Cell Membranes
Osmosis and Diffusion
Functions of Membranes
1. Protect cell
2. Maintain homeostasis
3. Selectively permeable - allows some
molecules in, others are kept out
Homeostasis
• Balanced internal condition of
cells
• Also called equilibrium
• Maintained by cell membrane
controlling what enters & leaves
the cell
Phospholipid Bilayer
Polar heads are hydrophilic “water loving”
Nonpolar tails are hydrophobic “water fearing”
Makes membrane “Selective” in what crosses
Fluid Mosaic Model:
“Proteins floating in a sea of lipids”
Proteins Are Critical to
Membrane Function
Blood-Brain Barrier
• Allows some substances into the
brain, but screens out toxins
and bacteria
• Substances allowed to cross
include:
water, CO2, Glucose, O2, Amino
Acids, Alcohol, and
antihistamines. HIV and
bacterial meningitis can cross the
barrier.
Solutions
• Solutions are made of solute and a
solvent
• Solvent – The liquid part. It does
the dissolving.
• Solute – The thing being dissolved.
Salt and sugar are solutes.
Methods of Transport
Across Membranes
1. Diffusion
2. Osmosis
3. Facilitated Diffusion
4. Active Transport
Diffusion
• Movement of molecules from an
area of high concentration to an
area of low concentration. (Passive
Transport)
• Movement from one side of a
membrane to another, without help.
Diffusion
Diffusion of Liquids
Diffusion through a Membrane
Solute moves DOWN concentration gradient (HIGH to
LOW)
Cell membrane
Facilitated Diffusion
• Does NOT require
energy
• Uses transport
proteins to move things
from high to low
concentration
Examples: Glucose or amino
acids moving from blood into a
cell.
Facilitated Diffusion
Molecules will randomly move through
the pores in Channel Proteins.
Facilitated Diffusion
• This is a helper.
Active Transport
• Requires energy (ATP)
• Moves materials from
LOW to HIGH concentration
AGAINST the concentration
gradient.
Osmosis
• Diffusion of water
across a membrane.
• Moves from HIGH
water amount (low
solute) to LOW
water amount (high
solute).
Diffusion across a membrane
Semipermeable
membrane
• Hypertonic- more water EXITS a cell
• Isotonic- Equal amounts of water enter
and exits.
• Hypotonic- more water ENTERS a cell
You could think of it this way…
• In osmosis, water
FOLLOWS salt!!
This is sometimes
an easier way to
remember this
concept.
Where is the water moving?
• The water is
moving out of the
cell.
• Why?
• Remember
wherever salt is
water follows!
Salt
SALT
SALT
Light blue = Salt
Dark blue = water
There is a lot of salt outside of the
cell. What will happen? Why?
What will happen if there is a
lot of salt inside of the cell?
• Normal
Red Onion Cells
• In Salt Water
• What happened to
the cytoplasm and
cell membrane?
Diffusion of H2O Across A
Membrane
High H2O potential
(amount) and
Low solute concentration
Low H2O potential
(amount) and
High solute concentration
Osmosis: Draw this
Three Forms of Transport Across the Membrane
Question
• Why does eating
popcorn make you
thirsty?
• Popcorn is very
salty and may cause
water to leave the
cells of your mouth
and through due to
diffusion. This
makes a person
thirsty.
When a sea urchin egg is removed from the
ocean and placed in freshwater, the egg
swells and bursts. Which of these causes
water to enter the egg?
A.
B.
C.
D.
Coagulation
Sodium Pump
Active Transport
Osmosis
D
The picture shows a cell model and the
solutions associated with it. In this situation
the cell model will —
A. Gain mass
B. Shrink
C. Increase in solute
content
D. Start to vibrate
B
Which of these best explains why a
freshwater aquarium would be a dangerous
habitat for saltwater fish?
A The tissues of the saltwater fish would absorb too much acid.
B The cells of the saltwater fish would lose too much water.
C The organ systems of the saltwater fish would consume too much energy.
D The cells of the saltwater fish would gain too much water.
D
On a hot summer day, a road-crew worker
perspires and then feels thirsty as her body
temperature increases. This response is an
example of –
A.
B.
C.
D.
Releasing enzymes
Maintaining homeostasis
Decreasing respiration
Assimilating proteins
B
Think/Pair/Share
• Work with a partner to
create different situations
that can be classified as
diffusion.
• Do the same for osmosis.
Active Transport Video:
http://www.iteachbio.com/Life%20Science/LifeFunction
sandTheCell/ActiveTransport.mov
Diffusion Video:
http://www.iteachbio.com/Life%20Science/LifeFunctio
nsandTheCell/Diffusion.mov
Another Video:
http://www.youtube.com/watch?v=ML-SWe5bRaE