Transcript Document

Membrane Function, Structure
&
Transport
Cell Membrane (Plasma
Membrane)
QuickTime™ and a
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Fluid Mosaic Model of the cell
membrane
http://www.susanahalpine.c
om/anim/Life/memb.htm
About Cell Membranes (continued)
• Cell membranes have
pores (holes) in it
a.Selectively
permeable: Allows
some molecules in
and keeps other
molecules out
b.The structure helps
it be selective
http://phschool.co
m/science/biology
_place/biocoach/b
iomembrane1/per
meability.html
Structure of the Cell Membrane
Outside of cell
Lipid
Bilayer
Inside of cell
(cytoplasm)
Membrane Function
• Six major functions of membrane proteins:
– Transport
– Enzymatic activity
– Signal transduction
– Cell-cell recognition
– Intercellular joining
– Attachment to the cytoskeleton and extracellular matrix
(ECM)
a. Transport
b. Enzymatic activity
c. Signal transduction
d. Cell-cell recognition
e. Intercellular joining
f. Attachment to the
cytoskeleton and extracellular matrix (ECM)
Transport Proteins
• Cell membranes are permeable to specific ions and polar
molecules, including water. These hydrophilic substances avoid
contact with the lipid bilayer by passing through transport
proteins that span the membrane
• Some transport proteins function by having a hydrophilic channel
that certain molecules or atomic ions use as a tunnel through the
membrane.
• Other transport proteins hold onto their passengers and physically
move them across the membrane.
• In both cases, the transport protein is specific for the substances it
translocates (moves), allowing only a certain substance or class of
closely related substances to cross the membrane.
• For example, glucose carried in blood to the human liver enters
liver cells rapidly through specific transport proteins in the plasma
membrane. The protein is so selective that it even rejects fructose,
a structural isomer of glucose.
• The selective permeability of a membrane depends on both the
discriminating barrier of the lipid bilayer and the specific transport
proteins built into the membrane.
Proteins
• Integral proteins penetrate the hydrophobic core of the lipid
bilayer.
• Many are transmembrane proteins, which completely span the
membrane. The hydrophobic regions of an integral protein
consist of one or more stretches of nonpolar amino acids usually
coiled into a helices
• The hydrophilic parts of the molecule are exposed to the
aqueous solutions on either side of the membrane.
• Peripheral proteins are not embedded in the lipid bilayer at all;
they are appendages loosely bound to the surface of the
membrane, often to the exposed parts of integral proteins
Membrane carbohydrates are
important for cell-cell recognition
•
•
Cell-cell recognition, a cell’s ability to distinguish one type of neighboring cell
from another, is crucial to the functioning of an organism. It is important, for
example, in the sorting of cells into tissues and organs in an animal embryo. It is
also the basis for the rejection of foreign cells (including those of transplanted
organs) by the immune system, an important line of defense in vertebrate
animals. The way cells recognize other cells is by keying on surface molecules,
often carbohydrates, on the plasma membrane.
Membrane carbohydrates are usually branched oligosaccharides with fewer
than 15 sugar units. (Oligo is Greek for "few"; an oligosaccharide is a short
polysaccharide.) Some of these oligosaccharides are covalently bonded to
lipids, forming molecules called glycolipids. (Recall that glyco refers to the
presence of carbohydrate.) Most, however, are covalently bonded to proteins,
which are thereby glycoproteins.
Membrane carbohydrates are important for
cell-cell recognition
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are needed to s ee this pic ture.
•
The oligosaccharides on the external side of the plasma membrane vary from
species to species, among individuals of the same species, and even from one
cell type to another in a single individual. The diversity of the molecules and their
location on the cellﾕs surface enable oligosaccharides to function as markers
that distinguish one cell from another. For example, the four human blood
groups designated A, B, AB, and O reflect variation in the oligosaccharides on
the surface of red blood cells.
Diffusion and Osmosis
Def: Diffusion - movement of solute from an area of higher
concentration to a area of lower concentration
http://highered.mcgrawhill.com/sites/0072495855/student_vie
w0/chapter2/animation__how_diffusio
n_works.html
http://www.indiana.edu/~phys215/lecture/l
ecnotes/lecgraphics/diffusion.gif
Diffusion Through Membranes
1. Freely Permeable
1M
glucose
Iain McKillop, Ph.D.
