The Cell
Membrane
Ms. Napolitano
Honors Biology
12/10 – Do Now
 Please
get your clickers!
 Turn in your webquest if you did not hand
it in last class.
 Think about the following question:
Why are cells small?
The Cell Membrane
 Selectively
permeable – allows some
substances to cross more easily than others
 8 nm thick
 Fluid mosaic model
 Fluid
structure of phospholipids with a mosaic of
various proteins embedded in it
 Cholesterol
maintains fluidity – acts as a
temperature buffer
 less
fluid as temp increases (restricts movement)
 More fluid as temp decreases (maintains
space)
The Structure of the Cell
Membrane
Membrane Proteins
 Different
types of cells contain different
types of membrane proteins
 Integral
proteins – embedded into the
hydrophobic core of the lipid bilayer

Can go all the way
through the membrane
 Peripheral
proteins –
on the surface of the
membrane
Types of Membrane Proteins

Transport




Enzymatic
Signal Transduction (chemical messages)


ID tags – glycolipids or glycoproteins
Intercellular Joining


Receptor proteins transmit information from outside of
the cell to inside of the cell
Cell Recognition


Channel/carrier proteins allow polar molecules & ions to
pass through hydrophobic layer
Channel & carrier proteins are substance specific
Join together adjacent cells
Cell Support

Attach to cytoskeleton or extracellular matrix for stability
What type of membrane
protein is this?
1.
2.
Integral
Peripheral
0%
1
0%
2
A cell that has been frozen will
have a more fluid membrane than
a cell at room temperature.
1.
2.
True
False
0%
1
0%
2
What would happen to a cell that
lacks cholesterol in the membrane?
1.
2.
3.
4.
It would not be able to
transport materials into/out
of the cell
The membrane would
change drastically with
temperature
It would not be able to use
enzymes effectively
Chemical messages would
not be able to get into the
cell
0%
1
0%
2
0%
3
0%
4
What is the function of membrane
carbohydrates?
1.
2.
3.
4.
Transport
Signal
transduction
Cell recognition
Intercellular
joining
0%
1
0%
2
0%
3
0%
4
Passive Transport

Passive Transport – movement across the cell
membrane that does not require energy

Diffusion – movement of particles from an area of high
concentration to an area of low concentration until an
equilibrium is reached

Said to go with/down its concentration gradient

Concentration gradient – the difference in the
concentration of a substance across a space

Equilibrium – concentration of a
substance is equal throughout a
space

Doesn’t stop moving
Osmosis
 Osmosis
– the diffusion of free water across
a selectively permeable membrane
 Water
diffuses across the cell membrane
from a region of low solute concentration
to that of a higher solute
concentration until it
reaches equilibrium
Really cool video!
http://www.youtube.com/watch?v=EsJ73x4
ycp0
Osmoregulation in Cells
without Cell Walls
 Osmoregulation
– control of water balance
 Tonicity – ability of a solution to cause a cell to
gain/lose water


Isotonic solution – no net movement of water across
the cell membrane
Hypertonic solution – more free water inside the cell
 Cell

loses water to environment and shrivels
Hypotonic solution – less free water inside the cell
 Water
will enter the cell and swell (and maybe lyse)
 Some cells have a contractile vacuole to pump water
out of the cell
Animal Cells
Osmoregulation of Cells with
Cell Walls
 Plant
cells are healthiest in a hypotonic
solution, osmotic pressure keeps cell walls
turgid (very firm)
 Plant
cells are flaccid (limp) in an isotonic
solution
 In
a hypertonic solution, the cell membrane
will shrink and pull away from the cell wall
 Called
plasmolysis (wilting)
Plant Cell
U – Tube (not YouTube!)
Which of the following can readily
diffuse across a lipid bilayer?
1.
2.
3.
4.
Glucose
Oxygen
Carbon dioxide
Both 2 & 3
0%
1
0%
2
0%
3
0%
4
If you immerse a living cell in a
hypotonic solution, water will…
1.
2.
3.
Diffuse into the
cell
Diffuse out of the
cell
Show no net
movement
0%
1
0%
2
0%
3
Plant 1 is in better shape than plant
2 because the cells of plant 1 are…
1.
2.
3.
4.
Turgid
Flaccid
Undergoing
plasmolysis
The same as
plant 2
Plant 1
Plant 2
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1
0%
2
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3
0%
4
The image below is a U-Tube with a
semi-permeable membrane. How
will water flow?
1.
2.
3.
It won’t
Left to right 
Right to left 
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1
0%
2
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3
12/ 16 Do Now:
 Get
Your Clicker!
 Compare/Contrast Active & Passive
Transport
 Compare/Contrast Diffusion & Osmosis
Facilitated Diffusion
 Facilitated
diffusion: passive transport aided by
proteins
 Channel proteins – hydrophilic passageways





