The Cell Membrane and
Diffusion
Cell Membranes
• Think back to the cell city activity from
yesterday
• Remember how there was an outer wall
bordering your city?
• This is similar to the cell membrane
– The cell membrane (aka
(
plasma membrane)) is a
border that separates the interior of the cell from
its surroundings
– Plant cells also have a supporting structure
around their cell membranes, known as a cell
wall, which provides support and protection for
plant cells
Cell Membranes
Cell Walls
• Cell walls are commonly made up of
cellulose, a strong structural carbohydrate
Cell Membranes
• Cell membranes regulate what enters and
leaves the cell, and also provides
protection and support
– The cell wall even more so…
• Our cell membranes are known as lipid
bilayers as there are two lipid layers that
make up our cell membranes
Cell Membrane – Lipid Bilayer
Phospholipids
p
p
1st Layer
2nd Layer
Phospholipids
p
p
• Consist of a
phosphate head
and a fatty acid
tail
• The tails, when
arranged
d iin a
bilayer, is the
actual
t l “b
“barrier”
i ” off
the cell
Phosphate Head
Fatty Acid Tails
Phospholipid Bilayer
• Think of oil and water. What happens
after you pour oil into water?
Phospholipid Bilayer
• A similar phenomenon occurs with the
phospholipid bilayer
– The inner layer is consists of lipids, or “fat/oil
molecules”.
– This
Thi creates
t a barrier
b i against
i t polar
l molecules
l
l
LIPID CENTER (Hydrophobic)
Regulation of Water
• As water is a polar
molecule, regulation of
water by the cell is done
th
through
h aquaporins,
i
small protein channels
that allow water to flow in
and out of a cell
Osmosis and Diffusion
• Because of aquaporins and other protein
channels,
h
l we consider
id th
the cellll membrane
b
tto b
be
selectively permeable as certain molecules can
easily cross it
• When these molecules move from an area of
higher concentration to that of a lower
concentration we call this diffusion.
ed
diffusion
us o o
of water
ate ac
across
oss a se
selectively
ect e y
• The
permeable membrane is known as osmosis
– In other words, osmosis refers to the diffusion of
water and only water
Diffusion and Osmosis
Diffusion and Osmosis
Concentration refers to the number of solute
molecules in a given volume of solvent. There
are more water molecules on the right side than
the left! So the concentration of water is higher
on the right.
Therefore, when given the
opportunity, there is a net movement
of water from the right side to the left!
Diffusion and Osmosis
• Both diffusion and osmosis stop once
q
or
both sides have reached equilibrium,
have the same concentration.
• Note that molecules will still move around,
but they are now evenly spaced apart
Hyper-, Hypo-, and Isotonic
Solutions
Solute Molecules
Keep in mind the terms refer to the
solution outside of the cell!
Hyper-, Hypo-, and Isotonic
Solutions
A hypotonic solution will have An isotonic solution will
fewer solute molecules than have the same number of
solute molecules as its
its reference
reference
A hypertonic solution will
have the more solute
molecules as its reference
Which Way Would Water
Molecules Move?
No net
movement!
Effect on Cells
Membrane Transport
• Cell membranes have a variety of proteins
to facilitate molecules other than water
across their membrane
• Passive transport does not use any
energy while active transport does
• Simple diffusion is the first method cells
use to facilitate molecular transport
– Molecules simply diffuse from a higher
concentration to a lower concentration and go
through
g the p
plasma membrane
– These are normally restricted to very small
molecules
Membrane Transport
• Facilitated diffusion is another form of passive
transport
• Involves the use of carrier proteins to move specific
molecules across the membrane
Facilitated
diffusion
(Channel
Protein)
Diffusion
(Lipid
Bilayer)
Carrier Protein
Facilitated Diffusion
Glucose
molecules
High Concentration
High
Cell Membrane
Low Concentration
Through a 
Transport
Protein
Low Protein
channel
Active Transport
• All forms of active transport require
gy
energy
• This is because they are able to “pump”
molecules from an area of low
concentration to an area of high
concentration
• In other words, this form of transport often
goes against the direction diffusion would
occur, known as the concentration
gradient
Protein Pumps
Protein Pumps transport proteins that
require
i energy to d
do
work
Protein
P
t i changes
h
shape to move
molecules: this
requires energy!
Endocytosis
•
Endocytosis: taking bulky
material
t i l iinto
t a cellll
• Uses energy
• Cell membrane in-folds
around food particle
• “cell
“ ll eating”
ti ”
• forms food vacuole &
di
digests
t ffood
d
• This is how white blood
cells eat bacteria!
Endocytosis
•
Phagocytosis: How a cell takes in “bulky” material
Endocytosis
•
Pinocytosis: Cell takes up liquid from the
surrounding
di environment
i
t
Exocytosis
• Exocytosis:
y
Forces
material out of cell
• membrane surrounding
g the
material fuses with cell
membrane
• Cell changes shape –
requires energy
• Hormones or wastes are
released from cell this
way
Endocytosis
Exocytosis
Membrane Transport
Membrane Transport
Passive Transport (No
E
Energy
R
Required)
i d)
Simple
Diffusion
Facilitated
Diffusion
Active Transport (Energy
R
Required)
i d)
Protein
Pumps
Endocytosis
Phagocytosis
g y
Pinocytosis
Exocytosis
Animal Cell
Membrane