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0.1M
glucose
0.55M
glucose
0.55M
glucose
Biol 3111
Diffusion Through Membranes
2. Cation Permeable
+
1M KCl
0.1M KCl
K+ Cl-
K+ Cl-
K+
K+
1M Cl-
<1M
K+
0.1M Cl-
>0.1M
K+
Water and K+ CAN move; Cl- CANNOT
Iain McKillop, Ph.D.
[email protected]
Biol 3111
Osmosis
Def: Movement of water from a high concentration to a low
concentration solution
Requires a water permeable
barrier
1M
glucose
0.1M
glucose
H20
H20
Iain McKillop, Ph.D.
[email protected]
http://zoology.okstate.edu/
zoo_lrc/biol1114/tutorials/
Flash/Osmosis_Animation
.htm
<1M
glucose
>0.1M
glucose
“Osmotic Pressure”
Biol 3111
Common Terms
Def: TONICITY
Def: OSMOLALITY
Def: TURGIDITY
- Measure of the concentration of
solutes in a solution
- Measure of osmotic pressure
- Description of “swollen” state of cells
Def: HYPERTONIC
- Higher concentration of solute ( dissolved
substances) and less solvent
Def: HYPOTONIC - Higher concentration of solvent (dissolving
substances) and less solute
Def: ISOTONIC
- Same concentration of solute & solvent
Def: plasmolysis
-loss of water from a plant
Water Balance of Cells Without Walls
•
•
•
•
If an animal cell is immersed in an environment that is isotonic to the cell, there
will be no net movement of water across the plasma membrane.
Water flows across the membrane, but at the same rate in both directions. In an
isotonic environment, the volume of an animal cell is stable.
A hypertonic solution to the cell. The cell will lose water to its environment,
shrivel, and probably die. This is one reason why an increase in the salinity
(saltiness) of a lake can kill the animals there.
A cell in a solution that is hypotonic to the cell, water will enter faster than it
leaves, and the cell will swell and lyse (burst) like an overfilled water balloon.
QuickTime™ and a
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Osmosis in Red Blood Cells
•
Observe sheep RBCs via a wet
mount of the sample
•
Aliquot one drop the following
solutions with a ½ drop of RBC
to a slide
Isotonic
Crenation
 0.9% saline
 10% NaCl
 Distilled water
Hypertonic
•
Hypotonic
Water Balance of Cells Without Walls
Cont,,,
• A cell without rigid walls can tolerate neither excessive uptake
nor excessive loss of water.
• This problem of water balance is automatically solved if such a
cell lives in isotonic surroundings.
• Seawater is isotonic to many marine invertebrates.
• The cells of most terrestrial (land-dwelling) animals are bathed
in an extracellular fluid that is isotonic to the cells.
• Animals and other organisms without rigid cell walls living in
hypertonic or hypotonic environments must have special
adaptations for osmoregulation, the control of water balance.
Contractile Vacuole
• Animals and other organisms without rigid cell walls have
osmotic problems in either a hypertonic or hypotonic
environment
• To maintain their internal environment, such organisms
must have adaptations for osmoregulation, the control of
water balance
• The protist Paramecium, which is hypertonic to its pond
water environment, has a contractile vacuole that acts as a
pump
•
Video: Chlamydomonas
Video: Paramecium Vacuole
Water Balance of Cells with Walls
• The cells of plants, prokaryotes, fungi, and some protists have
walls. When such a cell is in a hypotonic solution the wall helps
maintain the cell’s water balance.
• However, the elastic wall will expand only so much before it
exerts a back pressure on the cell that opposes further water
uptake. At this point, the cell is turgid (very firm), which is the
healthy state for most plant cells.
• Plants that are not woody, such as most house plants, depend
for mechanical support on cells kept turgid by a surrounding
hypotonic solution. If a plant’s cells and their surroundings are
isotonic, there is no net tendency for water to enter, and the cells
become flaccid (limp), causing the plant to wilt.
Water Balance of Cells with Walls
• A wall is of no advantage if the cell is immersed in a
hypertonic environment. A plant cell, like an animal
cell, will lose water to its surroundings and shrink. As
the plant cell shrivels, its plasma membrane pulls
away from the wall, (plasmolysis), is usually lethal.
The walled cells of bacteria and fungi also
plasmolyze in hypertonic environments.
Video: Plasmolysis
Video: Turgid Elodea
Animation: Osmosis
Membrane Transport
1 Simple Diffusion
2 Facilitated Diffusion
3 Active Transport
Iain McKillop, Ph.D.