Some always open for diffusion
Rate of movement is determined by concentration
gradient
(+) charged ions more likely to diffuse into the cell
(-) charged ions more likely to diffuse out of the cell
Some ion channels have gates and can be opened
by:
 Stretching
of the cell membrane
 Change in electrical charge
 Binding of specific molecules
Voltage Gated Channels
 Nerve
cells transmit electrical signals by
opening a series of Na+ gated channels
1.
2.
3.
4.
5.
Channel is closed
Area changes voltage
Channel opens briefly
Na+ flood into cell – voltage changes
Channel closes and electrical signal passes on
to the next voltage channel
Chemically (Ligand) Gated
Channels
 Nerve
cells send out a neurotransmitter called
acetylcholine (ACh) to nearby muscle cells to
signal muscles to contract
1.
2.
3.
ACh binds to ACh receptor proteins
Receptor gate opens for a microsecond to allow
Na+ in
Na+ sets off muscle
contraction
Facilitated Diffusion
 Carrier
1.
2.
3.
Proteins
Specific substance
binds to carrier protein
Protein changes shape
and transports
substances across cell
membrane
Molecule is released
into the cell, and
carrier protein returns
to its original shape
Which image best represents
facilitated diffusion?
1.
2.
3.
4.
1
2
3
4
3
87%
1
2
4
13%
0%
1
2
3
0%
4
____are transported by a particular carrier
or pass through a particular channel.
1. All water soluble
40%
ions or molecules
2. Certain water
soluble molecules
27%
or ions
20%
3. All insoluble
molecules or ions
13%
4. Certain insoluble
ions or molecules
5. Only the smallest
molecules or ions
1
2
3
4
0%
5
Active Transport
 Active
Transport – uses energy to move
solutes against the concentration gradient
 Carrier
by ATP
proteins act as “pumps” powered
 Examples:
 Sodium
Potassium Pump
 Proton Pump
 Cotransport
Sodium Potassium Pump
 Pumps
3 Na+ out of the cell and 2 K+ into the cell
 Actively transports both ions against their
concentration gradient, powered by ATP
 Prevents Na+ from accumulating in the cell
 Steps:
1.
2.
3.
4.
3 Na+ and a P (from ATP) bind to inside protein
pump
Pump changes shape transporting 3 Na+ across
membrane and out
2 K+ bind to pump and are transported across
membrane
2 K+ and P are released inside of cell
Sodium Potassium Pump
The sodium-potassium pump
passes
1.
2.
3.
4.
More Na+ out than K+
in
K+ out and Na+ in on
a one-for-one basis
Na+ out and K+ in on
a one-for-one basis
K+ and Na+ in the
same direction
87%
7%
7%
0%
1
2
3
4
Proton Pump
 Actively
transports protons (H+) through
the internal membranes of mitochondria
and chloroplasts
Cotransport (Coupled Transport)
 Cotransport
– an ATP-powered pump that
transports a specific solute


Can indirectly drive the active transport of several
other solutes
Ex: as proton pumps pump
H+ out, H+ diffuse back into
the cell pulling sucrose
molecules into the cell
with it
By a mechanism called
cotransport
1.
2.
3.
4.
5.
sugar moves down (with) its
concentration gradient while
sodium moves up (against) its
concentration gradient
sodium moves outward against is
concentration gradient while
sugar moves inward down its
concentration gradient
sugar and sodium move inward
down their concentration
gradient
sodium moves inward down its
concentration gradient while
sugar moves inward up its
concentration gradient
sodium and sugar move outward
against their concentration
gradient
47%
27%
20%
7%
0%
1
2
3
4
5
Bulk Transport
 Substances
that are too large to be transported
by carrier proteins
 Exocytosis (export)

Secretion of macromolecules by fusion of vesicles
with membranes, releasing the contents outside of
the cell
 Endocytosis


(import)
Cell membrane engulfs particles and pinches off to
form vesicles inside the cell. Vesicle may fuse with
lysosomes or other organelles
3 Types:
 Phagocytosis
– engulf another organism
 Pinocytosis – engulf fluid
 Receptor-mediated - specific
Exocytosis (top)/Endocytosis (bottom)
A cell engaged in
phagocytosis must be
1.
2.
3.
4.
5.
engulfing a live
organism
acquiring a liquid
engulfing a dead
organism
transporting bulk
dissolved nutrients
transporting bulk
solid material
80%
20%
1
0%
0%
2
3
0%
4
5
Substances transported by
facilitated diffusion
1.
2.
3.
4.
5.
move passively through specific
channels from an area of greater
concentration to one of lower
concentration
are limited to solvents
must have movements coupled to
those of other substances
may flow to a region of higher
concentration by the expenditure of
energy
are restricted to only one direction
through the membrane
87%
13%
1
0%
0%
2
3
0%
4
5
A contractile vacuole is an organelle that pumps excess water
out of many freshwater protozoan cells. A freshwater
protozoan was placed in solution A and observed to form
contractile vacuoles at a rate of 11 per minute. The same
protozoan was then placed in solution B and observed to form
contractile vacuoles at a rate of 4 per minute. Based on this
information, which of the following statements is correct?
1.
2.
3.
4.
5.
Solution A is hyperosmotic to
solution B.
Solutions A and B are isosmotic.
Solution B is hyperosmotic to
solution A.
Solutions A and B are isosmotic
to the protozoan cell.
Both a and b
47%
40%
7%
7%
0%
1
2
3
4
5
The methods of membrane transport that
don't require protein channels or carriers are
1.
2.
3.
4.
5.
osmosis
Diffusion
Phagocytosis
Exocytosis
all of the above
87%
13%
1
0%
0%
2
3
0%
4
5