Download Cell Membranes: Chapt. 6 - University of New England

Cell Membranes
Chapt 5
www.cellsalive.com/
The Cell
Membrane
Cell Membrane:
At Very High
Magnification
& in color
Membrane Structure
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellMembranes.html
Cell Membrane
Every cell is encircled by a membrane and most cells
contain an extensive intracellular membrane system.
Membranes fence off the cell's interior from its
surroundings. Membranes let in water, certain ions and
substrates and they excrete waste substances. They act
to protect the cell.
Without a membrane the cell contents would diffuse
into the surroundings, information containing molecules
would be lost and many metabolic pathways would
cease to work:
The cell would die!
www.biologie.uni-hamburg.de/b-online/e22/22.htm
Cell Membranes:
• Surround all cells
• Fluid-like composition…like soap bubbles
• Composed of:
– Lipids in a bilayer
– Proteins embedded in lipid layer (called
transmembrane proteins)
– And, Proteins floating within the lipid sea
(called integral proteins)
– And Proteins associated outside the lipid
bilayer (peripheral).
Membrane Lipids
• Composed largely of phospholipids
• Phospholipids composed of….glycerol and
two fatty acids + PO4 group
• P-Lipids are polar molecules…
P-Lipids are represented like this
Text pg. 81
Membrane Lipids
form a Bilayer
Outside layer
Inside Layer
Quiz
• If Phospholipids are polar, which end seeks
out water and which avoids water?
Phospholipid Molecule
Model
phosphate (hydrophilic)
glycerol
fatty acids (hydrophobic)
Membrane Proteins
• Integral: embedded within bilayer
• Peripheral: reside outside hydrophobic
region of lipids
Text pg. 80
Membrane Proteins
Text pg 80
Integral membrane proteins
Peripheral membrane proteins
Integral
Membrane Models
Fluid Mosaic Model - lipids arranged in
bilayer with proteins embedded or
associated with the lipids.
Fluid Mosaic Membrane
Text pg 80
Evidence for the Fluid Mosaic
Model (Cell Fusion)
More Evidence for the Fluid Mosaic
Model
Membrane Functions
• allows for different conditions between
inside and outside of cell
• subdivides cell into compartments with
different internal conditions
• allows release of substances from cell via
vesicle fusion with outer membrane:
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/exocyt.gif
Membrane Permeability
• Biological membranes are physical
barriers..but which allow small uncharged
molecules to pass…
• And, lipid soluble molecules pass through
• Big molecules and charged ones do NOT
pass through
How to get other molecules
across membranes??
There are two ways that the molecules typically move through
the membrane:
passive transport and active transport
•Active transport requires that the cell use energy that it has
obtained from food to move the molecules (or larger particles)
through the cell membrane.
•Passive transport does not require such an energy
expenditure, and occurs spontaneously.
Membrane Transport Mechanisms
I. Passive Transport
• Diffusion- simple movement from regions
of high concentration to low concentration
• Osmosis- diffusion of water across a semipermeable membrane
• Facilitated diffusion- protein transporters
which assist in diffusion
Text pg 89
Membrane Transport Mechanisms
II. Active Transport
• Active transport- proteins which transport
against concentration gradient.
• Requires energy input
Text pg 89
Diffusion
Movement generated by random motion of
particles. Caused by internal thermal
energy.
Movement always from region of high free
energy(high concentration) to regions of
low free energy (low conc.)
Text pg 86
Osmosis
Movement of water across
a semi-permeable
barrier.
Example: Salt in water,
cell membrane is barrier.
Salt will NOT move
across membrane, water
will.
Text pg 87
Osmosis in Hypertonic medium
cell
Hypertonic solutions- shrink cells
Osmosis in Hypotonic medium
Hypotonic solutions- swell cells
Endocytosis
• Transports macromolecules and large
particles into the cell.
• Part of the membrane engulfs the particle
and folds inward to “bud off.”
– Fig. 5.16
Endocytosis
Putting Out the Garbage
• Vesicles (lysosomes, other secretory
vesicles) can fuse with the membrane and
open up the the outside…
Exocytosis
(Cellular Secretion)
Movies!
Membrane Permeability
1) lipid soluble solutes go through faster
1) smaller molecules go faster
1) uncharged & weakly charged go faster
2) Channels or pores may also exist in
membrane to allow transport
1
2
Cellular Membranes
REVIEW
•
•
•
•
•
•
Importance of Membranes
Membrane Structure
Proteins
Fluid Mosaic model
Permeability
Types of Transport
– Passive and Active
Types of Protein Transporters:
Ion Channels
• work by facilitated diffusion No E!
• deal with small molecules... ions
• open pores are “gated”- Can change shape.
– How?
– How much gets in?
• important in cell communication
Ion Channels
• Work fast: No conform. changes needed
• Not simple pores in membrane:
– specific to different ions (Na, K, Ca...)
– gates control opening
– Toxins, drugs may affect channels
• saxitoxin, tetrodotoxin
• cystic fibrosis
Toxins…how they work
Cystic Fibrosis
• Fatal genetic disorder
• Mucus build-up results in lung and liver
failure
• Patients die between 4 and 30 yrs.
• Single gene defect
• 1 in 25 Caucasians carry 1 bad gene copy
• 1 in 2500 kids has it in Canada
• Testing
CF Cont…
• ~Proteins for diffusion of salt into
the airways don't work.
•
~Less salt in the airways means
less water in the airways.