[email protected]
Biol 3111
Membrane Transport
Simple Diffusion
1. Small, nonpolar e.g. Oxygen, ethanol
2. Small, polar
e.g. H2O
Iain McKillop, Ph.D.
[email protected]
Biol 3111
Membrane Transport
Facilitated Diffusion
*Utilize passive carriers - e.g. channels
Ion channels
Water channels
*Specific binding site for molecules
*Two thermodynamically equivalent shapes
–one facing outside, one facing inside
*Conformation not affected by solute
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Biol 3111
Model of Facilitated Diffusion
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[email protected]
Biol 3111
Ion Channels
General Properties
Selective
Fast
Passive
Gated - i.e. Open or Closed
Iain McKillop, Ph.D.
[email protected]
Biol 3111
Facilitated Diffusion of Ions
• The transmembrane channels that permit facilitated diffusion
can be opened or closed. They are said to be "gated".
• Some types of gated ion channels:･ligand-gated･
• Mechanically-gated - examples:
•
•
•
•
Sound waves bending the cilia-like projections on the hair cells of the
inner ear open up ion channels leading to the creation of nerve
impulses that the brain interprets as sound
･Mechanical deformation of the cells of stretch receptors opens ion
channels leading to the creation of nerve impulses.
Voltage-gated- In so-called “ excitable cells” like neurons and muscles
cells, some channels open or close in response to changes in the
charge (measured in volts) across the plasma membrane.Example: As
an impulse passes down a neuron, the reduction in the voltage opens
sodium channels in the adjacent portion of the membrane. This allows
the influx of Na+ into the neuron and thus the continuation of the nerve
impulse. Some 7000 sodium ions pass through each channel during
the brief period (about 1 millisecond) that it remains open. This was
learned by use of the patch clamp technique.
Light-gated- These are triggered by the presence of light. Phototaxis in
plants is a result of this type of diffusion.
Types of Gated Ion Channels
Iain McKillop, Ph.D.
[email protected]
Biol 3111
Ligand-gated ion channels
•
•
•
•
•
•
•
Many ion channels open or close in response to binding a small signaling
molecule or "ligand".
Some ion channels are gated by extracellular ligands; some by intracellular
ligands. In both cases, the ligand is not the substance that is transported when
the channel opens.
External ligandsExternal ligands (shown here in green) bind to a site on the
extracellular side of the channel.
Examples:
Acetycholine (ACh).
The binding of the neurotransmetter acetylcholine at certain synapses opens
channels that admit Na+ and initiate a nerve impulse or muscle contraction.
･Gamma amino butyric acid (GABA). Binding of GABA at certain synapses designated GABAA - in the central nervous system admits Cl- ions into the cell
and inhibits the creation of a nerve impulse.
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Internal ligands
• Internal ligands bind to a site on the channel protein exposed to
the cytosol.
• Examples:
• "Second messengers", like cyclic AMP (cAMP) and cyclic
GMP (cGMP), regulate channels involved in the initiation of
impulses in neurons responding to odors and light respectively.
• ATP is needed to open the channel that allows chloride (Cl-)
and bicarbonate (HCO3-) ions out of the cell. This channel is
defective in patients with cystic fibrosis. Although the energy
liberated by the hydrolysis of ATP is needed to open the
channel, this is not an example of active transport; the ions
diffuse through the open channel following their concentration
gradient.
Ach Receptor
Ligand-gated
Iain McKillop, Ph.D.
[email protected]
Biol 3111
Auditory Hair Cells
Stress-gated
Iain McKillop, Ph.D.
[email protected]
Biol 3111
Channel Specificity
O
H
O
Iain McKillop, Ph.D.
[email protected]
H
-
+
K
O
H
H
O
Biol 3111
Channel Specificity
H
O
Iain McKillop, Ph.D.
[email protected]
H
-
O
+
K
O
H
H
O
Biol 3111
Channel Specificity
H
O
+
O
O K O
H
H
Iain McKillop, Ph.D.
[email protected]
H
-
Biol 3111
H
-
O
O
H
H
O
O
H
+
K
H
Iain McKillop, Ph.D.
[email protected]
-
O
H
O
H
H
Biol 3111
Channel Specificity
H
-
O
O
H
H
H
+
Na
O
Iain McKillop, Ph.D.
[email protected]
O
Biol 3111
Channel Specificity
H
-
O
O
H
H
H
+
Na
O
Iain McKillop, Ph.D.
