Download The Cell Membrane

The Cell Membrane
AP Biology
AP Biology
Helpful links as you go through this
chapter…
 http://student.ccbcmd.edu/courses/bio1
41/lecguide/unit3/eustruct/cmeu.html
Cell membrane defines the perimeter of the cell
 Cell membrane separates living cell from
aqueous environment

thin barrier = 8nm thick
 Controls traffic in & out of the cell

allows some substances to cross more
easily than others
Movement across cell membrane
 Cell membrane is the boundary between
inside & outside…

separates cell from its environment
Can it be an impenetrable boundary?
NO!
OUT
IN
food
carbohydrates
sugars, proteins
amino acids
lipids
salts, O2, H2O
OUT
IN
waste
ammonia
salts
CO2
H2O
products
cell needs materials in & products or waste out
Selective Permeability
 Barrier allows SOME substances to
cross more easily than others
Also called SEMI-Permeable
 Key aspect of membrane structure and
function

 Remember: selectively permeable
membranes are important in many
organelles as well as the plasma
membrane of the cell itself.
Membrane Structure
 Primary components

Lipids
 Foundation molecules
 Primarily PHOSPHOLIPIDS

Proteins
 Embedded within the lipids of the membrane
 Important in carrying out most membrane
functions
 Remember: It’s the PROTEINS that do all the
WORK!
Phospholipids
 Phosphate head

“attracted to water”
hydrophilic
 Fatty acid tails

Phosphate
hydrophobic
 Arranged as a bilayer
Fatty acid
“repelled by water”
Aaaah,
one of those
structure–function
examples
Why the bilayer?
 Water is attracted to hydrophilic heads
and NOT to hydrophobic tails
 Water pushes tails away and they end
up pointed toward each other and
AWAY from water.
 Results in Bilayer
•Animation –
Click HERE
Bilayer as Selective Barrier
 Because of the close placement and
polarity of the phosphate heads, there
are some molecules that cannot
penetrate and some that can.
What CANNOT penetrate
Phospholipid Bilayer?
What CAN penetrate
Phospholipid Bilayer?
• Anything NONPOLAR
•Lipids
•Oxygen
•Carbon dioxide
NONPOLAR molecules do
NOT get attracted and
Stuck to polar heads; thus
They pass through.
• POLAR / Charged things
•Ions (H+, Cl-, Na+)
•Water
•Glucose
•Polar amino acids
Ions get large hydration
Shells in aqueous
Solution which makes them
Too large to fit b/t
Phospholipids
Also, polar/charged molecues
Become attracted and
stuck to polar heads
______ attracted to nonpolar center
of bilayer
Arranged as a Phospholipid bilayer
 Serves as a cellular barrier / border
sugar
H 2O
salt
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
impermeable to polar molecules
polar
hydrophilic
heads
waste
lipids
How, then, can these polar, but necessary
molecules pass through the membrane?
 PROTEINS

Embedded in the bilayer
Permeability to polar molecules?
 Membrane becomes semi-permeable via
protein channels


specific channels allow specific material across
cell membrane
Thus control is also maintained. Not just
anybody can get in.
inside cell
NH3
salt
H 2O
aa
sugar
outside cell
Aquaporin – A Protein Channel
 Used to be thought that


HI water
Conc.
water could penetrate the
phospholipid bilayer
unaided
Now we know that a
protein channel called
aquaporin is found in
nearly all cell membranes
in large amounts
This is the channel that
allows the passage of
water.

