Exocytosis and Endocytosis

How do endocytosis and exocytosis work?

Before we delve into the specifics of endocytosis and exocytosis, we must first discuss the selective permeability of the plasma membrane and the ways by which substances are transported across it.

What is the plasma membrane?

Cells are enclosed in a selectively permeable plasma membrane. Selective permeability means some molecules are able to pass through while others cannot. The plasma membrane is selectively permeable because it is composed of a phospholipid bilayer.

A phospholipid is a lipid molecule made of glycerol, two fatty acid chains, and a phosphate-containing group. The phosphate group makes up the hydrophilic ("water-loving") head, and the fatty acid chains make up the hydrophobic ("water-fearing") tails.

The phospholipid bilayer acts as a stable boundary between two water-based compartments. The hydrophobic tails attach to one another. Together, they form the interior of the membrane. On the other hand, the hydrophilic heads face outward, so they are exposed to aqueous fluids inside and outside the cell (Fig. 1).

Figure 1. This diagram shows the phospholipid bilayer of the plasma membrane.

Some small, non-polar molecules, such as oxygen and carbon dioxide, can pass through the phospholipid bilayer because the tails that form the interior are non-polar. But other larger, polar molecules like glucose, electrolytes, and amino acids cannot pass through the membrane because they are repelled by the non-polar hydrophobic tails.

How do substances pass through the plasma membrane?

Molecules are in constant motion and, as a result, they have thermal energy that causes them to move towards any available space. They tend to move in the direction of the concentration gradient. That is, from a region of higher concentration to a region of lower concentration, until a dynamic equilibrium is reached.

When substances move across a membrane according to the direction of the concentration gradient–a process called passive transport –energy is not required. This includes processes like diffusion, osmosis, and facilitated diffusion.

Besides its concentration gradient, the movement of substances can also be affected by its electrical gradient. Electrical gradient refers to the difference in charge across the plasma membrane. The combination of concentration and electrical gradients is called the electrochemical gradient. For substances to move against the electrochemical gradient, energy must be expended. Transport processes that require energy are called active transport.

Active transport involves carrier proteins. Carrier proteins bind to solute molecules and change shape in a way that enables them to carry the solutes across the membrane (Fig. 2).

Figure 2. This diagram shows how carrier proteins facilitate diffusion by changing their shape according to the specific solute being transported.

Bulk transport endocytosis and exocytosis

Some substances like protein and polysaccharides are too large to passively diffuse or to pass through channels. Such substances require bulk transport mechanisms.

Endocytosis and exocytosis are bulk transport mechanisms. Through endocytosis and exocytosis, substances are packaged into vesicles and moved across the plasma membrane in bulk. Vesicles are tiny fluid-filled sacs enclosed by a membrane. They form using the plasma membrane inside or outside the cell to transport or store substances.

Difference between endocytosis and exocytosis

In this section, we will delve into endocytosis and exocytosis. We will discuss what they are and how they work. We will also discuss their similarities and differences using a Venn diagram.

What is endocytosis?

The cell membrane of the cell would fold over the substance until it is fully enclosed by the membrane, forming a vesicle. Then, the vesicle that surrounds the foreign material would break off from the membrane and will be transported within the cell.

There are three types of endocytosis (Fig. 3):

• phagocytosis,
• pinocytosis, and
• receptor-mediated endocytosis.

Phagocytosis (“cellular eating”)

Phagocytosis is a type of endocytosis in which the cell engulfs solid large molecules outside the cell, particularly bacteria, for the immune response of the cell.

Extensions in the cytoplasm called pseudopodia would extend from the cytoplasm and trap the particle inside a cellular vesicle. After engulfing the particle, the vesicle containing it would pinch off from the cellular membrane. This vesicle would then attach to the lysosomes. Lysosomes are organelles in a cell that digest or break down engulfed molecules.

Microorganisms, apoptotic cells, and other molecules that are larger than 0.5 µm in diameter are eliminated through phagocytosis.

Pinocytosis (“cellular drinking”)

Pinocytosis is a type of endocytosis in which cells take up the fluid with dissolved substances called extracellular fluid. Dissolved substances usually include nutrients such as hormones, enzymes, and ions. Then, a cell membrane containing the extracellular fluid would invaginate (fold in such a way that a cavity or a pouch is created) and form what we call a pinosome.

Receptor-mediated endocytosis

Receptor-mediated endocytosis is a type of endocytosis in which cells take in specific molecules bound to receptors on the outside surface of the cell. For example, receptor-mediated endocytosis is utilized by human cells to ingest cholesterol for the creation of steroids.

Figure 3. This diagram shows the different mechanisms of endocytosis.

What is exocytosis?

If cells take in nutrients through cellular eating and cellular drinking, they also need to discard and release other molecules inside the cell. How are they able to do it? They do this through exocytosis, where materials are moved from the interior of the cell to the exterior (Fig. 4). It is essentially the reverse of endocytosis.

Vesicles inside the cell containing materials for release would be packed and then transported where it fuses with the cell membrane. Once the vesicle is fused to the cell membrane, the contents of the cell membrane are expelled outside.

There are two types of exocytosis (Fig. 4):

• constitutive secretion and
• regulated secretion.

Constitutive secretion

Constitutive secretion takes place continuously in all cells. By default, substances from the Golgi apparatus--including newly-synthesized membrane lipids and proteins--that are on their way to the plasma membrane undergo constitutive secretion. Constitutive secretion vesicles pinch off from the Golgi apparatus and fuse to the cell membrane, expelling their contents.

Regulated secretion

In contrast to constitutive secretion which takes place continuously in cells, regulated secretion occurs when specific conditions are met. Some examples in which regulated exocytosis takes place are the release of hormones and neurotransmitters. Regulated secretory vesicles also pinch off from the Golgi apparatus, but unlike constitutive secretory vesicles, they do not fuse with the cell membrane unless the cell has received a signal prompting it to secrete.

Figure 4. This diagram shows the process of exocytosis.

Exocytosis is used by the cell for various reasons. It can be used to release toxins, release secretions in sweat glands, or communicate with other cells through the release of neurotransmitters and other signaling molecules.

Venn diagram of Endocytosis and exocytosis

To sum up, both endocytosis and exocytosis are bulk transport mechanisms where large molecules are transported across the plasma membrane using vesicles. Specifically, endocytosis moves particles from the outside to the inside of the cell while exocytosis moves particles from the inside to the outside of the cell.

As forms of active transport, both endocytosis and exocytosis require energy. These similarities and differences are shown in the Venn diagram below (Fig. 5).

Figure 5. This Venn diagram compares and contrasts endocytosis and exocytosis.

Importance of endocytosis and exocytosis

Endocytosis and exocytosis are both important mechanisms to bring substances in and out of the cell. Let's briefly discuss some functions.

What is the importance of endocytosis?

Endocytosis plays an important role in:

• Taking up nutrients that are needed for cellular growth and repair (for example, taking up nutrients through intestinal villi of the small intestine).

• Engulfing foreign pathogens that can harm the cell (for example, immune cells that engulf bacteria)

• Disposing of old and apoptotic cells (cells that are undergoing programmed death).

What is the importance of exocytosis?

Exocytosis plays an important role in:

• Removing waste products from inside the cell like carbon dioxide and water during aerobic respiration.

• Release of signals like hormones and neurotransmitters for cellular communication.

• Transportation of proteins and lipids that are essential for maintenance and repair of cell membrane.