2.14: Vesicular Transport — Endocytosis and Exocytosis

 Vesicular transport is also a form of active transport that moves substances into or out of the cell using membrane-bound vesicles.

Master this section and you'll be able to

• Describe the role of vesicular transport as a form of active transport.
• Outline the similarities and differences of endocytosis and exocytosis.
• Compare the different types of endocytosis, specifically phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Vesicular transport is another type of active transport in which materials are moved into or out of the cell using small, membrane-bound sacs called vesicles. The process requires energy (usually from ATP) and allows the cell to transport large molecules, fluids, or even particles that cannot cross the membrane directly. It includes both endocytosis (bringing substances into the cell) and exocytosis (releasing substances out of the cell). Endocytosis takes up particles into the cell by invaginating the cell membrane, resulting in the release of the material inside of the cell.
 

Endocytosis: Bringing Stuff into the Cell

This type of active transport moves liquids and large substances — including big molecules, cell fragments, or even entire cells — into a cell.

There are several variations of endocytosis, but they all share one key feature:
the plasma membrane folds inward (invaginates) to form a pocket around the material to be transported.

This pocket then pinches off from the membrane, forming a vesicle inside the cell that contains the material. The vesicle is made from the very same plasma membrane that surrounded the cell to begin with — a clever bit of cellular recycling!

There are three main types of endocytosis:

1) Phagocytosis: Cell Eating 🍽️

Figure \(\PageIndex{1}\): Phagocytosis. In phagocytosis, the cell membrane surrounds the particle and engulfs it.
 

Phagocytosis is a form of endocytosis often described as “cell eating.” It is how special cells — Phagocytes — take in very large particles, such as cell debris, foreign invaders, or even entire cells.

For example, when harmful microorganisms enter the body, neutrophils and macrophages (both phagocytic cells or phagocytes) respond by surrounding and engulfing the invader. The engulfed material is then destroyed inside the phagocyte. This process is a critical part of your immune defense system.
 

How Does Phagocytosis Work?

Before phagocytosis begins, a section of the inner surface of the plasma membrane becomes coated with a special protein called clathrin. This protein helps stabilize and shape the forming vesicle.

The clathrin-coated membrane reaches out around the target particle and eventually encloses it, forming a vesicle inside the cell. Once the vesicle is fully inside:

• Clathrin is released, and

• The vesicle fuses with a lysosome  and is now called a "phagolysosme". Remember, lysosomes are organelles filled with digestive enzymes. The engulfed material is then broken down.

Once useful nutrients are absorbed, the remaining waste is expelled when the phagolysosome releases its contents by fusing with the plasma membrane. The vesicle membrane is then recycled, becoming part of the cell’s surface once again.
 

2) Pinocytosis: Cell Drinking 🥛

Figure \(\PageIndex{2}\): Pinocytosis. In pinocytosis, the cell membrane invaginates, surrounding a small volume of fluid and pinching it off.
 

"Pino-" = Greek for "drink". So, pinocytosis literally means “cell drinking.” It got this name when scientists first thought that cells were purposefully sipping extracellular fluid.

What is actually happening? The cell is bringing in small amounts of fluid from its surroundings, along with any dissolved nutrients or molecules it might need.

Unlike phagocytosis (which brings in big particles), pinocytosis creates smaller vesicles, and these do not fuse with lysosomes. Instead, they help the cell absorb useful substances directly from the fluid outside.

 

3) Receptor-mediated Endocytosis

Figure \(\PageIndex{3}\): Receptor-Mediated Endocytosis. In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type of substance that binds to the receptor on the external surface of the cell membrane.
 

Sometimes a cell wants to take in something very specific — like a VIP guest at a party. That is where receptor-mediated endocytosis comes in. This is a special form of vesicular transport where the cell uses receptor proteins on its surface to "catch" and bring in particular molecules from the extracellular fluid. Once the right ligand (molecule) binds to the receptor, a clathrin-coated pit forms on the inside of the membrane, creating a vesicle that pulls the desired molecules inside the cell.

This process is highly selective — and very important. If receptor-mediated endocytosis does not work properly, the cell cannot remove certain substances from the blood or tissues, and their levels can rise to dangerous levels. A real-life example? LDL, or "bad" cholesterol, is normally cleared from the blood this way. But in people with a genetic disorder called familial hypercholesterolemia, the LDL receptors are faulty or missing. That means the LDL cannot be removed, leading to dangerously high cholesterol levels and an increased risk of heart disease.

Even though this system is designed to be selective and secure, some crafty invaders have figured out how to exploit it. Certain viruses and bacterial toxins — like the flu virus, diphtheria toxin, and cholera toxin — can mimic the shape of the molecules normally taken in, tricking the receptors into letting them inside. It is like a sneaky intruder slipping past security by pretending to be on the guest list.

 

 

Exocytosis: Sending Stuff Out of the Cell

Figure \(\PageIndex{4}\): Exocytosis. Exocytosis is a form of active transport that releases materials from the cell by fusing an internal vesicle with the cell membrane, allowing its contents to exit into the extracellular space.

This form of active transport is how cells get rid of materials — or send important products like enzymes and hormones to other parts of the body.

In exocytosis, substances are packaged inside membrane-bound vesicles within the cell. These vesicles travel to the cell’s outer membrane, where they fuse with the plasma membrane.

Once the vesicle fuses, its contents are released into the extracellular fluid, and the vesicle’s membrane becomes part of the cell membrane — a perfect example of biological upcycling!

Cells use exocytosis for all sorts of critical tasks:

• Digestive cells in the stomach and pancreas release enzymes this way.

• Endocrine cells secrete hormones like insulin using exocytosis.

• Immune cells release histamine to help trigger inflammation and immune responses.

Without exocytosis, your body could not digest food, regulate blood sugar, or fight off infections.

Figure \(\PageIndex{5}\): Enzyme Products of Pancreatic Cells. Pancreatic acinar cells play a crucial role in producing and secreting numerous enzymes responsible for digesting food. The small black granules in the figure represent secretory vesicles filled with enzymes. These enzymes are destined for export from the cells through the process of exocytosis. LM × 2900. (Micrograph provided by the Regents of University of Michigan Medical School ©