•
~ Less water in the airways means
mucus layer is very
sticky (viscous).
• ~Sticky mucus cannot be easily
moved to clear particles from the
lungs.
•
~Sticky mucus traps bacteria and
causes more lung infections.
http://www.the-aps.org/education/lot/cell/HotT.htm
Transport Proteins
Facilitated Diffusion
& Active Transport
• move solutes faster
across membrane
• highly specific to
specific solutes
• can be inhibited by
drugs
Types of Protein
Transporters
A. Facilitated Diffusion
Assist in diffusion process.
Solutes go from High conc
to Low conc.
Examples: Glucose transporters
Text pg 88
http://bio.winona.msus.edu/berg/ANIMTNS/FacDiff.htm
Facilitated Diffusion
The Glucose Transporters
• Transport of glucose into cells mediated by proteins in the
GLUT (GLUcose Transport) family of transporters. There
are 7 different, but related, proteins. But, only four
(GLUT1-4) are known to be involved in glucose transport.
• All GLUT proteins share a set of similar structural
features and are all about 500 amino acids in length (giving
them a predicted molecular weight of about 55,000
Daltons)
• Glucose uptake shows saturation and glucose uptake can
be inhibited by drugs
A classic Membrane Transport protein
Glucose Transporter
Characteristics:
•
•
•
•
integral protein: spans the membrane
12 alpha helices woven into membrane
55,000 mol. wt.
Text pg. 88
Glucose Transporter:
How it works..
• glucose binds to outside
of transporter (exterior
side with higher glucose
conc.)
• glucose binding causes a
conform. change in
protein
• glucose drops off inside
cell
• protein reassumes 1st
configuration
Types of Protein Transporters:
Active Transport
• carrier proteins
• go against the concentration gradients Low
to High
• require Energy to function (ATP, PEP, light
energy, electron transport)
Membrane Transport:
Active transport
• Movement from region of low free
energy(low concentration) to regions of
high free energy (high conc.)
• Requires energy input
Active Transport:
Sodium-Potassium Pump
Na+
high
Na+
low
K+
low
K+
high
Balance of the two ions goes hand-in-hand
ATP required for maintenance of the pump
The sodium/potassium pump
• All nerve and muscle cells have a high internal potassium ion
concentration and a low internal sodium ion concentration.
[Ki=166 mM; Ko=5 mM; Nai=18 mM; Nao=135 mM].
• Early on, it was thought that the nerve and muscle membranes
were relatively impermeable to these ions and that the difference
in ionic concentration was set up in early development of the
cells. The membrane then became impermeable.
• The later availability and use of radioactive Na and K ions
showed that this was not true and that there was a metabolic
pump that pumped Na out of the cell and K in; the ratio being 3
Na pumped out of the cell for every 2 K pumped into the cell.
Is a Protein Involved ?
• Experiments showed a dependency of both
Na and K ions for pump to work
• Pump was inhibited by ouabain (a drug)
• 1957: an ATPase enzyme was found to be
associated with Na/K pumping
• Studies showed this ATPase capable of
pumping Na/K ions
• Text pg 90
Sodium/Potassium
ATPase Protein
• Made of 2 large and 2 small subunits
• 2 large units span membrane
– inside region: contains ATP binding site
– inside: binding sites for Na
– outside: binding site for K
• How does it work??
Sodium-Potassium Pump
• Text pg. 90
http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/sppump.html
Na-K Pump Model: Part I
• 3 Na+ bind to inner region of protein
• Na+ binding triggers phosphorylation of
protein. ATP
ADP + Pi
• Phosphorylation causes conformation change
and Na+ binding site faces outside
• 3 Na+ released to outside
Na-K Active Pump: Part II
• 2 K+ ions on outside are able to bind
• K+ binding causes dephosphorylation and
new conformation change
• 2K+ ions exposed to inside and released
Cyclic process uses ATP energy to drive Na &
K ion transport against conc. Gradient
Cell Junctions
• Allow specific types of cells to stay together
to perform special jobs
• Layers of these types of cells…
– Line body cavities
– Cover body surfaces
3 Types of Cell Junctions
1.
Tight Junctions
2.
Desmosomes
3.
Gap Junctions
Tight Junctions
• Seal tissues and
prevent leaks
• Link epithelial cells
together
• Prevent things from
moving through the
intercellular space
• Restrict migration of
proteins and
phospholipids
Desmosomes
• Like spot welds!
• Dense plaques with
fibers attachedAnchor cells together
from one side to the
other.
• These cells withstand
lots of abuse!
Gap Junctions
• Cell to cell
communication.
• Protein channels(what type?)
“connexons”
• Plasmodesmata
Put Them All Together…
Membrane Structure
The cell is highly organized with many functional units or organelles inside. Most of
these units are limited by one or more membranes. To perform the functions of an
organelle, the membrane is specialized in that it contains specific proteins and lipid
components that enable it to perform its unique roles.
In essence membranes are essential for the integrity and function of the cell.
Membrane functions:
• be protective
• regulate transport in and out of cell or organelle
• allow selective receptivity and signal transduction by providing transmembrane
receptors that bind signaling molecules
• allow cell recognition
• provide anchoring sites for cytoskeletal components. This allows the cell to
maintain its shape and perhaps move to distant sites.
• provide a stable site for the binding and catalysis of enzymes.
• regulate the fusion of the membrane with other membranes in the cell via
specialized junctions
• provide a passageway across the membrane for certain molecules
• allow directed cell or organelle motility