Cell Membranes and
Transports
Passive and Active Transport
Mechanisms
Ch 5
Membranes within a human cell
All living cells are encased within a lipid
membrane that few water-soluble
substances can pass through.
The membrane contains protein passageways which allows specific substances to
move into & out of the cell
This delicate skin of lipids with embedded
proteins is called a plasma membrane
Plasma Membrane
AKA: the cell membrane has several
functions:
1. Only allows certain molecules to enter &
leave the cell
2. Separates internal metabolic reactions
from the external environment
3. Allows the cell to excrete waste & to
interact with its environment
Phospholipids
Have a polar, hydrophilic (water loving)
phosphate head and 2-nonpolar,
hydrophobic (water fearing) fatty acid tails
Water molecules surround the plasma
membrane
The phospholipids line up so their heads
point outward toward the water & their tails
point inward away from the water
The result: phospholipid bilayer
Bilayer of the Phospholipids
The polar water molecules repel the
nonpolar tails packing the tails closely
together and as far as possible from the
water
Every phospholipid molecule orients its
polar head toward water with the tail facing
each other..no tail ever comes in contact
with the water.
Remember, Phospholipids
Have 2 rather than 3 fatty acids attached
to a molecule of glycerol
They have a phosphate group attached to
a third carbon of the glycerol
The cell membrane is made of 2 layers of
phospholipids called the lipid bilayer
The inability of lipids to dissolve in water
allows the membranes to form a barrier
between the inside and outside of the cell
Phospholipid
Sterols
The cell membrane of eukaryotes contain
lipids called sterols
Found between the tails of the
phospholipids
The major membrane in animal cells is
cholesterol
Sterols in the plasma membrane make the
membrane more firm & prevent freezing in
low temperatures
The Lipid Bilayer is Fluid
Lipid bilayer is stable because water’s
desire for H bonding never stops
https://homepages.abdn.ac.uk/p.marsto
n/pages/flash/samples/liposomes.swf
Membrane Protein
Often contain specific proteins embedded within
the lipid bilayer called integral proteins
Classes of Membrane Proteins
1. Cell Surface markers: emerge from only 1 side
of the membrane
2. Receptor Proteins & Transport Proteins: extend
across the plasma membrane and are exposed
to both the cell’s interior and exterior
environment. It can detect environmental signals
& transmit them inside the cell
3. Peripheral Proteins or enzymes: lie on only 1
side of the membrane & are not embedded in it.
Structures of a bilayer
Integral Proteins Uses
Exposed to the cell’s external environments have
carbohydrates attached
The carbohydrates act as labels on the cells
surface. The labels:






Help cells recognize each other & stick together
Viruses can use them as docks for entering & infecting
cell
Play important roles in active transport
Act as channels or pores to all certain substances to
pass through
Bind to a molecule on the outside of the cell & then
transport it through the membrane
Act as sites where chemical messengers (hormones)
can attach
Fluid Mosaic Model
The Fluid Mosaic Model states:
that the phospholipid bilayer behaves like
a fluid more than it behaves like a solid
https://www.youtube.com/watch?v=37KNBEf
cbsA&feature=related
Fluid Mosaic Model
Is a plasma membrane is made of lipids and
globular proteins
The lipids & proteins can move within the bilayer
smoothly, resulting in a pattern or “mosaic” of
lipids & proteins in the cells membrane
constantly changing.
 (moving around like a boat {lipids} on a pond
{proteins})
History
Scientists though for years that the proteins
covered the inner & outer surfaces of the
phospholipid bilayer like a coat of paint
1935: Davson-Danielli model: portrayed the
membrane as a sandwich
a phospholipid between 2 layers of globular proteins
1972: S. Singer & G.J. Nicolson: revised the
model in a simple but profound way:
globular proteins are inserted into the lipid bilayer with
their nonpolar segments in contact with the polar interior
of their polar portions protruding out from the membrane
surface
This model is called the Fluid Mosaic Model
Types of Membrane Proteins
6 key classes of membrane proteins and
how they move into and out of the cell
Membrane proteins acts as transporters,
enzymes, cell surface receptors, and cell
surface markers, as well as aiding in cell-tocell adhesion and securing the
cytoskeleton.
