Transcript Membrane Structure and Transport

Topic - Membranes
Cell Membranes are made of
PHOSPHOLIPIDS & PROTEINS
Amphipathic –
Molecules with
both hydrophilic
and hydrophobic regions
phosphate
hydrophilic
lipid
hydrophobic
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…
• Expanding our view of cell membrane
beyond just a phospholipid bilayer barrier
– phospholipids plus…
Cell Membrane Micrograph
2.4.2
A membrane is a collage of different proteins
embedded in the fluid matrix of the lipid
bilayer
Structures in Cell Membrane (animation)
2.4.1
Membrane Proteins
• Proteins determine most of membrane’s specific
functions
– enzymes, receptors and transport
• Membrane proteins:
– peripheral proteins = loosely
bound to surface of membrane
– integral proteins = penetrate into
lipid bilayer, often completely
spanning the membrane =
transmembrane protein
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
HYDROPHILIC/HYDROPHOBIC areas
determine positions of molecules in cell
membranes
hydrophobic amino acids
– stick in the lipid membrane
– anchors the protein
in membrane
hydrophilic amino acids
– stick out in the watery
fluid in or out of cell
Glycocalyx
• Fuzzy, sticky carbohydrate rich substance
at the cell surface.
• aid in cell communication
• gives support to cell
Cell Junctions
• Most cells live in tight knit communities
• (but some are “Footloose”)
• 3 factors act to bind cells together
– Glycoproteins in glycocalyx
– Contours fit together in tongue-and-groove fashion
– Special cell junctions
Tight Junctions
• Series of proteins in cellular membrane
which fuse together
• Impermeable junctions – nothing can
pass between the cells
Desmosomes
• “Anchoring junction” – connections
between cells that prevent cells from
separating.
Gap Junctions
• Communicating junction between cells.
• Adjacent plasma membranes
are very close and the cells
are connected by hollow
cylinders
Membranes provide a variety of cell
functions
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
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
What molecules can get through directly?
Small non-polar molar molecules (O2 & 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?
Diffusion
Diffusion is the
passive movement
of particles form a
region of high
concentration to a
region of low
concentration
Molecules of dye
Membrane (cross section)
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
2.4.4
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
Osmosis
Osmosis is the passive
movement of water
molecules across a
permeable membrane from
lower solute to higher solute
concentration
Hypotonic = lower [solute]
Hypertonic = higher [solute]
Isotonic = equal [solute]
Hypo -> Hypertonic
2.4.4
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
Same concentration
of sugar
Selectively
permeable membrane: sugar molecules cannot pass
through pores, but
water molecules can
Water molecules
cluster around
sugar molecules
More free water
molecules (higher
concentration)
Fewer free water
molecules (lower
concentration)
Osmosis

Water moves from an area of higher
free water concentration to an area
of lower free water concentration
2.4.4
2.4.4
Animal cells
= CYTOLYSIS
= CRENATION
http://www.stchs.org/science/courses/sbioa/metenergy/bloodcells.gif
Simple diffusion is the tendency of molecules of a substance
to spread out evenly in an available space. Substances diffuse
down their gradient (high to low concentration).
Small, non-ionic, non-polar particles are able to pass through
the phospholipids is the membrane is permeable to them.
2.4.5
Facilitated diffusion
• Move from HIGH to LOW concentration with aid
of membrane transport proteins
– passive transport
– no energy needed
– facilitated = with help
Facilitated diffusion is the movement of molecules across a
membrane with the aid of channel proteins.
Channel proteins create a bridge for particles to cross between
the membrane. Their size and chemical properties makes
them specific to one molecule.
2.4.5
Carriers and Channels are specific
inside cell
H2O
aa
sugar
NH3
salt
outside cell
. . . BUT STILL MOVES FROM [HIGHER] to [LOWER]
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
___________
Active transport involves moving substances AGAINST
their concentration gradients (from low to high
concentration). This is done by protein pumps embedded
in the membrane.
In contrast to
passive transport,
active transport
requires energy
in the form of
ATP.
2.4.6
Active Transport with ATP
2.4.6
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
2.4.6
Vesicles can be to used to
transport materials within the
cytoplasm of the cell without
mixing their components.
They are small sacs of membrane
that can change shape and move
in and out of the cell
Made in the rough Endoplasmic
Reticulum by pinching off small
regions of membrane
ATP supplies energy
2.4.7
Their membranes are
made by the rough
Endoplasmic Reticulum,
which later merge with the
plasma membrane via
fusion.
1
Transmembrane
glycoproteins
Secretory
protein
Glycolipid
Golgi
apparatus
2
Vesicle
3
Plasma membrane:
Cytoplasmic face
4
Secreted
protein
Extracellular face
Transmembrane
glycoprotein
Membrane carbohydrates /
lipids that are synthesized
in the rER are modified in
the Golgi apparatus
Membrane glycolipid
2.4.7
Important Steps for Vesicles
1. Move material around in
cell
2. Protein is synthesized by
ribosomes on the rER
3. Protein is stored in the
cisternae of the rER
4. Vesicles bud off cisternae
and carry protein to the
golgi apparatus
2.4.7
Important Steps for Vesicles
5. Protein is processed in
golgi apparatus
6. Protein is released in
more vesicles and
moved to the plasma
membrane
7. Vesicle fuses with
membrane
8. Protein is released
2.4.7
Phospholipids in the plasma membrane are able to move
within the bilayer, which results in its fluidity.
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)
This fluidity allows proteins embedded in the cell surface to
‘float’ around and between plasma membranes
2.4.8
Membrane Fluidity
2.4.8
Bulk transport across the plasma membrane occurs by
exocytosis and endocytosis. In exocytosis, transport vesicles
migrate to the plasma membrane, fuse with it, and release
their contents
In endocytosis, the cell takes in macromolecules by forming
new vesicles from the plasma membrane
2.4.8
Bulk transport across the plasma membrane occurs by
exocytosis and endocytosis. In exocytosis, transport vesicles
migrate to the plasma membrane, fuse with it, and release
their contents
In endocytosis, the cell takes in macromolecules by forming
new vesicles from the plasma membrane
2.4.8
2.4.8
Example in cells:
WHITE BLOOD CELL ENGULFING BACTERIA
using Phagocytosis
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
http://fig.cox.miami.edu/~cmallery/255/255ion/fig14x26.jpg
Transport summary
Passive Transport
http://programs.northlandcollege.edu/biology/bio
logy1111/animations/passive1.swf
Active Transport
http://programs.northlandcollege.edu/biology/Bi
ology1111/animations/active1.swf
2.4.8