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Exocytosis and Endocytosis

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Exocytosis and Endocytosis

How do cells protect themselves from foreign invaders such as bacteria? How do they communicate? How do they "eat" or "drink" substances? In the following article, we will discuss the bulk transport mechanisms endocytosis and exocytosis: what they are, how they work, and what their significance is to the cell.

Definition of endocytosis and exocytosis

Cells are enclosed by a selectively permeable plasma membrane that allows for the passage of different kinds of molecules. If molecules are small enough, they can readily pass through and passively diffuse across the membrane. On the other hand, other molecules require the help of channels and other proteins to pass through the membrane.

However, some materials are too large to passively diffuse or pass through channels. Such materials can be moved across the plasma membrane using bulk transport mechanisms.

Endocytosis and exocytosis are bulk transport mechanisms that allow large molecules to pass through the plasma membrane in bulk while packed in transport or storage sacs called vesicles. Endocytosis transports substances from the outside to the inside of the cell while exocytosis transports substances from the inside to the outside of the cell.

Endocytosis and exocytosis are active transport processes which means that they require energy to be expended.

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).

Exocytosis and Endocytosis Fig. 1: Phospholipid bilayer | StudySmarterFigure 1. This diagram shows the phospholipid bilayer of the plasma membrane. Source: OpenStax, CC BY 4.0, via Wikimedia Commons.

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.

A concentration gradient is created when there is a difference in the amounts of a substance on the two sides of a membrane. One side will have a higher concentration of this substance than the other.

Dynamic equilibrium refers to the point where the concentration of the molecules is equal on either side of the membrane. Molecules continue to move but their concentration at either side of the barrier doesn't change.

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).

Exocytosis and Endocytosis Fig. 2: Carrier proteins | StudySmarterFigure 2. This diagram shows how carrier proteins facilitate diffusion by changing their shape according to the specific solute being transported. Source: OpenStax, CC BY 3.0, via Wikimedia Commons.

To learn more about the other modes of transport we have mentioned in this article, please refer to our articles Diffusion, Osmosis and Active Transport.

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.

Bulk transport mechanisms are active transport processes that allow molecules that are too large for simple or facilitated diffusion to pass through the plasma membrane.

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?

Endocytosis is the process by which a cell engulfs material from outside the cell.

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.

Exocytosis and Endocytosis Fig. 3: Types of endocytosis | StudySmarterFigure 3. This diagram shows the different mechanisms of endocytosis. Source: Source: LadyofHats, Public domain, via Wikimedia Commons.

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.

Exocytosis and Endocytosis Fig. 4: Types of Exocytosis | StudySmarterFigure 4. This diagram shows the process of exocytosis. Source: LadyofHats, Public domain, via Wikimedia Commons.

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).

Exocytosis and Endocytosis Fig. 5: Venn Diagram | StudySmarter

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.

Exocytosis and Endocytosis - Key takeaways

  • Endocytosis and exocytosis are bulk transport mechanisms that allow large molecules to pass through the plasma membrane in bulk while packed in transport or storage sacs called vesicles.
  • Endocytosis transports substances from the outside to the inside of the cell.
  • The cell membrane folds over the substance until it is fully enclosed, forming a vesicle. Then, the vesicle that surrounds the foreign material would break off from the membrane and will be transported within the cell.
  • Exocytosis transports substances from the inside to the outside of the cell.
  • 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.

References

  1. Advanced Placement for AP Courses Textbook by Texas Education Agency
  2. Campbell Biology Eleventh Edition by Person Higher Education
  3. https://www.open.edu/openlearn/science-maths-technology/science/biology/intracellular-transport/content-section-2.4
  4. https://wikispaces.psu.edu/display/Biol230WFall09/Intracellular+Compartments-+Exocytosis%2C+Endocytosis%2C+and+the+Lysosome

Frequently Asked Questions about Exocytosis and Endocytosis

Endocytosis and exocytosis are both important mechanisms to bring substances in and out of the cell. 

  • One of the functions of endocytosis is taking up nutrients needed for cell growth and repair. 
  • One of the important functions of exocytosis is the release of signals like hormones and neurotransmitters.

Endocytosis and exocytosis are bulk transport mechanisms that allow large molecules to pass through the plasma membrane in bulk while packed in transport or storage sacs called vesicles

  • Endocytosis transports substances from the outside to the inside of the cell. 
  • Exocytosis transports substances from the inside to the outside of the cell.

Endocytosis and exocytosis differ from diffusion in terms of energy expenditure. Endocytosis and exocytosis are forms of active transport so they require energy. Diffusion is a form of passive transport so it does not require energy.

Endocytosis and exocytosis are both bulk transport mechanisms where large molecules are transported across the plasma membrane using vesicles.

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.  

Final Exocytosis and Endocytosis Quiz

Question

What is the process by which a cell engulfs material from outside the cell?

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Answer

Endocytosis

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Question

What is the process where materials are moved from the interior of the cell to the exterior through vesicles?

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Answer

Exocytosis

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Question

How does exocytosis take place?

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Answer

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.

Show question

Question

How does endocytosis take place?

Show answer

Answer

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. 

Show question

Question

What is a pseudopodium?

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Answer

Pseudopodia are extensions in the cytoplasm that extend from the cytoplasm and trap the particle inside a cellular vesicle.  

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Question

What role does the lysosome play in phagocytosis?

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Answer

The lysosome digests or breaks down molecules engulfed via phagocytosis.

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Question

What is a pinosome?

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Answer

A pinosome is a vesicle filled with extracellular fluid. It is formed during pinocytosis.

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Question

What type of exocytosis occurs by default?

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Answer

Constitutive secretion

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Question

What type of exocytosis occurs as a response to specific conditions?

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Answer

Regulated secretion

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Question

What are the similarities between endocytosis and exocytosis?

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Answer

Both endocytosis and exocytosis are bulk transport mechanisms where large molecules are transported across the plasma membrane using vesicles. As active transport processes, they require the use of energy.

Show question

Question

What are some important functions of exocytosis?

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Answer

Exocytosis serves some important functions including the release toxins, secretions, neurotransmitters, and other signaling molecules.  

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Question

What are some important functions of endocytosis?

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Answer

Some important functions of endocytosis include: taking up nutrients that are needed for cellular growth and repair; engulfing foreign pathogens that can harm the cell; and disposing old and apoptotic cells

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Question

Describe what happens to newly-synthesized membrane lipids and proteins undergoing exocytosis.

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Answer

Newly-synthesized membrane lipids and proteins are transported from the Golgi Apparatus to the Plasma membrane via vesicles. The vesicle attaches to the plasma membrane and then its contents are expelled.

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Question

How do endocytosis and exocytosis differ from diffusion?

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Answer

Diffusion involves the movement of substances in the direction of the concentration gradient. When substances diffuse across a plasma membrane, energy is not used.


Endocytosis and exocytosis transport molecules that are too large to diffuse or pass through channels by packaging them into vesicles. Endocytosis and exocytosis are active transport processes that require energy.

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Question

What is a vesicle and how is it involved in exocytosis and endocytosis?

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Answer

Vesicles are tiny fluid-filled sacs enclosed by a membrane. They form inside or outside the cell to transport or store substances during endocytosis and exocytosis.

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