Cells have to protect themselves from harmful bacteria and communicate with one another. They also need to take in nutrients and get rid of waste. Endocytosis and exocytosis are two ways cells carry out these functions. Endocytosis is when cells take in substances by wrapping them in a membrane. Exocytosis is when cells release substances by fusing a membrane with the cell wall. These mechanisms are important for the cell's survival and overall health. In this article, we will dive into the details of endocytosis and exocytosis and explore their significance.

Definition of endocytosis and exocytosis

Cells have a special membrane that only lets certain molecules in and out. Some small molecules can easily pass through while others need special help from proteins. But what about large molecules? They can't just sneak in. That's where bulk transport comes in. Endocytosis and exocytosis are two ways cells move large molecules in and out. Endocytosis brings things into the cell while exocytosis moves things out. Both of these processes need energy to work, which makes them active transport mechanisms. So, cells have some pretty cool tricks up their sleeves to get the job done!

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 have a special membrane called the plasma membrane that surrounds them. This membrane is "selectively permeable," which means it only lets certain molecules in and out. The plasma membrane is made up of something called a phospholipid bilayer. A phospholipid is a type of fat with a special head and tail. The heads are attracted to water, while the tails are not. This means that the phospholipids arrange themselves in a way that keeps the heads facing the water and the tails away from it. This arrangement creates a strong barrier between the inside and outside of the cell. So, the plasma membrane is like a bouncer, only letting in the right molecules and keeping the wrong ones out!

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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 always moving and have energy that makes them move from areas of high concentration to areas of low concentration. This is called the concentration gradient. When there is a difference in concentration on either side of a membrane, a concentration gradient is created. Eventually, the concentrations will become equal, and this is called dynamic equilibrium. Passive transport is when substances move across the membrane according to the concentration gradient, and this doesn't require energy. Examples of passive transport include diffusion, osmosis, and facilitated diffusion. However, substances can also be affected by their electrical gradient, which is the difference in charge across the membrane. The combination of concentration and electrical gradients is called the electrochemical gradient. When substances move against the electrochemical gradient, it requires energy, and this is called active transport. Carrier proteins are involved in active transport, and they change shape to carry solutes across the membrane. So, the movement of substances across the membrane is a complex process that involves both concentration and electrical gradients, carrier proteins, and energy!

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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 molecules are too large to pass through the plasma membrane through simple or facilitated diffusion. For these larger molecules, bulk transport mechanisms are required. These mechanisms are active transport processes that move large molecules across the membrane. Endocytosis and exocytosis are two types of bulk transport mechanisms that use vesicles to transport molecules.Endocytosis is the process by which cells take in substances from outside the cell. The plasma membrane forms a pocket around the substance and then pinches off to form a vesicle inside the cell. This process can be either phagocytosis, where solid particles are engulfed, or pinocytosis, where liquid particles are engulfed. Exocytosis, on the other hand, is the process by which cells release substances to the outside of the cell. The vesicle containing the substance fuses with the plasma membrane, and the substance is released.Both endocytosis and exocytosis involve the formation and movement of vesicles. The main difference between the two is the direction of movement of the vesicles. Endocytosis involves movement of vesicles towards the cell, while exocytosis involves movement of vesicles away from the cell. A Venn diagram can be used to illustrate the similarities and differences between endocytosis and exocytosis.

What is endocytosis?

Endocytosis is a process by which a cell takes in material from outside the cell. There are three types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis. Phagocytosis is used by the cell to engulf solid large molecules, particularly bacteria, for the immune response of the cell. Pinocytosis is used by the cell to take up fluid with dissolved substances like nutrients such as hormones, enzymes, and ions. Receptor-mediated endocytosis is used by the cell to take in specific molecules that are bound to receptors on the outside surface of the cell. For example, human cells use receptor-mediated endocytosis to ingest cholesterol for the creation of steroids. In all three types of endocytosis, the cell membrane folds over the material to be engulfed, forming a vesicle that is transported within the cell. The vesicle then attaches to the lysosomes where the material is digested or broken down.

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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. 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.

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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).

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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?

In summary, exocytosis plays a crucial role in removing waste products from inside the cell, releasing signals like hormones and neurotransmitters for cellular communication, and transporting proteins and lipids essential for maintenance and repair of the cell membrane. It is the process by which materials are moved from the interior of the cell to the exterior, essentially the reverse of endocytosis. There are two types of exocytosis: constitutive secretion, which takes place continuously in cells, and regulated secretion, which occurs when specific conditions are met, such as the release of hormones and neurotransmitters. Overall, 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.

References

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

Exocytosis and Endocytosis

What is the function of endocytosis and exocytosis?

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.

What is endocytosis and exocytosis?

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. 

How do endocytosis and exocytosis differ from diffusion?

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.

What do the endocytosis and exocytosis have in common? 

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

What are the differences between endocytosis and exocytosis? 

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.

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