Remember, water is polar
and would NOT be
attracted past the polar
heads
LO water
Conc.
Cell membrane is more than lipids…
 Transmembrane proteins embedded in
phospholipid bilayer

create semi-permeabe channels
protein channels
in lipid bilyer membrane
AP Biology
Why are
proteins the perfect
molecule to build structures
in the cell membrane?
AP Biology
2007-2008
Classes of amino acids
What do these amino acids have in common?
nonpolar & hydrophobic
Classes of amino acids
What do these amino acids have in common?
I like the
polar ones
the best!
polar & hydrophilic
Protein’s regions anchor molecule
 Within membrane

Polar areas
of protein
nonpolar amino acids
 hydrophobic
 anchors protein
into membrane
 On outer surfaces of
membrane in fluid

polar amino acids
 hydrophilic
 extend into
extracellular fluid &
into cytosol
Nonpolar areas of protein
H+
Examples
Retinal
chromophore
NH2
aquaporin =
water channel
Porin monomer
H 2O
b-pleated sheets
Bacterial
outer
membrane
Nonpolar
(hydrophobic)
a-helices in the
cell membrane
COOH
H
HH+++
Cytoplasm
proton pump channel
in photosynthetic bacteria
H 2O
function through
conformational change =
protein changes shape
Membrane Proteins do LOTS of different jobs!!
“Channel”
Outside
Plasma
membrane
Inside
Transporter
Enzyme
activity
Cell surface
receptor
Cell adhesion
Attachment to the
cytoskeleton
“Antigen”
Cell surface
identity marker
Membrane Proteins
 Proteins determine membrane’s specific functions

cell membrane & organelle membranes each have
unique collections of proteins
 Classes of membrane proteins:

peripheral proteins
 loosely bound to surface of membrane
 ex: cell surface identity marker (antigens)

integral proteins
 penetrate lipid bilayer, usually across whole membrane
 transmembrane protein
 ex: transport proteins
 channels, permeases (pumps)
Cell membrane must be more than
lipids…
 In 1972, S.J. Singer & G.

Nicolson proposed that
membrane proteins are
inserted into the
phospholipid bilayer
FLUID MOSAIC MODEL



Made up of lots of little
pieces – like a mosaic
Pieces can slide past
each other – like a
soap bubble – like a
fluid
Click here for an
animation
Don’t forget Cholesterol
 Embedded in
membrane b/t
phospholipids
 Keeps membrane
firm yet fluid
 Important for cell
membrane structure

Even though we
often think of
cholesterol as
being “bad”…
Membrane is a collage of proteins & other molecules
embedded in the fluid matrix of the lipid bilayer
Glycoprotein
Extracellular fluid
Glycolipid
Phospholipids
Cholesterol
Peripheral
protein
Transmembrane
proteins
Cytoplasm
Filaments of
cytoskeleton
1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model
Membrane carbohydrates
 Play a key role in cell-cell recognition

ability of a cell to distinguish one cell
from another
 antigens
important in organ &
tissue development
 basis for rejection of
foreign cells by
immune system

Getting through cell membrane
 Passive Transport

Simple diffusion
 diffusion of nonpolar, hydrophobic molecules
 lipids
 HIGH  LOW concentration gradient

Facilitated diffusion
 diffusion of polar, hydrophilic molecules
 through a protein channel (“door”)
 HIGH  LOW concentration gradient
 Active transport

pump against concentration gradient
 LOW  HIGH


uses a protein pump
requires ATP
ATP
Transport summary
HI
HI
LO
LO
LO
HI
simple
diffusion
facilitated
diffusion
active
transport
NONPOLAR cmpds.
Lipids; oxygen;
Carbon dioxide
POLAR cmpds.
Water; Ions – Na+, Cl-;
Glucose, etc.
ATP
REQUIRED
How about large molecules?
 Moving large molecules into & out of cell


through vesicles & vacuoles
Endocytosis – taking in
 phagocytosis = “cellular eating” - solids
 pinocytosis = “cellular drinking” - liquids

Exocytosis
 Expelling / secreting
substances
exocytosis
Endocytosis
Solids
fuse with
lysosome for
digestion
phagocytosis
Liquids
pinocytosis
Both are
non-specific
processes
Cholesterol is taken
Into the cell
This way.
receptor-mediated
endocytosis
LIGAND – substance
that fits receptor
SPECIFIC
The Special Case of Water
Movement of water across
the cell membrane
AP Biology
2007-2008
Osmosis is just diffusion of water
 Water is very important to life,
so we talk about water separately
 Diffusion of water from
HIGH concentration of water to
LOW concentration of water

across a
semi-permeable
membrane
Concentration of water
 Direction of osmosis is determined by comparing total
solute concentrations
 Hypertonic - more solute, less water