1. Transporters
Membrane very
selective
Allows only certain
substances to leave
or enter the cell
through channels or
carriers
Each has a channel
selective for a
particular molecule
2. Enzymes
Moves Enzymesinvolved in chemical
reactions inside the
cell.
Cells carry out many
chemical reactions on
the inside surface of
the plasma
membrane
3. Cell Surface Receptors
Works to detect
chemical messages,
with receptor proteins
on their surfaces
4. Cell Surface Identity Markers
Membranes carry
markers that identify
them to other cells
Most have their own
ID Tags {specific
combinations of
proteins}
5. Cell Adhesion Proteins
Cells use specific
proteins to “glue”
themselves to one
another.
Some is temporary,
but other are
permanent bonds
6. Attachments to the
Cytoskeleton
Surface proteins that
interact with other
cells often anchor to
the cytoskeleton by
linking proteins
Passive Transport
Passive Transport
Passive transport: is substances moving
across the cell membrane without any
input of energy by the cell
3 Types of Passive Transport
1. Simple Diffusion
2. Facilitated Diffusion
3. Osmosis
1. Simple Diffusion
Diffusion is the movement of molecules
from an area of higher concentration to an
area of lower concentration
The simplest type of passive transport
This difference in the concentration of
molecules across a distance is called a
concentration gradient
Sugar molecules, initially in
high concentration at the
bottom of a beaker
EXAMPLE
1. Will move about randomly
through diffusion
2. And eventually reach
equilibrium
3. At equilibrium the sugar
concentration will be the
same through out the
beaker.
Diffusion occurs naturally because of the
kinetic energy The molecules possess
Equilibrium
Is driven entirely by the molecules’ kinetic
energy
Molecules tend to move from areas where they
are more concentrated to areas where they are
less concentrated or “down” their concentration
gradient
Diffusion will eventually become equilibriumwhere the concentration of molecules will be
equal throughout the space
If the beaker of water is left undisturbed, at some
point the concentration of sugar molecules will be
the same throughout the beaker
The sugar concentration will then be at equilibrium
Simple diffusion
Cell membranes allow some molecules to
pass through, but not others –vsDiffusion only allows certain molecules to
pass through the membrane
Key factor in the exchange of substances
between cells and tissue fluids

Ex: Oxygen and carbon dioxide exchange
between lungs and blood
Factors that Affect the Rate of
Diffusion
1. The steeper the gradient the faster the
rate of diffusion
2. Temperature (the higher the faster)
3. Molecular size (the smaller the faster)
4. Difference in charge between 2 region
(electric gradient)
5. Difference in pressure (pressure gradient)
Membrane Transport is
Selective
Molecules that cells require are polar and cannot
pass through the nonpolar interior of the
phospholipid bilayer-so they enter through
specific channels in the plasma membrane
The inside of the channel is polar which moves
them across the membrane
Each channel (which fits the molecule) is
selective that that type of molecule and is said to
be selectively permeable
Simple Diffusion
Video Clip
http://www.stolaf.edu/people/giann
ini/flashanimat/transport/diffusion.
swf
http://www.coolschool.ca/lor/BI12/
unit4/U04L03/diffusion.swf
2. Facilitated Diffusion
Another type of Passive Transport
Used by molecules that cannot easily diffuse
through cell membranes, even when there is a
concentrated gradient across the membrane
Molecules may not be soluble in lipids or too
large to pass through the pores in the cell
Assistance is given by specific proteins called
carrier proteins
Protein Carriers
Transport molecules from an area of higher
concentration on one side of the membrane to
an area of lower concentration on the other side
Carrier Proteins
http://www.stolaf.edu/people/giannini/flash
animat/transport/caryprot.swf
1. A molecules (glucose)
binds to a carrier
protein on one side of
the cell membrane
2. The carrier protein
changes shape,
shielding the molecule
from the interior of the
membrane
3. The molecule is
released on the other
side of the membrane
4. The carrier protein then
returns to its original
shape
According to this
model…4 steps:
Example of facilitated diffusion
The transport of glucose:
Many cells depend on glucose for their
energy needs
Glucose is too large to diffuse easily
across the cell membrane
When the level of glucose outside the cell
is lower than the level of glucose inside
the cell carrier proteins transport glucose
into the cell
Diffusion through Ion Channels
A type of transport that involves membrane
proteins known as ion channels in the form of
passive transport
Ions: Sodium Na+, Potassium K+, Calcium Ca++, and Chlorine
Cl-
Do not move well through the membrane b/c
ions interact with polar molecules (water) but are
repelled by non-polar portion of the bilayer.

