Transcript Ch. 6 - Crestwood Local Schools

Chapter 6
Membrane Structure
and Function
Plasma Membrane
The
membrane at the
boundary of every cell.
Functions as a selective
barrier for the passage of
materials in and out of cells.
Membrane
Composition
phospholipids
Proteins
Question:
How are the materials
arranged?
Phosphophospholipid
Bilayer
Phospholipids
Hydrophilic
heads
Hydrophobic
tails
Membrane Models
Davson-Danielli Model
1935
phospholipid
bilayer.
Proteins coat the surfaces.
Sometimes called the
“sandwich” model.
Evidence
Biochemical
work.
TEM pictures show the
membrane as a double line.
Problems
Not
all membranes in a cell
were the same.
How could the proteins stay
in place?
Result - the model was
questioned and tested by
scientific process.
Fluid Mosaic Model
1972
New
model to fit the new
evidence with membranes.
Example of
“Science as a Process”.
Fluid Mosaic Model
Refers
to the way the
phospholipids and proteins
behave in a membrane.
“Fluid”
Refers
to the phospholipid
bilayer.
Molecules are not bonded
together, so are free to shift.
 Must remain "fluid" for
membranes to function.
Ways to keep the
membrane “fluid”
phospholipid
changes or shifts:
Cold hardening of plants
(shift to unsaturated fatty acids).
Hibernating animals
(Cholesterol increase).
“Mosaic”
Proteins:
float in a sea of
phospholipids.
Proteins form a collage or
mosaic pattern that shifts
over time.
Evidence
TEM
pictures of fractured
membranes.
Cell fusion studies.
Tagging of membrane
proteins by antibodies.
Protein Function in
Membranes
Transport.
Enzymatic
activity.
Receptor sites for signals.
Cell adhesion.
Cell-cell recognition.
Attachment to the cytoskeleton.
Types of Membrane
Proteins
Integral
- inserted into the
phospholipid bilayer.
Peripheral - not embedded in
the phospholipid bilayer, but
are attached to the membrane
surface.
Question?
How
do the integral proteins
stick to the membrane?
By the solubility of their
amino acids.
Hydrophilic
Amino Acids
Hydrophobic
Amino Acids
Hydrophilic
Amino Acids
Membranes are Bifacial
The
phospholipid
composition of the two layers
is different.
The proteins have specific
orientations.
Carbohydrates are found
only on the outer surface.
Carbohydrates
Membrane
Carbohydrates
Branched
oligosaccharides
form glycophospholipids and
glycoproteins on external
surface.
Function - recognition of
"self" vs "other”.
Question
How
do materials get across
a cell's membrane?
Problems
phospholipid
bilayer is
hydrophobic. Hydrophilic
materials don't cross easily.
Large molecules don't cross
easily. Too big to get through
the membrane.
Mechanisms
1. Passive Transport
2. Active Transport
Passive Transport
Movement
across
membranes that does NOT
require cellular energy.
Types of Passive
Transport
1. Diffusion
2. Osmosis
3. Facilitated Diffusion
Diffusion
The
net movement of atoms,
ions or molecules down a
concentration gradient.
Movement is from:
High
Low
Equilibrium
When
the concentration is
equal on both sides.
There is no net movement of
materials.
Factors that Effect
Diffusion
1.
2.
3.
4.
5.
Concentration
Temperature
Pressure
Particle size
Mixing
Osmosis
Diffusion
of water.
Water moving from an area if
its high concentration to an
area of its low concentration.
No cell energy is used.
Tonicity
The
concentration of water
relative to a cell.
1. Isotonic (same)
2. Hypotonic (below)
3. Hypertonic (above)
Isotonic
Isosmotic
solution.
Cell and water are equal in
solute concentration.
No net movement of water in
or out of the cell.
No change in cell size.
Hypotonic
Hypoosmotic
solution
Cell's water is lower than the
outside water (more solutes).
Water moves into the cell.
Cell swells, may burst or the
cell is turgid.
Hypertonic
Hyperosmotic
solution
Cell's water is higher than the
outside water (less solutes)
Water moves out of the cell.
Cell shrinks or plasmolysis
occurs.
Facilitated Diffusion
Transport
protein that helps
materials through the cell
membrane.
Doesn't require energy (ATP).
Works on a downhill
concentration gradient.
Aquaporins
Newly
found channels for
osmosis.
GFP labeled
Aquaporins
Active Transport
Movement
across
membranes that DOES
require cellular energy.
Types of Active
Transport
1. Carrier-Mediated
2. Endocytosis
3. Exocytosis
Carrier-Mediated
Transport
General
term for the active
transport of materials into
cells AGAINST the
concentration gradient.
Movement is: low
high
Examples
1. Na+- K+ pump
2. Electrogenic or H+ pumps
3. Cotransport
+
Na -
+
K
pump
Na+ ions out of cells
while moving K+ ions in.
Moves
Electrogenic or
H+ pumps
Also
called Proton pumps.
Create voltages across
membranes for other cell
processes.
Used by plants, fungi and
bacteria.
Cotransport
of H+ that allows
other materials to be
transported into the cell as
the H+ diffuses back across
the cell membrane.
Example - Sucrose transport
Movement
Exocytosis
Moves
bulk material out of
cells.
Example - secretion of
enzymes.
Endocytosis
Moves
bulk materials into
cells.
Several types known.
Types
1. Pinocytosis - liquids
2. Phagocytosis - solids
3. Receptor Mediated - uses
receptors to "catch" specific
kinds of molecules.
Carbohydrates
Forming vesicles
Summary
Know
membrane structure.
Be able to discuss the
various methods by which
cells move materials through
membranes.
Be able to solve problems in
osmosis.
AP Lab 1
Please
come in tomorrow
with your prelab done and
ready to go!

We will start lab at 7:30!!