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Plasma Membrane
Structure and Function
Image from: http://www.biologie.uni-hamburg.de/b-online/ge22/03.gif
Cell Size Limitations
• The size of cells is limited by the surface
area to volume ratio of cells
– As cells grow the volume inside of the cell
grows faster than the surface area available
to diffuse nutrients into the cell and expel
wastes out of the cell
• Surface area determines rate molecules enter and
leave cell
• Volume determines the demand for resources
needed by the cell (increase in volume requires
more resources to keep the cell functioning
properly)
• Smaller cells have more favorable surface
area to volume ratios for exchange with
the environment
Root hairs
Cells of alveoli:
Lungs
Villi: Small Intestines
• You should be
able to solve
basic surface
area and
volume
equations given
to you on the
AP Biology
exam.
• In order for cells to grow, reproduce and
maintain dynamic homeostasis, cells must
create and maintain an internal
environment that is different from the
external environment
– Cell membranes are selectively permeable to
accomplish this goal
– Cell membranes separate the internal
environment from the external environment
FLUID MOSAIC MODEL
Animation from: http://www.sp.uconn.edu/~terry/images/anim/fluidmem.gif
Click here to
See Fluidity
Click here to See
FLUIDITY
1972- S.J. Singer and G. Nicolson propose membrane is a
“mosaic” of proteins and phospholipids that are constantly
moving and changing
More than just a barrier…
• The membranes selective permeability is a
direct consequence of the membranes
structure consisting of phospholipids,
embedded proteins, cholesterol,
glycoproteins and glycolipids
A membrane is a collage of different proteins
embedded in the fluid matrix of the lipid
bilayer
Molecules need to move across
membranes in cells
OUT
waste
ammonia
salts
CO2
H2O
products
IN
food
carbohydrates
sugars,
proteins
amino acids
lipids
salts, O2, H2O
Image modiified from: http://www.accessexcellence.org/AB/GG/importProt.html
Membranes provide a variety of cell
functions
Cell Membranes are made of
PHOSPHOLIPIDS & PROTEINS
Amphipathic –
Molecules with
both hydrophilic
and hydrophobic regions
phosphate
hydrophilic
lipid
hydrophobic
Membrane Proteins
• Proteins determine most of membrane’s specific
functions
– cell membrane & organelle membranes each have
unique collections of proteins
• Membrane proteins:
– peripheral proteins = loosely
bound to surface of membrane
– integral proteins = penetrate into
lipid bilayer, often completely
spanning the membrane =
transmembrane protein
HYDROPHILIC/HYDROPHOBIC areas
determine positions of molecules in cell
membranes
hydrophobic amino acids
– Those with nonpolar side groups
– stick in the lipid membrane
– anchors the protein
in membrane
hydrophilic amino acids
– Amino acids with charge
and polar side groups
– stick out in the watery
fluid in or out of cell
Membrane Carbohydrates
• Attached to proteins (glycoproteins)
or lipids (glycolipids)
• Play a key role in cell-cell recognition
– ability of a cell to distinguish neighboring cells
from another
– important in organ &
tissue development
– basis for rejection of
foreign cells by
immune system
http://faculty.southwest.tn.edu/rburkett/GB1-osmosis.htm
Semi-permeable membrane
• Need to allow passage through the
membrane
• But need to control what gets in or out
– membrane needs to be semi-permeable
sugar
aa
lipid
H 2O
salt
NH3
So what makes a membrane semi permeable?
See a movie
PHOBIC TAILS in center determine
what can pass through
What molecules can get through directly?
Small non-polar (uncharged) molecules (O2, N2, &
CO2) and hydrophobic molecules (fats & other
lipids)
can slip directly through the phospholipid cell
membrane, but…
inside cell
NH3
lipid
O2
outside cell sugar aa
salt
H 2O
What about
other stuff?
Types of Cell Transport
Diffusion
• 2nd Law of Thermodynamics
governs biological systems
– Universe tends towards disorder
Animatioin from: http://www.biologycorner.com/resources/diffusion-animated.gif
• Diffusion
– movement from [higher]  [lower] concentration
Diffusion will also
happen across a
cell membrane as
long as there is a
difference in
concentration and
the membrane
will let the
molecule pass
through.
Example: DIFFUSION IN CELLS
http://facstaff.bloomu.edu/gdavis/links%20100.htm
O2 automatically moves from
HIGHER concentration (in lungs) to
LOWER concentration (in blood)
CO2 automatically moves from
HIGHER concentration (in blood)
to LOWER concentration (in lungs)
http://www.le.ac.uk/pa/teach/va/anatomy/case2/2_2.html
Diffusion of 2 solutes
• Each substance diffuses down its own
concentration gradient, independent of
concentration gradients of other
substances
What if cell needs to move a
AGAINST the
molecule _________
CONCENTRATION
GRADIENT?
