The Cell Membrane and
Passive Transport
Plasma Membrane
• a.k.a the cell membrane.
• It is the boundary that separates the inside
of the cell from the outside environment
and acts as a “gatekeeper” allowing only
certain molecules to enter and leave the
cell.
• This ability to control movement into and
out of the cell is called selective
permeability.
What is the cell membrane made
of?
The cell membrane is
a lipid bilayer. It has
two layers of
molecules called
phospholipids.
Each phospholipid
has a polar head and
two non polar tails
Hydrophobic and Hydrophillic
The non-polar tails are fatty
acids and they do not
dissolve in water. They
are called hydrophobic
(or water fearing).
The polar head is a
phosphate group and it
dissolves easily in water.
It is called hydrophillic
(or water loving)
How are the phospholipids
arranged in the cell membrane?
There are two layers of
phospholipids in the
cell membrane
(bilayer). The heads
point towards the
water and the tails
point towards each
other.
Membrane Proteins
Scattered throughout this
lipid bilayer are proteins
called glycoproteins that
provide a channel for
substances to move in
and out of the cell.
They can sit on the surface
or be embedded in the
bilayer depending on if
they are polar, non-polar
or both.
• The plasma membrane is in constant flux with
the proteins drifting around in the lipid bilayer.
Since the membrane does not remain static but
instead shifts and changes it is referred to as the
fluid mosaic model.
• Cell Signatures
– The membrane glycoproteins are not only channels
into and out of the cell.
– They also contain sugar molecules that extend to the
surface of the cell and act as unique “signatures” for
that cell.
– Some of these sugar molecules code for a certain
blood type (Type A, B, AB, O)
– Others are completely unique to each individual.
– These aid in matching tissues, organs or blood types
for donation/reception.
Transport across the membrane
•
How do substances get across the
cell/plasma membrane?
– 2 methods:
1. Passive transport
2. Active transport
•
Passive Transport
– The movement of materials across the cell
membrane without the expenditure of
energy is called passive transport.
- There are 2 types of passive transport
1. Diffusion
2. osmosis
Diffusion
• Molecules are always in motion, constantly
bouncing off each other when they collide.
The random movement of molecules is
called Brownian Motion. This causes them
to disperse into a larger area.
• Diffusion is the movement of molecules
from an area of high concentration to an
area of low concentration.
– Example: drop of food coloring in water
• What affects the rate of diffusion?
– Temperature – increasing temperatures
causes more collisions thus faster reactions.
– Pressure - increasing pressure also causes
more collisions thus faster reactions.
– Concentration – higher concentration = more
collisions and faster reactions
Which substances move into the
cell by diffusion?
• Oxygen
– Diffuses out of blood (an area of high
concentration) to cells (area of low
concentration)
– Cells constantly use oxygen thus levels in the
cell will always be low.
• Carbon dioxide
– Is a waste product inside the cell (high) and
will diffuse out into the blood (low) to be
carried to the lungs to exhale.
Facilitated Diffusion
• If a substance cannot diffuse directly
through the phospholipid bilayer it can
travel through transport proteins
(membrane proteins) embedded in the cell
membrane. This type of passive transport
is called facilitated diffusion. Because the
molecules are still traveling from an area
of high concentration to low, energy does
not have to be used.
Osmosis
• Is the diffusion of water through a semipermeable membrane.
• The movement of water through living
cells is vital to life processes.
• Cells are bathed (ideally) in isotonic
solutions.
– The solute concentration outside the cell is
equal to that inside the cell.
– H2O moving out = H2O moving in
• Other times the fluid
environment is a
hypotonic solution.
– Higher concentration of
water outside the cell
than inside
– H2O moves in to even out
the concentration
• In hypertonic
solutions, water
concentrations are
greater inside the cell
than outside. Water is
drawn outside of the
cell and the cell
shrinks.
• This is why salt on the
sidewalk in winter will
kill the grass around
it.