Endocytosis — Definition & Types

What Is Endocytosis?

It is a form of active transport. This means that it requires energy to occur. It doesn't just happen on its own.

The best way to remember what this word means is to break it down. Cyto means having to do with cells, and endo means inside.

So, endocytosis involves bringing stuff from the extracellular fluid to the inside of the cell that is generally too large to cross the cell membrane on its own. This is a very important concept in cell biology.

There are three types of endocytosis:

Phagocytosis

In phagocytosis, the cell brings in solid particles. This is often referred to as cellular eating.

Phagocytosis always makes me think of the cell like Pac-Man, eating his way along.

In phagocytosis, a solid particle approaches the cell. This can include nutrients, proteins, and even bacteria. The cell membrane begins to indent.

It keeps indenting and indenting until it surrounds the solid particle.

Eventually, the cell membrane pinches off, forming a little bubble around the solid particle (shown in red in the image above). This bubble is called an endocytic vesicle, vacuole, or phagosome.

The food particle is surrounded by a phospholipid bilayer (since it was pinched off from the cell membrane). This is an example of the fluidity of the cell membrane. Parts can pinch off, and no harm is done.

The solid particle is eventually digested (broken down) by the cell, and the waste products and remaining phospholipids are sent back to the cell surface.

Pinocytosis

The overall process of pinocytosis is really similar to phagocytosis. There's the same pinching in of the membrane, only this time it's around a liquid particle.

Pinocytosis is often called cellular drinking.

To remember that pinocytosis is cellular drinking, I think of piña coladas. Piña coladas are drinks, and pino sounds like piña.

Receptor-Mediated Endocytosis

Luckily, receptor-mediated endocytosis (RME) isn't too different from phagocytosis and pinocytosis. The difference, though, is that RME is a much more specific process.

There are receptors on the outside of the cell membrane specifically designed to recognize and bind to certain substances.

Once this binding occurs, the cell membrane indents and it's the same old story as before.

The key with RME is that the substance (called a ligand in this case) matches the receptors. The receptors don't bond to just any old thing. They recognize a specific ligand and bring it into the cell in vesicles formed from the membrane.

In the picture above, the receptors are shown in green. These receptors are specific to the star-shaped ligand. They do not match up with the blue square ligands.

This vesicle is still made of the cell membrane, but in addition to phospholipids it now includes the receptors, as shown in the image above.

(Video) Active Transport

by Teacher's Pet

Do cells eat? Sort of. Endocytosis is when the plasma membrane is used to bring substances into the cell, as if the cell were eating. This video explains the specifics and shows how it works.

Summary

Endocytosis is one type of vesicle movement, which is a type of active transport. This process is how cells ingest external fluid, macromolecules, or other large particles (like other cells). Phagocytosis is like when a cell eats. Pinocytosis is like when a cell drinks. The cell membrane pinches around the substances it wants to take in, forming a transport vesicle. This vesicle can then go anywhere it's needed in the cell.

(Video) In Da Club - Membranes and Transportation

by CrashCourse

Watch this video to learn how cells devour molecules and liquid. There are three specific ways cells do this: phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Three Types of Endocytosis

Active transport is any activity that moves molecules and requires energy. Most active transport is used to move molecules against their concentration gradient. Active transport moves ions from areas of low concentration to areas of high concentration.

Endocytosis is a form of active transport that is used to bring large molecules into the cell. There are three kinds, which we will explore below.

Phagocytosis

Phagocytosis is when a cell surrounds an incoming particle with its plasma membrane. This form of active transport can be used to bring large particles of food into the cell and is used by white blood cells to surround harmful bacteria so that they can be destroyed. The plasma membrane is fluid and stretchy, so when a cell senses an incoming particle, the plasma membrane engulfs it. It then pinches closed around the particle. This leaves the particle in a plasma membrane sac called a phagosome inside of the cell. The phagosome then attaches to lysosomes within the cell so that the digestive enzymes within the lysosome digest the particle.

Pinocytosis

Pinocytosis is entirely identical to phagocytosis, except that it is not specific to what it absorbs. It is used to engulf liquids and small, already broken down food particles into the cell.

Receptor-mediated Endocytosis

Receptor-mediated endocytosis is used for the absorption of very specific molecules. On the surface of the cell, you can find receptors. When these receptors bind to their specific signal protein, they will cause a folding of the plasma membrane to absorb proteins, steroids, and cholesterol.

Transport Into the Cell

Endocytosis is the cell's way of capturing macromolecules and particles outside the cell and engulfing them with their cell membrane. In other words, it's how the cell eats and drinks. This is a critical process for living cells. It is a kind of active transport, so it requires energy input in the form of ATP.

In less technical terms, this is the process of transporting substances into the cell inside of bubbles formed from the plasma membrane. There are three different types of endocytosis, but they all function in basically the same way.

Let's break down the parts of the word since it might make it easier to remember. The prefix endo- means within, absorbing, or containing. The middle part, -cyto- means cell, while the suffix -sis means the process of doing something. So, the endocytic pathway brings things into the cell.

Phagocytosis is also known as "cellular eating". During phagocytosis, the cell will surround a solid particle, like macromolecules or sometimes even whole other cells, then engulf it with the cell membrane. The membrane tightens around it and pulls it into the cell, forming a food vacuole, also known as a phagosome. This is a non-specific process, so the cell doesn't pick and choose what it consumes.

Pinocytosis is also known as "cellular drinking". For this type of endocytosis, the cell uses its membrane to capture a small drop of liquid. Once the liquid is fully engulfed, the membrane forms a pocket known as a vesicle. This is a non-selective process because the cell takes in everything dissolved in the liquid.

Receptor-mediated endocytosis, unlike phagocytosis and pinocytosis, is incredibly selective. It is able to be selective because of receptor proteins embedded in the cell membrane that can identify a variety of particles. The receptor proteins are located in "coated pits" (they look like sinkholes) on the outside of the cell membrane. If the receptor protein binds to the particle it was made for, then the cell membrane constricts the pit. This causes a bubble-like vesicle to form inside the cell. This is especially useful for collecting particles that are in very low concentrations.

$$\color{#8925ae}{\text{Vocabulary Review:}}$$

$\color{#8925ae}{\text{Plasma Membrane}}$ : A semi-permeable phospholipid bilayer

$\color{#8925ae}{\text{Active Transport}}$ : Moving materials across the cell membrane. It requires the input of energy

$\color{#8925ae}{\text{Pumps}}$ : Pumps are proteins embedded in the plasma membrane that only allow specific materials to pass through

$\color{#8925ae}{\text{Endocytosis}}$ : A cell surrounds and takes in materials from its environment

$\color{#8925ae}{\text{Exocytosis}}$ : How a cell gets rid of waste