Hypotonic - less solute, more water

Isotonic - equal solute, equal water

Water moves from a HYPOTONIC (HIGH Concentration of
water) solution to a HYPERTONIC (LOW Concentration of
water) solution
LESS water
MORE solute
MORE water
LESS solute
Yellow dot =
water
SOLUTE
Blue = WATER
hypotonic
hypertonic
net movement of water
Managing water balance
 Cell survival depends on balancing
water uptake & loss
freshwater
balanced
saltwater
1
Managing water balance
 Hypotonic

a cell in fresh water

high concentration of water around cell
 problem: cell gains water,
swells & can burst (lysis)
KABOOM!
 example: Paramecium
 ex: water continually enters
Paramecium cell
 solution: contractile vacuole
 pumps water out of cell
ATP
 ATP

plant cells
No problem,
here – I love it!
 turgid = full
 cell wall protects cell from bursting
freshwater
Pumping water out
 Contractile vacuole in Paramecium
ATP
2
Managing water balance
 Hypertonic


I’m shrinking,
I’m shrinking!
a cell in salt water
low concentration of water
around cell
 problem: cell loses water & can die
 example: shellfish
 solution: take up water or pump out
salt

plant cells
Water…
 plasmolysis = wilt = flaccid cells
 Cell membrane collapses away
from cell wall
 can recover, but may die
 Salt on roadsides after snow…
saltwater
3
Managing water balance
 Isotonic
That’s
perfect!

animal cell immersed in
mild salt solution

no difference in concentration of
water between cell & environment
 problem: none
 no net movement of water
flows across membrane equally, in
both directions
OK..but I could
 cell in equilibrium
be better…

 volume of cell is stable
 example:
blood cells in blood plasma
 slightly salty IV solution in hospital
balanced
1991 | 2003
Aquaporins
 Water moves rapidly into & out of cells

evidence that there were water channels
 protein channels allowing flow of water
across cell membrane
Peter Agre
Roderick MacKinnon
John Hopkins
Rockefeller
Do you understand Osmosis…
Remember:
It’s all
RELATIVE
.05 M
.03 M
Cell (compared to beaker)  hypertonic or hypotonic
Beaker (compared to cell)  hypertonic or hypotonic
Which way does the water flow?  in or out of cell
‘PUMPS’: Special Active Transport
 Sodium Potassium Pump



Na+/K+ Pump
Classic example
Important in nerve cell
and nerve impulse
transmission (among
others)
 Generates voltage
difference across the
membrane of the neuron

For animations click
 Here or Here
Cotransport
 Membrane proteins couple the
transport of one solute to another
 A combination of both active transport
and facilitated diffusion (passive).
Pump (active)
 Transport channel (passive)

Cotransport – Example – in plants
 Proton Pumps pump H+
against the
concentration gradient
and OUT of the cell.


Creates a very hi
concentration of H+
outside the cell.
ACTIVE transport
 That means, if given an
opportunity, H+ would
flow WITH its
concentration gradient
back INSIDE the cell.
Cotransport – Example – in plants
 In plants, sucrose is
in HIGH
concentration inside
the cells, but plants
need to get even
even MORE sucrose
into cells for
storage.
 Thus, sucrose must
move into a plant
cell AGAINST its
concentration
gradient.
Cotransport – Example – in plants
 “Downhill” (WITH concentration)



movement of H+ is coupled with the
“uphill” (AGAINST concentration)
movement of sucrose into the cell.
Sucrose MUST bind to an H+ ion.
“Rides” with the H+ as H+ flows INTO
the cell through the cotransport
channel WITH it’s concentration
gradient.
Remember, the H+ concentration
gradient is maintained by the proton
pump
The only way that sucrose can enter is
if it is bound to H+. Then the very
specific cotransport channel will allow
both to “diffuse” through.


PUMP to maintain H+ conc. =
ACTIVE
Cotransport channel = facilitated
diffusion = PASSIVE