[email protected]
O
Biol 3111
Channel Specificity
H
-
O
O
H
H
H
+
Na
O
Iain McKillop, Ph.D.
[email protected]
O
Biol 3111
Chemical Composition of Cells
Intra and extracellular compositions are different
Very important in removing waste and letting required molecules in
Unequal distribution of ions is critical to cell function
Ion
[Intracellular]
[Extracellular]
Na+
K+
5-15 mM
140 mM
145 mM
5 mM
H+
7x105 mM
4x105 mM
Mg2+
0.5 mM
1-2 mM
Ca2+
10-7 mM
1-2 mM
ClFixed Anions
5-15 mM
High
5-15 mM
Low
Iain McKillop, Ph.D.
[email protected]
Biol 3111
Active Transport
*Active transport moves molecules AGAINST a gradient
*Essential for cell function - Moves molecules IN and OUT
*Requires energy
Pumps
Coupled Transport
Iain McKillop, Ph.D.
[email protected]
Biol 3111
Active Transport
1. Pumps
Energy provided by LIGHT or ATP
http://www.wisc-online.com/objects/index_tj.asp?objID=AP11203
Active Transport
1. Pumps
Na+/K+ Exchange pump is VERY important
Acts as a pump AND an enzyme
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Iain McKillop, Ph.D.
[email protected]
Biol 3111
The sodium-potassium pump: a
specific case of active transport.
•
This transport system pumps ions against steep concentration
gradients. The pump oscillates between two conformational states in a
pumping cycle that translocates three Na+ ions out of the cell for every
two K+ ions pumped into the cell. ATP powers the changes in
conformation by phosphorylating the transport protein (that is, by
transferring a phosphate group to the protein).
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TIFF ( Unc ompres s ed) dec ompr es sor
are needed to s ee this pic tur e.
Active Transport
Protein Pumps -transport
proteins that require
energy to do work
• Example: Sodium /
Potassium Pumps
are important in
nerve responses.
Protein changes shape to
move molecules: this
requires energy!
Some ion pumps generate voltage
across membranes
• All cells have voltages across their plasma
membranes. Voltage is electrical potential
energy--a separation of opposite charges. The
cytoplasm of a cell is negative in charge
compared to the extracellular fluid because of
an unequal distribution of anions and cations
on opposite sides of the membrane. The
voltage across a membrane, called a
membrane potential, ranges from about -50 to
-200 millivolts. (The minus sign indicates that
the inside of the cell is negative compared to
the outside.)
Voltage cont..
• The membrane potential acts like a battery, an energy source
that affects the traffic of all charged substances across the
membrane.
• Because the inside of the cell is negative compared to the
outside, the membrane potential favors the passive transport
of cations into the cell and anions out of the cell.
• Two forces drive the diffusion of ions across a membrane: a
chemical force (the ion’s concentration gradient) and an
electrical force (the effect of the membrane potential on the
ion’s movement).
• This combination of forces acting on an ion is called the
electrochemical gradient.
• An ion does not simply diffuse down its concentration
gradient, but diffuses down its electrochemical gradient. For
example, the concentration of sodium ions (Na+) inside a
resting nerve cell is much lower than outside it. When the cell
is stimulated, gated channels that facilitate Na+ diffusion open.
Sodium ions then "fall" down their electrochemical gradient,
driven by the concentration gradient of Na+ and by the
attraction of cations to the negative side of the membrane.
Voltage Cont..
• Some membrane proteins that actively transport ions contribute
to the membrane potential.
• An example is the sodium-potassium pump.
• The pump does not translocate Na+ and K+ one for one, but
actually pumps three sodium ions out of the cell for every two
potassium ions it pumps into the cell. With each crank of the
pump, there is a net transfer of one positive charge from the
cytoplasm to the extracellular fluid, a process that stores energy
in the form of voltage.
• A transport protein that generates voltage across a membrane is
called an electrogenic pump. The sodium-potassium pump
seems to be the major electrogenic pump of animal cells. The
main electrogenic pump of plants, bacteria, and fungi is a
proton pump, which actively transports hydrogen ions (protons)
out of the cell. The pumping of H+ transfers positive charge from
the cytoplasm to the extracellular solution
• http://www.wiley.com/legacy/college/boyer/0470003790/a
nimations/membrane_transport/membrane_transport.htm
An electrogenic pump
• Proton pumps, the main electrogenic pumps of plants, fungi,
and bacteria, are membrane proteins that store energy by
generating voltage (charge separation) across membranes.