Ex: Na+ channels only allow Na+ ions to go through &
will not let Ca+2 or Cl-1 enter the cell
Ion channels provide small passageways across
the cell membrane so ions can diffuse
Each type of ion channel is usually specific for one
type of ion
Some channels are always open
Other have “gates” that open to let ions pass or close
to stop their passage
The gates may open or close in response to 3
kinds of stimuli:
1. Stretching of the cell membrane
2. Electrical signals
3. Chemicals in the Cytosol or external
environment
These stimuli control the ability of specific ions to
cross the cell membrane
Play an essential role in signaling by the nervous system
Example: As an impulse passes down a neuron, the reduction in the
voltage opens sodium channels in the adjacent portion of the membrane.
This allows the influx of Na+ into the neuron and thus the continuation of
the nerve impulse
PASSIVE TRANSPORT
VIDEO
https://www.youtube.com/watch?v
=JShwXBWGMyY
3. Osmosis
The process where water molecules
diffuse across a cell membrane from an
area of higher concentration to an area of
lower concentration
Passive transport of water
Osmosis video segment
http://www.coolschool.ca/lor/BI12/
unit4/U04L03/osmosis.swf
Direction of Osmosis
The net direction of osmosis depends on
the relative concentration of solutes on the
two sides of the membrane
3 Directions
1. Hypotonic
2. Hypertonic
3. Isotonic
Tonicity
The relative concentrations of SOLUTES
of two fluids
Isotonic-equal concentrations
Hypertonic-higher concentration
Hypotonic-lower concentration
Water tends to move from hypotonic
solutions to hypertonic solutions
Hypotonic
Contain a low concentration of solute
relative to another solution
(e.g. the cell's cytoplasm).
When a cell is placed in a Hypotonic
solution, the water diffuses into the cell,
causing the cell to swell and possibly
explode.
Hypotonic solution
https://video.google.com/videopla
y?docid=1157403886129286147&
ei=sV3tSOe4HIGE_AGiicSjAw&q
=hypotonic+solutions&vt=lf&hl=en
Hypotonic
Hypertonic
Hypertonic Solutions
• Hypertonic Solutions: contain a high
concentration of solute relative to another
solution
• (e.g. the cell's cytoplasm).
• When a cell is placed in a Hypertonic solution,
the water diffuses out of the cell, causing the cell
to shrivel.
Hypertonic solution
http://video.google.com/videoplay
?docid=1157403886129286147&e
i=sV3tSOe4HIGE_AGiicSjAw&q=
hypotonic+solutions&vt=lf&hl=en
Hypertonic
Hypotonic
Isotonic Solution
• Isotonic Solutions: contain the same
concentration of solute as an another solution
• (e.g. the cell's cytoplasm).
• When a cell is placed in an isotonic solution, the
water diffuses into and out of the cell at the
same rate.
• The fluid that surrounds the body cells is
isotonic.
Isotonic Solution
http://video.google.com/videoplay
?docid=1157403886129286147&e
i=sV3tSOe4HIGE_AGiicSjAw&q=
hypotonic+solutions&vt=lf&hl=en
Isotonic
Isotonic
If the concentration of solute (salt) is equal on both
sides, the water will move back in forth but it won't have
any result on the overall amount of water on either side.
"ISO" means the same
The word "HYPO" means less, in
this case there are less solute (salt)
molecules outside the cell, since
salt sucks, water will move into the
cell.
The cell will gain water and grow
larger. In plant cells, the central
vacuoles will fill and the plant
becomes stiff and rigid, the cell wall
keeps the plant from bursting
In animal cells, the cell may be in
danger of bursting, organelles
called CONTRACTILE VACUOLES
will pump water out of the cell to
prevent this.
The word "HYPER" means more, in
this case there are more solute (salt)
molecules outside the cell, which
causes the water to be sucked in that
direction.
In plant cells, the central vacuole
loses water and the cells shrink,
causing wilting.
In animal cells, the cells also shrink.
In both cases, the cell may die.
