endocytosis is a specialized type of pinocytosis. During receptor-mediated endocytosis, macromolecules bind to receptors along the surface of the cell’s plasma membrane. Cholesterol uptake is an example of receptor-mediated endocytosis.
The steps of endocytosis
The following is an outline of the basic steps of the two types of endocytosis.
Two types of endocytosis: phagocytosis and pinocytosis.
- A particle or substance binds to receptors on the cell’s surface, stimulating the release of pseudopodia (extensions of the plasma membrane filled with cytoplasm).
- Pseudopodia surround the object until their membranes fuse, forming a phagocytic vesicle.
- The phagocytic vesicle pinches off from the cell membrane, entering the cell.
- The phagocytic vesicle fuses with lysosomes, which recycle or destroy the vesicle’s contents.
- Molecules bind to receptors located along the surface of the cellular membrane.
- The plasma membrane folds in, forming a pinocytic vesicle that contains the molecules and the extracellular fluid.
- The pinocytic vesicle detaches from the cell membrane inside the cell.
- The vesicle fuses with early endosomes where the contents found within are sorted.
Macrophages are a type of white blood cell that play a central role in protecting mammals against pathogens like bacteria and viruses. When a macrophage comes into contact with a virus, say a cold virus in the bloodstream, it can bind to the virus’s cell surface.
Next, the macrophage will form a vesicle around the virus, completely ingesting it. The vesicle then travels to the cytosol and fuses with the lysosome, where the virus is broken down. Some viruses replicate by “tricking” host cells into endocytosing them, at which point the cell is hijacked by the virus and is instructed to replicate the virus genome and capsid.
What is exocytosis? Exocytosis definition and purposes
Exocytosis is the process by which cells move materials from within the cell into the extracellular fluid. Exocytosis occurs when a vesicle fuses with the plasma membrane, allowing its contents to be released outside the cell.
Exocytosis serves the following purposes:
- Removing toxins or waste products from the cell’s interior: Cells create waste or toxins that must be removed from the cell to maintain homeostasis. For instance, in aerobic respiration, cells produce the waste products carbon dioxide and water during ATP formation. Carbon dioxide and water are removed from these cells via exocytosis.
- Facilitating cellular communication: Cells create signaling molecules like hormones and neurotransmitters. They are delivered to other cells following their release from the cell through exocytosis.
- Facilitating cellular membrane growth, repair, signaling and migration: When cells absorb materials from outside the cell during endocytosis, they use lipids and proteins from the plasma membrane to create vesicles. When certain exocytotic vesicles fuse with the cellular membrane, they replenish the cell membrane with these materials.
Types of exocytosis
Regulated ExocytosisMost exocytotic vesicles contain substances created within the endoplasmic reticulum for use elsewhere in the body, such as neurotransmitters or hormones. These molecules are then packaged within a layer of membrane called a vesicle.
Once excreted from the endoplasmic reticulum, these vesicles are transported to the Golgi apparatus (also known as the Golgi complex) for further modification. The molecules are then packaged once again in a vesicle that makes its way to the plasma membrane.
The release of these molecules from the cell is termed regulated exocytosis because the expulsion of the materials is controlled, or regulated, by extracellular signals that cause membrane depolarization.
Constitutive exocytosis, in contrast, doesn’t require any extracellular signals. The majority of molecules traveling to the plasma membrane do so using this pathway.
After exocytosis, some exocytotic vesicles are incorporated into the plasma membrane (full vesicle fusion), while others return to the interior of the cell after their contents have been released (this is termed the “kiss-and-run” pathway). Others remain docked to the membrane, where they can be used multiple times (the “kiss-and-stay” pathway).
Exocytosis involves the passage of a vesicle from the endoplasmic reticulum or Golgi apparatus, through the cytoplasm to the cell membrane, where it fuses and releases its contents.
The steps of exocytosis
Below is an outline of the basic steps of exocytosis.
- A vesicle is formed, typically within the endoplasmic reticulum and the Golgi apparatus or early endosomes.
- The vesicle travels to the cell membrane.
- The vesicle fuses to the plasma membrane, during which the two bilayers merge.
- The vesicle’s contents are released into the extracellular space.
- The vesicle either fuses with or separates from the cell membrane.
Let’s take the macrophage that we discussed in our endocytosis example. Once the white blood cell has engulfed a foreign pathogen eliminate it, certain parts of the pathogen are no longer needed. The macrophage gets rid of this waste material through exocytosis, during which vesicles carry out the unwanted pathogen material.
Endocytosis vs exocytosis: a comparison
|Definition||The process of taking a particle or substance from outside of the cell and transferring it inside the cell using a vesicle. ||The process of taking a substance or particle from inside of the cell and transferring it to outside the cell using a vesicle. |
|Examples||White blood cells engulfing a virus and eliminating it.||Releasing a neurotransmitter for cellular communication.|