Using ATP for power, a proton pump translocates positive
charge in the form of hydrogen ions. The voltage and H+
gradient represent a dual energy source that can drive other
processes, such as the uptake of nutrients.
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Neuron Signaling and Structure
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• http://outreach.mcb.harvard.edu/animations/action
potential.swf
Exocytosis & Endocytosis
• Water and small solutes enter and leave the cell by passing
through the lipid bilayer of the plasma membrane or by being
pumped or carried across the membrane by transport proteins.
• Large molecules, such as proteins and polysaccharides,
generally cross the membrane by a different mechanism
involving vesicles.
• The cell secretes macromolecules by the fusion of vesicles with
the plasma membrane; this is exocytosis.
• A transport vesicle that has budded from the Golgi apparatus
moves along fibers of the cytoskeleton to the plasma
membrane. When the vesicle membrane and plasma
membrane come into contact, the lipid molecules of the two
bilayers rearrange themselves so that the two membranes
fuse. The contents of the vesicle then spill to the outside of the
cell
Exocytosis & Endocytosis cont..
•
•
•
•
•
•
Many secretory cells use exocytosis to export their products
In endocytosis, the cell takes in macromolecules and particulate matter
by forming new vesicles from the plasma membrane. The steps are
basically the reverse of exocytosis. A small area of the plasma membrane
sinks inward to form a pocket. As the pocket deepens, it pinches in,
forming a vesicle containing material that had been outside the cell.
There are three types of endocytosis: phagocytosis ("cellular eating"),
pinocytosis ("cellular drinking"), and receptor-mediated endocytosis. The
cell "gulps" droplets of extracellular fluid into tiny vesicles. Because any
and all solutes dissolved in the droplet are taken into the cell, pinocytosis
is unspecific in the substances it transports.
phagocytosis, a cell engulfs a particle by wrapping pseudopodia around
it and packaging it within a membrane-enclosed sac large enough to be
classified as a vacuole.
The particle is digested after the vacuole fuses with a lysosome containing
hydrolytic enzymes.
•
•
•
•
Exocytosis & Endocytosis
Receptor-mediated endocytosis is very specific. Embedded in the
membrane are proteins with specific receptor sites exposed to the
extracellular fluid. The extracellular substances that bind to the receptors
are called ligands, a general term for any molecule that binds specifically
to a receptor site of another molecule. The receptor proteins are usually
clustered in regions of the membrane called coated pits, which are lined on
their cytoplasmic side by a fuzzy layer of protein. These coat proteins
probably help deepen the pit and form the vesicle. After the ingested
material is liberated from the vesicle for metabolism, the receptors are
recycled to the plasma membrane.
Receptor-mediated endocytosis enables the cell to acquire bulk quantities
of specific substances, even though those substances may not be very
concentrated in the extracellular fluid.
For example, human cells use the process to take in cholesterol for use in
the synthesis of membranes and as a precursor for the synthesis of other
steroids. Cholesterol travels in the blood in particles called low-density
lipoproteins (LDLs), complexes of lipids and proteins. These particles bind
to LDL receptors on membranes and then enter the cells by endocytosis.
In humans with familial hypercholesterolemia, an inherited disease
characterized by a very high level of cholesterol in the blood, the LDL
receptor proteins are defective, and the LDL particles cannot enter cells.
Cholesterol accumulates in the blood, where it contributes to early
atherosclerosis (the buildup of fat deposits on blood vessel linings).
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http://www.wisconline.com/objects/index_tj.asp?objID=AP11203
Kidney’s and Osmosis and Diffusion
• http://kvhs.nbed.nb.ca/gallant/biology/nephron_str
ucture.html
• http://users.rcn.com/jkimball.ma.ultranet/BiologyP
ages/K/Kidney.html
• http://www.sumanasinc.com/webcontent/animatio
ns/content/kidney.html
• http://www.kscience.co.uk/animations/kidney.swf
Digestion and Osmosis and
Diffusion
• http://eatwellgetwell.wordpress.com/2006/05/
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter
26/animation__organs_of_digestion.html
• http://www.mhhe.com/biosci/genbio/animation_qu
izzes/animate_100.htm
Circulation and Diffusion
• http://www.nhlbi.nih.gov/health/dci/Disease
s/hlw/hlw_respsys.html
• http://www.airinfonow.org/html/lungattack/l
ungplay.htm