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Bis2A 09.3 Endocytosis
∗
Mitch Singer
Based on Active Transport† by
OpenStax College
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 4.0‡
Abstract
By the end of this section, you will be able to:
• Understand how electrochemical gradients aect ions
• Describe endocytosis, including phagocytosis, pinocytosis, and receptor-mediated endocytosis
• Understand the process of exocytosis
1 Endocytosis
Endocytosis
is a type of active transport that moves particles, such as large molecules, parts of cells,
and even whole cells, into a cell.
There are dierent variations of endocytosis, but all share a common
characteristic: The plasma membrane of the cell invaginates, forming a pocket around the target particle.
The pocket pinches o, resulting in the particle being contained in a newly created vacuole that is formed
from the plasma membrane.
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‡ http://creativecommons.org/licenses/by/4.0/
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†
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Figure 1: Three variations of endocytosis are shown. (a) In one form of endocytosis, phagocytosis, the
cell membrane surrounds the particle and pinches o to form an intracellular vacuole. (b) In another
type of endocytosis, pinocytosis, the cell membrane surrounds a small volume of uid and pinches o,
forming a vesicle. (c) In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a
single type of substance that binds at the receptor on the external cell membrane. (credit: modication
of work by Mariana Ruiz Villarreal)
Phagocytosis is the process by which large particles, such as cells, are taken in by a cell.
For example,
when microorganisms invade the human body, a type of white blood cell called a neutrophil removes the
invader through this process, surrounding and engulng the microorganism, which is then destroyed by the
neutrophil (Figure 1).
A variation of endocytosis is called
pinocytosis.
This literally means cell drinking and was named at
a time when the assumption was that the cell was purposefully taking in extracellular uid. In reality, this
process takes in solutes that the cell needs from the extracellular uid (Figure 1).
A targeted variation of endocytosis employs binding proteins in the plasma membrane that are specic
for certain substances (Figure 1). The particles bind to the proteins and the plasma membrane invaginates,
bringing the substance and the proteins into the cell.
receptor-mediated endocytosis
If passage across the membrane of the target of
is ineective, it will not be removed from the tissue uids or blood.
Instead, it will stay in those uids and increase in concentration. Some human diseases are caused by a failure
of receptor-mediated endocytosis.
For example, the form of cholesterol termed low-density lipoprotein or
LDL (also referred to as bad cholesterol) is removed from the blood by receptor-mediated endocytosis. In
the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely.
People with this condition have life-threatening levels of cholesterol in their blood, because their cells cannot
clear the chemical from their blood.
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See receptor-mediated endocytosis in action and click on dierent
:
1
parts for a focused animation
to learn more.
2 Exocytosis
In contrast to these methods of moving material into a cell is the process of exocytosis.
Exocytosis
is
the opposite of the processes discussed above in that its purpose is to expel material from the cell into
the extracellular uid. A particle enveloped in membrane fuses with the interior of the plasma membrane.
This fusion opens the membranous envelope to the exterior of the cell, and the particle is expelled into the
extracellular space (Figure 2).
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Figure 2: In exocytosis, a vesicle migrates to the plasma membrane, binds, and releases its contents to
the outside of the cell. (credit: modication of work by Mariana Ruiz Villarreal)
3 Section Summary
The combined gradient that aects an ion includes its concentration gradient and its electrical gradient.
Living cells need certain substances in concentrations greater than they exist in the extracellular space.
Moving substances up their electrochemical gradients requires energy from the cell. Active transport uses
energy stored in ATP to fuel the transport. Active transport of small molecular-size material uses integral
proteins in the cell membrane to move the materialthese proteins are analogous to pumps. Some pumps,
which carry out primary active transport, couple directly with ATP to drive their action.
In secondary
transport, energy from primary transport can be used to move another substance into the cell and up its
concentration gradient.
Endocytosis methods require the direct use of ATP to fuel the transport of large particles such as
macromolecules; parts of cells or whole cells can be engulfed by other cells in a process called phagocytosis.
In phagocytosis, a portion of the membrane invaginates and ows around the particle, eventually pinching
o and leaving the particle wholly enclosed by an envelope of plasma membrane. Vacuoles are broken down
by the cell, with the particles used as food or dispatched in some other way. Pinocytosis is a similar process
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on a smaller scale. The cell expels waste and other particles through the reverse process, exocytosis. Wastes
are moved outside the cell, pushing a membranous vesicle to the plasma membrane, allowing the vesicle to
fuse with the membrane and incorporating itself into the membrane structure, releasing its contents to the
exterior of the cell.
4 Multiple Choice
Exercise 1
(Solution on p. 6.)
Active transport must function continuously because __________.
a. plasma membranes wear out
b. cells must be in constant motion
c. facilitated transport opposes active transport
d. diusion is constantly moving the solutes in the other direction
5 Free Response
Exercise 2
Where does the cell get energy for active transport processes?
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(Solution on p. 6.)
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Solutions to Exercises in this Module
to Exercise (p. 5)
D
to Exercise (p. 5)
The cell harvests energy from ATP produced by its own metabolism to power active transport processes,
such as pumps.
Glossary
Denition 1: active transport
the method of transporting material that requires energy
Denition 2: electrochemical gradient
a gradient produced by the combined forces of the electrical gradient and the chemical gradient
Denition 3: endocytosis
a type of active transport that moves substances, including uids and particles, into a cell
Denition 4: exocytosis
a process of passing material out of a cell
Denition 5: phagocytosis
a process that takes macromolecules that the cell needs from the extracellular uid; a variation of
endocytosis
Denition 6: pinocytosis
a process that takes solutes that the cell needs from the extracellular uid; a variation of endocytosis
Denition 7: receptor-mediated endocytosis
a variant of endocytosis that involves the use of specic binding proteins in the plasma membrane
for specic molecules or particles
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