Chapter 3
Cell Transport -- Active
Slide 162-184
Active Transport
http://bcs.whfreeman.com/webpub/Ektron/pol1e/Animated%20Tutorials/at0502/at_0502_active_trans.html
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The two main types of transport into and out of cells include passive transport and active transport.
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PASSIVE TRANSPORT involves the transport of molecules from regions of high concentration to regions of low
concentration. No energy is used in this process.
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Three main types of passive transport:
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1. SIMPLE DIFFUSION
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Diffusion is the tendency of molecules to spread from regions of high concentration to regions of low
concentration. This movement is due to a concentration gradient. This is the difference in
concentration between two different regions. An example - when you spray perfume in the corner of
a room and eventually you can smell it everywhere.
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2. FACILITATED (HELPED) DIFFUSION
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Facilitated diffusion is similar to simple diffusion, but it involves molecules diffusion quickly across a
cell membrane, through transport proteins. Facilitated diffusion only works from high to low
concentration. A transport protein binds to the substance on one side of the cell membrane and then
releases it on the other side.
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3. OSMOSIS
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Osmosis is the diffusion of water across a selectively permeable membrane from a region of high
water concentration to a region of low water concentration.
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ACTIVE TRANSPORT involves the transport of molecules from regions of low concentration to regions of high
concentration. Energy is needed for this type of transport.
There are two main types of active transport:

1. TRANSPORT PROTEINS - the same as facilitated diffusion, except a molecule goes from low
concentration to high
concentration across a cell membrane and it needs energy to take place.
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2. MEMBRANE VESICLES - Substances entering the cell become enclosed by an inpocketing of the cell
membrane to form
a vesicle. This process is called endocytosis.
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Endocytosis (two forms ):
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a) Pinocytosis - a form of endocytosis that transports liquids into the cell.
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b) Phagocytosis - a form of endocytosis that transports solids into the cell.
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Exocytosis is the movement of a substance out of a cell via vesicles. Exocytosis is the reverse of
endocytosis.
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Bulk transport across the plasma membrane
occurs by exocytosis and endocytosis

Small molecules and water enter or leave the
cell through the lipid bilayer or via transport
proteins
 Large molecules, such as polysaccharides and
proteins, cross the membrane in bulk via
vesicles
 Bulk transport requires energy
 Larger materials are brought into the cell
through endocytosis and moved out through
exocytosis.
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Exocytosis and Endocytosis

Both mechanisms employ vesicles, the
membrane-lined enclosures that alternately
bud off from membranes or fuse with them.
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Exocytosis

In exocytosis, a transport vesicle moves
from the interior of the cell to the plasma
membrane and fuses with it, at which point
the contents of the vesicle are released to
the environment outside the cell.
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Exocytosis

Exocytosis is a cellular process where cells eject
waste products or chemical transmitters (such as
hormones) from the interior of the cell.

In exocytosis, transport vesicles migrate to the
membrane, fuse with it, and release their contents

Many secretory cells use exocytosis to export their
products

Exocytosis is similar in function to endocytosis
but working in the opposite direction.
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There are five steps to exocytosis:
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Exocytosis
Five Steps of Exocytosis:
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Vesicle Trafficking
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
Vesicle Tethering
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In this step, the vesicle comes in contact with the cell membrane, where it begins to
chemical and physically merge with the proteins in the cell membrane.
Vesicle Priming
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As the vesicle approaches the cell membrane, it is secured and pulled towards the
part of the cell from which it will be eliminated.
Vesicle Docking


In this first step, the vesicle containing the waste product or chemical transmitter is
transported through the cytoplasm towards the part of the cell from which it will be
eliminated.
In those cells where chemical transmitters are being released, this step involves the
chemical preparations for the last step of exocytosis.
Vesicle Fusion

In this last step, the proteins forming the walls of the vesicle merge with the cell
membrane and breach, pushing the vesicle contents (waste products or chemical
transmitters) out of the cell. This step is the primary mechanism for the increase in
size of the cell’s plasma membrane.
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Exocytosis
(a) Exocytosis
extracellular fluid
transport vesicle
(b) Micrograph of exocytosis
protein
cytosol
Figure 5.9
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Endocytosis

Endocytosis is a cellular process where cells
absorb molecules or substances from outside the
cell by engulfing it with the cell membrane.

This technique is critical to the survival of the cell,
since most molecules important to the cell cannot
normally pass through the cell’s hydrophobic
plasma membrane.

Endocytosis is the opposite of
exocytosis.

There are three types of endocytosis:
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Endocytosis

In endocytosis, the cell takes in macromolecules
by forming vesicles from the plasma membrane
 Endocytosis is a reversal of exocytosis, involving
different proteins
 There are three types of endocytosis



Phagocytosis (“cellular eating”)
Pinocytosis (“cellular drinking”)
Receptor-mediated endocytosis
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Endocytosis
Three Types of Endocytosis:
 Phagocytosis


Pinocytosis



In this process, the cell membrane folds around the molecule or
substance, isolating it for later use in a vacuole called a phagosome.
(Phagocytosis is literally translated as “cell eating.”)
In this process, the cell membrane folds around the molecule or
substance, and it is dissolved into the interior of the cell.
Molecules such as proteins and other water-soluble substances are
thus ingested this way, which could not otherwise breach the cell
membrane. (Pinocytosis is literally translated as “cell drinking.”)
Receptor-Mediated Endocytosis



This process is similar to phagocytosis, except that the cell uses
receptor proteins embedded within the cell membrane.
These proteins target specific molecules or substances, attracting then
seizing hold of them and pulling them into the cell within a fold of the cell
membrane.
Cholesterol is absorbed from our bloodstream by cells using receptormediated endocytosis.
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In phagocytosis

when certain cells use pseudopodia or “false feet” to surround and
engulf whole cells, fragments of them, or other large organic materials.

a cell engulfs a particle in a vacuole

The vacuole fuses with a lysosome to digest the particle
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In pinocytosis

is the movement of moderate-sized molecules into a cell by means of the
creation of transport vesicles produced through an infolding or
“invagination” of a portion of the plasma membrane.

molecules are taken up when extracellular fluid is “gulped” into tiny vesicles
that bud off from the plasma membrane.

In receptor-mediated endocytosis, binding of ligands to receptors
triggers vesicle formation

A ligand is any molecule that binds specifically to a receptor site of
another molecule
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Phagocytosis
http://www.stolaf.edu/people/giannini/flashanimat/cellstructures/phagocitosis.swf
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Endocytosis
(a) Pinocytosis
receptors
captured
molecules
coated
pit
vesicle
Formation of a pinocytosis vesicle.
(b) Phagocytosis
bacterium
(or food particles)
pseudopodium
vesicle
A human macrophage (colored blue) uses
phagocytosis to ingest an invading yeast cell.
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Endocytosis
http://www.youtube.com/watch?feature=player_embedded&v=4gLtk8Yc1Zc
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Endocytosis and Exocytosis
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter5/endocytosis_and_exocytosis.html
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Passive transport
Diffusion
Facilitated diffusion
Active transport
ATP
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https://www.youtube.com/watch?v=SUyMRfuPQ_w
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