_______________
(LOWER  HIGHER)
Cell example:
Want to put MORE glucose
into mitochondria when there is
already glucose in there
Image from: http://www.biologyclass.net/mitochondria.jpg
What if a cell needs to
LARGE or ______
POLAR
move _____
molecules
that can’t get
through the
membrane?
http://www.d.umn.edu/~sdowning/Membranes/membraneImages/jpegimages/diffusionmedium.jpg
What if cell needs to move
FAST
molecules really _______?
(can’t wait for it to diffuse)
Cell example:
Movement of
Na + & K+ ions
required to send
nerve signals
http://www.steve.gb.com/images/science/neuron.png
WAY to
Cells need a ____
HELP molecules across
____
cell membranes that
can’t go across by
_______
themselves
___________
Facilitated diffusion
• Move from HIGH to LOW concentration with
aid of membrane transport proteins
– passive transport
– no energy needed
– facilitated = with help
– Examples:
• Way glucose enters cells
• Also how charged and polar molecules enter the cell
Facilitated Diffusion
Animation from: http://bio.winona.edu/berg/ANIMTNS/facdifan.gif
Carrier Proteins
Grab molecule,
undergo conformational
change, flip to other side
Channel Proteins
Create passageway for
substances to pass
through
May be gated or not:
open in response to
chemical or electrical
signals
Animation from: http://www2.uic.edu/~myilma1/ionchannel.gif
Gated channels
• open only in presence of stimulus (signal)
– stimulus usually different from transported
molecule
• ex: ion-gated channels
when neurotransmitters bind to a specific gated
channels on a neuron, these channels open
= allows Na+ ions to enter nerve cell
• ex: voltage-gated channels
change in electrical charge across nerve cell
membrane opens Na+ & K+ channels
Carriers and Channels are specific
inside cell
H2O
aa
sugar
NH3
salt
outside cell
. . . BUT STILL MOVES FROM [HIGHER] to [LOWER]
Active transport
Uses energy from ATP to move molecules
against concentration gradient
– Moves from [lower] → [higher]
– Uses protein pumps
OR
vesicles
Animations from:
http://academic.brooklyn.cuny.edu/biology/bio4fv/page/cell-movement.html
http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/images/sppump.gif
PROTON PUMP
Moves Protons (H+)
across membrane
EXAMPLES:
- Creates acidic condition inside lysosomes
- Photosynthesis/respiration use H+ gradients to
generate ATP
See a movie
proton pump
Active transport
SODIUM-POTASSIUM PUMP
Sets up difference in charge across membranes
Pumps 3 Na+ out
and 2 K+ in
Makes cells
more + outside
more - inside
See a movie
about Na+ - K+ pump
Animation from: http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/images/sppump.gif
All cells have voltages across their membranes
= membrane potential
• Cytoplasm inside cell is more negative
than extracellular fluid outside
• TWO FORCES drive diffusion
• Concentration gradient
• Electrical force
• Ions move DOWN the
electrochemical gradient
Favors the passage of cations (+) into cells
MEMBRANE POTENTIAL created by
electrogenic pumps (proteins that generate
voltage by pumping ions)
PROTON PUMP
Main electrogenic pump in
plants, fungi, and bacteria
http://fig.cox.miami.edu/~cmallery/150/memb/electrogenic.jpg
Na+- K+ PUMP
Main electrogenic pump
in animal cells
Animation from: http://www.lionden.com/cell_animations.htm
Electrogenic pumps can be coupled to
actively transport other substances
= COTRANSPORT
Ex: Cells pump H+
out of cell then use
the diffusion of H+
back into cell down
its gradient to drive
the uptake of
sucrose into cells
http://faculty.southwest.tn.edu/rburkett/GB1-os29.jpg
Na+ - K+ pump sets up MEMBRANE
POTENTIAL
http://hyperphysics.phy-astr.gsu.edu/hbase/biology/nervecell.html#c2
BULK TRANSPORT
• ACTIVE transport
• Requires energy (ATP)
• Uses vesicles
Watch two video clips about
endo/exocytosis
endo/exocytosis
Animation from: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBooktransp.html
Endocytosis
phagocytosis
pinocytosis
receptor-mediated
endocytosis
“Cell eating”
large molecules;
whole cells
“Cell drinking”
Fluids;
Small molecules
triggered by
ligand signal
Example in cells:
WHITE BLOOD CELL ENGULFING
BACTERIA
SEE
PHAGOCYTOSIS
MOVIE
http://fig.cox.miami.edu/~cmallery/255/255ion/fig14x28.jpg
EXOCYTOSIS
• Active transport (requires ATP)
• Uses vesicles
• Releases substances to outside
INSULIN being released by pancreas cells using exocytosis
Video: http://www.southtexascollege.edu/tdehne/BC_ShockwaveAnimations/07SWF-TourOfTheCell/07-16-EndomembraneSystem.swf
GOLGI BODIES USE EXOCYTOSIS
Animation from: http://www.franklincollege.edu/bioweb/A&Pfiles/week04.html
See a Golgi movie
http://fig.cox.miami.edu/~cmallery/255/255ion/fig14x26.jpg
Transport summary
Water Transport
The Special Case of Water
The Special Case of Water
Movement of water across
the cell membrane =
OSMOSIS
Osmosis is diffusion of water
• Water is very important, so we talk about
water separately
• Diffusion of water from high concentration
of water to low concentration of water
– across a
semi-permeable
membrane
– Passive
– Uses no energy
Aquaporins
1991 | 2003
• Transport proteins that move water rapidly
into & out of cells
– evidence that there were water channels
Peter Agre
Roderick MacKinnon
John Hopkins
Rockefeller
TONICITY- ability of a solution to cause
a cell to lose or gain water
• Refers to the concentration of SOLUTES
• Is a RELATIVE term, comparing two
different solutions
Solute-substance that is dissolved
Solvent- substance solute in dissolved in
Solution = solute + solvent
What if there is a difference in
concentration but solute
molecules can’t move across a
membrane?
