Chapter 3
Cell Passive Transport
(Facilitated Diffusion)
Slide 106-116
Facilitated Diffusion: Passive Transport
Aided by Proteins

In facilitated diffusion, transport proteins speed
the passive movement of molecules across the
plasma membrane

Channel proteins provide corridors that allow a
specific molecule or ion to cross the membrane

Channel proteins include
 Aquaporins, for facilitated diffusion of water
 Ion channels that open or close in response
to a stimulus (gated channels)
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Facilitated Diffusion
 In
facilitated diffusion, transport
proteins function as channels for
larger hydrophilic substances—
substances that, because of their
size and electrical charge, cannot
diffuse through the hydrophobic
portion of the plasma membrane.
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How Facilitated Diffusion Works

How can cells control diffusion? One way
in which cells can control diffusion is through
the control of movement of particular types of
molecules. Some molecules can pass freely
through a cellular membrane. Large polar
molecules and ions cannot diffuse through the
lipid bilayer. Carrier proteins can move
molecules across the membrane. Cells will
have carrier proteins for specific molecules.
These molecules will pass freely across the
membrane while other molecules will not. The
types of carrier proteins found on a cell are
related to cell function.
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Facilitated Diffusion

Move from HIGH to LOW through a channel
sugar
sugar
inside cell
sugar
sugar
sugar
sugar
LOW
Which way will
sugar move?
HIGH
outside cell
sugar
sugar
sugar
sugar
sugar
sugar
sugar
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Facilitated Diffusion
glucose
outside
cell
plasma
membrane
inside cell
1. The transport protein has a binding
site for glucose
that is open to the
outside of the cell.
2. Glucose binds
to the binding
site.
3. This binding causes
the protein to change
shape, exposing
glucose to the inside
of the cell.
4. Glucose passes
into the cell and
the protein returns
to its original shape.
Figure 5.8
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Diffusion

Move from HIGH to LOW concentration

directly through membrane
simple diffusion
 no energy needed


help through a protein channel
facilitated diffusion (with help)
 no energy needed HIGH

LOW
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Simple vs. Facilitated diffusion
simple diffusion
facilitated diffusion
lipid
inside cell
inside cell
H2O
protein channel
outside cell
outside cell
H2O
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Facilitated diffusion

Facilitated diffusion is still passive because the
solute moves down its concentration gradient, and
the transport requires no energy

Carrier proteins undergo a subtle change in shape
that translocates the solute-binding site across the
membrane

Some diseases are caused by malfunctions in
specific transport systems, for example the kidney
disease cystinuria
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FACILITATED DIFFUSION
https://www.youtube.com/watch?v=1T2Zwh3N200
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How Facilitated Diffusion Works
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter5/how_facilitated_diffusion_works.html
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