This is why it is dangerous to drink
sea water - its a myth that drinking
sea water will cause you to go insane,
but people marooned at sea will
speed up dehydration (and death) by
drinking sea water.
Terms Associated with Osmosis
Plasmolysis-cell loses water and shrinks
Cytolysis-cell gains water and swells
Hydrostatic pressure-pressure created by water
–cytoplasm pushes against cell membrane
Osmotic Pressure: pressure created by water as
it moves into or out of a cell
Osmotic pressure in plant cells is called turgor-it
makes a plant rigid and up right rather than
wilted
Osmosis Problems:
What will happen to a cell (1% salt) that is
placed in a 5% solution?
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Convert salt concentrations into water concentrations
by subtracting the salt concentration from 100%.
(A 5% salt solution = a 95% water solution)
Remember osmosis moves down the concentration
gradient
So, a cell that is 99% water in a 95% water
environment will cause water to move down the
gradient (99%  95%) – in this case, out of the cell
Hypo to hyper---the cell would shrink
http://www.stolaf.edu/people/giannini/flashanimat/transport/osmosis.swf
Active Transport
Active Transport
Movement of molecules against (or up) the
concentration gradient (from low concentration
to high concentration)
Requires the use of energy (ATP)
Involves highly selective protein carriers within
the membrane
Ions, sugar, amino acids, nucleotides
Sodium-potassium pump is an example
Liver storing excess glucose is an example
Sodium Potassium Pump
Is a carrier protein in animal cells
Requires ATP  ADP
Transports Na+ & K+ ions up their
concentration gradients
To function properly, some animal cells must have
a higher concentration of Na+ ions outside the cell
and a higher concentration of K+ ions inside the
cell
The sodium potassium pump maintains these
concentration differences
Na-K Pump Step 1
Three Na+ ions bind
to the carrier protein
on the cytosol side of
the membrane
Na-K Pump Step 2
At the same time, the
carrier protein
removes a phosphate
group from a
molecule of ATP 
The phosphate group
from the ATP
molecule binds to the
carrier protein
Na-K Pump Step 3
The removal of the
phosphate group from ATP
supplies the energy needed
to change the shape of the
carrier protein
With the new shape, the
protein carries 3 Na+ ions
through the membrane to
the outside of the cell where
the Na+ concentrations
must remain high
Na-K Pump Step 4
The carrier protein
has the shape
needed to bind with 2K+ ions outside the
cell
When the K+ ions
bind, the phosphate
group is released
Na-K Pump Step 5
When the K+ ions
bind, the phosphate
group is released and
the carrier protein
restores its original
shape releasing 2-K+
ions inside the cell
Now the carrier
protein is ready to
begin the process
again
Sodium-Potassium
Pump
http://www.stolaf.edu/people/giann
ini/flashanimat/transport/atpase.s
wf
Sodium-Potassium
Pump in Action
http://www.coolschool.ca/lor/BI12/
unit4/U04L03/active%20transport
_jeffedit.swf
Why a Na-K pump?
The exchange of 3-Na+ for 2K+ ions
creates an electrical gradient across the
cell membrane
Outside –positively charged; inside –
negatively charged (like a battery)
Is important for the conduction of electrical
impulses along nerve cells
Endocytosis
Plasma membrane extends outward and
envelopes or engulfs food particles
Folds into itself forming a pouch, then
pinches off from the cell and becomes a
vesicle

Some vesicles fuse with lysosomes & are
digested
3 types of endocytosis:
1. phagocytosis “cell eating” (ingest
bacteria or viruses)
2. pinocytosis “cell drinking” (fluids)
3. receptor-mediated endocytosis:
molecules bind to receptors on
membrane and then form vesicle (how
cholesterol is brought into a cell)
Endocytosis is a process by which a cell
surrounds and takes in material from its
environment
Nucleus
Digestion
Endocytosis
Wastes
Exocytosis
Exocytosis
Reverse of endocytosis
Discharge of materials from vesicles at the
cell’s surface
Examples: cells secreting hormones,
neurotransmitters, digestive enzymes
Exocytosis
Exocytosis is the expulsion or secretion of
materials from a cell.
Endocytosis and exocytosis both move masses of
material and both require energy
Nucleus
Wastes
Digestion
Endocytosis
Exocytosis