WATER will move
until concentrations
reach equilibrium
Animation: http://www.ouhscphysio.org/humanphys/animations/osmosis1.swf
See an animation
Osmosis1
http://faculty.etsu.edu/currie/images/osmosis1.jpg
Animation
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
hypotonic
hypertonic
net movement of water
• Pressure potential:
– pressure exerted by the rigid cell wall
that limits further water uptake.
• Solute potential
– The effect of solute concentration.
– Pure water at atmospheric pressure
has a solute potential of zero.
– As solute is added, the value for solute
potential becomes more negative. This
causes water potential to decrease
also.
– As solute is added, the water potential
of a solution drops, and water will tend
to move into the solution.
In beaker B, what is the water potential of the
distilled water in the beaker, and of the beet
core?
– Since water potential = solute potential (-0.4) + pressure
potential (0.2),
– water potential = -0.2 (water left the beet core)
Solute concentration
Lower outside
than inside
Equal outside
and inside
Greater outside
than inside
HYPOTONIC
ISOTONIC
HYPERTONIC
What will happen to an animal cell
placed in different solutions?
Animation from: http://www.ouhscphysio.org/humanphys/animations/osmosis3.swf
OSMOSIS
See an animation
Osmosis3
Video
Choose Blood
Hypotonic link
HYPOTONIC:
Concentration outside cell is
________________
inside the cell
LESS THAN
More water enters than leaves cell so cell
will ___________________
swell and possibly burst
Animation from: http://www.ouhscphysio.org/humanphys/animations/osmosis4.swf
OSMOSIS
See an animation
OSMOSIS 4
HYPERTONIC: Concentration outside cell is
____________________
inside cell
GREATER THAN
More water leaves cell than enters
so cell ____________
shrinks
Video
Choose Blood
Hypertonic link
Remember:
Cells try to “maintain stable
internal conditions =
HOMEOSTASIS
____________________
http://bioweb.wku.edu/courses/biol121/Osmosis/Osmosis.asp
So an animal cell in
ISOTONIC conditions
stays same size
Water entering = water leaving
Video
Choose Blood Isotonic link
Animal cells
= CYTOLYSIS
= CRENATION
http://www.stchs.org/science/courses/sbioa/metenergy/bloodcells.gif
http://www.stchs.org/science/courses/sbioa/metenergy/aplantturgor.gif
Plant cells
Cell wall keeps plant cell from bursting made of cellulose
rather than phospholipids and is external the cell
membrane. Provides structural barrier for some
substances to the internal environment.
Managing water balance
• Isotonic
– animal cell immersed in isotonic
solution
• blood cells in blood
• no net movement of water across
plasma membrane
• water flows across membrane, at
same rate in both directions
• volume of cell is stable
Osmosis…
.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
http://www.biology4kids.com/files/cell_vacuole.html
Loss of water from central vacuole= plants “wilt”
Vacuole full of water gives plant support
(turgor pressure)
Turgid = very firm
Flaccid = limp
HYPOTONIC
Sitting in the bathtub
makes your fingers
plump up and get “pruny”
Grocery stores spray
water on their veggies
to “plump them up”
If cells can’t maintain
“stable internal conditions” . . .
damage can result and cells can die.
http://www.the-aps.org/education/lot/cell/Quiz.htm
Cell survival depends on balancing water
uptake & loss = OSMOREGULATION
http://www.microscopy-uk.org.uk/mag/imgjun99/vidjun1.gif
Paramecium vs. pond
water
Paramecium is
hypertonic
H2O continually enters
cell
to solve problem,
specialized organelle,
contractile vacuole
Drink salt water
ACTIVELY pump ions OUT;
Urinate less frequently
Gills ACTIVELY pump ions in;
Urinate frequently
Kidspiration by: Riedell