Download facilitated diffusion

Introduction –
Membrane Transport
One of the most important
functions of a cell is to keep its
internal environment relatively
constant
This is done by regulating the
movement of solutes and water
from of side of the membrane
to the other
How do substances move into and
out of cell?
• Two major ways:
– Passive transport
• Examples of passive transport:
–Simple diffusion
–Facilitated Diffusion
»Osmosis
– Active transport
• Examples of active transport
–Protein pumps
–Endocytosis
–Exocytosis
Solute Movement Across Membranes
Cellular cytoplasm consists of many
different substances dissolved in water
(these substances are called solutes)
Sometimes there is a higher concentration
of solutes on one side of the membrane than
the other
This is called a concentration gradient:
where there is difference in concentration of
a substance on the inside and outside of the
cell
Movement of Substances & the
Concentration Gradient
Solutes are constantly in motion & collide with
one another and spread out randomly until they
are spread out evenly, or are at equilibrium
Solutes naturally move from areas
where they are more concentrated to
areas where they are less
concentrated until equilibrium is
reached
This is called moving with or down
the concentration gradient
Diffusion
• The process by which
solutes move from
areas of higher
concentration to lower
concentration is called
diffusion
One type of Diffusion:
Simple diffusion
Simple diffusion means that
solutes move directly through the
lipid bilayer cell membrane
Because diffusion works by random
solute movement down the
concentration gradient, solutes
diffuse across the membrane
without requiring energy
Another type of diffusion:
Facilitated diffusion
Solutes which easily pass through
the lipid bilayer membrane are
generally small and/or uncharged
O2 and CO2
But many ions, like Ca2+, and larger
molecules, like glucose, also pass through
easily and quickly…how is this possible?
Proteins in the cell membrane act as
channels making it easy for certain solutes to
diffuse easily
– Each type of channel lets only a specific
solute through
Facilitated Diffusion
 The process by which molecules which can
not diffuse directly through the membrane
pass through special protein channels is
called facilitated diffusion
 Facilitated diffusion does not require
energy: solutes still move from areas of
high concentration (more concentrated) to
areas of low concentration (less
concentrated), like simple diffusion, it’s just
that they move through protein channels
instead of directly through the membrane
• http://www.biosci.ohiou.edu/introbioslab/Bios170
/diffusion/Diffusion.html
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapte
r2/animation__how_diffusion_works.html
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapte
r2/animation__how_facilitated_diffusion_works.h
tml
Factors that Affect How Fast
Diffusion Occurs
•What environments
would result in
diffusion occurring
faster or slower?
Homework 11/12
• Due Monday: Complete
osmosis worksheet
• Quiz TUESDAY 11/16 on:
(1) Membrane structure
and (2) Simple and
facilitated diffusion
Factors Which Affect The Rate of
Diffusion (how fast diffusion occurs)
Surface area to volume ratio
(size of cell)

Smaller is better!
Temperature

Diffusion occurs faster at higher
temperatures!
Concentration gradient

The more concentrated the
solutes, the faster diffusion
occurs!
Osmosis – An Example of
Facilitated Diffusion
Is the lipid bilayer hydrophobic or
hydrophillic?
How then… does water pass
through?
Water passes through the cell
membrane through specialized
proteins channels called
aquaporins
Osmosis is similar but different
than other types of diffusion
• Similar:
–Molecules still move from [high] to
[low] (called “down” or “with” the
concentration gradient)
•This means that osmosis is still
passive transport (like simple and
facilitated diffusion): does not
require energy
• Different: in the case of osmosis, it is
water which moves across the lipid
bilayer, not solutes
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/
chapter2/animation__how_osmosis_works
.html