Movement across the Cell Membrane
2006-2007
Diffusion
 Diffusion
movement from HIGH  LOW concentration
Down the concentration gradient
What did it take to
make the
concentration
gradient?
The concentration gradient
stores potential energy
It takes energy to generate the
gradient
The process is still considered
passive
2006-2007
3
Facilitated Diffusion
• Diffusion through protein
channels
– highly specific
– passive process
HIGH
what sort of
molecules
will use this
process?
LOW
“The Bouncer”
Active Transport
• Cells may need to move
molecules against concentration
gradient
– conformational shape change
transports solute from one
side of membrane to other
– protein “pump”
LOW
– “costs” energy = ATP
Once completed,
where has the
energy for the
active transport
gone?
ATP
HIGH
“The Doorman”
Active transport
• Many models & mechanisms
ATP
ATP
antiport
symport
Getting through cell membrane
• Passive Transport
– Simple diffusion
• nonpolar, hydrophobic molecules
–HIGH  LOW concentration gradient
– Facilitated transport
• polar, hydrophilic molecules
• through a protein channel
–HIGH  LOW concentration gradient
• Active transport
– against concentration gradient
ATP
• LOW  HIGH
– uses a protein pump (requires ATP)
Transport summary
simple
diffusion
facilitated
diffusion
active
transport
ATP
What about large molecules?
• Moving large molecules into & out of cell
– through vesicles & vacuoles
– endocytosis
• phagocytosis = “cellular eating”
• pinocytosis = “cellular drinking”
– exocytosis
exocytosis
Endocytosis
phagocytosis
fuse with
lysosome for
digestion
pinocytosis
non-specific
process
receptor-mediated
endocytosis
triggered by
molecular
signal
What about Osmosis?
• Diffusion of water from
HIGH concentration of water to
LOW concentration of water
– across a
semi-permeable
membrane
Why does
osmosis need
a membrane?
Concentration of water
• Direction of osmosis is determined by
comparing total solute concentrations
– Hypertonic - more solute, less water
– Hypotonic - less solute, more water
– Isotonic - equal solute, equal water
water
hypotonic
hypertonic
net movement of water
Hypotonic , Hypertonic, Isotonic
How does sugar
concentration lead to
water movement?
13
Let’s watch
the video
Which way would
water move if oil was
added to the left side
of the tube?
14
Managing water balance
• Hypotonic
• Animal cell/protist cell
problem?
• Solutions?
• Plant cells....
KABOOM!
• turgid = full
• cell wall protects from
bursting
No problem,
here
freshwater
Managing water balance
• Hypertonic
• Animal cells:
I’m shrinking,
I’m shrinking!
• How can they adjust:
• Take up water or pump out
solutes.
– plant cells
• plasmolysis = wilt
Why do people make
beef jerky, and add
syrup to preserve
canned fruit?
I will
survive!
saltwater
Managing water balance
Isotonic
–Animal cell problems:
–none
–solutes are equally concentrated
That’s
on both sides of membranes perfect!
–Plant cell problems
–flaccid cells lose structure.
I could
be better…
balanced
Aquaporins
• Water moves rapidly into & out of cells
– evidence that there were water channels
• protein channels allowing flow of water across
cell membrane
Peter Agre
Roderick MacKinnon
John Hopkins
Rockefeller
Which way will water
move?
.05 M
.03 M
Hey...watch this